3.5 Biosynthesis of Coumarin and Furanocoumarin Glycosides
It has been established experimentally that by using grafts of Melilotus alba on Trigonella foenum graecum, practically no coumarin is formed in the shoots of M. alba; therefore, it may be inferred that the roots are absolutely essential for coumarin synthesis, perhaps since they provided an important precursor (Reppel and Wagenbreth, 1958)**. However, more recently it has been shown with the aid of reciprocal grafting experiments involving parsnip (Pastinacia sativa) that furanocoumarinsin this species are usually generated in the fruits where they accumulate, and of course, no evidence for translocation could be observed (Beyrish, 1967)***.
There are two experimentally demonstrated pathways whereby natural products incorporating the bezopyran nucleus are usually formed, namely:
(a) In 3- and 4-phenylcoumarins, the aromatic components of this nucleus is derived from
polyketide, wherein the 3 aliphatic carbon atoms and the phenyl substitute is found to originate from shikimic acid via a phenylpropanoid intermediate, and
(b) The coumarin originates via the shikimate-chorismate pathway leading to phenylpyruvic acid, from which arise L-phenylalanine by transmination and trans-cinnamic acid in turn by the action of phenylalanine ammonia lyase. Generally, two types of furanocoumarins are recognised, namely: Linear furanocoumarin and Angular furanocoumarin as shown below:
In (A), the furan ring is fused at C-6 & C-7 positions of the benzopyran nucleus (eg., psoralens), whereas in (B) the fusion is between C-7 and C-8. However, the latter is less widely distributed than the former.
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* Kreuzaler, F., and K., Hahlbrock, Phytochemistry, 12, 1149-1152, 1973.
** Reppel L., and D. Wagenbreth, Flora (Jena), 146, 212-227, 1958.
*** Beyrish, T., Planta Med. 15, 306-310, 1967.
