Ene-Ene-Retroene Conversion of (-)-β-Pinene to (+)-β-Selinene

Article abstract of DOI:10.1021/jo00169a028

(-)-β-Pinene undergoes an ene reaction with acryloyl chloride at 70 deg C to afford 6,6-dimethylbicyclo<3.1.1.>hept-2-ene-2-butanoyl chloride (2) in better than 80percent yield.Cyclization of 2 to 10,10-dimethyltricyclo<7.1.1.0^2,7>undec-2(7)-en-6-one (4) by way of an intramolecular ene reaction involving a ketene intermediate occurs on heating with tributylamine at 150 deg C.Unsaturated ketone 4 undergoes a clean retroene reaction to yield (+)-7-(2-propenyl)-Δ⁹-decal-1-one (11) on brief heating at 265 deg C.Lithium dimethylcuprate addition to 11 yields a mixture of four isomeric 7-(2-propenyl)-10-methyl-1-decalones (15-18) where the isomers having the desired cis relationship between the angular methyl and the 2-propenyl groups comprise ca. 75percent of the product.Treatment of the ketone mixture with methylenetriphenylphosphorane completes the synthesis of (+)-β-selinene.