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The labile tert-butyldimethylsilyl esters of the azetidinones 6-8b served as the crucial synthons in the preparation of the potentially useful ylide pyridyl thio esters 18-20. These intermediates were utilized to synthesize a host of title carbapenems 25-30d, 32, and 49-53. The antimicrobial properties and DHP-I susceptibility of these carbapenems were studied with reference to thienamycin.

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1) Total Synthesis of &Lactam Antibiotics. VI: R. A. Firestone, N. S. Macie-(2) R. W. Ratcliffe and B. G. Christensen, Tetrahedron Lett., 4645, 4649, (3) (a) We propose the name l-oxacephalothin for7&hienybcetamiloxadethiicephalosporanic acid. (b) A l-oxadethiicepham has been reported.