A NOVEL ROUTE TO ISOANNULATED HETEROAROMATIC-COMPOUNDS .1. SYNTHESIS AND REACTIONS OF FUROFURANS, THIENOFURANS, FUROBENZOFURANS, AND BENZOTHIENOFURANS

A general method for the synthesis of furo- and thienofurans 4 - 7 has been developed. The reaction principle is based on the thermal transformation of suitably structured epoxyhex-enynes (15 - 19) following the general reaction sequence 1 --> 2 --> 3 (annulation type A). Derivatives of the so far unknown diheteropentalenes furo[3,4-b]furan (4b, c, d, g, h), furo[3,4-b]benzofuran (6b, d, e, 45) as well as benzo[4,5]thieno[2,3-c]furan (7b, d) are obtained by short-time thermolysis. Likewise two representatives of the previously reported thieno[2,3-c]furan system are prepared (5b, c). By flash vacuum thermolysis the benzo-annulated epoxyhexenyne 19b rearranges in poor yield to the isobenzofuran 52 (identified as dimethyl acetylenedicarboxylate adduct 47) and 2-cyano-alpha-naphthol (46). The structure of the furo[3,4-b]furan 4b has been established by X-ray structural analysis. A mechanistic explanation of the transformation of the epoxyhexenynes to diheteropentalenes is proposed. Indications for the occurrence of carbenes as product-determining species are obtained with the phenylcyano-substituted oxiranes 15c, h and 17e which - in additon to the furofurans 4c, h and 6e - lead to the furylindenes 31 c, h/32 c, h and 41, resp. The Diels-Alder reactivity of the furo- and thienofurans 4g, 5b, c, and 6b has been examined.