Synthesis of [1]Benzothieno[3,2- b][1]benzothiophene Derivatives via Successive Iodocyclization/Photocyclization of Alkynes.

A new synthetic method for [1]benzothieno[3,2-b][1]benzothiophene derivatives (BTBTs) was developed. The present method consists of iodocyclization of 1,2-bis(2-methylthiophenyl)ethynes and photolysis of 3-iodo-(2-methylthiophenyl)benzo[b]thiophenes. With 1,2-bis(2-methylthiophenyl)ethynes treated with I-2/PhI(OAc)(2) in CH2Cl2 at room temperature, selective cyclization at sulfur took place to give 3-iodo-(2-methylthiophenyl)benzo[b]thiophenes in good yields. Irradiation of the iodinated benzo[b]thiophenes with a high-pressure Hg lamp (>290 nm) provided BTBTs in good yields. Furthermore, the present method was applied to the synthesis of bis[1]benzothieno[2,3-d:2',3'-d']benzo[1,2-b;4,5-b']dithiophene (BBTBDT).