The application of a mechanistic model leads to the extension of the sharpless asymmetric dihydroxylation to allylic 4-methoxybenzoates and conformationally related amine and homoallylic alcohol derivatives

Article abstract of DOI:10.1021/ja00149a003

The scope and utility of the Sharpless asymmetric dihydroxylation has been expanded to include the use of allylic 4-methoxybenzoates as precursors of a wide variety of substituted chiral glycerol derivatives. The allylic 4-methoxybenzoyl group was found to be superior to other allylic alcohol protecting groups with respect to both yield and enantiomeric purity of the product. For example, asymmetric dihydroxylation of allyl 4-methoxybenzoate (6a) using the (DHQD)₂PYDZ·OsO₄ (1·OsO₄) catalyst system affords (S)-3-(4-methoxybenzoyloxy)-1,2-propanediol (7a) in >99% yield and 98% ee. The 4-methoxybenzoates of a variety of other allylic alcohols also serve as excellent substrates, in contrast to the parent alcohols themselves. The efficient asymmetric dihydroxylation of homoallylic 4-methoxyphenyl ethers (12a and 15), allyl 9-fluorenimine (18b), bis(homoallyl) 4-methoxybenzoate (14) and other structurally related substrates is also described. This methodology was developed under mechanistic guidance from the transition state model advanced earlier by us for the bis-cinchona alkaloid catalyzed asymmetric dihydroxylation reaction. The 4-methoxybenzoyl group functions not only to selectively protect one of the hydroxy groups of the product triol for subsequent synthetic manipulation but also to provide an extended binding group that participates in hydrophobic and aryl-aryl interactions with the U-shaped binding pocket of the (DHQD)₂PYDZ·OsO₄ catalyst (1·OsO₄), thereby enhancing enantioselectivity.