Reactions of pyridinium or isoquinolinium ketene dithioacetals with aromatic N ‐imines and S ‐imines

Reactions of ketene dithioacetals, 1‐[1‐substituted 2,2‐bis(methylthio)ethenyl]pyridinium 1a‐i or ‐isoquinolinium 2a,b iodides with aromatic N ‐imines, 1‐aminopyridinium 3a‐1,1 ‐aminoquinolinium ( 4 ), and 2‐amino‐isoquinolinium ( 5 ) mesitylene sulfonates gave the corresponding 2‐methylthioimidazo[1,2‐ a ]pyridines 9a‐k , 2‐methylthiopyrazolo[1,5‐ a ]pyridines 11a‐q , 2‐methylthioimidazo[2,1‐ a ]isoquinoline derivatives 10a,b and 2‐methylthiopyrazolo[1,5‐ a ]quinoline ( 12 ). The benzoyl compounds, 1‐[1‐benzoyl‐2,2‐bis(methylthio)ethenyl]‐pyridinium iodides 1g,h,i reacted with N ‐imine 3a to give the 3‐benzoyl‐2‐methylthioimidazo[1,2‐ a ]pyridines 9h‐k . The reaction of pyridinium ketene dithioacetals 1a,f,g (R 1 = COOE t , COPh, and CN) with substituted pyridinium N ‐imines having an electron‐withdrawing group on the pyridine ring afforded only the corresponding pyrazolo[1,5‐ a ]pyridine derivatives 11j‐r in good yields. Reactions of ketene dithioacetals with various S ‐imines are also described. Possible mechanisms for the formation of 9 and 11 are described.