The chemistry of terpenes. Part XIV. Syntheses of (±)- and (+)-fenchone, and (+)-cis-2,2,5-trimethyl-3-vinylcyclopentanone, a photoisomer of (–)-trans-caran-4-one

Abstract

Treatment of (+)-α-2,3-epoxypinane or (–)-trans-pinocarveol with hydrogen bromide gives a mixture in which (–)-2-endo-bromo-6-exo-hydroxy-1,3,3-trimethylbicyclo[2,2,1]heptane is the major product. With silver acetate this affords (–)-2,2,4-trimethylcyclopent-3-enylacetaldehyde, which gives (–)-4-(2-hydroxyethyl)-1,3,3-trimethylcyclopentene on reduction. The alcohol reacts with peracetic acid giving an almost quantitative yield of (–)-β-1,2-epoxy-4β-(2-hydroxyethyl)-1,3,3-trimethylcyclopentane, which with boron trifluoride yields (+)-cis-3-(2-hydroxyethyl)-2,2,5-trimethylcyclopentanone. Treatment of the corresponding chloro-ketone with base affords (+)-fenchone.

Pyrolysis of (+)-cis-3-(2-acetoxyethyl)-2,2,5-trimethylcyclopentanone yields (+)-cis-2,2,5-trimethyl-3-vinyl-cyclopentanone, a photoisomer of (–)-trans-caran-4-one.

(±)-Fenchone is synthesised from 2,2,4-trimethyl-3-oxocyclopentanecarboxylic acid by chain extension to the largely cis-1-acetic acid, diborane reduction of which gives solely, (±)-cis-3-(2-hydroxyethyl)-2,2,5-trimethyl-cyclopentanone. The corresponding chloro-ketone affords (±)-fenchone on treatment with base.