NOVEL ANTIOXIDANTS - UNEXPECTED REARRANGEMENTS IN THE RADICAL CYCLIZATION APPROACH TO 2,3-DIHYDROBENZO[B]THIOPHENE-5-OL DERIVATIVES

The novel alpha-tocopherol analogue ,6,7-pentamethyl-2,3-dihydrobenzo[ b]thiophene-5-ol (4a) was prepared by cyclodehydration of the aryl 2-h ydroxyalkyl sulfide 6 in concentrated sulfuric acid. Aromatic brominat ion and dehydration of alcohol 6 furnished the 2-bromoaryl methallyl s ulfide 10a as the kinetic product and the 2-bromoaryl 2-methyl-2-prope nyl sulfide 11a as the thermodynamic one. Radical cyclization of these compounds afforded the desired 2,3-dihydrobenzo[b]thiophene derivativ e 12 in addition to a rearranged isomer 13 thereof. It is proposed tha t the transformation of compound 10a to 13 occurs via 6-endo cyclizati on, beta-scission, and subsequent 5-exo cyclization. The radical cycli zation of the 2-bromoaryl allyl sulfide 16, unsubstituted in the allyl ic moiety, furnished only 2,3-dihydrobenzo[b]thiophene-5-ol derivative 17.