A-Terpinyl cation

The biosynthesis of different classes of monoterpenes formed from a-terpinyl cation and respective precursors are illustrated in Fig. 4.3. 

Pinane monoterpenes are bicyclic monoterpenes resulting from intramolecular rearrangement of the a-terpinyl cation yielding the [3.1.1] bicyclic system (Fig. 4.3). a-Pinene 45 and -pinene 46 (Structure 4.11) are the main constituents of turpentine oil from pines. They occur widely in essential oils. 

H,14C]MVA was incorporated into la,2a,3p-trihydroxy-p-menthane by a Fusicoccum sp., and the labelling pattern suggested that p-menth-2-en-l-ol was generated from an a-terpinyl cation by a 1,3-hydride shift, followed by formation of the a-epoxide and cleavage.896 Ips beetles converted [2H]myrcene (the isotopically normal substrate being obtained in nature from Pinus spp.) into ipsdienol and... 

Figure 1. Postulated ionic mechanism for the formation of monoterpenes via the a-terpinyl cation (a) and the terpinen-4-yl cation (b). Regular (head-to-tail) structures are divided into isoprene units and the labeling patterns from Cl-labeled acyclic precursor are illustrated.

Treatment of geraniol, nerol, linalool, and their acetates and phosphates with 85% phosphoric acid results in essentially similar mixtures of predominantly cyclic hydrocarbons (a-terpinene, y-terpinene, isoterpinolene, limonene, p-cymene, and p-menth-3-ene) the results are rationalized in terms of carbonium ion structures and stabilities with the interesting observations (i) that limonene can neither be derived from the a-terpinyl cation nor from (67), into which the... 

Apparent from Scheme 4 is the necessity of the ionization-isomerization step to the cisoid linalyl intermediate for cyclization to the a-terpinyl cation, a reaction not directly possible from the geranyl substrate because of the frans-C2,C3 bond. A notable feature of all monoterpene cyclases is the abUity to utilize LPP... 

Table 2. Summation of mechanisms involved in the biosynthesis of selected monoterpenes derived from the a-terpinyl cation. Numbering is based on the a-terpinyl cation shown in Scheme 3...

LPP to undergo the 1,6-ring closure to the (5)- or (f )-a-terpinyl cation (40). A hypothetic antf-Markovnikov 1,7-cyclization has never been reported as it would imply the formation of a less stable secondary cycloheptenyl cation. 

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