Ring transformations in reactions of heterocyclic compounds with nucleophiles (XIII) Degenerate ring transformations of N‐methylpyrimidinium salts into 2‐substituted pyrimidines

On treatment of the quaternary pyrimidinium salts i.e. 1‐methyl‐4‐phenylpyrimidinium iodide and 1‐methyl‐5‐phenylpyrimidinium iodide with cyanamide, O‐methylisouronium chloride or bis[S‐methylisothiouronium] sulfate in basic media, 2‐amino‐4‐phenylpyrimidine and 2‐amino‐5‐phenylpyrimidine are formed respectively. A ring transformation is involved in which the two‐atom fragment N(1)‐C(2) of the pyrimidine ring is replaced by an N‐C fragment of the reagent. On reacting 1‐methylpyrimidinium iodide with benzamidinium chloride or pivalamidinium chloride in a solution of sodium ethoxide in ethanol, 2‐phenylpyrimidine and 2‐tert‐butylpyrimidine are formed respectively. It is proved by 15N‐labelling that this nucleophilic substitution occurs via a ring transformation in which the N(1)‐C(2)‐N(3) fragment of the pyrimidine is replaced by the N‐C‐N fragment of the amidine. These reactions are new examples of a nucleophilic substitution occurring according to an SN(ANRORC) mechanism.