Cyclisation of squalene

The application of this procedure to the fused polycyclic compound E, which already has a linear dual and only the last two steps (iii-iv) apply to it, leads to a linear acyclic structure F which may be traced back to the biogenetic cyclisation of squalene to lanosterol via cationic intermediates, as well as to the stereospecific cationic cyclisation of polyolefins studied by Johnson [18]. 

Currently there is no experimentally determined three-dimensional structural information available for OSCs, although studies with a related enzyme, squa-lene-hopene cyclase (SC EC 5.4.99.7) have proved informative. SCs are involved in the direct cyclisation of squalene to pentacyclic triterpenoids known as hopanoids, which play an integral role in membrane structure in prokaryotes [ 51 ]. A number of SC genes have been cloned from bacteria [52 - 54]. The SC and OSC enzymes have related predicted amino acid sequences, and so should have similar spatial structures [55]. The crystal structure of recombinant SC from the Gram-positive bacterium Alicyclobacillus acidocaldarius has established that the enzyme is dimeric [55]. Each subunit consists of two a-a barrel domains that assemble to form a central hydrophobic cavity [55,56]. 

Although cycloartenol (44) belongs to the tetracyclic triterpenoids as the parent compound, it is implicated as an important biosynthetic intermediate both in that class and in plant steroids and is thus a relevant introduction to the latter group which have been so significant in semi-synthetic applications. Following the enzyme-catalysed cyclisation of squalene epoxide, it is the first intermediary compound encountered and in the plant series it has a comparable position to lanosterol (45) in the animal... 

Cyclisation of squalene 2,3-epoxide (10) yielded, in addition to tricyclic products, the bicyclic alcohol (11) which presumably arises from the bicyclic carbonium ion (12) by a series of hydride and methyl migrations. Sharpless and van Tamelen have suggested that one of the functions of the cyclase enzyme is to prevent this type of process at the bicyclic level. 

The in vivo formation and cyclisation of squalene are discussed in detail in Part II, Chapter 8. 

Three distinct methods have been demonstrated for the cyclisation of squalene. Although most triterpenoids follow a similar route to lanosterol [(7)— (71)— (72) (73)] the 3-desoxytriterpenoids such as femene (98) are formed from squalene (7) not from 2,3-oxidosqualene (71). Presumably a proton-initiated cyclisation is followed by elimination of a hydrogen atom, with, or without, rearrangement of the carbonium ion . When [2- C,3R,4R- H]mevalonic acid... 

Tetrahymanol (99) shows a variant process whereby the carbonium ion from proton-initiated cyclisation of squalene (7) is quenched by the medium. Again... 

The triterpenoids constitute a large diverse group of active natural products derived from squalene. In excess of 4000 triterpenoids have been isolated so far and more than 40 skeletal types have been identified. In general, triterpenes arise by cyclisation of squalene or the 3S isomer of 2,3-epoxy-2,3 -dihydrosqualene. Cyclisation of this isomer leads to 3p-hydroxytriterpenoids which, by oxidation and reduction, can be transformed into 3a-hydroxyterpenoids. 

The enantioselective polyene cyclisation of squalene to the pentacydic hopene and hopanol ranks as one of the most complicated biochemical reactions. In a single step, nine stereogenic centres are formed. Hopene is one of 512 (2 ) possible stereoisomers. 

CoRNFORTH, J. W., R. H. CoRNFORTH, A. Pelter, M. G. Horning, and G. PopjAK Studies on the biosynthesis of cholesterol. 7. Rearrangements of methyl groups during enzymic cyclisation of squalene. Tetrahedron 5, 311 (1959 b). 

The isoprenoid polyenes famesyl acetate, geranyl acetate and squalene underwent oxidative poly cyclisation to bis-, tris- and penta-tetrahydrofurans with RuO /aq. Na(IO )/CH3CN-EtOAc [185]-[188]. This oxidative polycyclisation of squalene with RuO was shown to lead to the cis-threo-cis-threo-trans-threo-trans-threo-trans penta-tetrahydrofuranyl diol product, this configuration being determined by 2D-NMR (Fig. 3.14) [185]-[188] cf mech. Fig. 1.8 [185]. 

The synthesis of tricyclic compounds based on the cyclisation of methyl geranyl-geranoate was described eighteen years ago. However, the discovery of expoxy-squalene as an intermediate in steroid biosynthesis, and the corresponding in vitro cyclisation experiments, have re-awoken interest in this area of diterpene synthesis. The structural and stereochemical course of the in vitro cyclisation of the epoxy-trans-olefin (138) has been studied. Two A/B-trans-fused tricyclic compounds (139) and (140) have been isolated. The levantenolides (142) and (143) have been obtained by two closely related routes. In the first, the butenolide (141), derived from monocyclofamesyl bromide, afforded the a- and P levantenolides (142) and (143) on cyclisation with stannic chloride. In the second, the corresponding acyclic butenolide derived from famesyl bromide was cyclised. 

Stefan Neumann and Helmut Simon (from the Technical University of Munich) had discovered already in 1986 that the squalene-hopene cyclase from Alicydo-bacillus acidocaldarius [260] carries out the cyclisation, not only of squalene to hopene andhopanol, but also ofhomofarnesolto A/wfcrojc . [261] It may therefore be assumed, that this latter cydisation proceeds in a very similar way as in case of squalene. 

Q Bloch had to flee as a Jew from the Nazi regime in 1936 via Switzerland to the USA. At Columbia University in New York, he was able to show together with David Rittenberg in 1942, using isotopically labelled acetate, that this was a precursor for the cholesterol synthesis in animals. The polyene cyclisation of the triterpene squalene produces lanosterol, which is degraded to cholesterol, the central intermediate of all human steroids. [350]... 

The idea that such cyclisations should be highly stereoselective derives from the Stork-Eschenmoser hypothesis, according to which the double bonds in polyalkenes of the squalene type are properly arranged to undergo cyclisation to fused polycyclic systems with the natural trans-anti-trans configuration. 

Squalene is bound into the active site in a chair-chair conformation. [259] The cyclisation begins with a protonation at C-3 of the future hopene by aspartic acid D376. Carbocation n-interactions with various aromatic amino acids, such as tyrosine, phenylalanine and tryptophan, stabilise the cationic intermediates, until eventually deprotonation produces hopene and hydroxylation gives hopa-nol (Fig. 3.56). 

Figure 20.21 shows more clearly the cydisation process in which squalene is firstly oxidised via squalene epoxi-dase to form 2,3-squalene epoxide. This then cyclises to produce the basic steroid nucleus, resulting in the formation of lanosterol. This is why all steroids have an oxygen atom at the 3 position. 

Barton, D.H.R., G. Mellows, and D.A. Widdowson Biosynthesis of Terpenes and Steroids. Part III. Squalene Cyclisation in the Biosynthesis of Terpenoids. The Biosynthesis of Fern-9-ene in Polypodium vulgare Linn. J. Chem. Soc. (C) 1971, 110. 

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