ON THE REACTION OF (VINYLIMINO)PHOSPHORANES .17. PREPARATION OF N-VINYLCARBODIIMIDES AND THEIR [4+2] CYCLOADDITION WITH SEVERAL DIENOPHILES TO GIVE PYRIDINE RING-SYSTEM

The aza-Witting reaction of (vinylimino)triphenylphosphorane and its several derivatives was examined to give N-phenyl-N'-vinylcarbodiimide and its derivatives, all of which underwent a [4 + 2] cycloaddition reaction with electron-rich dienophiles (enamines) and/or electron-deficient dienophiles (activated acetylenes), resulting in the formation of a pyridine ring system. The regioselectivity of the cycloaddition reaction could be rationalized on the basis of a frontier orbital consideration.