Direction of the Reaction of 6-Methylpyrimidine-2,4(1H,3H)-dione with 2-Chloromethylthiirane: N 1- or N 3-Thietanyl Derivative?

The reaction of 6-methylpyrimidine-2,41H,3H-dione with 2-chloromethylthiirane gave 6-methyl-N-thietan-3-ylpyrimidine-2,41H,3H-dione. Its oxidation subsequent reaction of the resulting N-1,1-dioxo-lambda(6)-thietan-3-yl derivative with ethyl chloroacetate afforded the corresponding ethyl pyrimidinylacetate. The structure of the latter was determined by X-ray analysis, which confirmed the formation of N (3)-thietan-3-yl derivative rather than its N1-substituted isomer in the title reaction. According to the results of B3LYP/6-31G++d,p, PBE/3 zeta, MP2/6-31G++d,p quantum chemical calculations,the N (3)-thietanyl derivative is more stable than the N (1)-isomer. It was also found that the calculated barrier to internal rotation of the thietanyl group about the N-C bond in 6-methyl-3-thietan-3-yl-pyrimidine-2,41H,3H-dione is lower than in the N (1)-isomer.