Studies in [3,4] sigmatropic rearrangements: a novel synthesis of 9-allenyl-10-substituted anthracenes from 10,10-disubstituted 9-anthrols
A series of 10,10-disubstituted anthrones (3a–f) have been prepared from anthrone (1) and 10-prop-2-ynylanthrone (2) by both phase-transfer catalysed alkylation and classical alkylation. Reduction of these compounds with sodium borohydride in methanol gave a series of 10,10-disubstituted anthrols (6a–f). These when heated in acetic acid underwent smooth rearrangement to give 9-allenyl-10-alkyl (and related alkenyl and alkynyl) anthracenes (7a, b, e, f) except for 10,10-dibut-2-ynyl (6c) and 10,10-dibut-2-enyl-9-anthrol (6d) where the corresponding acetates (8c, d) were obtained. This reaction was also studied in the presence of a small amount of trifluoroacetic acid (TFA) and toluene-p-sulphonic acid (TPSA). In the presence of the former in carbon tetrachloride the reaction was complete in 5 min at 0–5 °C. The migratory aptitude of groups and the mechanism of the reaction is commented on.