Efficient Syntheses of 2-Functionalized Thiophenes, Cyclopent[b]thiophenes, and Polysubstituted Benzo[b]thiophenes from 2-(Benzotriazol-1-ylmethyl)thiophenes

Diverse 2-(functionalized-alkyl)- and 2-alkenylthiophenes 2a, b, 4a, b, and 6a − d are prepared via the side chain elaboration of 2-(benzotriazol-1-ylmethyl)thiophenes 3a, b, themselves readily available from the condensation of 1-(hydroxymethyl)benzotriazole with thiophenes 1a, b . Treatment of 2-(benzotriazol-1-ylmethyl)thiophenes 3b and 5g, j with styrenes in the presence of zinc bromide results in formal [3 + 2] cycloaddition to give in good yields substituted cyclopent[ b ]thiophenes 16a / 17a, 16b / 17b, and 18 . Lithiation and 1,4-addition to a variety of α,β-unsaturated ketones and aldehydes, and subsequent acid-catalyzed intramolecular cyclization followed by debenzotriazolylation−dehydration converts 3 and 5 to a range of polysubstituted benzo[ b ]thiophenes 19a − d and 25a − e in moderate to excellent yields. NOE difference spectroscopy and NMR 1 H− 13 C long-range correlation support structures of types 19 and 25 and exclude those of type 26, thus confirming the cyclization pathway.