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-(benzotriazo l-1-ylmethyl)thiophenes 3a,b, themselves readily available from the co ndensation of 1-(hydroxymethyl)benzotriazole with thiophenes 1a,b. Tre atment of 2-(benzotriazol-1-ylmethyl)thiophenes 3b and 5gj with styren es in the presence of zinc bromide results in formal [3 + 2] cycloaddi tion to give in good yields substituted cyclopent[b]thiophenes 16a/17a , 16b/17b, and 18. Lithiation and 1,4-addition to a variety of alpha,b eta-unsaturated ketones and aldehydes, and subsequent acid-catalyzed i ntramolecular cyclization followed by debenzotriazolylation-dehydratio n converts 3 and 5 to a range of polysubstituted benzo[b]thiophenes 19 a-d and 25a-e in moderate to excellent yields. NOE difference spectros copy and NMR H-1-C-13 long-range correlation support structures of typ es 19 and 25 and exclude those of type 26, thus confirming the cycliza tion pathway.