of a chiral auxiliary. The selection of dihydroquinidine (DHQD) or dihydroquinine (DHQ) as a chiral auxiliary dictates the facial selectivity of the olefin, since the absolute configuration of the ligands are opposite. The catalyst, oxidant, and chiral auxiliary can be purchased premixed for selective dihydroxylation. AD-mix-α contains the chiral auxiliary (DHQ)2PHAL, which positions OsO4 on the alpha-face of the olefin; AD-mix-β contains (DHQD)2PHAL and delivers hydroxyl groups to the beta-face. The Sharpless asymmetric dihydroxylation has a large scope for substrate selectivity by changing the chiral auxiliary class. Other variants As mentioned above, the ability to synthesize anti-diols from allylic alcohols can be achieved with the use of NMO as a stoichiometric oxidant. The use of tetramethylenediamine (TMEDA) as a ligand produced syn-diols with a favorable diastereomeric ratio compared to Kishi’s protocol; however, stoichiometric osmium is employed. Syn-selectivity is due to the hydrogen bond donor ability of the allylic alcohol and the acceptor ability of the diamine. This has since been applied to homoallylic systems. Other dihydroxylation methods Since osmium tetroxide is expensive and toxic, other metals have been used to prepare vicinal diols from olefins. Another popular metal used in dihydroxylation is ruthenium. Although it is highly oxidative, ruthenium has been used because of its short reaction time and its cost-effectiveness. Typically, the ruthenium tetroxide is created in situ from ruthenium trichloride, and a secondary oxidant NaIO4 is used to regenerate the catalyst. The turnover-limiting step of the reaction is the hydrolysis step; therefore, sulfuric acid is added to increase the rate of this step. Manganese is also used in dihydroxylation and is often chosen when osmium tetroxide methods yield poor results. Similar to ruthenium, the oxidation potential of manganese is high, leading to over-oxidation of substrates. Potassium permanganate is often used as the oxidant for dihydroxylation; however, due to its poor solubility in organic solvent, a phase-transfer catalyst (such as benzyltriethylammonium chloride, TEBACl) is also added to increase the number of substrates for dihydroxylation. Mild conditions are required to avoid over-oxidation. In particular, a solution that is too warm, acidic, or concentrated will lead to cleavage of the glycol. Prévost and Woodward dihydroxylation Unlike the other methods described that use transition metals as catalyst, the Prévost and Woodward methods use iodine and a silver salt. However, the addition of water into the reaction directs the cis- and trans- addition of the hydroxyl groups. The Prévost reaction typically uses silver