New synthetic routes to pyrrolo-[1,2-a]- and -[1,2-c]-imidazol-5-ones by flash vacuum pyrolysis

1-Substituted and 1,3-disubstituted pyrrolo[1,2-c]imidazol-5-ones 25-27 have been made in fair to excellent yield by flash vacuum pyrolysis (FVP) of the Meldrum's acid precursors 11-13, FVP of the appropriate propenoate precursor 23, 22 and 18-20 gives pyrrolo[1,2-c]imidazol-5-one 2, pyrrolo[1,2-a]-imidazol-5-one 3 and their 6-methyl derivatives 28-30 respectively in 32-90% yield, The mechanism of the propenoate pyrolyses involves rate determining E to Z-isomerisation of the alkene followed by elimination of an alcohol and electrocyclisation to the fused ring products.