A Novel Route to Isoannulated Heteroaromatic Compounds, 1. – Synthesis and Reactions of Furofurans, Thienofurans, Furobenzofurans, and BenzothienofuransA general method for the synthesis of furo‐and thienofurans4–7has been developed. The reaction principle is based on the thermal transformation of suitably structured epoxyhexenynes (15–19) following the general reaction sequence1→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 epoxyhexenyne19brearranges in poor yield to the isobenzofuran52(identified as dimethyl acetylenedicarboxylate adduct47) and 2‐cyano‐α1‐naphthol (46). The structure of the furo[3,4‐b]furan4bhas 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 oxiranes15c, hand17ewhich – in additon to the furofurans4c, hand6e– lead to the furylindenes31c, h/32c, hand41, resp. The Diels‐Alder reactivity of the furo