## Abstract

A straightforward and industrially feasible synthesis of 7<latex>$\beta$</latex>-acylamino-7<latex>$\alpha$</latex>-methoxy-3-[(1-methyl-1H-tetrazol-5-ylthio) methyl]-1-oxa-3-cephem-4-carboxylic acid (35) from penicillins is described. Reaction of 6<latex>$\alpha$</latex>-acylaminopenicillanate S-oxides with triphenyl-phosphine gave epioxazolines (16) which were transformed into allylic alcohols (28) via allylic chlorides (22) and iodides (27). Intramolecular etherification of 28 gave 7<latex>$\alpha$</latex>-acylamino-3-exomethylene-1-oxacepham-4<latex>$\alpha$</latex>-carboxylate (30), a versatile key intermediate. 7<latex>$\alpha$</latex>-Methoxylation of 30, followed by substitution with sodium 1-methyl-1<latex>$H$</latex>-tetrazol-5-ylthiolate at C<latex>$_3$</latex> and side chain cleavage, afforded the methoxyamine (33), the desired 1-oxa-3-cephem nucleus, which on acylation and subsequent deprotection gave 35 (6059-S free acid, where the acyl is <latex>$p$</latex>-hydroxyphenylmalonyl).