Unusual Reactivity of (Vinylimino)phosphoranes and Their Utility in the Preparation of Pyridine and Dihydropyridine Derivatives.

New reactions of (vinylimino)phosphoranes with aldehydes involving an initial nucleophilic attack of the beta-carbon atom of the vinyl side chain on the carbonyl carbon atom are reported. Iminophosphorane 4 derived from ethyl beta-azidoacrylate reacts with substituted cinnamyl aldehydes to give a mixture of 2-arylpyridine and 4-styryldihydropyridine derivatives, whereas the reaction with substituted benzaldehydes provides 4-aryldihydropyridine derivatives. However, the iminophosphorane 16 derived from the diethyl azidofumarate reacts with cinnamyl aldehydes through the expected aza-Wittig fashion to give 4-arylpyridine derivatives after dehydrogenation of the resulting dihydropyridine.