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

Abstract

AbstractPhotocyclization 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 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 stereo‐selectively. 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.