Intramolecular cyclizations of o-acylbenzyllithiums. Formation of benzocyclobuten-1-ol derivatives and their thermal isomerization

The formation of benzocyclobutenol derivatives by intramolecular cyclizatio ns of o-acylbenzyllithiums is described. Treatment of o-(trialkylsilylmethy l)phenyl ketones with lithium diisopropylamide (LDA) followed by quenching of the resulting benzylic carbanions with chlorotrialkylsilane resulted in stereoselective formation of the corresponding 1 -trialkylsiloxy-2-(trialky lsilyl)benzocyclobutenes in good yields. Subsequently, o-acyl-m-methoxybenz yllithiums were found to work well in cyclization to benzocyclobuten-1-ol d erivatives. The reaction of 2-benzoyl-3,4,5-trimethoxybenzyllithium, genera ted in situ by deprotonation of 6-methyl-2,3,4-trimethoxybenzophenone with LDA, with chlorotrimethylsilane afforded the corresponding 1 -(trimethylsil oxy)benzocyclobutene. Cyclization of 2-pivaloyl-3-methoxybenzyllithiums, ge nerated in situ from tert-butyl 2-methyl-6-methoxyphenyl ketones upon depro tonation with LDA, proceeded spontaneously even at -78 degrees C to give th e corresponding bentocyclobuten-1-ols. We also describe the results of ther mal isomerization of the se 1-trimethylsiloxy-2-(trialkylsilyl)benzocylobut enes.