Intramolecular 4+3 cycloadditions, vinylthionium ions from allylic alcohols

Article abstract of DOI:10.1016/0040-4039(95)02391-7

Aldehyde 9 can be prepared from ethyl pyruvate in several steps. Treatment of 1 with various diene-containing Grignard reagents results in the formation of the corresponding allylic alcohol in good yield. Exposure of these alcohols to triflic anhydride results in the formation of 4+3 cycloadducts in good to excellent yields. Furan and simple butadiene trap the intermediate allylic cation efficiently in the formation of 5,7-fused ring systems. A tethered thiophene undergoes only intramolecular Friedel-Crafts alkylation.