Bisabolyl cation

In the sesquiterpene series, similar cyclizations lead to an amazing variety of products. After the initial unfavourable allylic rearrangement of the pyrophosphate group, farnesyl pyrophosphate can give a six-membered ring cation known as the bisabolyl cation. 

The bisabolyl cation (P10.1) is the key intermediate in the biosynthesis of the bergamotane, a- and (3-santalane families of terpenes. The biogenetic reactions involved in converting (P10.1) to the final products, are all enzyme mediated, but their chemistry follows the basic principles of carbocation mechanisms. Draw mechanisms to account for the formation of a-bergamotene (P10.2), a-santalene (P10.3) and (3-santalene (P10.4) from the cation (P10.1) and propose a structure for the intermediate (X) (Figure P10). 

The positive charge of the bisabolyl cation can be trapped by the double bond in the six-membered ring and this can happen in one of two ways. On the left of Figure S10, we see it being trapped so as to leave a tertiary... 

Biosynthesis Farnesyl diphosphate is converted to trails- fi-hergamotene by cyclization of an intermediate bisabolyl cation. Oxidative ring opening followed by three specific oxidation reactions leads to O.. ... 

The recombinant enzyme has been functionally expressed to greatly facilitate the understanding of structure-function relationships. This cyclase catalyzes initial syn isomerization of FPP to NPP, with rotation about the C2-C3 bond and anti-allylic attack leading to C6-C1 bond closure to generate the bisabolyl cation. This sequence is followed by C11-C7 closure, a 1,4-hydride shift, two suc-... 

Several terpene synthases appear to be multi-functional. For example, a sesquiterpene synthase from Zea mays produces a complex mixture of terpene volatiles. The closely related TPS4 and TPS5 from different maize varieties each synthesize the same complements of sesquiterpenes from FPP, but in different proportions as a result of the different ratios of S)- versus (il)-bisabolyl cation formation. This difference in stereoselectivity is determined by four amino acid residues in the active site [35]. 

The simplest biogenesis of the tricyclic cedrane sesquiterpenes (Scheme 20) involves initial cyclization of the P-bisabolyl cation (211) to the spiro[4.5]decane nucleus of the acoranes followed by a second cyclization upon the remaining double bond in the cyclohexene ring (108, 285). The P-bisabolyl cation may arise less directly from (—)-y-curcumene via additional deprotonation-protonation step, a distinction which is unnecessary for the present purposes (110). An alternative biogenesis of the requisite acorenyl cation intermediate has also been proposed in which the cationic precursor to the muurolenes undergoes 1,2-hydride shift, ring contraction and finally 1,4-hydride shift (110). 

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