N-(3-benzo[b]thienyl)iminophosphoranes toward the synthesis of benzo[b]thieno[3,2-b]pyridines:: Reactivity and alternative regioselectivity with α,β-unsaturated ketones and aldehydes

The novel N-(3-benzo[b]thienyl)iminophosphoranes 1b-d react with alpha,beta-unsaturated aldehydes and ketones 2a-e to give varying mixtures of the regioisomeric benzothieno[3,2-b]pyridines 3a-d and 4a-d as a result of preferential attack of either imino nitrogen or alpha-thienyl carbon at the enone carbonyl group. The findings indicate that the progressive replacement of phenyl with methyl P-substituent greatly enhances the reactivity of the phosphorane 1 and concomitantly enhances the propensity of the phosphorane itself for addition to the enone by the alpha-thienyl carbon. (C) 2000 Elsevier Science Ltd. All rights reserved.