Cycloadditions of allylic cations to conjugated dienes: stereochemistry

Abstract

The 3-methylbicyclo[3,2,1]oct-6-en-3-yl cation (8) has been generated from cyclopentadiene and the 2-methyl-allyl cation, and trapped under conditions which favour (i) a long and (ii) a short carbonium ion lifetime. In case (i)(solvent liquid sulphur dioxide; nucleophile: partly suspended trichloroacetate ion) a near statistical mixture of endo- and exo-substituted epimers (9a) and (10a) arises, while in case (ii)(solvent dichloromethane–acetonitrile; nucleophile:acetonitrile) only a single epimer, namely the endo-substituted derivative (15), is formed. On these and theoretical grounds it is suggested that a chair-like transition state (2) is favoured over the boat alternative (4) in cycloadditions of allylic cations to conjugated dienes.