3-Hydroxypyrroles and 1H-pyrrol-3(2H)-ones. Part 14.Pyrolysis of oxazolidinylmethylene derivatives of Meldrum's acid – synthesis of N-alkenyl-3-hydroxypyrroles and related reactions

Abstract

Flash vacuum pyrolysis (FVP) of the title compounds 14 and 17 at 600–625 °C (0.005 Torr) gives the N-alkenylpyrrolones 22 and 23 respectively. The mechanism is shown to involve hydrogen transfer and cyclisation of the methyleneketene intermediate (e.g.25) to a fused pyrrolone (e.g.28). This species fragments to create an azomethine ylide which provides the alkenyl substituent by a further hydrogen transfer. Similar reactions are shown by the thiazolidine derivatives 20 and 21, though in the latter case the initial bicycles 35 and 37 can be observed by NMR spectroscopy. When the normal sites for hydrogen transfer are blocked by substituents (as in compound 16) an alternative hydrogen transfer–cycloaddition sequence leads to the fused pyridin-4-one 43.