Cyclocondensation of 2-lithiomethylthiazoles and 2-lithiomethylthiazoline with α-oxoketene dithioacetals: synthesis of substituted and annelated thiazolo- and dihydrothiazolo[3,2-a]pyridinium compounds

Abstract

The oxoketene dithioacetals 1 undergo regioselective 1,2-addition with 2-lithiomethyl-4-methylthiazole (2a) and 2-lithiomethylthiazoline (3) to afford the carbinol acetals 4 which on borontrifluoride etherate assisted cyclization yield the corresponding substituted and annelated 3-methyl-5- methylthiothiazolo[3,2-a.] pyridinium tetrafluoroborates (5a- 8, 8a , b, 10, 12) and the dihydro derivatives (23 - 25) respectively in good yields. The corresponding 2-methyl-5-phenylthiazole (16) however underwent both nuclear and side chain deprotonation and yielded a mixture of products under similar reaction conditions.