Synthesis of 2,3-dihydroimidazo[1,2-a]pyrimidin-5(1H)-ones by the domino Michael addition retro-ene reaction of 2-alkoxyiminoimidazolidines and acetylene carboxylates

2-Alkoxyiminoimidazolidines react with acetylene dicarboxylates and ethyl phenylpropiolate to give 8-alkoxy-imidazo[1,2-a]pyrimidin-5(3H)-ones , which subsequently undergo a sterically induced multihetero-retro-ene fragmentation to give imidazo[1,2-a]pyrimidin-5(1H)-ones together with formaldehyde or benzaldehyde. On the other hand, a similar reaction of with ethyl propiolate gives corresponding 8-alkoxy-imidazo[1,2-a]pyrimidin-5(3H)-ones . The unsubstituted imidazo[1,2-a]pyrimidin-5(1H)-one can be prepared by retro-ene reaction of upon prolonged heating in refluxing ethanol. A direct synthetic approach to 1-formyl-7-phenyl-imidazo[1,2-a]pyrimidine-5(1H)-one is reported using DMF/sulfonyl chloride as a new Vilsmeier-type N-formylating reagent.