Studies related to penicillins. Part 18. Epimerisations of benzylpenicillin sulphones

Abstract

In the presence of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN), methyl benzylpenicillinate 1,1-dioxide (9) equilibrates with methyl benzyl-6-epipenicillinate 1,1-dioxide (10). Under corresponding conditions, methyl benzyl-5-epipenicillinate 1,1-dioxide (11) forms a mixture of methyl benzyl-3,5,6-triepipenicillinate 1,1-dioxide (12), methyl benzyl-3,5-diepipenicillinate 1,1-dioxide (13), and (3S,4R)-1-(1-methoxycarbonyl-2-methylprop-1-enyl)-2-oxo-3-phenylacetamidoazetidine-4-sulphinic acid (14); further treatment with DBN converts the derivatives (12) and (13) into the oxoazetidinesulphinic acid (14).

It is suggested that the epimerisations of penicillin sulphones at positions 6 and 3 occur by way of carbanionic species and that intermediates derived by β-elimination reactions do not intervene.