Regio- and Stereoselective Dihydroxylation

Besides the opening of the previously described epoxides with oxygen nucleo-philes, which would result in the formation of a second diol unit with the trans configuration, an approach towards a diol unit with cis configuration is achievable by dihydroxylation reactions. The osmium tetroxide-catalyzed dihydroxylation of the unprotected diol 4 gives a diastereomeric mixture of 19 and 20 in a ratio of 5:1 (Scheme 6.4.9; Eq. 1). Compound 21 is obtained as a single diastereomer when using protected trans-CHD 7 as starting material [25]. Proof of the relative config-


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