Diastereoselective dihydroxylations, osmium tetroxide

A catalytic enantio- and diastereoselective dihydroxylation procedure without the assistance of a directing functional group (like the allylic alcohol group in the Sharpless epox-idation) has also been developed by K.B. Sharpless (E.N. Jacobsen, 1988 H.-L. Kwong, 1990 B.M. Kim, 1990 H. Waldmann, 1992). It uses osmium tetroxide as a catalytic oxidant (as little as 20 ppm to date) and two readily available cinchona alkaloid diastereomeis, namely the 4-chlorobenzoate esters or bulky aryl ethers of dihydroquinine and dihydroquinidine (cf. p. 290% as stereosteering reagents (structures of the Os complexes see R.M. Pearlstein, 1990). The transformation lacks the high asymmetric inductions of the Sharpless epoxidation, but it is broadly applicable and insensitive to air and water. Further improvements are to be expected. 

Asymmetric induction also occurs during osmium tetroxide mediated dihydroxylation of olefinic molecules containing a stereogenic center, especially if this center is near the double bond. In these reactions, the chiral framework of the molecule serves to induce the diastereoselectivity of the oxidation. These diastereoselective reactions are achieved with either stoichiometric or catalytic quantities of osmium tetroxide. The possibility exists for pairing or matching this diastereoselectivity with the face selectivity of asymmetric dihydroxylation to achieve enhanced or double diastereoselectivity [25], as discussed further later in the chapter. 

The ratio of diols 61 62 from dihydroxylation with osmium tetroxide alone is 2.8 1, whereas with (DHQD)2-PHAL the ratio is 39 1, and with (DHQ)2-PHAL the ratio is 1 1.3, which shows the cumulative effect that can be achieved by matching diastereoselectivities and also reveals that AD may not be as reliable as is asymmetric epoxidation (Chapter 6A) for attaining good results when diastereoselectivities are mismatched. 

In the synthesis of polyhydroxylated polyenes, osmium tetroxide induced dihydroxylation of an alkene adjacent to a coordinated iron atom has been used as the key synthetic step115. The reaction has been shown to be highly diastereoselective for E-alkenes and diastereospecific for Z-alkenes. With both types of alkene, the reaction occurs anti to the coordinated iron group (equation 18). 

Sulfoxide groups direct the dihydroxylation of a remote double bond in an acyclic system perhaps by prior complexation of the sulfoxide oxygen with osmium tetroxide (eqs 16 and 17). Chiral sulfoximine-directed diastereoselective osmylation of cy-cloalkenes has been used for the synthesis of optically pure di-hydroxycycloalkanones (eq 18). Nitro groups also direct the osmylation of certain cycloalkenes, resulting in dihydroxylation from the more hindered side of the ring. In contrast, without the nitro group the dihydroxylation proceeds from the less hindered side (eq 19). ... 

Acetylide 241 combines smoothly with electrophiles, in particular, aldehydes (Scheme 1-186). The adducts can be subjected to enzymatic racemate resolution and then converted by Lindlar hydrogenation or reduction with sodium bis(2-methoxyethoxy)-aluminum hydride ("Red-Al") into the cis- and tram-allylic alcohol, respectively. After diastereoselective dihydroxylation with osmium tetroxide and protection of the 1-hydroxyl group 2,6-dideoxy-6,6,6-trifluoro sugars are obtained (Scheme 1-186). ... 

Dihydroxylation of olefins attached to the tricarbonyl(diene)iron system with osmium tetroxide affords the respective diols in good diastereoselectivity. (Methylenecyclohexadienejiron complexes can undergo various stereoselective cyclo-... 

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