A Ring Transformation of 6,7-Dihydro-4H-Pyrido-[1,2-a]Pyrimidin-4-Ones

摘要

Under the conditions of Vilsmeier-Haack formylation, nitrogen bridgehead ring systems containing a 6,7,8,9-tetrahydro-4H-pyrido[1,2-a]pyrimidin-4-one moiety (which can isomerize into a tautomeric 6,7-dihydro form under the reaction conditions) or a 6,7-dihydro-4H-pyrido[1,2-a]-pyrimidin-4-one moiety undergo a ring transformation to afford the same nitrogen bridgehead nitrogen ring systems containing an unsaturated 4H-pyrido[1,2-a]pyrimidin-4-one moiety. Some details of the ring transformation were investigated by using deuterated and optically active derivatives. In the first step, a 7-dimethylaminomethylene-6,7-dihydro-4H-pyrido[1,2-a]pyrimidin-4-one species is formed, which is involved in the ring transformation.