Synthesis of 2‐amino‐5,6‐dihydro‐4H‐1,3‐thiazines and related compounds by acid catalyzed cyclization of allylic isothiuronium salts

Abstract

AbstractSeveral allylic isothiuronium salts 6 have been shown to yield 4‐substituted 2‐amino‐5,6‐dihydro‐4H‐1,3‐thiazines (2; R3 = H) and the 2‐phenylimino compound 10 on exposure to trifluoroacetic acid or trifluoroacetic acid‐stannic chloride. The overall process constitutes a convenient transformation of readily available ketones or aldehydes 3 into the target hetero‐cycles. The scope and limitations of this approach are discussed in terms of carbenium ion intermediates (7). Whereas this method successfully led to the imidazo[2,l‐b][1,3]thiazine 13 (93% from 12), its application to the synthesis of 2‐amino‐2‐thiazolines (eg. 14→ 15) or imidazo‐[2,1‐b]thiazoles (eg. 16 → 17) was unsuccessful. The 2‐acetylimino derivatives 9 were found to be useful for purification of the 2‐aminothiazines.