these molecules possess the capacity to increase 
 the cardiac output through their inhibitory interaction with the extracellular 
 surface of the membrane - bound sodium pump ( na / k  atpase ) through stabilization in the e2-p transition state , 
 resulting in the increase of intracellular sodium concentration and 
 the buildup of intracellular calcium concentration in the sarcoplasmic 
 reticulum . cardiac glycosides possess several characteristic 
 features ( figure 1a ) : ( i ) glycosylation , 
 if any , is found at the c3 position of the steroidal framework ; ( ii ) 
 in contrast to the common steroidal skeleton , both their a / b and c / d 
 rings are of cis configuration ; ( iii ) a -configured 
 tertiary alcohol is present at c14 ( and often at c5 ) ; and finally , 
 ( iv ) an unsaturated lactone ring is found at the c17 position . the 
 c17 lactone domain further defines the subclass of cardiac glycosides : 
 those with an unsaturated butyrolactone moiety , typically afforded 
 by plant sources , are called cardenolides and those with an unsaturated 
 2-pyrone moiety , typically afforded by animal sources , are called 
 bufadienolides . ( a ) structures of cardiotonic 
 steroid ouabagenin ( 1 ) and its parent glycoside ouabain 
 ( 2 ) . ( b ) interaction 
 of ouabain ( 2 ) with borosilicate glassware . in 1888 , a highly oxidized member of the cardenolide 
 family , ouabain 
 ( 2 ) , was isolated by arnaud from the roots and barks of the african ouabaio tree . its aglycone , 
 ouabagenin ( 1 ) , was later isolated in 1942 by mannich 
 and siewert , who also proposed the correct 
 structure for the aglycone and the parent glycoside . these molecules 
 have attracted considerable attention due to the discovery of naturally 
 occurring ouabain ( or a ouabain - like compound ) in mammals . in fact , 
 buildup of endogenous ouabain has recently been proposed as one genetic 
 molecular mechanism for hypertension in animal models . from a chemical synthesis point of view , the predominant 
 -orientation of the hydroxyl groups of ouabagenin and ouabain 
 presents an additional layer of complexity as ouabain ( 2 ) has been reported to undergo facile complexation with borosilicate 
 glassware ( figure 1b ) . our laboratory became enamored with cardiotonic steroids for 
 both 
 chemical and biological reasons . chemically , there was no scalable 
 solution ( semisynthetic or fully synthetic ) to the synthesis of highly 
 oxygenated steroid systems such as ouabain ( 2 ) , thus 
 presenting an opportunity for innovation . from a biological vantage 
 point , we were interested in the medicinal chemistry of highly oxygenated 
 steroids that would result from such an effort . this full account 
 traces the evolution of our synthetic strategy that ultimately led 
 to a scalable solution to the puzzle posed by the ouabain problem 
 and enabled initial medicinal chemistry explorations of uniquely oxidized 
 steroid derivatives . semisynthesis 
 campaigns for the preparation of the cardenolides and the related 
 bufadienolides date back to the early 1970s with the landmark synthesis 
 of batrachotoxinin by werhli and co - workers . since then , numerous synthetic efforts have resulted in the semisynthesis 
 of strophanthidol and its parent glycoside ( strophanthidin ) by yoshii , the semisynthesis of digitoxigenin by wiesner and kabat , and 
 bufalin by wiesner and yoshii . in addition , the first total synthesis of digitoxigenin was reported by stork and co - workers in 1996 , 
 and the total synthesis of rhodexin a was achieved by jung and co - workers in 2011 . more recently , an elegant 
 total synthesis of 19-hydroxysarmentogenin starting from carvone was 
 disclosed by inoue and co - workers . synthetic studies toward the most oxidized members of the cardenolide 
 family , ouabagenin ( 1 ) and ouabain ( 2 ) , 
 have also been conducted , culminating in a landmark total synthesis 
 of these compounds in 2008 by deslongchamps and co - workers featuring 
 a polyanionic cascade as the key skeletal construction step ( scheme 1 ) . in addition , other 
 elegant approaches have been developed , including a novel heck annulation 
 by overman and co - workers , and a diels  alder 
 approach by jung and co - workers . given the success of pharmaceutical 
 research on semisynthetic steroids , 
 we anticipated that a semisynthetic approach would be the most viable 
 means to achieve a scalable synthesis of ouabagenin ( 1 ) . look - ahead  approach commencing with readily available cortisone acetate ( 9 ; figure 2 ) . this starting material was chosen 
 because of ( i ) its bulk availability at a reasonable cost ( $ 1.2/g ) ; 
 ( ii ) the existence of  pre - built  oxidations at c3 and 
 c11 , which would facilitate access to the requisite hydroxyl groups 
 at these two carbons ; and ( iii ) the facile cleavage of the c17c20 
 bond to give a ketone moiety at c17 , which 
 would provide us with a functional handle for the butenolide appendage . an overwhelming 
 amount of 
 literature evidence has suggested that the a ring of a steroid skeleton 
 exhibits higher reactivity than the rest of the tetracyclic framework . thus , we posited that the a ring be fully elaborated 
 before the rest of the molecule in the approach toward ouabagenin 
 ( 1 ) . several scenarios exist for the elaboration of the 
 a ring using the putative steroid intermediate 10 derived 
 from cortisone acetate ( 9 ) following c17c20 cleavage 
 ( figure 2 ) . pre - built  oxidation state 
 within the a ring would be desirable . in pathway a , it is envisioned that either the c1 or c5 hydroxyl group could 
 be introduced via selective hydration of the dienone moiety . however , 
 introduction of the hydroxyl group at c19 would still be problematic 
 due to the geometrically unfavorable 1,4-relationship to the requisite 
 hydroxyl group at either c1 or c5 . the 
 same problem would also be encountered with the use of the hydroxyl 
 group at c3 to direct oxidation onto the c19 position , as they exist 
 in a 1,6-relationship ( pathway c ) . literature evidence 
 suggested that c19 functionalization by way of alkoxy radical generation 
 at c3 proceeded only in poor yield . thus , 
 it is apparent that the oxidation at the c19 carbon is unlikely to 
 be accessed via the use of other functionalities on the a ring as 
 directing groups , and would have to be attained via other means ( pathway b ) , preferably early in the synthesis . installation of functionality at c19 is a classic unsolved 
 problem 
 in steroid chemistry dating back to the days of woodward . a representative panel of strategies that have 
 been developed to solve this problem is shown in scheme 2 . while the bromohydrin functionalization route ( scheme 2a ) is a well - established method to access the c19 
 oxidation in steroid semisynthesis literature , our studies 
 thus commenced with the reinvestigation of an intriguing report of 
 metalloporphyrin - catalyzed hydroxylation of the steroidal 
 c19 position as this would represent 
 the most direct access to the desired c19-hydroxyl compound ( scheme 2b ) . the described reaction utilized iron - porphyrin 18 in the presence of n - methylimidazole and 
 cumyl hydroperoxide to furnish hydroxylated product 19 in moderate yield among other oxidation products . in our hands , 
 this result could not be reproduced , as 17 proved to 
 be unreactive toward oxidation under the reported conditions . disappointingly , 
 the use of a more reactive perfluorinated iron - porphyrin catalyst resulted only in epoxidation of the c4c5 
 olefin of 17 . attention was then turned to a report 
 from rindone and co - workers 
 ( scheme 2c ) of a direct dichloromethylation 
 of estrone under aerobic reaction conditions with cobalt(ii ) salen 
 as catalyst . this report was viewed as 
 an alternative solution for the installation of the requisite oxidation 
 state at the c19 position . in our hands , formation of two products 
 was observed with nmr spectra identical to the ones reported by the 
 authors . x - ray analysis , however , established that the reported dichloromethyl 
 adduct 22 was instead an oxidized dimer of estrone ( 23 ) . in the original publication , the structure of product 22 was established based on the presence of a singlet at 5.39 
 ppm by h nmr spectroscopy . however , the actual structure 
 of the product 23 suggests that this singlet belongs 
 to the o - quinone proton . in addition , a multitude 
 of conditions were attempted to effect a para - functionalization 
 of estrone but most conditions only resulted in the formation of ortho - functionalized products . the preference for ortho - functionalization could be rationalized by the high 
 thermodynamic penalty associated with breaking the aromaticity of 
 the a ring in the para - functionalization pathway . these failures led us to retool our synthesis plan and consider 
 the possibility of introducing the hydroxyl group at c19 with the 
 use of an appropriate functionality embedded at the c11 carbon . literature 
 evidence suggested that hypoiodite photochemistry would lead to nonselective 
 functionalizations of both c19 and c18 carbons . on the other hand , a norrish type ii photochemistry could 
 pave the way for selective functionalization of the c19 methyl group 
 ( scheme 2d ) . jeger and co - workers were able 
 to introduce a hydroxyl moiety at c19 by first effecting a norrish 
 type ii photochemistry to generate cyclobutanol 25 in 
 modest yield , followed by an oxidative fragmentation with pb(oac)4 . encouraged by this finding , 
 we arrived at the final retrosynthesis for ouabagenin as depicted 
 in figure 3 . the general premise of this strategy 
 was to relay the c17 and c19 ketone oxidation states to neighboring 
 and distal carbon atoms . this so - called  redox - relay  
 approach would potentially simplify the installation of four hydroxyl 
 groups ( c1 , c5 , c14 and c19 ) and minimize functional group incompatibilities 
 along the way . butenolide installation at a late stage would result 
 in target 27 and allow divergent exploration of d - ring 
 substituents . subsequent hydroxyl - directed bis - oxygenation ( c1 and c5 ) 
 simplifies the target to c19-oxygenated steroid 29 , which 
 is essentially one oxidation away from readily available cortisone 
 acetate ( 9 ) . oxidative cleavage of the 
 c17c20 bond of cortisone acetate 
 ( 9 ) generated adrenosterone ( 30 ) in a straightforward 
 manner ( scheme 3 ) . preliminary photochemical 
 investigations suggested that the presence of ketone moieties at c3 
 and c17 led to the formation of a complex mixture of products , presumably 
 via competitive photoexcitation . thus , we elected to convert these 
 two ketone moieties to the corresponding ketals . although 31 could be converted to the desired cyclobutanol 32 , 
 the reaction gave only a modest yield of desired product ( 43% ) and 
 suffered from formation of undesired side products , most notably compound 33 , which arises from norrish type i cleavage of the c9c11 
 bond . reagents and conditions : ( a ) 
 nabh4 ( 0.6 equiv ) , 0 c , 135 min ; then naio4 ( 5 equiv ) , 23 c , 12 h , 86% ; ( b ) p - tsoh ( 0.1 
 equiv ) , ethylene glycol ( 30 equiv ) , phch3 , 135 c , 
 6 h , 81% ; ( c ) h , vycor filter , 48120 
 h ; ( d ) pb(oac)4 , c6h6 , 80 c , 
 1 h. in 2004 , a total synthesis of herbertenolide was accomplished by garcia - garibay and co - workers 
 and featured 
 for the first time the application of a solid - state norrish type i 
 photoreaction in total synthesis . remarkably , the authors reported 
 a highly chemoselective formation of cyclopentane 35 , which stood in stark contrast to the nonselective outcome obtained 
 with conventional solution photochemistry ( scheme 4a ) . in addition , scheffer and co - workers also reported a highly 
 differential outcome of a norrish type ii photoreaction when conducted in solution and in solid - state : 
 significant improvement in the formation of cyclobutanols 37 and 38 was observed when solid - state photochemistry 
 was employed in place of conventional solution photochemistry ( scheme 4b ) . encouraged by these precedents , we elected to 
 conduct our photochemistry in the solid state . gratifyingly , the use 
 of solid - state photochemistry led to an appreciable increase in the 
 yield of cyclobutanol formation ( 43% to 68% ) while minimizing the 
 formation of undesired -cleavage product 33 ( see 
 scheme 3 ) . unfortunately , the 
 oxidative fragmentation protocol described by 
 jeger proved unsuccessful on 32 as a complex mixture 
 of products was observed , none of which corresponded to the desired 
 c19-functionalized product 29 . presumably , the c5c6 
 olefin of 32 is incompatible with the strong oxidant / harsh 
 conditions employed , leading to the formation of various undesired 
 products . following precedents by snider , phillips , and several 
 other research 
 groups , a range of inorganic oxidants 
 was also screened to effect the oxidative fragmentation of the c11c19 
 bond , but none led to formation of the desired product ( scheme 5a ) . it was then reasoned that a hypervalent iodine 
 species could undergo a ligand exchange reaction with cyclobutanol 32 and the resulting adduct could then fragment oxidatively 
 to give either a hydroxyl or an acetate group at c19 . unfortunately , 
 no reaction was observed with both ibx and dmp even at elevated temperatures , 
 and undesired fragmentation of the c9c11 bond to hemiketal 40 and 41 was observed with phi(oac)2 and hypoiodite photolysis conditions ( i2/phi(oac)2 ) , respectively . reagents and conditions : ( a ) 
 phi(oac)2 ( 1.5 equiv ) , ac2o , 80 c , 3 h , 
 68% ; ( b ) h , i2 ( 1.5 equiv ) , phi(oac)2 ( 2 equiv ) , ch2cl2 , 23 c , 20 min , 
 61% ; ( c ) pd(oac)2 ( 0.2 equiv ) , phi(oac)2 ( 1.5 
 equiv ) , ac2o , 80 c , 3 h. building on precedents set by uemura and co - workers , we next envisaged a palladium - mediated fragmentation 
 reaction ( scheme 5b ) . it was shown that cyclobutanols 
 can be engaged in a -carbon elimination from the initial palladium(ii ) 
 alcoholate adduct . thus , it was hoped that the resulting metalloketone 
 intermediate could be intercepted by an oxidant to generate a pd species , which would in turn undergo reductive elimination 
 to furnish acetate 45 . to this end , 32 was heated with pd(oac)2 and phi(oac)2 , with 
 ac2o as solvent at 80 
 c . the desired c19 functionalized compound 45 was 
 indeed formed , albeit in minor amounts , validating our hypothesis 
 ( scheme 5c ) . the major product from this reaction 
 turned out to be hemiketal 40 , indicating significant 
 background reaction of the cyclobutanol with the oxidant itself . unfortunately , 
 despite extensive experimentations , we were not able to improve the 
 yield of formation of acetate 45 . eventually , the 
 desired c19 functionalization was effected by using 
 either barluenga s reagent or n - iodosuccinimide under photolytic 
 conditions , with the latter being preferred simply due to the lower 
 cost of the reagent . a proposed mechanism for this transformation 
 is put forth in figure 4 . in addition , since 
 both conditions b ( see scheme 5 ) and d / e ( see figure 4 ) should generate 
 an identical hypoiodite intermediate , it is entirely possible that 
 additives from the latter conditions exert subtle geometrical changes 
 in the reaction transition state such that the o i bond is 
 anti - periplanar to the c11c19 bond , leading to a chemoselective 
 fragmentation to iodide 47 . with iodide 47 now in hand , selective deketalization 
 at c3 was achieved by using ticl4 , and the iodide at c19 
 was hydrolyzed with agoac to furnish enone alcohol 49 ( scheme 6 ) . in contrast to a similar epoxidation 
 with a simpler c19 methyl substrate , epoxidation 
 of the c4c5 olefin proceeded in a completely diastereoselective 
 fashion . dehydrogenation of the c1c2 bond was then achieved 
 with seo2 to give 50 , as the use of other 
 reagents such as ibx or hio3 led to formation of over - oxidized products . lastly , another diastereoselective 
 epoxidation furnished diepoxide 51 , a landmark intermediate en route to full elaboration of the a ring of ouabagenin 
 ( 1 ) . reagents 
 and conditions : ( a ) h , n - iodosuccinimide 
 ( 3 equiv ) , 
 li2co3 ( 3.5 equiv ) , meoh , phch3 , 
 23 c , 20 min , 85% ; ( b ) ticl4 ( 1 m in ch2cl2 , 1 equiv ) , ch2cl2 , 10 
 c , 15 min ; agoac ( 1.5 equiv ) , thf , h2o , 50 c , 
 2 h , 71% ; ( c ) h2o2 ( 35 wt% in h2o , 
 6 equiv ) , 3 m naoh ( 1 equiv ) , meoh , 0 c , 75 min ; ( d ) seo2 ( 1.1 equiv ) , phcl , 90 c , 10 h ; ( e ) h2o2 ( 35 wt% in h2o , 6 equiv ) , 3 m naoh ( 1 equiv ) , 
 meoh , 0 c , 75 min , 50% ( over three steps ) ; ( f ) conc . hcl ( 1 
 equiv ) , ch2cl2 , 23 c , 1 h , 87% . as was observed by 
 jung and co - workers in their model study , reductive opening of diepoxide 51 proved to be daunting . 
 a wide range of conditions ( table 1 ) were tried , but most conditions 
 resulted only in the formation of regioisomeric mixtures of a - ring 
 enones . hydrogenation conditions ( entry 10 ) , despite multiple precedents 
 by porco , led to reduction of the c3 
 ketone while leaving the two epoxides intact . gratifyingly , 
 triol 54 was accessible via treatment 
 of 51 with in situ - generated aluminum 
 amalgam . poor conversion to 54 was observed when this 
 reaction was run in the more conventional biphasic solvent mixture , 
 leading to an extensive solvent screening for the reaction ( entries 
 1120 ) , which eventually revealed saturated nahco3 as the optimal medium ( entries 14 and 15 ) . it is worth noting that 
 with these  on - water  conditions , we observed formation of over - reduced product 57 , suggesting that this aqueous medium provided a higher reduction 
 potential for the reagent than conventional organic solvents . cognizant of the ligating ability of the a ring of ouabain ( 2 ) , we elected to introduce an 
 appropriate protecting group on the a ring . while attempts to tie 
 the three hydroxyl groups in 54 as an orthoformate were 
 unsuccessful , clean acetonide formation could be effected in acidic 
 acetone to form 58 . reduction of the c3 ketone of 58 with libet3h ( scheme 7 ) proceeded in a completely diastereoselective fashion and 
 also protected the remaining two free hydroxyl moieties as the boronate 
 ester to give 59 . the ketone 
 moiety of the c ring was next reduced under thermodynamic conditions 
 ( li / nh3 ) to furnish an -configured alcohol at c11 . following deketalization , the c17 ketone moiety of 60 was subjected to saegusa oxidation to provide the corresponding 
 conjugated enone . isomerization of the olefin at c15c16 out 
 of conjugation could be effected with a sio2/ipr2etn mixture to give 61 , although this necessitated the use of octafluorotoluene 
 as solvent in order to minimize undesired epimerization of the c14 
 stereocenter ( the c14 -hydrogen epimer of the conjugated enone 
 can not be isomerized to give 61 ) . to introduce the last 
 hydroxyl group at c14 , the c14c15 olefin was subjected to 
 mukaiyama hydration . once again , solvent 
 optimization leading to the use of dioxane was necessary in order 
 to achieve a satisfactory diastereomeric ratio of hydration products . 
 with the completion of this step , we arrived at the protected form 
 of the ketonic core of our target molecule ( 62 , termed 
  ouabageninone  ) , and to date , more than 500 mg of this 
 compound has been prepared . in addition , as a testament to the versatility 
 of this route for further diversification , we demonstrated that 61 could undergo radical fluorination to produce 63 ( scheme 8) . in 
 light of recent advances in radical functionalization of alkenes , compound 61 could be viewed as 
 a viable platform for further generation of new steroidal skeletons , 
 and ultimately , novel analogues of the cardenolides and bufadienolides . reagents 
 and conditions : ( a ) 
 ppts ( 0.2 equiv ) , caso4 ( 2.5 equiv ) , me2co , 
 23 c , 20 h ; ( b ) libet3h ( 1 m in thf , 1.1 equiv ) , 
 78 c , 30 min , 92% ; ( c ) li ( 60 equiv ) , nh3 , thf , 78 c , 30 min ; ( d ) ppts ( 1.5 equiv ) , me2co , 70 c , 16 h , 69% over 2 steps ; ( e ) tmsotf ( 3 equiv ) , et3n ( 4 equiv ) , ch2cl2 , 0 to 23 c , 
 30 min ; pd(oac)2 ( 1.2 equiv ) , mecn , 23 c , 3 h , then 
 fecl3 ( 1 equiv ) , 0 c , 10 min ; ( f ) sio2 , ipr2etn ( 55 equiv ) , c6f5cf3 , 23 c , 45 min , 55% over 2 steps ; ( g ) 
 co(acac)2 ( 0.2 equiv ) , phsih3 ( 3 equiv ) , o2 ( 1 atm ) , dioxane , 23 c , 3 h , 86% . reagents and conditions : fe2(oxalate)3 ( 4 
 equiv ) , nabh4 ( 6.4 equiv ) , 
 selectfluor ( 4 equiv ) , 3:4:2 mecn : thf : h2o , 0 c , 20 
 min , 51% . with a scalable 
 route to ouabageninone secured , attention was then turned to introduction 
 of the butenolide moiety with the correct stereochemical orientation . 
 to this end , a model study employing estrone as starting material 
 was conducted ( scheme 9 ) . using a similar route 
 to scheme 7 , the tertiary hydroxyl group at 
 c14 was introduced to afford 64 ( see the supporting information for its preparation ) . it was initially envisioned that the butenolide moiety would be appended 
 via a palladium - catalyzed cross - coupling , followed by a chemo- and 
 diastereoselective reduction of the c16c17 olefin . thus , 
 ketone 64 was first converted to the corresponding vinyl 
 iodide using barton s protocol . a wide range of conditions were tried for the butenolide attachment , 
 but initial results were discouraging as the desired product 68 could only be obtained in very poor yield ( table 2 ) . eventually , it was found that the use of frstner s 
 modified stille coupling protocol led 
 to the formation of dienoate 68 in a synthetically useful 
 yield . reagents and conditions : ( a ) 
 n2h4 ( 10 equiv ) , et3n ( 10 equiv ) , 
 etoh , 50 c , 5 h ; i2 ( 3 equiv ) , et3n ( 4 
 equiv ) , thf , 23 c , 10 min ; ( b ) ra - ni ( 10 wt equiv ) , thf , 23 
 c , 14 h , 90% . compound 65 was used 
 without further purification from the previous step . with sufficient quantities of 68 in hand , no reaction 
 was observed when palladium on carbon was used as the hydrogenation 
 catalyst , and when the more active platinum catalyst was employed , 
 nonselective hydrogenation of both olefins occurred . a chemoselective 
 hydrogenation was eventually effected by using ra - ni , but this led to the wrong stereochemical outcome of the 
 reduction , giving 69 . this result stood in stark contrast 
 to hydrogenation conducted on steroids possessing a more conventional trans c / d ring configuration , where reduction from the  
 face is typically observed . clearly , the cis configuration 
 of 68 rendered its convex face more accessible for hydrogenation . alternatively , a directing effect of the c14 hydroxyl group could 
 also be invoked to explain the stereochemical outcome . a radical - based 
 coupling approach employing iodide 71 and lactones 66 , 72 , and 73 was also attempted but only deiodination of 71 was observed ( scheme 10 ) . reagents 
 and conditions : ( a ) 
 o2 ( 1 atm ) , n2h4 , etco2h , etoh , 100 c , 2.5 h. another approach 
 was then developed based on the idea of using 
 a butenolide anion equivalent to effect a nucleophilic addition onto 
 the c17 carbon from the more accessible -face . a hydrazone - boronic 
 acid coupling protocol disclosed by barluenga 
 and co - workers in 2009 was viewed as a highly attractive option owing 
 to the minimal number of concession steps to be performed either before 
 or after the coupling , and thus , the potential brevity of the overall 
 sequence . a c2-substituted furanboronic acid was identified as a suitable 
 butenolide equivalent ( scheme 11 ) , and this 
 compound was accessed by subjecting furan 74 to ir - catalyzed 
 c  h silylation employing a procedure developed by falck and 
 co - workers . gratifyingly , 
 coupling of this boronic acid and tosylhydrazone 77 proceeded to give 78 with the correct stereochemical 
 configuration , albeit in modest yield . conversion of the substituted 
 furan moiety to butenolide 70 could be readily achieved 
 by treatment with basic acooh solution . reagents and conditions : 
 ( a ) 
 [ ir(ome)(cod)]2 ( 0.05 equiv ) , dtbpy ( 0.1 equiv ) , 2-norbornene 
 ( 1.5 equiv ) , phme2sih ( 1.5 equiv ) , 80 c , 8 h , 43% ; 
 ( b ) naio4 ( 3 equiv ) , hcl ( 1 m , 0.7 equiv ) , 4:1 thf : h2o , 23 c , 2 h , 91% ; ( c ) tsnhnh2 ( 1.5 equiv ) , 
 dioxane , 110 c , 5 h , 64% ; ( d ) k2co3 ( 2.2 
 equiv ) , dioxane , 110 c , 5 h , ca . 20% ; ( e ) acooh ( 32 wt % in 
 h2o ) , naoac ( 5 equiv ) , ch2cl2 , 23 
 c , 24 h , ca . having identified 
 a viable method for installation of the butenolide moiety , we turned 
 our efforts back to the synthesis of ouabagenin ( 1 ) . 
 in the conversion of ouabageninone to the corresponding tosylhydrazone , 
 we encountered the first hurdle : concomitant removal of all the protecting 
 groups was observed upon conversion to the corresponding hydrazone . this problem was rectified by the use of trisnhnh2 , which 
 allowed formation of hydrazone 79 at ambient temperature 
 ( scheme 12 ) . submitting this hydrazone to the 
 coupling conditions with boronic acid 76 , however , led 
 to formation of a complex mixture of products , none of which could 
 be identified as the desired coupled product 81a . reagents 
 and conditions : ( a ) 
 trisnhnh2 ( 1.5 equiv ) , ch2cl2 , 23 
 c , 10 h , 80% ; ( b ) 76 ( 2 equiv ) , k2co3 ( 2.2 equiv ) , dioxane , 110 c , 5 h ; ( c ) tmsotf ( 3 equiv ) , 
 et3n ( 4 equiv ) , 23 c , 1 h ; ( d ) ac2o ( 3 
 equiv ) , pyridine ( 7 equiv ) , dmap ( 1 equiv ) , dmf , 40 c , 20 h , 
 66% ; ( e ) momcl ( 2.4 equiv ) , ipr2etn ( 3.5 
 equiv ) , dmap ( 1 equiv ) , ch2cl2 , 0 to 23 c , 
 24 h , 62% . positing that the free hydroxyl 
 group at c11 is incompatible with 
 the transient diazo species , we decided to convert it to the corresponding 
 tms ether ( 80 ) , acetate ( 82 ) and mom ether 
 ( 83 ) . unfortunately , subjection of these compounds to 
 the barluenga coupling conditions again led to formation of a complex 
 mixture of products . several additives , notably fluoride salts as well as different protecting groups at the 
 c11 hydroxyl moiety were also tried , but none of these strategies 
 led to any improvement to the outcome of the reaction . these 
 failures led us to reinvestigate the original stille coupling / chemoselective 
 reduction route ( scheme 13 ) . thus , ketone 62 was converted to the corresponding vinyl iodide and then 
 subjected to the modified stille cross - coupling conditions to deliver 
 dienoate 85 . as was observed in the estrone model system , 
 hydrogenation of 85 resulted in reduction from the undesired 
 convex face of the molecule . use of other conditions led to a nonselective 
 reduction outcome , none of which could be identified as the desired 
 product . we eventually discovered that the use of in situ - generated co2b led to chemoselective 
 formation of tetrasubstituted olefin 86 . extensive screening 
 of organic bases and superbases ( table s1 , supporting 
 information ) again led to the formation of an epimeric mixture 
 of enoates , where our desired product was only present in minor amounts . gratifyingly , we discovered that heating 86 in the presence 
 of barton s base ( btmg ) leads 
 to the formation of a 3:1 mixture of enoates in favor of the desired 
 product . to complete the synthesis of ouabagenin ( 1 ) , 
 this product overall , the synthesis of ouabagenin ( 1 ) was achieved in 20 steps and in 0.6% yield from adrenosterone 
 ( 30 ) or in 21 steps and in 0.5% yield from cortisone 
 acetate ( 9 ) . reagents and conditions : ( a ) 
 n2h4 ( 10 equiv ) , et3n ( 10 equiv ) , 
 4:1 ch2cl2:etoh , 50 c , 5 h ; i2 ( 3 equiv ) , et3n ( 4 equiv ) , thf , 10 min ; ( b ) 66 ( 4 equiv ) , [ ph2po2][nbu4 ] ( 4 equiv ) , 
 pd(pph3)4 ( 0.15 equiv ) , cutc ( 3 equiv ) , dmf , 
 23 c , 2 h , 42% over two steps ; ( c ) cocl26h2o ( 2.5 equiv ) , nabh4 ( 5 equiv ) , etoh , 0 to 23 c , 
 20 min ; ( d ) btmg ( 1.5 equiv ) , c6h6 , 100 c , 
 10 min , 70% over two steps ; hcl ( 2 equiv ) , meoh , 23 c , 
 30 min , 90% . as an extension of this work , we applied some of the 
 methodologies developed in this campaign to the preparation of c19-hydroxylated 
 analogues of corticosteroid drugs . topical glucocorticoids represent 
 the treatment of choice for inflammatory dermal diseases and when used properly , only limited systemic 
 adverse events are observed , which make them a safe and effective 
 therapy . however , long - term use of topical treatment on large body 
 surface areas can be associated with systemic side effects partly 
 due to mineralocorticoid antagonism of the glucocorticoids , 
 which causes fluid - electrolyte imbalance and hypertension . all therapeutically used glucocorticoids 
 possess this undesired mineralocorticoid receptor ( mr ) antagonism 
 in addition to their glucocorticoid receptor ( gr ) agonism , and 
 some sar information on these two receptors has been reported for 
 a range of steroids . however , only very 
 limited sar knowledge is available for glucocorticoids modified 
 at the c19 position , presumably due to the difficulty in procuring 
 meaningful quantities of these c19-modified compounds . to investigate 
 the pharmacological impact of installing a c19-hydroxy function in 
 the glucocorticoid scaffold , analogue 90 became 
 our initial target compound ( figure 5a ) . its 
 synthesis commenced with global protection of the ketone moieties 
 of cortisone acetate ( 9 ) to afford 87 . following 
 the procedure delineated in the synthesis of ouabagenin ( 1 ) , iodide 88 was prepared . a chemoselective reduction of the c11 ketone followed by deprotection 
 of the side chain under forcing conditions furnished the desired analogue 90 ( for details , see the supporting information ) . to probe the overall effect of the hydroxyl moieties at the c19 
 and c11 positions , analogues bearing the epimeric hydroxyl group ( 91 ) and no hydroxyl group ( 92 ) at c11 were also 
 synthesized from intermediate 89 ( for details , see the supporting information ) . as can be seen 
 in figure 5b , the gr binding 
 data for 92 showed a total lack of activity ( ic50 > 10  000 nm ) , suggesting that introduction of a hydroxyl 
 group at c19 alone can not compensate for loss of the critical c11 
 hydroxyl . however , the data for 90 confirm that introduction 
 of the c19 hydroxyl while keeping the c11 hydroxyl  in the same 
 orientation as in hydrocortisone  only results in a 10-fold 
 loss of affinity compared to the latter . this proves that the c19 
 hydroxyl is generally tolerated in steroidal gr agonists . the fact 
 that 91 , the c11 epimer of 90 , is completely 
 inactive in the gr binding assay further underscores the critical 
 role of the presence and orientation of the c11 hydroxyl moiety . we 
 were then very pleased to see that the gr binding of 90 also translated into anti - inflammatory efficacy in a cellular assay 
 ( lps - induced il-12b released from primary human peripheral blood mononuclear 
 cells ) , albeit with low potency . ( a ) syntheses of glucocorticoid 
 analogues 90 , 91 , 92 , 97 , and 98 . reagents 
 and conditions : ( a ) hcho 
 ( 37% ) , hcl ( conc . ) , chcl3 , 23 c , 61% ; ( b ) ethylene 
 glycol , tsoh ( cat . ) , benzene , reflux , 60% ; ( c ) h , 
 vycor filter , 60 h , 57% ; ( d ) h , n - iodosuccinimide ( 3.0 equiv ) , li2co3 ( 3.5 equiv ) , 
 meoh : phch3 = 1:20 , 23 c , 20 min ; ( e ) 2:10:1 tfa : ch2cl2:h2o , 0 c , 2 h , 67% over 2 
 steps ; ( f ) agf ( 3.8 equiv ) , 10:1 mecn : h2o , 23 c , 
 98% ; ( g ) nabh4 ( 0.9 equiv ) , 1:1 ch2cl2:meoh , 0 c , 2 h , 72% ; ( h ) hcl ( 6 n ) , tfa , 23 c , 30 min , 
 53% ; ( i ) mscl ( 1.3 equiv ) , et3n ( 2.0 equiv ) , dmap ( 0.1 
 equiv ) , ch2cl2 , 0 to 23 c , 40 min ; licl 
 ( 1.3 equiv ) , dmf , 60 c , 3 h , 81% ; ( j ) tmscl ( 4.0 equiv ) , nai 
 ( 40.0 equiv ) , ac2o , 0 c , 1 h ; selectfluor ( 1.3 equiv ) , 
 mecn , 0 c , 1 h , 49% ; for 97 , ( k ) ( etco)2o ( 1.5 equiv ) , tmsotf ( 0.45 equiv ) , ch2cl2 , 
 0 c , 1 h ; ( l ) hcl ( 6 n ) , 75 c , 5 h , 44% over 2 steps , 
 and for 98 , ( k ) ( etco)2o ( 1.5 equiv ) , tmsotf 
 ( 0.45 equiv ) , ch2cl2 , 0 c , 1 h , 91% ; ( l ) 
 hcl ( 6 n ) , w , 120 c , 15 min , 31% . ( b ) gr binding and 
 anti - inflammatory efficacy of steroid analogues and reference compounds . indicates that examination is not 
 done . in light of these data and the 
 1000-fold difference in 
 cellular potency ( il-12b ) between clobetasole propionate and hydrocortisone , 
 we next elected to explore the effect of incorporating two d - ring 
 clobetasol side chains ( chloroketone and propionate ester moieties ) 
 on 90 . a chemoselective 
 reduction of the c11 ketone in the presence of the a - ring enone moiety 
 initially proved to be challenging . a preliminary solution was found 
 by first globally reducing the two ketones , followed by allylic oxidation 
 of the c3 hydroxyl group to regenerate the enone moiety . eventually , 
 it was found that a modified nabh4 reduction protocol could 
 effect a chemoselective c11 ketone reduction while leaving the 
 a - ring enone intact to arrive at 93 . it was serendipitously 
 found that a methylene transfer to the c11 and c19 diol from the side 
 chain of 93 could be effected under acidic conditions 
 to afford 94 , which interestingly achieved the desired 
 protection by concomitant deprotection . with diol 94 in 
 hand , installation of a propionate ester at c17 and deprotection with hcl then completed 
 the synthesis of analogue 97 . we were encouraged to see 
 the increase in gr binding affinity ( 15-fold ) and cellular 
 potency ( 30-fold ) of 97 compared to 90 ( figure 5b ) . furthermore , 97 showed an interesting selectivity profile against the mineralocorticoid 
 receptor where it exhibited neither antagonistic nor agonistic effect 
 when tested up to 1 m ( data not shown ) , suggesting that the 
 c19 position potentially could be a trigger region for obtaining selectivity 
 for the glucocorticoid receptor over the mineralocorticoid 
 receptor . finally , further refinement of 97 by introduction 
 of a fluorine atom at c6 ( to give analogue 98 ) provided a two - fold improvement in both the gr affinity 
 and cellular potency , which is in agreement with published sar for 
 compounds bearing no hydroxyl group at c19 . stereochemically defined functional adornment 
 of a steroid system 
 was accomplished by following a  redox - relay  approach . this simplifying strategy allowed for the sequential installation 
 of four hydroxyl units that underscore the complexity of ouabagenin 
 ( 1 ) . first , solid - state photochemistry and cyclobutanol 
 fragmentation achieved hydroxylation at the c19 position of 
 a steroid skeleton , which is a key motif in both ouabagenin ( 1 ) and c19-hydroxylated steroid analogues of cortisone 
 acetate ( 9 ) . then , many redox - relay events were conducted , 
 involving directed epoxidation , diepoxide fragmentation , and a saegusa 
 oxidation / mukaiyama hydration sequence . unexpected difficulty was 
 encountered when installing the requisite butenolide moiety in 1 , which was overcome by careful examination of an estrone 
 model system . finally , adapting the c19-hydroxylation method 
 to related steroid systems has resulted in a series of hydroxylated 
 analogues for structure  activity relationship studies . evaluation 
 of these steroid analogues for glucocorticoid receptor ( gr ) 
 agonism has shown that the c19-hydroxylated steroid skeleton 
 is a promising scaffold for identifying new anti - inflammatory drug 
 candidates with improved properties , which will motivate our continued 
 efforts in this field .
the 
 natural product ouabagenin is a complex cardiotonic steroid 
 with a highly oxygenated skeleton . 
 this full account describes the 
 development of a concise synthesis of ouabagenin , including the evolution 
 of synthetic strategy to access hydroxylation at the c19 position 
 of a steroid skeleton . 
 in addition , approaches to install the requisite 
 butenolide moiety at the c17 position are discussed . 
 lastly , methodology 
 developed in this synthesis has been applied in the generation of 
 novel analogues of corticosteroid drugs bearing a hydroxyl group at 
 the c19 position .