Synthesis and Photoreaction of a-Methylene-γ-Butyrolactone-Psoralen Heterodimers

Alpha-methylene-gamma-butyrolactones: versatile skin bioactive natural products

Natural products containing an alpha-methylene-gamma-butyrolactone moiety, mainly of the sesquiterpene type, are widely observed in plants, which upon coming into contact with skin, will induce major skin toxicological side effects or phytodermatitis. Indeed two main dermatological pathologies have been associated with a skin exposure to molecules containing an alpha-methylene-gamma-butyrolactone moiety: allergic contact dermatitis (ACD) and chronic actinic dermatitis (CAD). ACD is an immunologically based disease resulting from modifications of epidermal proteins by sensitizers or haptens. Indeed, alpha-methylene-gamma-butyrolactones are highly electrophilic structures that can act as Michael acceptors towards nucleophilic residues of proteins. Cysteine and lysine are the most modified residues leading, in the case of enantiomerically pure lactones, to the formation of diastereomeric adducts. This chemical enantioselectivity induces an enantiospecificity of the allergic reaction, i.e., an individual sensitized to one enantiomer will not develop clinical symptoms when exposed to the other enantiomer and vice versa. Sesquiterpene lactones have been also associated with another pathology that involves UV irradiation and DNA modifications. Interestingly, it was found that alpha-methylene-gamma-butyrolactones, in addition to their electrophilic properties, were highly photoreactive molecules able to react with thymine/thymidine to form [2 + 2] photoadducts in very high yields. In all cases a syn regioselectivity was observed, probably associated with the polarization of the exomethylenic bond. This high photoreactivity of alpha-methylene-gamma-butyrolactones towards thymidine could be an explanation of the progressive evolution of allergic contact dermatitis towards chronic actinic dermatitis.

Synthesis and photocytotoxic activity of new alpha-methylene-gamma-butyrolactone-psoralen heterodimers

The synthesis of a new alpha-methylene-gamma-butyrolactone-psoralen heterodimer 2 is reported. Its photoantiproliferative activity and skin phototoxicity were compared with that of 5-methoxypsoralen (5-MOP) and another heterodimer 1. Both derivatives show a significant phototoxicity toward malignant cell lines including melanoma cells A375 compared to their intrinsic cytotoxicity in the dark. Both compounds were found to be nonphototoxic on mice skin and therefore could be active potential drugs in photochemotherapy.

Evidence for [2 + 2] photoreaction of alpha-methylene-gamma-butyrolactones with thymine: an explanation for chronic actinic dermatitis to sesquiterpene lactones?

The photoreactivity of isoalantolactone, a natural sesquiterpene lactone, toward thymine was studied. After 313 nm irradiation of a deoxygenated acetone solution of isoalantolactone (2 x 10(-3) M) and thymine (4 x 10(-3) M), two intermolecular [2 + 2] photoadducts, 3 and 4, were isolated with respective yields of 30% and 18%. The structures of these two photoadducts were determined by a combination of NMR experiments. Compound 3 was identified as a cis-syn-exo intermolecular [2 + 2] photoadduct involving the 5,6 double bond of thymine and the exomethylenic double bond of the lactone, while compound 4 was identified as an intermolecular [2 + 2] photoadduct involving the same bonds but with the cis-syn-endo conformation. This high photoreactivity of sesquiterpene lactones toward thymine could be an explanation of the progressive evolution of allergic contact dermatitis toward chronic actinic dermatitis.