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

Abstract

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.