The chemistry of terpenes. Part 25. A synthesis of car-2-, car-3-, and car-3(10)-ene (β-carene)

Reaction of dibromocarbene with 4,4-dimethoxycyclohexene (7) affords 3,3-dimethoxy-7,7-dibromobicyclo-[4.1.0]heptane (8), which with lithium dimethylcuprate(II) and methyl iodide, and treatment of the product with acid, yields 7,7-dimethylbicyclo[4.1.0]heptan-3-one, ±(2). In the absence of methyl iodide, 7-exo-methylbicyclo[4.1.0]heptan-3-one, ±(12), is the major product. Treatment of the ketone (2) with methylenetriphenylphosphorane gives car-3(10)-ene (β-carene) (11) and reaction of the ketone ±(2) with methylmagnesium iodide affords the epimeric caran-3-ols (13) and (14). On dehydration these yield a mixture of carenes.