STEREOSLECTIVE PHOTOCHEMICAL ELECTROCYCLIC VALENCE ISOMERIZATIONS OF ALPHA-PHELLANDRENE CONFORMATIONAL ISOMERS

The primary products from the unsensitized photolysis of α-phellandrene are the cis,trans- and cis,cis-3,7-dimethylocta1,3,5-trienes; the trans,trans and trans,cis isomers are formed through photoisomerizations of the primary products. The temperature dependence of the ratio of the two primary products, when plotted as log (ratio) vs. T−1, gives a linear correlation consistent with the postulate that the α-phellandrene conformer with a pseudoaxial isopropyl substituent is photoisomerized to the cis,cis-triene, while the conformer with a pseudoequatorial isopropyl group is converted only to the cis,trans-triene. The conversion of the pseudoaxial to the pseudoequatorial conformational isomer of α-phellandrene occurs with ΔH° = –0.46 kcal/mol. The data and interpretation correctly predict the sign reversal in the molecular rotation of α-phellandrene noted in independent ord and cd studies. © 1969, American Chemical Society. All rights reserved.