Steric inhibition of π-electron delocalization in 1, 3-dienes : Circular dichroism of methylisopulegene and 3, 8-p-menthadiene

The conjugated diene, “methylisopulegene” (1, 1-isopropenyl-2,4-dimethyl-1-cyclohexene), exhibits a CD Cotton effect (CE) with Δε₂₁₇= + 1.77 but no UV maximum down to 200nm; whereas, the structurally related conjugated diene, 3,-8-p-menthadiene (2, 1-isopropenyl-4-methyl-1-cyclohexene), exhibits a more normal UV maximum at 232 nm (ε = 17,300) and a CE with Δε₂₄₀ = +2.56.