Reductive Ring Contraction of Mesoionic Thiazol-4-ones to Azetidin-2-ones

Article abstract of DOI:10.1021/jo01299a026

A series of anhydro-2,3,5-triaryl-4-hydroxythiazolium hydroxides was prepared and desulfurized with Raney nickel.The reduction was stereospecific and gave cis-1,3,4-triphenylazetidin-2-ones.Desulfurization in the presence of triphenylphosphine gave the corresponding trans-azetidinones.Consideration of the possible mechanistic pathways led to the conclusion that the reaction proceeds through formation of biradical-dipole, hydrogenation to a 1,4-dipole, and ring closure.It was also concluded that in the preparation of β-lactams by the nonconcerted <2+2> cycloaddition of imines and ketenes the first step (dipole formation) is the stereochemistry-determining step.