Alkylation of the Sulfur- and Nitrogen-containing Compounds Analogous to Thiazoline Systems

Abstract N,N-Dimethyl-2-(alkylthio)ethylamines and 3-methylthiazolidines underwent N-methylation both with trimethyloxonium tetrafluoroborate (3a) and with methyl iodide. In the methylation of N,N-dimethyl-o-(methylthio)aniline, the main reaction was S-methylation. On the other hand, in the o-ethylthio derivative, S-methylated product was major with methyl iodide, while N-methylated product was major with 3a. In the N-ethyl or N-benzyl derivative, only S-methylation occurred with both reagents. 3-Methyl-2, 2-diphenyl-2,3-dihydro-benzothiazole gave S-methylated product with 3a, suggesting that S-methylation becomes major when the 2-substituent is bulkier. 4-Methyl-3,4-dmydro-2H-benzo[b][1,4]thiazine was methylated on the nitrogen with 3a, while it gave S-methylated product with methyl iodide. Phenothiazines and 1-dialkylamino-8-(methylthio)naphthalenes gave only S-methylated products. The selective S-methylation in the case of N-methylation was carried out through protonation on the nitrogen atom followed by methylation with 3a. Possibility of intramolecular interaction between sulfonio and amino groups has been discussed based on their NMR data.