Dammaranes biosynthesis

Triterpenoid saponins are synthesized via the isoprenoid pathway.4 The first committed step in triterpenoid saponin biosynthesis involves the cyclization of 2,3-oxidosqualene to one of a number of different potential products (Fig. 5.1).4,8 Most plant triterpenoid saponins are derived from oleanane or dammarane skeletons although lupanes are also common 4 This cyclization event forms a branchpoint with the sterol biosynthetic pathway in which 2,3-oxidosqualene is cyclized to cycloartenol in plants, or to lanosterol in animals and fungi. 

FIGURE 1.4 Proposed biosynthetic route for the biosynthesis of (A) squalene oxide (squalene-2,3-oxide) via the isoprenoid pathway and (B) triterpene saponins of the dammarane-type and oleanane-type from squalene oxide. PP, diphosphate group GPS, geranyl phosphate synthase FPS, farnesyl phosphate synthase NADPH, nicotinamide adenine dinucleotide phosphate. 

Alfhough bearing a sterane nucleus, fhe biosyntheses of cucurbitacins and dammaran-type saponins will not be reviewed here. Both groups belong to the triterpenoids, the chemistry, biosynthesis and biological activities of which have been described elsewhere (Charlwood and Banthorpe, 1991 ... 

Figure 3.34 Outline biosynthesis of the dammarane triterpenoids including ginsenosides and limonoids.

It is relevant to note that 2- C-mevalonate is incorporated specifically into the methyl group of the isopropenyl side chain of the lupane triterpene betulinic acid (348, R = C02H) (447). Hence, in the biosynthesis the two methyl groups in postulated intermediate (347) are not randomized. A related chemical observation is the acid-catalyzed equilibration of two C20 epimeric dammarane diol derivatives (12p-hydroxy-24,25-dihydro version of 320 and 345) (448). 

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