2,3-Oxidosqualene synthesis

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. 

HARALAMPIDIS, K, BRYAN, G Qi, X., PAPADOPOULOU, K., BAKHT, S, MELTON, R, OSBOURN, A.E., A new class of oxidosqualene cyclases directs synthesis of antimicrobial phytoprotectants in monocots, Proc. Natl. Acad. Sci., USA, 2001,98,13431-13436. 

The first committed step in the synthesis of triterpenoid saponins involves the cyclisation of 2,3-oxidosqualene to give one of a number of different potential products [9]. Most plant triterpenoid saponins are derived from oleanane or dammarane skeletons, although lupanes are also common [9]. This cycHsation event forms a branchpoint with the sterol biosynthetic pathway, in which 2,3-oxidosqualene is cyclised to lanosterol (in animals and fungi) or to cycloartenol (in plants) (Fig. 2). Sterols are important membrane constituents and also serve as precursors for hormone biosynthesis. 

Yeast does not synthesise cycloartenol or triterpenes, and so approaches to clone plant OSCs by complementation in S. cerevisiae are not feasible because of the lack of appropriate mutants. However, LS-deficient yeast mutants accumulate high levels of 2,3-oxidosqualene, favouring the synthesis of novel cycfisation products generated by heterologous expression of OSCs. The absence of lano-sterol also facilitates analysis of the reaction products. Corey and co-workers isolated a cDNA encoding Arabidopsis thaliana CS by transforming a plant cDNA expression library into such a yeast mutant and screening protein preparations derived from pools of transformants for the ability to synthesise cycloartenol by TLC [39]. 

The synthesis of saponins from the cyclisation product of 2,3-oxidosqualene involves a series of additional modifications. These may include a variety of oxidation and substitution events, and the addition of sugars at different positions on the skeleton [9]. Very little is known about the enzymes required for these... 

Reactions between a-thioallyllithiums and allyl halides, which usually proceed predominantly via OL-CL (head-to-head) coupling, have proved to be valuable in synthesis.They have been used successfully for example in the synthesis of 1,5-dienes such as squalene, (/ )-(-(-)-10,11-epoxyfamesol, (/ )-(+)- and (S)-(-)-2,3-oxidosqualene and mokupalide, dendrolasin, cembra-nolides, methyl ceriferate and Cecropia juvenile hormone (Scheme 25 and Scheme 26). ... 

Grosa, G., Viola, F., Ceruti, M., Brusa, P, Delprino, L., Dosio, F., and Cattel, L., Synthesis and biological activity of a squalenoid maleimide and other classes of squalene derivatives as irreversible inhibitors of 2,3-oxidosqualene cyclase, Eur. J. Med. Chem., 29, 17, 1994. 

Scheme 56 Synthesis of 3-aryl substituted benzisothiazoles as oxidosqualene cyclase inhibitors...

Figure 14.5-1. Synthesis of lanosterol analogues using 2,3-oxidosqualene lanosterol cyclase.

Initially, this process was thought to be concerted in nature, but it has been demonstrated that oxidation and cyclization are different steps by the synthesis and isolation of 2,3-oxidosqualene as an intermediate and the use of inhibitors of the cyclization reaction such as 2,3-iminosqualene [92]. The enzymes for the oxidation and cyclization differ in their susceptibiUty to solubilization by detergents. The partially purified cyclase M, 90000) aggregates and is inactivated upon removal of the detergent. The oxidation is Oj dependent while cyclization is anaerobic with no cofactors required. 

Squalene is converted into the first sterol, lanosterol, by the action of squalene epoxidase and oxidosqualene lanosterol cyclase. The catalytic mechanism for the cyclase s four cyclization reactions was revealed when the crystal stmcture of the human enzyme was obtained (R. Thoma, 2004). Oxidosqualene lanosterol cyclase is considered an attractive target for developing inhibitors of the cholesterol biosynthetic pathway because its inhibition leads to the production of 24,25-epoxycholesterol (M.W. Huff, 2005). This oxysterol is a potent ligand activator of the liver X receptor (LXR) and leads to expression of several genes that promote cellular cholesterol efflux, such as ABCAl, ABCG5, and ABCG8 (Section 4.1). Thus, inhibitors of oxidosqualene lanosterol cyclase could be therapeutically advantageous because they would reduce cholesterol synthesis and promote cholesterol efflux (M.W. Huff, 2005). 

Cholesterol is formed biosynthetically from isopentenyl pyrophosphate (active isoprene). The majority of cholesterol in the body derives from de novo biosynthesis in the liver [1,2]. Cholesterol synthetic pathway has been assumed to occur primarily in the cytoplasm and endoplasmic reticulum (ER). However, more recent evidences have suggested that the enzymes, except squalene synthase, squalene epoxidase and oxidosqualene cyclase, are partly localized in the peroxisomes, which are essential for normal cholesterol synthesis [11]. 

Brown and coworkers found use for the Meyer-Schuster rearrangement in their synthesis of quinuclidine inhibitors of 2,3-oxidosqualene cyclase/lanosterol synthase (OSC). The rearrangement was the last step in the synthesis of one such inhibitor, utilizing concentrated H2SO4 to carry out the transformation from 37. 

Corey and coworkers successfully applied this cross-coupling method to the synthesis of (3S)-2,3-oxidosqualene (Scheme 10.2 5) [3]. The utility of allylic barium reagents for nucleophilic substitution reactions was further demonstrated by the synthesis of cembrol A (7), in which ring closure of the epoxy compound occurred regioselectively (Scheme 10.3) [4]. 

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