Reactions of the hydrofluoroborate salts of open-chain analogues of Reissert compounds with some alpha,beta-ethylenic esters

The reaction of the hydrofluoroborate salt of an open-chain analogue of a R eissert compound with some a,B-ethylenic esters does not give a [4 + 2] cyc loadduct, as previously described in the case of ethyl acrylate. The reacti on starts with a 1,3-dipolar cycloaddition of a munchnone imine 5c, d, The [3 + 2] cycloadducts 13 evolve via a rearrangement-condensation sequence to give a substituted 2-pyridone derivative 18 or 19. The proposed mechanism has been verified by the isolation and structural X-ray analysis of some co mpounds of the reaction sequence.