Photocyclization reactions .6. Solvent and substituent effects in the synthesis of dihydrobenzofuranols using photocyclization of 2-alkoxybenzophenones and ethyl 2-benzoylphenoxyacetates

Photocyclization reactions were carried out on 2-alkoxybenzophenones la-h and ethyl 2-benzoylphenoxyacetates 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 of cis- and trans-isomers of 4b-f were 12:1 to 1:0, showing stereoselective formation of cis-isomers. On the other hand, irradiation of 1a-f in acetonitrile and methanol gave 4a-f in 68-81% and 7-75% yields, respectively. However, the ratios of cis- and trans-isomers of 4b-f were 3.5:1 to 1.3:1 in acetonitrile and 2.0:1 to 1:1.7 in methanol, showing decreased stereoselectivity. The decrease in stereoselectivity was attributed to intermolecular hydrogen bonding between the hydroxyl group of 1,5-biradicals and solvents (acetonitrile and methanol). Similarly, irradiation of 5a-e in benzene afforded cis-dihydrobenzofuranols cis-11a-e stereoselectively. In contrast, irradiation of 5a-e in acetonitrile and methanol gave a mixture of cis- and trans-isomers of 11a-e because of intermolecular hydrogen bonding between the hydroxyl group of 1,5-biradicals and solvents. The cis and trans ratios of 11a-e varied from 1.5:1 to 17.8:1 in acetonitrile and from 2.6:1 to 1:4.5 in methanol. Solvent and substituent effects on the cyclization of 1,5-biradicals and reaction pathways are discussed.