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 furanocoumarins
in 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.
___________________________________________
* 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.
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
Source:Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar
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