Stereoselective synthesis and X-ray crystallographic analysis of mixed 6,6-dihalo penicillanates

The pivaloyloxy methyl 6,6-dihalo penicillanates 1 a, 1 b, and 1 c have been stereoselectively prepared from the reaction of pivaloyloxy methyl 6-diazo penicillanate 2 with either N-halosuccinimide/halide or the interhalogens Xl (X = Cl, Br). The crystal structures of 3S,5R,6R pivaloyloxy methyl 6-bromo, 6-chloro penicillanate 1 a; 3S,5R,6R pivaloyloxy methyl 6-iodo, 6-bromo penicillanate 1 b, and 3S, 5R, 6R pivaloyloxymethyl 6-iodo, 6-chloro penicillanate 1 c have been determined by X-ray single crystal analysis. The stereochemistry of the displacement reaction is discussed.