Photocyclization reactions. Part<b>6</b>. Solvent and substituent effects in the synthesis of dihydrobenzofuranols using photocyclization of 2-alkoxybenzophenones and ethyl 2-benzoylphenoxyacetates
作者:Essam Mohamed Sharshira、Mutsuo Okamura、Eietsu Hasegawa、Takaaki Horaguchi
DOI:10.1002/jhet.5570340325
日期:1997.5
Photocyclization reactions were carried out on 2-alkoxybenzophenones 1a-h and ethyl 2-benzoyl-phenoxyacetates 5a-e in three solvents of different polarity (benzene, acetonitrile and methanol) to examine solvent and substituent effects on the cyclization of 1,5-biradical intermediates to dihydrobenzofuranols. Irradiation of 1a-f in benzene gave dihydrobenzofuranols 4a-f in 80–94% yields. The ratios
在不同极性的三种溶剂(苯,乙腈和甲醇)中,对2-烷氧基二苯甲酮1a-h和2-苯甲酰基苯氧基乙酸乙酯5a-e进行光环化反应,以研究溶剂和取代基对1,5-双自由基环化的影响二氢苯并呋喃醇的中间体。在苯中辐照1a-f可以得到80-94%的二氢苯并呋喃醇4a-f。4b-f的顺式和反式异构体的比例为12∶1至1∶0,显示了顺式异构体的立体选择性形成。另一方面,在乙腈和甲醇中照射1a-f得到4a-f分别达到68–81%和7–75%的产量。但是,4b-f的顺式和反式异构体之比在乙腈中为3.5:1至1.3:1,在甲醇中为2.0:1至1:1.7,显示立体选择性降低。立体选择性的降低归因于1,5-双自由基的羟基与溶剂(乙腈和甲醇)之间的分子间氢键。类似地,在苯中辐照5a -e可立体选择性地得到顺式-二氢苯并呋喃醇顺式-11a -e。相反,在乙腈和甲醇中辐照5a-e可以得到顺式和反式异构体的混合物。11a-e是