Alternatively, the compound [I] of the present invention may C.(decomp.) by the steps of reacting an O-protected-natural penicillin of the formula: ##STR8## wherein A is alkyl, alkenyl, phenyl or substituted phenyl, E is single bond, sulfur atom or oxygen atom and Y3 is a protecting group, with a phosphorus halide compound to give an iminohalide compound of the formula: ##STR9## wherein X is halog