1,5-Phosphonium betaines from N-triflylpropiolamides, triphenylphosphane, and active methylene compounds

N-Phenyl-N-(trifluoromethylsulfonyl)propiolamides react with triphenylphosphane in the presence of various active methylene compounds CH(2)XY in a 1:1:1 molar ratio to furnish 1-phosphonium-5-oxabetaines, Ph(3)P(+)–C(R)=CH–C(O(–))=CXY. These betaines are formed preferentially, but not exclusively, as E-diastereoisomers with respect to the vinylic double bond. In some cases, separation of the two diastereoisomers was achieved by fractionating crystallization. Structure determination by X-ray diffraction analysis revealed marked conformational differences around the CH–C(O(–)) single bond of E and Z-isomers and extended charge delocalization in the anionic part.