摘要：

α-Hydroxy- and α-mercapto-carboxylic acids are condensed with pivalaldehyde to give 2-t-butyl-5-substituted-l,3-dioxolanones or 1,3-oxathiolanones ( 2); the predominate CK-isomers are separated by crystallization. The cis-disubstituted heterocycles 2 derived from lactic, mandelic and malic acid furnish, after deprotonation with LDA, reaction with electrophiles such as alkyl halides, aldehydes and ketones, and hydrolysis α-branched α-hydroxy-carboxylic acids ( 3, 6, 8, 9, 10). These result from an overall substitution of the proton in the α-CO position with retention of configuration. The optically active carboxylic acids are α-alkylated without racemization and without employment of a chiral auxiliary ("self-reproduction of chirality". Scheme I). The diastereoselectivities (ds) are generally > 95% (Table 1, 2, and 20-25).

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