First ring contraction-desulfurization of 1-(arylcarbonyl)pyrido[2,1-c]-1,4-thiazines to 1-(arylcarbonyl)indolizines and its application to 3-arylthieno[3,2-a]indolizine synthesis

The ring contraction-desulfurization route from transient 1-(arylcarbonyl)pyridol2,1-c]-1,4-thiazine intermediates, generated in situ from the treatment of the corresponding pyridinium salts with a base and then a dehydrogenation agent, to 1-(arylcarbonyl)indolizines was first observed. By using this route three 1-arylcarbonyl-6,8-dimethylindolizines having the protected 2-thiol group were prepared and their transformation to 2-acyl-3-arylthieno[3,2-a]indolizine derivatives were performed in good yields.