Alcohols 4_ and s prepared from 3 underwent completely stereospecific etherification to give l-oxacephams 2 and s which were converted into the 1-oxacephem nucleus 2 via kaand lc. Functionalization at C-3' in 6 and &was unsuccessful. Since 7a-methoxy-1-oxacephem & (6059-S) was found to be a highly active f3-lactam antibiotic of a new type,2 we have been concerned for exploring efficient synthetic routes to 7amethoxy compounds Lfrom abundant penicillins. A previous synthesis,' though stereocontrolled and convenient on small scales, has a disadvantage of removing three carbon atoms of the starting penicillin which, if properly functionalized, are usable for constructing the I-oxacephem skeleton. Our recent finding4 that azetidinone-epioxazoline 2 can be easily prepared from 6epipenicillin S-oxide &has prompted us to examine new, stereocontrolled synthetic routes to ,& starting from this useful intermediate 2 and retaining all the carbon atoms of penicillins. We now report a convenient synthesis of the 3-methyl 7a-methoxy-1-oxacephem nucleus ,l& and attempted functionalization at C-3' of 3-methyl A3-1-oxacephem derivatives. b R' = R2 = H; d = CHPh3 5 R'=PhCO; R2=H; R3=CHPh2 Treatment of 2 with KC103/catalytic 0s04 in THF-H20 at 60 'C gave an isomeric mixture of diols 2,' which were used for the next step without separation. Cyclization of &was effected by treatment with a catalytic amount of BF3-Et20 in Et20-CH2C12 at 25 OC to give, after column chromatography, 6 3a-hydroxycepham &,7 its 3S-epimer 2, and a mixture of & and z in 37, 25, and 8% overall yields from 2, respectively. No undesired 6,7-cis isomers were isolated. This result indicates that the above intramolecular etherification proceeds in a completely