Structures of the Bergamottin Photoproducts

The extensive use of bergamot oil in perfumery and cosmetics led us to study the photoreactivity of 5‐geranoxypsoralen (bergamottin), the major chromophore in this oil. Structure determination of the bergamottin photoproducts was achieved using extensive NMR techniques including two‐dimensional NMR: 1H‐13C correlation by long‐range coupling, 1H‐1H correlation spectroscopy (COSY) and 1H‐1H COSY phase sensitive (COSYPH). Bergaptol is formed according to the well‐known fragmentation of allyl‐aryl ethers. The other photoproducts arise from an intramolecular [2 + 2] photocycloaddition and appeared as an equimolecular mixture of diastereomeric cyclobutyl derivatives. The relative configurations were determined by nuclear Overhauser effect analysis. These structures were further corroborated by X‐ray crystal analysis. The facile formation of an intramolecular cyclobutyl derivative in bergamottin might prevent the intermolecular cyclobutane adduct formation with pyrimidine DNA bases responsible, in part, for phototoxicity of psoralens.