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Farnesyl carbocation

In Figure 5.38, we saw the trans raws-farnesyl carbocation cyclise in the preferred way, as far as the bond polarisation is concerned, to the 10,11-double bond. However, this gave a very sterically crowded... [Pg.129]

Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl pyrophosphate produce a rich variety of structural types m the... [Pg.1090]

Geranyl diphosphate and farnesyl diphosphate are analogues of dimethylallyl diphosphate that contain two and three C5 subunits respectively they can undergo exactly the same SnI reactions as does dimethylallyl diphosphate. In all cases, a carbocation mechanism is favoured by the resonance stabilization of the allylic carbocation. Dimethylallyl diphosphate, geranyl diphosphate, and farnesyl diphosphate are precursors for natural terpenoids and steroids. [Pg.197]

A quantum chemical investigation of the biosynthesis of farnesyl pyrophosphate through the condensation of isopentenyl pyrophosphate and dimethylallyl pyrophosphate suggests that the mechanism is concerted, although the transition state has carbocationic character.164 Quantum chemical calculations were performed on the cyclization of the farnesyl cation to the sesquiterpene pentalenene.165 Two distinct pathways with similar activation barriers were identified, each differing from previous proposed mechanisms, and each involving unusual carbocationic intermediates. Mechanisms previously proposed for enzyme-catalysed formation of the sesquiterpene trichodiene involve carbocation intermediates with a 1,4-hydride transfer as the key step, e.g. (89) -> (90) - (91).166 Quantum chemical calculations, however, show a... [Pg.222]

In a process that is quite similar to the formation of the cyclic monoterpenes, isomerization about the double bond of farnesyl pyrophosphate, followed by carbocation... [Pg.1190]

Much attention has been focused on defining the transition state of FTase and the structural determinants of the chemical step. For FTase, there is evidence for both an electrophilic contribution to the transition state, obtained from studies with fluoromethyl FPP analogues, and a nucleophilic contribution, obtained from the metal-substitution and pH studies [31,40,41]. These results are supported by the inability to trap a carbocation intermediate, inversion of configuration at Cl of the farnesyl group during the reaction, and an a-secondary kinetic isotope effect near unity [31,42,43]. Taken together, the available data suggest that the transition state of FTase... [Pg.6]

All other terpenes are biologically derived from geranyl and farnesyl pyrophosphates by a series of reactions. Cyclic compounds are formed by intramolecular reactions involving nucleophilic attack of n bonds on intermediate carbocations. [Pg.1134]

Terpene cyclase enzymes catalyze the cychzation of allylic pyrophosphate substrates to form carbocyclic products via car-bocation reaction intermediates. One well-studied example is pentalenene synthase (11, 12), which catalyzes the cychzation of farnesyl pyrophosphate to give pentalenene, whose reaction mechanism is shown in Fig. 15. Cychzation of farnesyl pyrophosphate is proposed to form an 11-membered intermediate, humulene, which is followed by a five-membered ring closure to form a bicychc tertiary carbocation. 1,2-Hydride migration followed by an additional five-membered ring closure gives a tricyclic carbocation, which gives pentalenene, at elimination. [Pg.432]

Propose a mechanism for the biosynthesis of the sesquiterpene trichodiene from farnesyl diphosphate. The process involves cyclization to give an intermediate secondary carbocation, followed by several carbocation rearrangements. [Pg.1099]

Shibuya et al. (1990) and Gebler and Poulter (1992) provided experimental support that DMAT synthase catalyzes an electrophilic aromatic substitution (Fig. 4) similar to that of other prenyl transferases such as farnesyl diphosphate synthase. A positively charged alkyl intermediate—an allyl carbocation—is... [Pg.413]

When considering sesquiterpenoids, the number of possible reaction pathways increases significantly over those available in the monoterpenoid series. This results from the increased size of the precursor, farnesyl pyrophosphate, and the fact that it contains three double bonds. Both the 2,3- and 6,7-double bonds are initially formed in the trans configuration, but the 2,3-bond is part of an allylic carbocation system and therefore it can isomerise to the cis form. Initial ring closure can occur at either of the two remote double bonds and with the 2,3-bond in either of the cis or trans configuration. [Pg.38]

The essential oil of Echinops giganteus var. lelyi, a culinary herb from Cameroon, contains many sesquiterpenoids. The most abundant (26.9% of the oil) is silphiperfol-6-ene, (P24.1). The biogenesis of this material from farnesyl pyrophosphate is enzyme mediated but the chemistry of the enzymatic reactions follows the basic principles of carbocation mechanisms (Figure P24). Initial cyclisation of farnesyl pyrophosphate gives the cation (P24.2). Suggest a sequence of cationic reactions which could lead from (P24.2) to (P24.1). This is a particularly difficult problem. If you would like a hint (without having to look at the solution) one will be found at the end of the problems section. [Pg.363]

Of the carbocyclic sesquiterpenes found in Eremophila, the most elaborate are the tetracyclic enr-zizaenes (89,90,92,93). One possible sequence for the assembly of such a nucleus is given in Scheme 24. Cyclization of 2E,6Z-farnesyl pyrophosphate between the 1 and 6 position would generate the bisabolonium cation equivalent which, after a hydride shift, could further cyclize to a spiro carbocation. The tricyclic ring system can then be assembled by invoking alkylation of the cyclohexene double bond. The tertiary carbocation generated incorporates a bicyclo[3.2.1]octane system which can rearrange in two ways leading to the tricyclic sesquiterpenes metabolites found in Eremophila. [Pg.251]

Geranyl diphosphate is an allylic diphosphate and, like dimethylallyl diphosphate, can react with isopentenyl diphosphate. A 15-carbon carbocation is formed, which on deprotonation gives farnesyl diphosphate. Hydrolysis of farnesyl diphosphate gives the sesquiterpene farnesol. [Pg.1094]

Analogous processes involving cyclizations and rearrangements of carbocations derived from farnesyl diphosphate produce a rich variety of structural types in the sesquiterpene series. We will have more to say about the chemistry of higher terpenes, especially the triterpenes, later in this chapter. For the moment, however, let s return to smaller molecules to complete the picture of how isoprenoid compounds arise from acetate. [Pg.1096]

The taU-to-tail coupling of two farnesyl diphosphate molecules leads via a cyclopropane intermediate to squalene. The primary cyclopropane derivative, presqualene diphosphate, cleaves off the diphosphate residue, and the resulting cyclopropylmethyl carbocation opens the ring again. The aUyl cation is reduced by NADPH to squalene. [Pg.529]

Reaction mechanism of DMAT formation catalysed by DMAT synthase rotation around the C-l/C-2 bond of the allylic carbocation in a fraction of the molecules prior to bond formation with the indole. These findings characterise the prenyl transfer of DMAT synthase as an electrophilic aromatic substitution, mechanistically similar to the electrophilic alkylation catalysed by farnesyl diphosphate synthase (Song and Poulter, 1994). [Pg.109]


See other pages where Farnesyl carbocation is mentioned: [Pg.39]    [Pg.129]    [Pg.179]    [Pg.136]    [Pg.262]    [Pg.39]    [Pg.129]    [Pg.179]    [Pg.136]    [Pg.262]    [Pg.223]    [Pg.191]    [Pg.1191]    [Pg.377]    [Pg.6]    [Pg.33]    [Pg.76]    [Pg.77]    [Pg.86]    [Pg.33]    [Pg.55]    [Pg.317]    [Pg.33]    [Pg.200]    [Pg.296]    [Pg.297]    [Pg.298]    [Pg.180]    [Pg.181]    [Pg.58]    [Pg.89]    [Pg.56]    [Pg.78]    [Pg.263]   
See also in sourсe #XX -- [ Pg.262 ]




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