Ring transformations of heterocyclic compounds with nucleophiles. Part XVI. Degenerate ring transformations of 1,3‐diethyl‐1,4(3,4)‐dihydro‐4‐oxopyrimidinium tetrafluoroborates with ammonia

Treatment of 1,3‐diethyl‐1,4(3,4)‐dihydro‐4‐oxopyrimidinium tetrafluoroborate and its 2‐phenyl, 6‐phenyl, 6‐methyl and 6‐ethoxy derivatives with aqueous ammonia resulted in the formation of a mixture of open‐chain compounds i.e. N ‐formyl(acetyl,benzoyl)‐ N ‐ethyl‐3‐(ethylamino)‐acrylamides and N ‐ethyl‐3‐[formyl(acetyl,benzoyl)ethylamino]‐acrylamides. They are formed by cleavage of the pyrimidine ring between the N(1)‐C(2) and N(3)‐C(2) bond, respectively. In liquid ammonia the same ring cleavage generally occurs; however, in the case where a 6‐ethoxy group is present, recyclisation can take place, leading to 6‐(ethylamino)pyrimidine derivatives. This degenerate ring transformation has been observed also with the 2‐methyl and 2‐phenyl derivative of 1,3‐diethyl‐1,4(3,4)‐dihydro‐6‐ethoxy‐4‐oxopyrimidinium tetrafluoroborate. Evidence is presented by means of 1 H‐NMR and 13 C‐NMR spectroscopy that all these reactions are iniated by attack of NH 3 at the C(2)‐position. Some of the above‐mentioned open‐chain compounds underwent a ring closure to the initially used 1,3‐diethyl‐1,4(3,4)‐dihydro‐4‐oxopyrimidinium tetrafluoroborates on treating them with hydrofluoroboric acid in absolute ethanol.