Title of Invention: ANTI-CANCER AGENTS Abstract: Described herein are compounds that may be selectively activated to produce active anti-cancer agents in tumor cells. Also disclosed are pharmaceutical compositions comprising the compounds, and methods of treating cancer using the compounds. Detailed Description: CROSS-REFERENCE TO RELATED APPLICATIONS This application is a divisional of U.S. patent application Ser. No. 13/539,471, filed Jul. 1, 2012, which application claims priority to U.S. Provisional Patent Application No. 61/503,957, filed on Jul. 1, 2011. The entire contents of each of these applications are hereby incorporated by reference. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH This invention was made with United States government support awarded by the National Cancer Institute, Grant No. 1R15CA152914-01. The United States Government has certain rights in this invention. SEQUENCE LISTING The instant application contains a Sequence Listing which has been submitted in text format via EFS-Web and is hereby incorporated by reference in its entirety. Said text copy, created on Oct. 30, 2012, is named ASFILED_SequenceListing-Text and is 2,270 bytes in size. BACKGROUND Many cancer cells exhibit a disturbed intracellular redox balance, making them distinctively different from their “healthy” counterparts. Among these differences, some cancer cells are hypoxic and have an increase in bioreductive processes, while others have high intracellular concentrations of reactive oxygen species due to oxidative stress. There is a continuing need for new anti-cancer agents, particularly those that have fewer toxic side-effects. SUMMARY In one aspect, the disclosure provides a compound of formula (I): wherein: each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from optionally substituted alkyl, alkoxy, amino, halo, and —CH2—N(Ra)3⊕; each R3 is independently selected from: each R4a and R4b is independently selected from halo and —OSO2Ra; each Y is independently a bond or —CH2—; each R5 is independently C1-C4 alkyl; n is 0, 1 or 2; p is 1 or 2; each Ra is independently selected from optionally substituted alkyl; wherein if the compound of formula (I) bears a positive charge, it further comprises at least one counterion Z⊖. In another aspect, the disclosure provides a compound of formula (Ia): wherein: R1 is —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; R3 is: wherein each R4a and R4b is independently selected from halo and —OSO2Ra; and each Ra is independently selected from optionally substituted alkyl. In another aspect, the disclosure provides a compound of formula (II): wherein: each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from optionally substituted alkyl, alkoxy, amino, halo and nitro; each R3 is independently selected from: each R4a and R4b is independently selected from halo and —OSO2Rb; each R5 is independently optionally substituted alkyl; m is 1 or 2; n is 0, 1 or 2; p is 2; and wherein the compound further comprises at least one counterion Z⊖. In another aspect, the disclosure provides a compound of formula (IIa): each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from hydrogen, optionally substituted alkyl, alkoxy, amino, halo and nitro; each R3 is independently selected from: each R4a and R4b is independently selected from halo and —OSO2Rb; each R5 is independently optionally substituted alkyl; m is 1 or 2; n is 0, 1 or 2; p is 2; and wherein the compound further comprises at least one counterion Z⊖. In another aspect, the disclosure provides a compound of formula (III): wherein: each R1 is independently selected from selected from the group consisting of: wherein at least one R1 is other than —CHO; each R2 is independently selected from an electron-withdrawing group; each X is independently halo; each R4a and R4b is independently selected from halo and —OSO2Rb; and each R5 is independently selected from optionally substituted alkyl; wherein if the compound of formula (III) bears a positive charge, it further comprises at least one counterion Z⊖. In another aspect, the disclosure provides a compound of formula (IIIa): wherein: each R1 is independently selected from selected from the group consisting of; each R2 is independently selected from an electron-withdrawing group; each X is independently halo; each R4 is independently selected from the group consisting of —H and —COO(alkyl), and each R5 is independently selected from optionally substituted alkyl; wherein if the compound of formula (III) bears a positive charge, it further comprises at least one counterion Z⊖. In another aspect, the disclosure provides a compound of formula (IV): wherein: R1 is selected from the group consisting of: R2 is selected from the group consisting of —H and a hydroxy protecting group; R3 is selected from the group consisting of —H and —P(N(CH(CH3)2)2)(OCH2CH2CN); and each R4 is independently an electron withdrawing group. In another aspect, the disclosure provides a compound of formula (Va), (Vb), (Vc), (Vd), (Ve) or (Vf): wherein: each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from optionally substituted alkyl, alkoxy, amino, halo, and —CH2—N(Ra)3; and n is 0, 1 or 2. In another aspect, the disclosure provides a pharmaceutical composition comprising a compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf) as described herein. In another aspect, the disclosure provides a method of treating cancer in a subject in need of treatment, comprising administering the subject a therapeutically effective amount of a compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf) as described herein. In another aspect, the disclosure provides a method of reducing the proliferation of a cancer cell, comprising contacting the cancer cell with an effective amount of a compound described herein, such as a compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf) as described herein. Other aspects and embodiments will become apparent in light of the following description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts a denaturing PAGE gel of DNA samples treated with compounds described herein, in the presence or absence of hydrogen peroxide. FIG. 2 depicts mean growth percentages of various human cancer cell lines in the presence of 10 μM of a compound described in Example 12: (A) Compound 6; (B) Compound 7; (C) Compound 8; (D) Compound 10; and (E) Compound 17a. FIG. 3 depicts a denaturing PAGE gel of DNA samples treated with compounds described herein, in the presence or absence of hydrogen peroxide. FIG. 4 depicts: (A) a graph of the percentage of viable cells from four cancer cell lines following treatment with compounds described herein; and (B) a graph of the percentage of viable cells from three healthy donor cell lines in the presence of three compounds described in Example 14. FIG. 5 depicts graphs of the percentage of viable cells from three healthy donor cell lines following treatment with compounds described herein for: (A) 24 hr; (B) 48 hr; and (C) 72 hr. FIG. 6 depicts a denaturing PAGE gel of DNA samples treated with compounds described herein, in the presence or absence of hydrogen peroxide. FIG. 7 depicts a denaturing PAGE gel of DNA samples incorporating compounds described herein, following irradiation (350 nm). FIG. 8 depicts a denaturing PAGE gel of DNA samples incorporating compounds described herein, following irradiation (350 nm). FIG. 9 depicts graphs of the percentage of interstrand crosslinks (ICLs) of DNA samples incorporating compounds described in Example 17, following irradiation (350 nm). Black bar—aerobic conditions (air); grey bar—anaerobic conditions (Ar). (A) DNA duplexes incorporating compound 53a; (B) DNA duplexes incorporating compound 53b. DETAILED DESCRIPTION Described herein are compounds that may be selectively activated in cancer cells to release anti-cancer agents. For example, some compounds may be activated by reactive oxygen species, such as hydrogen peroxide, which may be found in elevated levels in certain types of cancer cells. Other compounds may be activated under hypoxic conditions, which are also found in certain types of cancer cells. Such compounds may not be activated in healthy cells, which may reduce toxicity associated with many known anti-cancer agents. 1. Definitions The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the specification and the appended claims, the singular forms “a,” “and” and “the” include plural references unless the context clearly dictates otherwise. Section headings as used in this section and the entire disclosure herein are not intended to be limiting. For the recitation of numeric ranges herein, each intervening number with the same degree of precision is explicitly contemplated. For example, for the range 6-9, the numbers 7 and 8 are contemplated in addition to 6 and 9, and for the range 6.0-7.0, the numbers 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9 and 7.0 are explicitly contemplated. As used herein, the term “about” is used synonymously with the term “approximately.” Illustratively, the use of the term “about” indicates that values slightly outside the cited values, namely, plus or minus 10%. Such values are thus encompassed by the scope of the claims reciting the terms “about” and “approximately.” “Administration” or “administering,” as used herein, refers to providing, contacting, and/or delivery of a compound or compounds by any appropriate route to achieve the desired effect. Administration may include, but is not limited to, oral, sublingual, parenteral (e.g., intravenous, subcutaneous, intracutaneous, intramuscular, intraarticular, intraarterial, intrasynovial, intrasternal, intrathecal, intralesional or intracranial injection), transdermal, topical, buccal, rectal, vaginal, nasal, ophthalmic, via inhalation, and implants. “Contacting,” as used herein as in “contacting a cell,” refers to contacting a cell directly or indirectly in vitro, ex vivo, or in vivo (i.e. within a subject, such as a mammal, including humans, mice, rats, rabbits, cats, and dogs). Contacting a cell, which also includes “reacting” a cell, can occur as a result of administration to a subject. Contacting encompasses administration to a cell, tissue, mammal, subject, patient, or human. Further, contacting a cell includes adding an agent to a cell culture. Other suitable methods may include introducing or administering an agent to a cell, tissue, mammal, subject, or patient using appropriate procedures and routes of administration as defined herein. “Effective amount,” as used herein, refers to a dosage of the compounds or compositions effective for eliciting a desired effect. This term as used herein may also refer to an amount effective at bringing about a desired in vivo effect in an animal, mammal, or human, such as reducing proliferation of a cancer cell. “Electron withdrawing group,” as used herein, refers to a group covalently linked to a carbon atom that forms a polarized bond, similar to a good leaving group, and can include groups such as halogens, nitro, cyano, carbonyl, and boronic acid or ester groups. A “hydroxy protecting group,” as used herein, is well known in the art and includes those described in detail in Protecting Groups in Organic Synthesis, T. W. Greene and P. G. M. Wuts, 3rd edition, John Wiley & Sons, 1999, the entirety of which is incorporated herein by reference. Exemplary hydroxy protecting groups may include, but are not limited to, acetyl (Ac), benzyl (Bn), benzoyl (Bz), ethers (e.g., methoxymethyl ether (MOM) and β-methoxyethoxymethyl ether (MEM)), silyl groups (e.g., trimethylsilyl (TMS), triethylsilyl (TES), triisopropylsilyl (TIPS), tri-iso-propylsilyloxymethyl (TOM) and β-butyldimethylsilyl (TBDMS)), trityl groups (e.g., dimethoxytrityl (DMT) and methoxytrityl (MMT)), and the like. “Reducing proliferation of a cell,” as used herein, refers to reducing, inhibiting, or preventing the survival, growth, or differentiation of a cell, including killing a cell. A cell can be derived from any organism or tissue type and includes, for example, a cancer cell (e.g., neoplastic cells, tumor cells, and the like). As used herein, the term “subject” is intended to include human and non-human animals. Exemplary human subjects include a human patient having a disorder, e.g., cancer, or a normal subject. The term “non-human animals” includes all vertebrates, e.g., non-mammals (such as chickens, amphibians, reptiles) and mammals, such as non-human primates, domesticated and/or agriculturally useful animals (such as sheep, dogs, cats, cows, pigs, etc.), and rodents (such as mice, rats, hamsters, guinea pigs, etc.). As used herein, the term “treat” or “treating” a subject having a disorder refers to administering a regimen to the subject, e.g., the administration of a compound or a composition comprising a compound, such that at least one symptom of the disorder is cured, healed, alleviated, relieved, altered, remedied, ameliorated, or improved. Treating includes administering an amount effective to alleviate, relieve, alter, remedy, ameliorate, improve or affect the disorder or the symptoms of the disorder. The treatment may inhibit deterioration or worsening of a symptom of a disorder. Definitions of specific functional groups and chemical terms are described in more detail below. For purposes of this invention, the chemical elements are identified in accordance with the Periodic Table of the Elements, CAS version, Handbook of Chemistry and Physics, 75th Ed., inside cover, and specific functional groups are generally defined as described therein. Additionally, general principles of organic chemistry, as well as specific functional moieties and reactivity, are described in Organic Chemistry, Thomas Sorrell, University Science Books, Sausalito, 1999; Smith and March March's Advanced Organic Chemistry, 5th Edition, John Wiley & Sons, Inc., New York, 2001; Larock, Comprehensive Organic Transformations, VCH Publishers, Inc., New York, 1989; Carruthers, Some Modern Methods of Organic Synthesis, 3rd Edition, Cambridge University Press, Cambridge, 1987; the entire contents of each of which are incorporated herein by reference. The term “acyl” refers to an alkylcarbonyl, cycloalkylcarbonyl, heterocyclylcarbonyl, arylcarbonyl or heteroarylcarbonyl substituent, any of which may be further substituted (e.g., with one or more substituents). The term “alkyl” refers to a straight or branched hydrocarbon chain, containing the indicated number of carbon atoms. For example, C1-C12 alkyl indicates that the alkyl group may have from 1 to 12 (inclusive) carbon atoms. The term “alkylene” refers to a divalent alkyl, e.g., —CH2—, —CH2CH2—, —CH2CH2CH2— or —CH2CH(CH3)CH2—. An alkyl or alkylene may be optionally substituted. The term “alkenyl” refers to a straight or branched hydrocarbon chain having one or more double bonds. Examples of alkenyl groups include, but are not limited to, allyl, propenyl, 2-butenyl, 3-hexenyl and 3-octenyl groups. One of the double bond carbons may optionally be the point of attachment of the alkenyl substituent. The term “alkenylene” refers to a divalent alkenyl, e.g., —CH═CH—, —CH═CH2CH2— or —CH═C═CH—. An alkenyl or alkenylene may be optionally substituted. The term “alkynyl” refers to a straight or branched hydrocarbon chain having one or more triple bonds. Examples of alkynyl groups include, but are not limited to, ethynyl, propargyl, and 3-hexynyl. One of the triple bond carbons may optionally be the point of attachment of the alkynyl substituent. The term “alkynylene” refers to a divalent alkynyl, e.g., —C≡C— or —C≡C—CH2—. An alkynyl or alkynylene may be optionally substituted. The term “amino” refers to a group of the formula —NR1R2, wherein R1 and R2 are each independently selected from, for example, hydrogen, alkyl, cycloalkyl, heterocyclyl, aryl and heteroaryl, or R1 and R2, together with the nitrogen to which they are attached, may form a ring structure. Examples of amino groups include, but are not limited to, —NH2, alkylamino groups such as —NHCH3, —NHCH2CH3 and —NHCH(CH3)2, dialkylamino groups such as —N(CH3)2 and —N(CH2CH3)2, and arylamino groups such as —NHPh. Examples of cyclic amino groups include, but are not limited to, aziridinyl, azetidinyl, pyrrolidinyl, piperidino, piperazinyl, perhydrodiazepinyl, morpholino, and thiomorpholino. The groups R1 and R2 may be optionally substituted. The term “aryl” refers to an aromatic monocyclic, bicyclic, or tricyclic hydrocarbon ring system, wherein any ring atom capable of substitution can be substituted (e.g., with one or more substituents). Examples of aryl moieties include, but are not limited to, phenyl, naphthyl, and anthracenyl. The term “arylalkyl” refers to an alkyl moiety in which an alkyl hydrogen atom is replaced with an aryl group. Arylalkyl includes groups in which more than one hydrogen atom has been replaced with an aryl group. Examples of arylalkyl groups include benzyl, 2-phenylethyl, 3-phenylpropyl, 9-fluorenyl, benzhydryl, and trityl groups. The term “cycloalkyl” as used herein refers to nonaromatic, saturated or partially unsaturated cyclic, bicyclic, tricyclic or polycyclic hydrocarbon groups having 3 to 12 carbons. Any ring atom can be substituted (e.g., with one or more substituents). Cycloalkyl groups can contain fused rings. Fused rings are rings that share one or more common carbon atoms. Examples of cycloalkyl groups include, but are not limited to, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cyclohexenyl, cyclohexadienyl, methylcyclohexyl, adamantyl, norbornyl and norbornenyl. The term “halo” or “halogen” as used herein refers to any radical of fluorine, chlorine, bromine or iodine. The term “haloalkyl” as used herein refers to an alkyl in which one or more hydrogen atoms are replaced with a halogen, and includes alkyl moieties in which all hydrogens have been replaced with halogens (e.g., perfluoroalkyl such as CF3). Haloalkyl groups may contain an indicated number of carbon atoms. For example, C1-C4 haloalkyl indicates that the haloalkyl group may have 1, 2, 3 or 4 carbon atoms. The term “heteroaryl” as used herein refers to an aromatic 5-8 membered monocyclic, 8-12 membered bicyclic, or 11-14 membered tricyclic ring system having 1-3 heteroatoms if monocyclic, 1-6 heteroatoms if bicyclic, or 1-9 heteroatoms if tricyclic, said heteroatoms independently selected from O, N, S, P and Si (e.g., carbon atoms and 1-3, 1-6, or 1-9 heteroatoms independently selected from O, N, S, P and Si if monocyclic, bicyclic, or tricyclic, respectively). Any ring atom can be substituted (e.g., with one or more substituents). Heteroaryl groups can contain fused rings, which are rings that share one or more common atoms. Examples of heteroaryl groups include, but are not limited to, radicals of pyridine, pyrimidine, pyrazine, pyridazine, pyrrole, imidazole, pyrazole, oxazole, isoxazole, furan, thiazole, isothiazole, thiophene, quinoline, isoquinoline, quinoxaline, quinazoline, cinnoline, indole, isoindole, indolizine, indazole, benzimidazole, phthalazine, pteridine, carbazole, carboline, phenanthridine, acridine, phenanthroline, phenazine, naphthyridines and purines. The term “heterocyclyl” as used herein refers to a nonaromatic, saturated or partially unsaturated 3-10 membered monocyclic, 8-12 membered bicyclic, or 11-14 membered tricyclic ring system having 1-3 heteroatoms if monocyclic, 1-6 heteroatoms if bicyclic, or 1-9 heteroatoms if tricyclic, said heteroatoms selected from O, N, S, Si and P (e.g., carbon atoms and 1-3, 1-6, or 1-9 heteroatoms of O, N, S, Si and P if monocyclic, bicyclic, or tricyclic, respectively). Any ring atom can be substituted (e.g., with one or more substituents). Heterocyclyl groups can contain fused rings, which are rings that share one or more common atoms. Examples of heterocyclyl groups include, but are not limited to, radicals of tetrahydrofuran, tetrahydrothiophene, tetrahydropyran, piperidine, piperazine, morpholine, pyrroline, pyrimidine, pyrrolidine, indoline, tetrahydropyridine, dihydropyran, thianthrene, pyran, benzopyran, xanthene, phenoxathiin, phenothiazine, furazan, lactones, lactams such as azetidinones and pyrrolidinones, sultams, sultones, and the like. The term “hydroxy” refers to an —OH radical. The term “alkoxy” refers to an —O-alkyl radical. The term “aryloxy” refers to an —O-aryl radical. The term “oxo” refers to an oxygen atom, which forms a carbonyl when attached to carbon, an N-oxide when attached to nitrogen, and a sulfoxide or sulfone when attached to sulfur. The term “mercapto” or “thiol” refers to an —SH radical. The term “thioalkoxy” or “thioether” refers to an —S-alkyl radical. The term “thioaryloxy” refers to an —S-aryl radical. The term “substituents” refers to a group “substituted” on an alkyl, alkenyl, alkynyl, cycloalkyl, cycloalkylalkyl, heterocyclyl, heterocyclylalkyl, aryl, arylalkyl, heteroaryl or heteroarylalkyl group at any atom of that group. Any atom can be substituted. Suitable substituents include, without limitation: acyl, acylamido, acyloxy, alkoxy, alkyl, alkenyl, alkynyl, amido, amino, carboxy, cyano, ester, halo, hydroxy, imino, nitro, oxo (e.g., C═O), phosphonate, sulfinyl, sulfonyl, sulfonate, sulfonamino, sulfonamido, thioamido, thiol, thioxo (e.g., C═S), and ureido. In embodiments, substituents on a group are independently any one single, or any combination of the aforementioned substituents. In embodiments, a substituent may itself be substituted with any one of the above substituents. The above substituents may be abbreviated herein, for example, the abbreviations Me, Et, Ph, Ac and Ts represent methyl, ethyl, phenyl, acetyl and tosyl (p-toluenesulfonyl), respectively. A more comprehensive list of the abbreviations used by organic chemists of ordinary skill in the art appears in the first issue of each volume of the Journal of Organic Chemistry; this list is typically presented in a table entitled Standard List of Abbreviations. The abbreviations contained in said list, and all abbreviations used by organic chemists of ordinary skill in the art, are hereby incorporated by reference. For compounds described herein, groups and substituents thereof may be selected in accordance with permitted valence of the atoms and the substituents, such that the selections and substitutions result in a stable compound, e.g., which does not spontaneously undergo transformation such as by rearrangement, cyclization, elimination, etc. Where substituent groups are specified by their conventional chemical formulae, written from left to right, they optionally encompass substituents resulting from writing the structure from right to left, e.g., —CH2O— optionally also recites —OCH2—. In accordance with a convention used in the art, the group: is used in structural formulas herein to depict the bond that is the point of attachment of the moiety or substituent to the core or backbone structure. 2. Compounds Compounds may have the following formula (I): wherein: each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from optionally substituted alkyl, alkoxy, amino, halo, and —CH2—N(Ra)3⊕; each R3 is independently selected from: each R4a and R4b is independently selected from halo and —OSO2Ra; each Y is independently a bond or —CH2—; each R5 is independently C1-C4 alkyl; n is 0, 1 or 2; p is 1 or 2; each Ra is independently selected from optionally substituted alkyl; wherein if the compound of formula (I) bears a positive charge, it further comprises at least one counterion Z⊕. In embodiments, compounds of formula (I) may have the following structures: In embodiments, R1 may be selected from the group consisting of: For example, in embodiments R1 is selected from the group consisting of: In some embodiments, n is 0. In some embodiments, n is 1. In some embodiments, R2 is methyl. In some embodiments, R3 is selected from the group consisting of: wherein each R4a and R4b is independently selected from chloro, bromo, and —OSO2CH3. For example, compounds of formula (I) may be selected from the group consisting of: Compounds may have the following formula (Ia): wherein: R1 is —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; R3 is: wherein each R4a and R4b is independently selected from halo and —OSO2Ra; and each Ra is independently selected from optionally substituted alkyl. In embodiments, R1 is selected from the group consisting of: For example, in embodiments R1 is selected from the group consisting of: In embodiments each R4a and R4b is independently selected from chloro, bromo, and —OSO2CH3. Compounds may have the following formula (II): wherein: each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from optionally substituted alkyl, alkoxy, amino, halo and nitro; each R3 is independently selected from: each R4a and R4b is independently selected from halo and —OSO2Rb; each R5 is independently optionally substituted alkyl; m is 1 or 2; n is 0, 1 or 2; p is 2; and wherein the compound further comprises at least one counterion Z⊖. In embodiments, compounds of formula (II) may have the following structures: In embodiments, R1 is selected from the group consisting of: For example, in embodiments R1 is: In embodiments, each R2 is independently selected from the group consisting of alkoxy (e.g., methoxy, ethoxy, propoxy), halo (e.g., fluoro), nitro, —NH2, alkylamino (e.g., —NHCH3, —NHCH2CH3) and dialkylamino (e.g., dimethylamino, diethylamino). In embodiments, R3 is selected from the group consisting of: wherein each R4a and R4b is independently selected from chloro, bromo, and —OSO2CH3. Compounds may have the following formula (IIa): each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from hydrogen, optionally substituted alkyl, alkoxy, amino, halo and nitro; each R3 is independently selected from: each R4a and R4b is independently selected from halo and —OSO2Rb; each R5 is independently optionally substituted alkyl; m is 1 or 2; n is 0, 1 or 2; p is 2; and wherein the compound further comprises at least one counterion Z⊕. In embodiments, each R1 is selected from the group consisting of: For example, in embodiments each R1 is: In embodiments, each R2 is independently selected from the group consisting of alkoxy (e.g., methoxy, ethoxy, propoxy), halo (e.g., fluoro), nitro, —NH2, alkylamino (e.g., —NHCH3, —NHCH2CH3) and dialkylamino (e.g., dimethylamino, diethylamino). In embodiments, each R3 is independently selected from the group consisting of: wherein each R4a and R4b is independently selected from chloro, bromo, and —OSO2CH3. Compounds may have the following formula (III): wherein: each R1 is independently selected from selected from the group consisting of: wherein at least one R1 is other than —CHO; each R2 is independently selected from an electron-withdrawing group; each X is independently halo; each R4a and R4b is independently selected from halo and —OSO2Rb; and each R5 is independently selected from optionally substituted alkyl; wherein if the compound of formula (III) bears a positive charge, it further comprises at least one counterion Z⊖. In embodiments, each R1 is —CH2X. In embodiments, each X is bromo. In embodiments, each R1 is —CH2—N(Ra)3⊕. In embodiments, each Ra is methyl. In embodiments, one R1 is —CHO and the other is —CH2—N(Ra)3⊕. In embodiments, each R1 is: wherein each R4a and R4b is independently selected from chloro, bromo, and —OSO2CH3. In embodiments, each R2 is independently selected from nitro, cyano and carboxy. In embodiments, the compound is selected from the group consisting of: Compounds may have the following formula (IIIa): wherein: each R1 is independently selected from selected from the group consisting of: each R2 is independently selected from an electron-withdrawing group; each X is independently halo; each R4 is independently selected from the group consisting of —H and —COO(alkyl), and each R5 is independently selected from optionally substituted alkyl; wherein if the compound of formula (III) bears a positive charge, it further comprises at least one counterion Z⊖. In embodiments, each R1 is —CH2X. In embodiments, each X is chloro. In embodiments, each R1 is —CH(X)—R4. In embodiments, each X is chloro. In embodiments, each R4 is —C(O)OC(CH3)3. In embodiments, each R1 is —CH2—N(Ra)3⊕. In embodiments, each Ra is methyl. In embodiments, each R2 is independently selected from the group consisting of nitro, cyano and carboxy. In embodiments, each R2 is nitro. In embodiments, the compound is selected from the group consisting of: Compounds may have the following formula (IV): wherein: R1 is selected from the group consisting of: R2 is selected from the group consisting of —H and a hydroxy protecting group; R3 is selected from the group consisting of —H and —P(N(CH(CH3)2)2)(OCH2CH2CN); and each R4 is independently an electron withdrawing group. In embodiments, R2 is —H. In embodiments, R2 is a hydroxy protecting group. In embodiments, R2 is a 4,4′-dimethoxytrityl group. In embodiments, R3 is —H. In embodiments, R3 is —P(N(CH(CH3)2)2)(OCH2CH2CN). In embodiments, each R4 is independently selected from the group consisting of nitro, cyano and carboxy. In embodiments, each R4 is nitro. In embodiments, the compound is selected from the group consisting of: Compounds may have the one of the following formulae (Va)-(Vf): wherein: each R1 is independently —B(XR′)2, wherein each X is independently selected from O and S, and each R′ is independently selected from hydrogen and alkyl, or two R′ are taken together to form an optionally substituted 5- to 8-membered ring; each R2 is independently selected from optionally substituted alkyl, alkoxy, amino, halo, and —CH2—N(Ra)3⊕; and n is 0, 1 or 2. In embodiments, R1 is selected from the group consisting of: For example, in embodiments R1 is: In some embodiments, the anti-cancer agent is an anti-cancer agent described herein. In some embodiments, the anti-cancer agent is linked to the remainder of the compound of formula (V) via a functional group such as a hydroxyl group, an amino group or a carboxy group. In some embodiments, the anti-cancer agent is modified with a functional group such that it can be linked to the remainder of the compound of formula (V). It will be understood by the skilled artisan that compound having a boronic acid group (i.e. —B(OH)2) may be in equilibrium with the corresponding boronic anhydride (also known as a boroxine). Accordingly, when compounds of the formulae herein have boronic acid groups, it is understood that the formulae are also intended to cover the boroxine forms. Compounds of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) and (Vf) include compounds that differ only in the presence of one or more isotopically enriched atoms. For example, compounds may have the present structures except for the replacement of hydrogen by deuterium or tritium, or the replacement of a carbon by a 13C- or 14C-enriched carbon. A compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf) can be in the form of a salt, e.g., a pharmaceutically acceptable salt. The term “pharmaceutically acceptable salt” includes salts of the active compounds that are prepared with relatively nontoxic acids or bases, depending on the particular substituents found on the compounds. Suitable pharmaceutically acceptable salts of the compounds of this invention include acid addition salts which may, for example, be formed by mixing a solution of the compound according to the invention with a solution of a pharmaceutically acceptable acid such as hydrochloric acid, sulfuric acid, methanesulfonic acid, fumaric acid, maleic acid, succinic acid, acetic acid, benzoic acid, oxalic acid, citric acid, tartaric acid, carbonic acid or phosphoric acid. Furthermore, where the compounds of the invention carry an acidic moiety, suitable pharmaceutically acceptable salts thereof may include alkali metal salts, e.g. sodium or potassium salts, alkaline earth metal salts, e.g. calcium or magnesium salts; and salts formed with suitable organic ligands, e.g. quaternary ammonium salts. Neutral forms of the compounds may be regenerated by contacting the salt with a base or acid and isolating the parent compound in a conventional manner. The parent form of the compound differs from the various salt forms in certain physical properties, such as solubility in polar solvents, but otherwise the salts are equivalent to the parent form of the compound for the purposes of this disclosure. Compounds of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) and (Vf) can be, for example, an enantiomerically enriched isomer of a stereoisomer described herein. Enantiomer, as used herein, refers to either of a pair of chemical compounds whose molecular structures have a mirror-image relationship to each other. For example, a compound may have an enantiomeric excess of at least about 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%. In some embodiments, a preparation of a compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf) may be enriched for isomers (subject isomers) which are diastereomers of the compound. Diastereomer, as used herein, refers to a stereoisomer of a compound having two or more chiral centers that is not a mirror image of another stereoisomer of the same compound. For example, the compound may have a purity corresponding to a compound having a selected diastereomer of at least about 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, or 99%. When no specific indication is made of the configuration at a given stereocenter in a compound, any one of the configurations or a mixture of configurations is intended. Compounds may be prepared in racemic form or as individual enantiomers or diastereomers by either stereospecific synthesis or by resolution. The compounds may, for example, be resolved into their component enantiomers or diastereomers by standard techniques, such as the formation of stereoisomeric pairs by salt formation with an optically active base, followed by fractional crystallization and regeneration of the free acid. The compounds may also be resolved by formation of stereoisomeric esters or amides, followed by chromatographic separation and removal of the chiral auxiliary. Alternatively, the compounds may be resolved using a chiral HPLC column. The enantiomers also may be obtained from kinetic resolution of the racemate of corresponding esters using lipase enzymes. A compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf) can also be modified by appending appropriate functionalities to enhance selective biological properties. Such modifications are known in the art and include those that increase biological penetration into a given biological system (e.g., blood, lymphatic system, central nervous system), increase oral availability, increase solubility to allow administration by injection, alter metabolism, and/or alter rate of excretion. Examples of these modifications include, but are not limited to, esterification with polyethylene glycols, derivatization with pivolates or fatty acid substituents, conversion to carbamates, hydroxylation of aromatic rings, and heteroatom substitution in aromatic rings. Preparation of Compounds Compounds described herein may be prepared according to a variety of methods. Representative syntheses of exemplary compounds of formula (I) are illustrated in Schemes 1-9. Compounds of general formula Ic (R═OMe) have previously been synthesized (Zysman-Colman, Eli et al. Canadian Journal of Chemistry, 2009, 87, 440-447;). Compound I can be converted into 2 as illustrated in Scheme 1 (Lin, Song-Wen et al. Bioorganic & Medicinal Chemistry Letters, 2011, 21, 940-943). The boronation of 3 can be catalyzed by PdCl2(dppf) using KOAc as base to form compound 4 (Wang, Lianhui et al. Advanced Synthesis & Catalysis, 2010, 352, 2002-2010). 4 can be converted into 5 by treatment with methanesulfonyl chloride (Ferlin, M. G. et al. Bioorganic Medicinal Chemistry, 2004, 12, 771-777). Compounds 6-9 can be obtained by treating 5 with lithium bromide or lithium chloride respectively and 6 can converted to 10. Compounds 3 can be converted to compounds 11-16 by the same procedures as 6-10. Boron acid 17 can be obtained by treatment of 11-16 with butyllithium followed by boron ester (White, James R. et al. Tetrahedron Letters, 2010, 51, 3913-3917). 6-10 can also be synthesized by the reaction of 17 and pinacol (White, James R. et al. Tetrahedron Letters, 2010, 51, 3913-3917). Bromination of 11-16 can produce 18 by reacting with NBS using AIBN as the catalyst. Boronic acid 19 and boronic ester 20 can be obtained with the procedures as illustrated in Scheme 3, Compound 21 can be converted into 23 as illustrated in Scheme 4. 23 can be converted into 24 by treatment with methanesulfonyl chloride followed by lithium bromide or lithium chloride respectively. 24 can be converted to 25 by treated with bis(2-chloroethyl)amine. 26 can be obtained by the boronation of 25 using butyllithium as base (Kevin R. Flower et al. Dalton Trans. 2011, 40, 11696-11697). Compound 27 can be converted into 28 as illustrated in Scheme 5. 28 can be converted to 29 by treatment with methanesulfonyl chloride followed by lithium bromide or lithium chloride respectively. 30 can be converted to 31 by treated with bis(2-chloroethyl)amine. 32 can be obtained by the boronation of 31 using butyllithium as a base. 32 can be converted to boronic ester 33 and salt 34 can be obtained by treating 33 with iodomethane. Compound 35 can be obtained as illustrated in Scheme 6. Compound 36 can be converted into 37 as illustrated in Scheme 7. SOCl2 treatment of 37 can produce boronic acid 38. Compound 40 can be obtained by the reaction of 36 and nitrogen mustards 39 which can be easily synthesized from 2,2′-(methylazanediyl)diethanol. Compounds 41 and 42 can be obtained as illustrated in Scheme 8. Compound 43 can be obtained as illustrated in Scheme 9 (Ferlin, M. G. et al. Bioorganic & Medicinal Chemistry, 2004, 12, 771-777). 43 can be converted to 44 when treated with formaldehyde and NaBH4 in the presence of H2SO4. The reaction of 44 and 36 can produce compound 45. Compounds of formula (II) can be synthesized as illustrated in Scheme 10, as generally described in Peng et al. Chem. Eur. J. 2012, 18, 3250. Compounds of formula (III) can be prepared as illustrated in Scheme 11, wherein reagents and conditions are as follows: i) PdCl2(dppf), K2CO3, bis(pinacolato)diboron, DMSO, 85° C., 3 h, 90%; ii) a) NBS, AIBN, CH3CN, reflux overnight; b) Me2NH, r. t. overnight, 3% for 3a, 55% for 3b and 15% for 3c; iii) MeI, CH2Cl2, r. t. 24 h, 85% for 4b and 70% for 4c; iv) NBS, AIBN, CH3CN, reflux 8 h, 65%; v) N-methyldiethanolamine, CH3CN, reflux 20 h, 85%; vi) SOCl2, r. t., 3 days, 74%; vii) KOAc, DMF, 70° C., 6 h, 88%; viii) 10% KOH, reflux 24 h, 50%. Compounds of formula (IIIa) can be prepared as illustrated in Scheme 12, wherein reagents and conditions are as follows: ix) BrCH2CH2Br, t-BuOK, DMF, r. t. 3 days, 45%; x) Cl2CHCOOtBu, t-BuOK, DMF, −25° C., 1 h, 30%; xi) HOAc, reflux, 4 h, 70%; xii) Me3N, EtOH, 40° C., 24 h, 60%. Compounds of formula (IV) such as compounds 55a,b can be prepared as illustrated in Scheme 13. Treatment of 50 with NBS can produce 51. After deprotection, 53 can be selectively protected by treatment with DMTC1 in the presence of pyridine to yield 54, which can then be converted to 55. Other methods of synthesizing the compounds of the formulae herein will be evident to those of ordinary skill in the art. Synthetic chemistry transformations and protecting group methodologies (protection and deprotection) useful in synthesizing the compounds are known in the art and include, for example, those such as described in R. Larock, Comprehensive Organic Transformations, VCH Publishers (1989); T. W. Greene and P. G. M. Wuts, Protective Groups in Organic Synthesis, 2d. Ed., John Wiley and Sons (1991); L. Fieser and M. Fieser, Fieser and Fieser's Reagents for Organic Synthesis, John Wiley and Sons (1994); and L. Paquette, ed., Encyclopedia of Reagents for Organic Synthesis, John Wiley and Sons (1995), and subsequent editions thereof. Evaluation of Compounds Compounds may be evaluated using standard methods such as NMR spectroscopy and mass spectrometry. To test the activities of compounds, standard assays may be used. For example, compounds of formulae (I), (Ia), (II), (IIa), (III), (IIIa) and (IV), and compounds of formulae (Va), (Vb), (Vc), (Vd), (Ve) and (Vf) wherein the anti-cancer agent is an alkylating agent, may be evaluated to determine abilities to promote DNA interstrand crosslinks (ICLs). Such assays involve the use of a sample oligonucleotide duplex, such as a 32P-labeled compound, and incubating it in the presence of a compound of interest in a suitable buffer. Denaturing polyacrylamide gel electrophoresis (PAGE) will separate cross-linked DNA from single-stranded DNA, and the gel can be visualized using appropriate techniques. Compounds of formulae (I), (Ia), (II) and (IIa), which may be activated in the presence of hydrogen peroxide, can be evaluated in the presence or absence of hydrogen peroxide to evaluate selectivity. Compounds of formulae (III), (IIIa) and (V), which may be activated under hypoxic conditions and under radiation (e.g., by UV light of about 350 nm), can be evaluated in the presence or absence of such conditions to evaluate selectivity. Compounds can also be evaluated for their activities in cancer cell lines. For example, compounds can be evaluated against a panel of 60 cell lines by the Developmental Therapeutics Program of the National Cancer Institute. Compound activities may further be compared to those in healthy cell lines. Compounds can also be evaluated in animal models. For example, compounds can be tested using mouse xenografts as generally described in Cheng et al. Cancer Research 2012, 72(10): 2634-44. Pharmaceutical Compositions While a compound described herein, such as a compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf), may be administered alone in the methods described herein, it may also be presented as one or more pharmaceutical compositions (e.g., formulations). A compound described herein may be formulated with one or more pharmaceutically acceptable carriers, adjuvants, excipients, diluents, fillers, buffers, stabilizers, preservatives, lubricants, or other materials well known to those skilled in the art and optionally other therapeutic or prophylactic agents. Accordingly, the disclosure may provide a pharmaceutical composition comprising at least one compound of formula (I), (Ia), (II), (IIa), (III), (IIIa), (IV), (Va), (Vb), (Vc), (Vd), (Ve) or (Vf). The methods described herein include administration of one or more pharmaceutical compositions, as discussed herein, in which a compound described herein is admixed together with one or more pharmaceutically acceptable carriers, excipients, buffers, adjuvants, stabilizers, or other materials, as described herein. Suitable carriers, excipients, etc. can be found in standard pharmaceutical texts, for example, Remington's Pharmaceutical Sciences, 18th edition, Mack Publishing Company, Easton, Pa., 1990. The pharmaceutical compositions may conveniently be presented in unit dosage form and may be prepared by any methods known in the art of pharmacy. Such methods include the step of bringing into association the active compound(s) with the carrier which constitutes one or more accessory ingredients. In general, the pharmaceutical compositions are prepared by uniformly and intimately bringing into association the active compound with liquid carriers or finely divided solid carriers or both, and then if necessary shaping the product. Pharmaceutical compositions may be in the form of liquids, solutions, suspensions, emulsions, elixirs, syrups, tablets, lozenges, granules, powders, capsules, cachets, pills, ampoules, suppositories, pessaries, ointments, gels, pastes, creams, sprays, mists, foams, lotions, oils, boluses, electuaries, or aerosols. Pharmaceutical compositions suitable for oral administration (e.g. by ingestion) may be presented as discrete units such as capsules, cachets or tablets, each containing a predetermined amount of the active compound; as a powder or granules; as a solution or suspension in an aqueous or non-aqueous liquid; or as an oil-in-water liquid emulsion or a water-in-oil liquid emulsion; as a bolus; as an electuary; or as a paste. A tablet may be made by conventional means, e.g., compression or molding, optionally with one or more accessory ingredients. Compressed tablets may be prepared by compressing in a suitable machine the active compound in a free-flowing form such as a powder or granules, optionally mixed with one or more binders (e.g. povidone, gelatin, acacia, sorbitol, tragacanth, hydroxypropylmethyl cellulose); fillers or diluents (e.g. lactose, microcrystalline cellulose, calcium hydrogen phosphate); lubricants (e.g. magnesium stearate, talc, silica); disintegrants (e.g. sodium starch glycolate, cross-linked povidone, cross-linked sodium carboxymethyl cellulose); surface-active or dispersing or wetting agents (e.g. sodium lauryl sulfate); and preservatives (e.g. methyl p-hydroxybenzoate, propyl p-hydroxybenzoate, sorbic acid). Molded tablets may be made by molding in a suitable machine a mixture of the powdered compound moistened with an inert liquid diluent. The tablets may optionally be coated or scored and may be formulated so as to provide slow or controlled release of the active compound therein using, for example, hydroxypropylmethyl cellulose in varying proportions to provide the desired release profile. Tablets may optionally be provided with an enteric coating, to provide release in parts of the gut other than the stomach. Pharmaceutical compositions suitable for parenteral administration (e.g. by injection, including cutaneous, subcutaneous, intramuscular, intravenous and intradermal), include aqueous and nonaqueous isotonic, pyrogen-free, sterile injection solutions which may contain anti-oxidants, buffers, preservatives, stabilizers, bacteriostats, and solutes which render the pharmaceutical composition isotonic with the blood of the intended recipient; and aqueous and non-aqueous sterile suspensions which may include suspending agents and thickening agents, and liposomes or other microparticulate systems which are designed to target the compound to blood components or one or more organs. Examples of suitable isotonic vehicles for use in such pharmaceutical compositions include Sodium Chloride Injection, Ringer's Solution, or Lactated Ringer's Injection. The pharmaceutical compositions may be presented in unit-dose or multi-dose sealed containers, for example, ampoules and vials, and may be stored in a freeze-dried (lyophilized) condition requiring only the addition of the sterile liquid carrier, for example water for injections, immediately prior to use. Extemporaneous injection solutions and suspensions may be prepared from sterile powders, granules, and tablets. Pharmaceutical compositions may be in the form of liposomes or other microparticulate systems which are designed to target the active compound to blood components or one or more organs. Pharmaceutical compositions suitable for topical administration (e.g. transdermal, intranasal, ocular, buccal, and sublingual) may be formulated as an ointment, cream, suspension, lotion, powder, solution, past, gel, spray, aerosol, or oil. Alternatively, a pharmaceutical composition may comprise a patch or a dressing such as a bandage or adhesive plaster impregnated with active compounds and optionally one or more excipients or diluents. Pharmaceutical compositions suitable for topical administration in the mouth include lozenges comprising the active compound in a flavored basis, usually sucrose and acacia or tragacanth; pastilles comprising the active compound in an inert basis such as gelatin and glycerin, or sucrose and acacia; and mouthwashes comprising the active compound in a suitable liquid carrier. Pharmaceutical compositions suitable for topical administration to the eye also include eye drops wherein the active compound is dissolved or suspended in a suitable carrier, especially an aqueous solvent for the active compound. Pharmaceutical compositions suitable for nasal administration, wherein the carrier is a solid, include a coarse powder having a particle size, for example, in the range of about 20 to about 500 microns which is administered in the manner in which snuff is taken, i.e. by rapid inhalation through the nasal passage from a container of the powder held close up to the nose. Suitable pharmaceutical compositions wherein the carrier is a liquid for administration as, for example, nasal spray, nasal drops, or by aerosol administration by nebulizer, include aqueous or oily solutions of the active compound. Pharmaceutical compositions suitable for administration by inhalation include those presented as an aerosol spray from a pressurized pack, with the use of a suitable propellant, such as dichlorodifluoromethane, trichlorofluoromethane, dichoro-tetrafluoroethane, carbon dioxide, or other suitable gases. Further pharmaceutical compositions suitable for inhalation include those presented as a nebulizer. Pharmaceutical compositions suitable for topical administration via the skin include ointments, creams, and emulsions. When formulated in an ointment, the active compound may optionally be employed with either a paraffinic or a water-miscible ointment base. Alternatively, the active compounds may be formulated in a cream with an oil-in-water cream base. If desired, the aqueous phase of the cream base may include, for example, at least about 30% w/w of a polyhydric alcohol, i.e., an alcohol having two or more hydroxyl groups such as propylene glycol, butane-1,3-diol, mannitol, sorbitol, glycerol and polyethylene glycol and mixtures thereof. The topical pharmaceutical compositions may desirably include a compound which enhances absorption or penetration of the active compound through the skin or other affected areas. Examples of such dermal penetration enhancers include dimethylsulfoxide and related analogues. When formulated as a topical emulsion, the oily phase may optionally comprise merely an emulsifier (otherwise known as an emulgent), or it may comprises a mixture of at least one emulsifier with a fat or an oil or with both a fat and an oil. Preferably, a hydrophilic emulsifier is included together with a lipophilic emulsifier which acts as a stabilizer. It is also preferred to include both an oil and a fat. Together, the emulsifier(s) with or without stabilizer(s) make up the so-called emulsifying wax, and the wax together with the oil and/or fat make up the so-called emulsifying ointment base which forms the oily dispersed phase of the cream pharmaceutical compositions. Suitable emulgents and emulsion stabilizers include Tween 60, Span 80, cetostearyl alcohol, myristyl alcohol, glyceryl monostearate and sodium lauryl sulfate. The choice of suitable oils or fats for the pharmaceutical composition is based on achieving the desired cosmetic properties, since the solubility of the active compound in most oils likely to be used in pharmaceutical emulsion pharmaceutical compositions may be very low. Thus the cream should preferably be a non-greasy, non-staining and washable product with suitable consistency to
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