Energetics of the 2+2 Cyclization of Limonene

The hydrocarbon 1,2-dimethyltricyclo[3.3.0.0^2,7]octane (4) was synthesized from carvone in two steps. Compound 4 is the product that would be obtained from the photochemical 2+2 intramolecular cyclization of limonene. The calculated enthalpy for the transformation of limonene to compound 4 is 7.2kcal/mol, the endothermicity of the reaction being attributed to ring strain in the product. This strain energy, accompanied by the increase in specific gravity obtained upon cyclization, gives compound 4 a larger heat of combustion, on a volumetric basis, than jet fuel. Other naturally occurring terpenes, which are inexpensive and abundant, bear consideration as starting materials for easily prepared high energy density materials.