A [2 + 2] photo-adduct of 8-methoxypsoralen and thymine: X-ray crystal structure; a model for the reaction of psoralens with DNA in the phototherapy of psoriasis

Ultaviolet irradiation of a mixture of 8-methoxypsoralen and thymine in an ice-methanol matrix in the presence of benzopenone affords a [2 + 2] adduct which is shown by X-ray crystallography to result from cis-bonding of the 4′- and 5′-(furan) positions of the 8-methoxypsoralen to, respectively, the 5- and 6-positions of the thymine.