Enantioselective syntheses of 3-substituted 4-(alkoxycarbonyl)-2-azetidinones from malic acid have been developed. Alkylation of diesters of D-malic acid provided primarily the erythro products 15RS. Base-mediated inversion gave racemic threo isomers 15. Saponification of 15 and selective monoesterification provided the correspondingly substituted 0-hydroxy acids 17, which could also be epimerized at the hydroxyl position. Separate conversion of the isomers 17 to the hydroxamates 18 followed by cyclization gave the desired /3-lactams 19 with complete control of stereochemistry.
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