Synthesis of furano[2,3-c]pyran-3-one and thieno[2,3-c]pyran-3-one derivatives through the coupling of 3-alkynyl-2-heteroaromatic carboxaldehydes with Fischer carbene complexes: total synthesis of a Baccharis-derived cadinene derivative.

The coupling of Fischer carbene complexes with 3-alkynyl-2-heteroaromatic carboxaldehyde derivatives has been examined. The reaction affords pyrones fused to furans or thiophenes in a single step. The compounds are stable enough for isolation. If the carbene complex features a remote alkene substituent, a subsequent Diels-Alder reaction can occur. This reaction has been used as the key step in the synthesis of a naturally occurring cadinene derivative.