EFFECT OF ORTHO SUBSTITUENTS ON THE DIRECTION OF 1,2-MIGRATIONS IN THE REARRANGEMENT OF 2-EXO-ARYLFENCHYL ALCOHOLS

A series of 8-exo-arylfenchyl alcohols (11a-k) was submitted to acid h ydrolysis in EtOH/10 M HCl (2:1-1:1, v/v) or with trifluoromethanesulf onic acid (TfOH) (1 equiv) in CHCl3 under varying conditions. In all c ases, 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 ary l 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 ano)-1,2,3,4,4a,9a-hexahydro-9a-methyl- 9H-fluorene (3b) and ano)-1,2,3,4,4a,9b-hexahydro-4a-methyldibenzofura n (3c), respectively. In the case of 3c bearing an oxygen heteroatom, equilibration to give the thermodynamic product hexahydr-1,4-methano-1 ,4a,9b-trimethyldibenzofuran (4a), arising from Nametkin rearrangement , took place via the intermediate 8-fenchyl cation. In contrast, when the aryl substituent was o-OCH3 11f, direct conversion of the cyclofen chene to the Nametkin product 4a occurred with no detectable prior for mation of the Wagner-Meerwein product. In the case of o-tolylfenchyl a lcohol 11k, cyclofenchene formation was facile, but subsequent convers ion to either the Wagner-Meerwein or Nametkin products was highly disf avored. The results indicate that ortho substitution disfavors Wagner- Meerwein rearrangement through adverse steric and electronic effects. However, when the ortho substituent is NH2 or OH it is proposed that a nchimeric assistance provides an intermediate 15a that is stereoelectr onically predisposed to Wagner-Meewein rearrangement.