Biosynthesis of triterpenes

Cyclization of squalene in eukaryotes is initiated by protonation of the epoxide derivative, squalene oxide 96, formed by flavin-mediated oxidation of squalene with molecular O2. A wide variety of polycychc structures with different ring sizes and stereochemistries 

The neem (margosa or Indian lilac) tree (Azadirachta indica) is particularly well known for producing a complex mixture of limonoids which can be extracted from the bark, leaves and seed oil. Extracts of neem have been employed for many years in the Indian subcontinent as insect repellents and folk medicines and can be used to preserve stored grains and pulses (Boeke et al. 2004). The best known of the neem limonoids is azadirachtin (105), 

As well as the free triterpenes and sterols, plant cells often accumulate significant quantities of triterpene derivatives such as glycosides (as shown above by the ginsenosides and 

R = H digitoxin (106) isolated from foxglove (Orgjfofis ourpuiaa) 

R = OH digoxin from D. lanata widely used Irealment for heart failure. 

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Despite the broad medical potentials reported so far, the total synthesis of triterpene QMs is yet to be reported. On the contrary, the biosynthesis of triterpene QMs has recently been validated as from the oxidosqualene 88 (Scheme 8.16) in the plants including Maytenus aquifolium and Salacia campestris.10S With the assistance of HPLC analysis and isotopic labeling, it was found that triterpene QMs 90 were formed only in the root of these plants from friedelin 89 and similar cyclized intermediates, which were synthesized in the leaves from oxidosqualene by cyclase. 

The biosynthesis of triterpenes by photosynthetic organisms proceeds via cyclization of 2,3-oxidosqualene (92) to yield cycloartenol (93), in contrast to non-photosynthetic organisms where the cyclization product is lanosterol (94) The last step in this... 

An additional limitation applies to in vitro enzyme assays described here and in all subsequent sections. Results of in vitro assays often contradict results obtained in in vivo studies. For example, although transcript and metabolite profiling indicated that a glycosyltransferase from Medicago truncatula is involved in the biosynthesis of triterpene saponins, this enzyme glycosylated certain phenolic compounds with higher efficiency than triterpenes in vitro [22]. 

The literature on the biosynthesis of triterpenes, steroids, and carotenoids for 1970 and 1971 has been reviewed in Volunne I. The reader is, therefore, referred to this for much basic information regarding biosynthetic pathways. This Report attempts to survey material related to the biosynthesis of triterpenes, steroids, and carotenoids published during 197Z Since much of the work which has been reported during that period has centred on enzymic aspects, this chapter of necessity reflects such emphasis. It is inevitable that there will be some overlap with chapters in another volume of the Specialist Periodical Reports series. Reviews centred on enzymic as well as stereochemical and mechanistic aspects of sterol biosynthesis have appeared. 

Rees, H. H., and T. W. Goodwin Biosynthesis of Triterpenes, Steroids and Carotenoids in Biosynthesis (Specialist Periodical Report), vol. 1, p. 100. London The Chemical Society. 1972. 

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