Syntheses of 1,3,3-trimethyl-2-oxabicyclo[2.2.2]oct-5-ene (2,3-didehydro-1,8-cineole), 6,6-dimethyl-2-methylene-7-oxabicyclo[3.2.1]octane (isopinol), 2,6,6-trimethyl-7-oxabicyclo[3.2.1]oct-2-ene (pinol), and 1-(3-lsopropylidenecyclopentyl)ethanone (pinolone) from hydroxy- and bromo-derivatives of cineole

Abstract

Treatment of (+)-1,3,3-trimethyl-2-oxabicyclo[2.2.2]octan-6-one p-tolylsulphonylhydrazone with n-butyl-lithium gave, in a low yield. 1,3,3-trimethyl-2-oxabicyclo[2.2.2]oct-5-ene (1), a terpene recently found in Laurus nobilis oil. Dehydration of the 1,3,3-trimethyl-2-oxabicyclo[2.2.2]octan-6-ols (an equilibrated endo–exo-mixture) with phosphoryl chloride–pyridine yielded mainly pinolone (5). The 1,3,3-trimethyl-2-oxabicyclo[2.2.2]octan-6-ols gave the 6-bromo-1,3,3-trimethyl-2-oxabicyclo[2.2.2]octanes (endo–exo mixtures) in 60 : 40 ratio with phosphorus tribromide–pyridine and in 80 : 20 ratio with triphenylphosphine dibromide. From treatment of the latter mixture with 1,5-diazabicyclo[5.4.0]undec-5-ene, (+)-pinol and a new terpenoid. (–)-isopinol, were obtained: with potassium t-butoxide in dimethyl sulphoxide, however, three products were obtained, namely the didehydrocineole (1), (+)-pinol, and (–)-isopinol. The structure of (–)-isopinol(6,6-dimethyl-2-methylene-7-oxabicyclo[3.2.1]-octane) and those of the other new compounds reported were determined from i.r., n.m.r., and mass spectral data.