Effect of Ortho Substituents on the Direction of 1,2-Migrations in the Rearrangement of 2-exo-Arylfenchyl Alcohols

A series of 2-exo-arylfenchyl alcohols (11a−k) was submitted to acid hydrolysis in EtOH/10 M HCl (2:1−1:1, v/v) or with trifluoromethanesulfonic acid (TfOH) (1 equiv) in CHCl3 under varying conditions. In all cases, initial formation of the cyclofenchene 12 took place, and after prolonged treatment with acid the reaction proceeded along one of two pathways depending on the nature of the aryl substituent. When the aryl substituent was o-NH2 11b or o-OH 11e, Wagner−Meerwein rearrangement took place to give a carbocationic intermediate 9 that was trapped by the N or O heteroatom to give (2R,4aR,9aR)-9-aza-2,4a-(10,10-dimethylmethano)-1,2,3,4,4a,9a-hexahydro-9a-methyl-9H-fluorene (3b) and (3R,4aR,9bR)-3,9b-(10,10-dimethylmethano)-1,2,3,4,4a,9b-hexahydro-4a-methyldibenzofuran (3c), respectively. In the case of 3c bearing an oxygen heteroatom, equilibration to give the thermodynamic product (1R,4S,4aR,9bR)-1,2,3,4,4a,9b-hexahydro-1,4-methano-1,4a,9b-trimethyldibenzofuran (4a), arising from Nametkin rearrangemen...