Photocyclization Reactions. Part 7 [1]. Solvent and Substituent Effects in the Synthesis of Dihydrobenzofuranols Using Photocyclization of α-(2-Acylphenoxy)toluenes and Ethyl 2-Acylphenoxyacetates

Article abstract of DOI:10.1002/jhet.5570340634

Photocyclization reactions were carried out on α-(2-acylphenoxy)toluenes 1a-e and 2-acylphenoxyacetates 2a-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-e in benzene gave cis-dihydrobenzofuranols cis-4b-e selectively in 14-84% yields along with rearranged products, 2-acylbenzophenones 5b-d (23-39% yields). Photocyclization of 1a-e in acetonitrile or methanol gave a mixture of cis- and trans-dihydrobenzofuranols 4a-e in 28-81% yields and small amount of 2-acylbenzophenones 5b-c in 6-12% yields. In polar acetonitrile or methanol, the cis and trans stereoselectivity of dihydrobenzofuranols decreased. The decrease in stereoselectivity was attributed to intermolecular hydrogen bonding between the hydroxyl group of 1,5-biradicals and solvents. On the other hand, irradiation of esters 2a-e in benzene afforded cis-dihydrobenzofuranols cis-13a-e selectively in 48-74% yields. Similarly, photocyclization of 2a-e in acetonitrile or methanol produced dihydrobenzofuranols 13a-e in a fair to good yields. In the photocyclization of 2b-d, not only in nonpolar benzene but also in polar acetonitrile or methanol, increasing the size of alkyl group from methyl (R = Me) to ethyl or isopropyl group (R = Et or i-Pr) gave cu-dibydrobenzofuranols cis-13b-d predominantly. Conformational, solvent and substituent effects on the cyclization of 1,5-biradicals and reaction pathways are discussed.