Enantioselective synthesis of 2-isocephem and 2-isooxacephem antibiotics

Abstract

The azido lactone 8 was prepared in a highly stereoselective manner by introduction of an azide function to the lactone 6 derived from L-aspartic acid, and then was converted into (2S,3S)-3-amino-2-azido-4-hydroxybutanoic acid 4. Four-component condensation of the amino acid 4, p-nitrobenzyl isocyanide and formaldehyde or 2,2-diethoxyacetaldehyde furnished the corresponding 3,4-cis-azetidinone 16 or 26 in excellent yield. 3-Methoxy-2-isooxacephalosporin was prepared by the intramolecular acylation of imidazolide 23 derived from compound 16. 3-Unsubstituted 2-isocephem and 2-isooxacephem analogues were prepared from the azetidinone 26.