Tandem Reactions in 4-Siloxy-1-benzopyrylium Salts: Introduction of Substituents and Cyclohexene and Cyclopentane Annulation in Chromones

Article abstract of DOI:10.1021/jo00006a018

Reactions of 4-<(tert-butyldimethylsilyl)oxy>-1-benzopyrylium triflates (2a-c) with silyl enol ethers (3a-d) or allyl organometallic reagents (5a-c) afforded the corresponding 2-substituted 4-siloxy-2H-1-benzopyrans (4a-d and 6a-d) along with 2,3-dihydrobenzopyrone derivatives (7a-c).An unexpected cyclopentane annulation to give 8a,b was observed in the reaction of 2a,b with 3-(trimethylsilyl)-1-butene (5d).Treatment of the products (4a and 6a) with electrophiles (iminium salt, NBS, and NCS) converted them into the corresponding 2,3-disubstituted 2,3-dihydrobenzopyrone derivatives (9a-c).Reaction of benzopyrylium salts (2a,b) with α,β-unsaturated ketones (10a-g) in the presence of tert-butyldimethylsilyl triflate and 2,6-lutidine gave cyclohexene annulation products (xanthone derivatives, 11a-j) in moderate to high yield.The reaction mechanisms are explained in terms of stereoelectronic and 1,3-allylic strain effects together with steric hindrance during the reaction.