The aza-ene reaction of heterocyclic ketene aminals with enones: an efficient and simple synthetic route to fused di- and tri-heterocycles

Heterocyclic ketene aminals bearing a secondary amino moiety acted as hetero-ene components to react with a number of enones under very mild conditions. The reaction of five- and six-membered heterocyclic ketene aminals 5 and 8 with methyl vinyl ketone proceeded effectively through the aza-ene addition, imine-enamine tautomerization and intramolecular cyclization to give good yields of 8-aroyl-5-hydroxy-5-methyl-1,2,3,5,6,7-hexahydroimidazo[1,2-a]pyridines 6 and 9-aroyl-6-hydroxy-6-methyl-1,2,3,4,7,8-hexahydroxy-6H-pyrido[1,2-a]pyrimidines 9, respectively. When methyl vinyl ketone was present in excess, preliminarily formed product 6 underwent a second aza-ene reaction followed by intramolecular cyclization and water-participating debenzoylation, to yield the cisoid-4,9-dihydroxy-4,9-dimethyl-1,2,5,6,6a,7,8,10 alpha-octahydro-4H,9H-imidazo[1,2,3-ij][1,8]naphthyridin-10 alpha-ylium benzoate 7 as the sole product. In refluxing acetonitrile, heterocyclic ketene aminals 5 underwent aza-ene addition and cyclocondensation reaction with a number of substituted alpha,beta-unsaturated ketones to afford 1,2,3,7-tetrahydroimidazo[1,2-a]pyridine derivatives 23 while tricyclic imidazo[1,2-a]quinoline derivatives were easily obtained when 2-benzylidene-cyclohexanone and -cyclohexane-1,3-dione were used as enophiles. The effect of the structures of heterocyclic ketene aminals, particularly of the intramolecular hydrogen bond, on the aza-ene reactivity is also discussed.