The 3-formyl-2-cephem 4, available from 7-aminocephalosporanic acid has been converted to C-3 vinylic cephems. The reactions involved are the Grignard addition to 4, the conversion of the resulting alcohols to mesylates, and the elimination of the mesyl group by LiCl. When ethylmagnesium iodide is used, only 3-[(E)-1-propenyl] cephem is obtained, which is not easily available by conventional Wittig reaction.