Total synthesis of (±)-fenchone, a synthesis of (+)-fenchone and of (+)-cis-2,2,5-trimethyl-3-vinylcyclopentanone, a photoisomer of (–)-trans-caran-4-one

(+)-cis-3-(2-Hydroxyethyl)-2,2,5-trimethyl-cyclopentanone and its racemic form are synthesised from (+)-α-2,3-epoxypinane or (–)-trans-pinocarveol, and (±)-2,2,5-trimethyl-1-oxocyclopentane-3-carboxylic acid, respectively; treatment with base of the chloroketone, derived from the (+)-hydroxyketone, affords (+)-fenchone, while pyrolysis of the (+)-acetoxyketone gives (+)-cis-2,2,5-trimethyl-3-vinylcyclopentanone, the latter and its trans-isomer being photo-products of (–)-trans-and (–)-cis-caran-4-one, respectively.