Ring transformations in reactions of heterocyclic compounds with nucleophiles Reactions of 4(6)‐alkoxy‐1‐ethyl‐ and 4,6‐dialkoxy‐1‐ethyl‐pyrimidinium tetrafluoroborates with liquid ammonia

On treatment with liquid ammonia at −33° the quaternary pyrimidinium salts i.e. 4‐ethoxy‐1‐ethyl‐ (1) and 4,6‐diethoxy‐1‐ethyl‐pyrimidinium tetrafluoroborate (3) undergo amino‐de‐ethoxylation, yielding 1,4‐dihydro‐1‐ethyl‐4‐imino‐pyrimidine hydrogen tetrafluoroborate (7) and a mixture of 1,4‐dihydro‐6‐ethoxy‐1‐ethyl‐4‐imino‐ (9) and 1,6‐dihydro‐4‐ethoxy‐1‐ethyl‐6‐imino‐pyrimidine hydrogen tetrafluoroborate (11), respectively. 1HNMR and 13CNMR spectroscopic evidence is presented for the fact that compounds 1 and 3 easily give σ‐adducts at position 2. Using 15N‐labelled ammonia it was shown that in these amino‐de‐ethoxylation reactions the substitution at C(4) or C(6) does not involve ring opening but probably occurs via an SN(AEn) process. Reaction of 4‐ethoxy‐1‐ethyl‐2‐phenyl‐ (2), 6‐ethoxy‐1‐ethyl‐4‐phenyl‐ (4), 4,6‐dimethoxy‐1‐ethyl‐2‐phenyl‐ (5) and 4,6‐dimethoxy‐1‐ethyl‐2‐methyl‐pyrimidinium tetrafluoroborate (6) with liquid ammonia gives besides the amino‐de‐ethoxylation product degenerate ring transformations leading to the N‐de‐ethylated products 14–16 and 4(6)‐(ethylamino)pyrimidines 17‐19. The salt 11 and 1,6‐dihydro‐1‐ethyl‐6‐imino‐4‐phenylpyrimidine hydrogen tetrafluoroborate (12) undergo, with potassium hydroxide, a Dimroth rearrangement to pyrimidines 20 and 17, respectively.