Iminium carbonic acid derivative salts.IX. Synthesis ofN,S-containing heterobicycles fromN-protected 2-methylthio-1,3-thiazinium and 2-methylthiothiazolium salts part 1. Preparation ofN-protected 2-methylthio-1,3-thiazinium and 2-methylthiothiazolium salts and their reaction with CH-acidic compounds

N-Boc-protected 2-methylthio-1,3-thiazinium 3 and 2-methylthiothiazolium salts 4,7 obtained from the corresponding 1,3-thiazine-2-thiones and thiazolidine-2-thiones by the action of methyl iodide or trimethyloxonium tetrafluoroborate were reacted with vinylogous CH-acidic compounds forming ketene N,S-acetals 5,6,8,9. The protection group was removed with trifluoroacetic acid whereupon the desired cyclisation to pyrido[2,1-b]-1,3-thiazines 10 and thiazolo[3,2-b]pyridines 11,12 took place.