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

Several 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.