Novel Antioxidants: Unexpected Rearrangements in the Radical Cyclization Approach to 2,3-Dihydrobenzo[b]thiophene-5-ol Derivatives

The novel α-tocopherol analogue 3,3,4,6,7-pentamethyl-2,3-dihydrobenzo[ b ]thiophene-5-ol ( 4a ) was prepared by cyclodehydration of the aryl 2-hydroxyalkyl sulfide 6 in concentrated sulfuric acid. Aromatic bromination and dehydration of alcohol 6 furnished the 2-bromoaryl methallyl sulfide 10a as the kinetic product and the 2-bromoaryl 2-methyl-2-propenyl sulfide 11a as the thermodynamic one. Radical cyclization of these compounds afforded the desired 2,3-dihydrobenzo[ b ]thiophene derivative 12 in addition to a rearranged isomer 13 thereof. It is proposed that the transformation of compound 10a to 13 occurs via 6-endo cyclization, β-scission, and subsequent 5-exo cyclization. The radical cyclization of the 2-bromoaryl allyl sulfide 16, unsubstituted in the allylic moiety, furnished only 2,3-dihydrobenzo[ b ]thiophene-5-ol derivative 17 .