Synthesis of some 1-oxa-1-dethiacephalosporins

Abstract
The penicillin-derived (3R,4R)-4-methylsulphonyl-3-triphenylmethylaminoazetidin-2-one (14) has been converted into various (6R, 7S)-1-oxa-1-dethiacephalosporins, the dihydro-oxazine ring being closed by an intramolecular Wittig reaction. The isomerisation and cyclopropanation of the double bond in (6R, 7S)-t-butyl 3-methyl-7-triphenylmethylamino-1-oxa-1-dethiaceph-3-em-4-carboxylate (21; R1= CMe3) have been investigated.

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