. Palladium-catalyzed syntheses of naturally occurring acetylenic thiophenes and related compounds

5-(3-Buten-l-ynyl)-2,2'-bithienyl (1a), a natural product first isolated from Tagetes roots which shows nematicidal and photo-induced fungicidal activity, and 2-phenyl-5-(3-buten-l-ynyl) thiophen (1b) have been synthesized using two different methods. The first one (Method A) involves the palladium-catalyzed cross-coupling of vinyl bromide with the Grignard reagents derived from 5-ethynyl-2,2'-bithienyl (6a) and 2-ethynyl-5-phenylthiophen (6b). The second method (Method B) utilizes the coupling reaction of vinyl bromide with 6a and 6b, respectively, in the presence of a catalytic amount of (PPh3)4Pd and CuI. Such reaction, which was carried out under phase-transfer conditions employing BnEt3N+Cl- as phase transfer agent and 2.5NaqNaOH as base, has been also employed to prepare a large number of heterocyclic acetylene derivatives including some naturally-occurring compounds. The experimetal conditions of Method B allow also the direct production of heterocyclic acetylene derivatives (1) starting from 1-alkynyltrimethylsilanes (5) and organic halides (2).