Studies related to cephalosporins. 61. Bromination of 3-exomethylene in cepham derivatives

The methyl 3-methylene-7-phthalimidocepham-4-carnoxylate 3, the corresponding (R)-oxide 2 and the 1,1-dioxide 1 add bromine affording stereoisomeric mixtures of dibromoderivatives. These results are at variance with previous reports regarding 3-exomethylene sulfides and sulfoxides. The stereochemistry of the dibromoderivatives was unambiguously determined via X ray crystallography of 5a, following 1H-NMR correlations. The single dibromo derivatives as well as the isomeric mixtures, can be dehydrobrominated to afford 3-bromomethyl cephem derivatives (7,8,9) in very good yields.