Alkylphosphonous acid diesters, novel reagents for the oxalimide cyclization to penems

Abstract Alkylphosphonous acid diesters MeP(OR) 2 3a-b were shown to be highly effctive and mild reducing reagents in the oxalimide cyclization of azetidinone-1-oxalyl-4-di- or tri-thiocarbonates or 4-thioesters 1a-t forming penems 2a-t at lower temperature shorter reaction times, and higher yields compared to classical phosphites P(OMe) 3 and P(OEt) 3 .