A,B-Diheteropentalenes by a Tandem Intramolecular Diels-Alder/Reverse Diels-Alder Reaction Sequence. Application to the Synthesis of Thieno[3,4-b]furan

Alkylation of 2-furylmethanethiol (28) with propargyl chloride gave the thioether (22) which on methoxycarbonylation afforded the acetylenic ester (30). On heating, this material underwent an intramolecular Diels-Alder reaction to give the tricyclic compound (32). In the presence of 3,6-di(pyridin-2'-yl)-s-tetrazine, (32) afforded methyl 4,6-dihydrothieno[3,4-b]furan-3-carboxylate (38) by a sequence involving a further Diels-Alder reaction followed by two reverse Diels-Alder reactions. The ester (38) could be dehydrogenated to give methyl thieno[3,4-b]furan-3-carboxylate (40) while hydrolysis of (38), followed by decarboxylation and dehydrogenation delivered the parent thieno[3,4-b]furan (5). 3-Methyl-4,6-dihydrothieno[3,4-b]furan (46) and 3-methylthieno[3,4-b]furan (47) were prepared; a comparison of the (4)J(Me-C=C-H) coupling constants in the H-1 n.m.r. spectra of (46) and (47) suggests that an increase in the C2-C3 furyl bond order accompanies the (46) --> (47) conversion. Methyl 4,6-dihydrofuro[3,4-b]furan-3-carboxylate (39), 4,6-dihydrofuro[3,4-b]furan (27) and methyl 4,6-dihydro-6-phenylfuro[3,4b]furan-3-carboxylate (53) were prepared by an analogous tandem reaction sequence. These compounds could not be dehydrogenated to the fully conjugated furo[3,4-b]furan ring system.