that attractive dispersion forces may be responsible for a change of the binding mode. The catalytic [2+2] photocycloaddition was shown to proceed on the triplet hypersurface with a quantum yield of 0.05. The positive effect of Lewis acids on the outcome of a given intramolecular [2+2] photocycloaddition was illustrated by optimizing the key step in a concise total synthesis of the sesquiterpene (±)‐italicene
oxidation of the allyl alcohols 9a or 9b afforded the pure ketones 3a or 3b MnO2 oxidation of the alcohols 10a or 10b furnished the aldehydes 4a or 4b. – Surprisingly, the italicene epoxides 2a or 2b rearranged with diluted HCl to the italicene ethers (epoxyacorenes) 5a or 5b, which previously were isolated from Lantana and Helichrysum oil. – The odor of 3a, 3b, 4a, 4b and 5a was evaluated.
From Helichrysum oil two newsesquiterpenehydrocarbons, italicene and isoitalicene were isolated. Their structure was confirmed by spectral data, degradation reactions and unambiguous synthesis starting from m-bromoanisole.