Intramolecular Cyclizations of o-Acylbenzyllithiums. Formation of Benzocyclobuten-1-ol Derivatives and Their Thermal Isomerization

• Published in: Journal of organic chemistry Vol. 64; no. 10; pp. 3557 - 3562
• Main Authors: Kobayashi, Kazuhiro, Kawakita, Masataka, Uchida, Masaharu, Nishimura, Koichi, Mannami, Tohru, Irisawa, Susumu, Morikawa, Osamu, Konishi, Hisatoshi
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 14.05.1999
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo982366o

The formation of benzocyclobutenol derivatives by intramolecular cyclizations of o-acylbenzyllithiums is described. Treatment of o-(trialkylsilylmethyl)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-(trialkylsilyl)benzocyclobutenes in good yields. Subsequently, o-acyl-m-methoxybenzyllithiums were found to work well in cyclization to benzocyclobuten-1-ol derivatives. The reaction of 2-benzoyl-3,4,5-trimethoxybenzyllithium, generated in situ by deprotonation of 6-methyl-2,3,4-trimethoxybenzophenone with LDA, with chlorotrimethylsilane afforded the corresponding 1-(trimethylsiloxy)benzocyclobutene. Cyclization of 2-pivaloyl-3-methoxybenzyllithiums, generated in situ from tert-butyl 2-methyl-6-methoxyphenyl ketones upon deprotonation with LDA, proceeded spontaneously even at −78 °C to give the corresponding benzocyclobuten-1-ols. We also describe the results of thermal isomerization of these 1-trimethylsiloxy-2-(trialkylsilyl)benzocyclobutenes.