Psoralen

Overview
Psoralen (also called psoralene) is the parent compound in a family of natural products known as furocoumarins. It is structurally related to coumarin by the addition of a fused furan ring, and may be considered as a derivative of umbelliferone. Psoralen occurs naturally in the seeds of Psoralea corylifolia, as well as in the common Fig, celery, parsley and West Indian satinwood. It is widely used in PUVA (=Psoralen +UVA) treatment for psoriasis, eczema and vitiligo. Although safe to mammals, it should be used with care since many furocoumarins are extremely toxic to fish, and some are indeed used in streams in Indonesia to catch fish.

Uses
An important use of psoralen is in PUVA treatment for skin problems such as psoriasis and (to a lesser extent) eczema and vitiligo. This takes advantage of the high UV absorbance of psoralen. The psoralen is applied first to sensitise the skin, then UVA light is applied to clean up the skin problem. Psoralen has also been recommended for treating alopecia.

Psoralea corylifolia is an important plant in the Indian Ayurveda system of medicine, and also Chinese medicine. The seeds of this plant contain a variety of coumarins including psoralen, and they have a variety of medicinal uses, but the specific role (if any) of psoralen in this is unknown.

Related furocoumarins
Another isomer of psoralen is angelicin, and most furocoumarins can be regarded as derivatives of psoralen or angelicin. Some important psoralen derivatives are Imperatorin, xanthotoxin, bergapten and nodekenetin.

Another important feature of this class of compounds is its ability in generating singlet oxygen.

Chemistry
The structure of psoralen was originally deduced using its degradation reactions. It exhibits the normal reactions of the lactone of coumarin, such as ring opening by alkali to give a coumarinic acid or coumaric acid derivative. Potassium permanganate causes oxidation of the furan ring, while other methods of oxidation produce furan-2,3-carboxylic acid.

Synthesis
Psoralen synthesis is challenging, due the fact that umbelliferone undergoes substitution at the 8-position rather than at the desired 6 position. Benzofuran reacts preferentially in the furan ring rather than in the benzene ring. However the 6-hydroxy derivative of 2,3-dihydrobenzofuran (also called coumaran) does undergo substitution as desired allowing the following synthesis of the coumarin system via a Gatterman-Koch reaction followed by a Perkin condensation using acetic anhydride. The synthesis is then completed by dehydrogenation of the five-membered ring to produce the furan ring.