Modified Triterpenoids

The triterpenoid skeletons may be subjected to a variety of structural modifications as already illustrated. However, the particular modifications considered in this section are those that lead to loss of several skeletal carbon atoms. Preeminent amongst such degraded triterpenoids are the steroids, and these are so important that they are considered separately. Other degraded triterpenoids include the limonoids (tetranortriter-penoids), in which four terminal carbons from the side-chain are removed, and the quassinoids, which have lost ten carbons, including one of the C-4 methyls. The quassinoids thus have a C20 skeleton which could be misinterpreted as a diterpene structure. Biosynthetic information is relatively sparse, but the relationship to precursors of the euphol type is outlined in Figure 5.66. Limonoids are found mainly in plants of the

Limonoids Formation Pathway

euphol(20r) tirucallol (205)

butyrospermol opening of epoxide ring generates tertiary cation and initiates 1,2-shift; loss of proton gives alkene euphol(20r) tirucallol (205)

butyrospermol

W-M 1,2-methyl shift

opening of epoxide ring generates tertiary cation and initiates 1,2-shift; loss of proton gives alkene

Tirucallol
limonoids (tetranortriterpenoids)

via cleavage of ring C

via cleavage of ring C

AcO'

azadirachtin

cleavage of 4 carbons from side-chain and formation of furan ring as indicated

- 10C (including one from C-4)

cleavage of 8 carbon side-chain and one from ring D; formation of lactone ring as indicated

W-M 1,2-methyl shift

Apotirucallol
apoeuphol (20R) apotirucallol (20S)

quassinoids

AcO'

OMe azadirachtin

quassinoids

quassin

quassin

families Rutaceae, Meliaceae, and Simaroubaceae. Azadirachtin (Figure 5.66) is probably one of the most complex limonoid structures to be encountered, but is currently of considerable interest. This material has potent insect antifeedant properties and is extracted commercially from seeds of the Neem tree (Azadirachta indica; Meliaceae) for use as an agricultural pesticide to prevent insect damage to crops. It is a relatively inexpensive and ecologically sound pesticide. Quassinoids are produced by many plants in the Simaroubaceae family, in particular Quassia. Quassin (Figure 5.66) from Q. amara (quassia wood) is a typical example. They have attracted considerable study because of their cytotoxic, antimalarial, and amoe-bicidal properties.

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