INTRAMOLECULAR 4-CYCLOADDITIONS - VINYLTHIONIUM IONS FROM ALLYLIC ALCOHOLS(3)

Aldehyde 9 can be prepared from ethyl pyruvate in several steps. Treat ment of 1 with various diene-containing Grignard reagents results in t he formation of the corresponding allylic alcohol in good yield. Expos ure of these alcohols to triflic anhydride results in the formation of 4+3 cycloadducts in good to excellent yields. Furan and simple butadi ene trap the intermediate allylic cation efficiently in the formation of 5,7-fused ring systems. A tethered thiophene undergoes only intramo lecular Friedal-Crafts alkylation.