Cycloartenol formation

One of the clear distinctions between higher animals and plants is in the products resulting from cyclization of squalene epoxide (76). Plants form cycloartenol (78) whereas animals form lanosterol (80). Moreover, animals are unable to metabolize cycloartenol. Further examples of cycloartenol formation are reported with a tissue culture of Rubus fructicosus and Pinus pineaf Cycloartenol and 24-methylenecycloartanol are recovered unchanged with microsomes from the Rubus tissue culture but cycloeucalenol (79) is metabolized... 

Fig. 5.1 Cyclization of 2,3-oxidosqualene to sterols and triterpenoids. The 2,3-oxidosqualene cyclase enzymes that catalyse the formation of the different products are indicated LS, lanosterol synthase CS, cycloartenol synthase LuS, lupeol synthase PAS, P-amyrin synthase aAS, a-amyrin synthase.

An additional feature of the protosteryl cation is that the C-10 methyl and H-5 also share an anti-axial relationship, and are also susceptible to Wagner-Meerwein rearrangements, so that the C-9 cation formed in the cycloartenol sequence may then initiate further migrations. This can be terminated by formation of a 5,6-double... 

The formation of cyclosteroids through intramolecular substitution of a sulphonate by a carbanion, used successfully in the preparation of 5a,7a-cyclosteroids, has provided a route to a variety of curious cyclic structures. The 9jS,i9-cyclo structure (13) found in cycloartenol was obtained [yi] when... 

As expected, lanosterol (70) but not cycloartenol (72) was converted in rats into cholesterol on the other hand both triterpenoids are utilized by Zea mays in the formation of C-24-alkylated sterols. Sterol formation has been demonstrated in the fern Polypodium vulgare, tobacco, and Calendula... 

First, we will take up cyclopropyl group formation by the rearrangement of carbon skeletons via cationic intermediates encountered in various mono- and sesquiterpenes, and also examine the illudin biosynthesis where contraction of a cyclobutyl cation to a cyclopropane has been invoked. We will then discuss the head-to-head condensation of isoprenoid alcohols at the C15 or C20 level to generate the cyclopropyl intermediates, presqualene pyrophosphate and prephytoene pyrophosphate, on the way to the C30 and C40 polyene hydrocarbons, squalene and phytoene respectively. Conversion of 2,3-oxidosqualene via common intermediate protosterol cation to cycloartenol or lanosterol represents an important pathway in which the angular methyl group participates in the three-membered ring formation. The cyclopropanation outcome of this process has been carefully studied. 

B. Cyclopropane Formation in Steroids by Participation of the Angular Methyl Group Squalene to Cycloartenol... 

The syntheses of labelled lanosterol, cycloartenol, and parkeol derivatives for use in biosynthetic studies have been described. Terminal labelling of the side-chain [25- C] or [26,27- H6] was achieved by the formation of Wittig intermediates with the trisnortriterpenoid units followed by reaction with labelled acetone. Methods for the removal of one or both methyl groups from 4,4-dimethyl-steroids have been published. Lanosterol has been degraded to the trimethyl-pregnenolone (54). 

The present state of knowledge of terpenoid biosynthesis does not allow many detailed conclusions to be reached on its taxonomic importance. However, some gross differences at the phyla level are apparent. This review has already commented on differences observed in the formation of steroidal A - and A -double bonds, 24-alkyl groups, and whether lanosterol or cycloartenol is formed from squalene epoxide. 

Fig. 7. The formation of 24-methylene cycloartanol, cyclolaudenol and cyclosadol by methylation of cycloartenol.

Fig. 10. Retention of configuration on formation of the cyclopropane ring of cycloartenol.

The scheme for steroid biosynthesis is the same in both plants and animals up to the formation of the carbocation 3-2. The biosynthesis diverges at this point in animals the methyl group at Cg migrates to afford lanosterol (4-1) as an isolable product (Scheme 2.4). The first steroidal product that can be isolated in plants, cycloartenol (4-2), features a cyclopropyl ring fused on to ring B at carbons 9,10. 

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