Photochemical transformations of α-phellandrene
U.v. irradiation of an ethereal solution of α-phellandrene (p-mentha-1,5-diene)(I) results in ring opening to give a mixture of geometrical isomers of 3,7-dimethylocta-1,3,5-triene (II) which, upon continued irradiation, forms at least six monomeric photoproducts. The major product (ca. 50%) is shown to be exo-6-isopropyl-2-methylbicyclo[3.1.0]hex-2-ene (XVa) on the basis of physical measurement and chemical degradation, and the second most important product (ca. 10%) is the endo-epimer (XVb). The configuration at C-6 in these products was determined by a stereospecific synthesis of dimethyl t-3-methylcyclopropane-r-1,c-2-dicarboxylate, (XIXa) which was identical with a degradation product of the main photoisomer (XVa) which is therefore the exo-6-isopropyl compound.