Cw-Verbenol

Both a- and P-pinenes are popular starting materials for the synthesis of other monoterpene chiral synthons such as carvone, terpineol, and camphor (vide infra). Reactions leading to other monoterpenes are briefly summarized in Figure 5.1. Treatment of a-pinene with lead tetraacetate followed by rearrangement gives trans-verbenyl acetate (7), which is hydrolyzed to yield trans-verbenol (8) 8 Subsequent oxidation of 8 gives verbenone (9), which can be reduced to give cw-verbenol... 

Adult male and female bark beetles (Ipsparaconfusus) convert (—)-a-pinene into ( + )-cis-verbenol and (-)-myrtenol whereas (+)-rran5-verbenol and ( + )-myrtenol are produced from (+)-a-pinene. Since ( + )-cw-verbenol is the aggregation pheromone for this beetle it would appear that production of the pheromone is controlled by the host tree. 

A similar phenomenon was reported by us in 1976.50 (13 ,43 ,53 )-dx-Verbenol (Figure 4.25) is a pheromone component of the bark beetle, Ips paraconfusus. At that time there was a confusion concerning the name of that pheromone component. Some people called (15 ,45 ,55 )-cw-verbenol as (+)-dx-verbenol, while others called it (-)-ds-verbenol. Our study of this compound revealed it to be levorotatory in chloroform, while dextrorotatory in methanol and acetone. 

Ipsenol (111, Figure 4.63), ipsdienol (112) and cw-verbenol (A) were isolated and identified in 1966 by Silverstein et al. as the components of the male-produced aggregation pheromone of a bark beetle, California five-spined ips (Ips paraconfusus). 

Fig. 12.1 Aggregation pheromones of (a) Ips paraconfusus, three male-produced synergistic components - (+ )2-methyl-6-methylene-2,7-octadien-4-ol (ipsdienol) (-)2-methyl-6-methylene-7-octen-4-ol (ipsenol) and cw-verbenol. (b) Dendroctonus brevicomisj three attractive components, cxo-7-ethyl-5-methyl-6,8-dioxabicyclo-[3.2.1] octane (cxo-brevicomin) from the female l,5-dimethyl-6,8-dioxabicyclo-[3.2.1] octane (frontalin) from the male and myrcene released from the host tree. Verbenone and trans-verbenol, released after a male locates the female, interrupts response to the attractant blend.

As with the previous discussions of the selective forces that mold the communication channel, it is difficult to arrive at either definitive proofs or absolute refutations of these models. Notwithstanding such difficulties, several cases provide illuminating evidence of the potential contribution of sex pheromones to the speciation process. In many of the bark beetles it is known that the aggregation pheromone is biosynthesized from precursors obtained by the feeding adult beetles from the phloem of the host trees. In Ips paraconfusus the pheromone component cw-verbenol is biosynthesized from the host terpene (-)-pinene. Ingestion of the opposite isomer produces the opposite optical form of the pheromone (Renwick et al., 1976). Clearly this suggests that beetles... 

The synthesis of the pheromon S(-)cw-verbenol in high enantiomeric excess based on a photoinduced catalytic reaction in the presence of a cyclodextrane-substituted iron(III) porphyrin complex (21) [43] may be... 

It is often difficult to know whether terpene compounds formd in insects are made de novo by the insect or whether the insect has altered some terpenes it finds in its food or environment. The boll weevil is known to alter plant terpenes from cotton. Ips beetles which bore under the bark of pine trees produce an aggregation pheromone consisting of ipsdienol or ipsenol (Figure 6.8) which were thought to be conversion products of myrcene from the pine tree. However studies with radio-labelled acetate and mevalonate showed that at least two species, Ips paraconfusus and Ips pini were able to synthesize their pheromones from scratch . On the other hand, both males and females of Ips paraconfusus convert a-pinene from the tree to cw-verbenol, which adds to the attraction of the male-produced pheromone. 

Show how a-pinene (Figure 6.8) can be derived from geranyl pyrophosphate. What kind of enzyme is required to convert it to cw-verbenol ... 

Optical rotatory dispersion (ORD) measurements have been made on some enantiomeric pheromones 49) and classical optical rotations are routinely determined. It should be noted that the sign of rotation may depend upon the solvent used (15, 45, 55)-cw-verbenol (234) is dextrorotatory in methanol and acetone 48, 51) and levorotatory in chloroform (52). 

Ips spp. bark beetles use a variety of terpene alcohols for communication. Among these are cw-verbenol (234, Scheme 46), ipsenol (240a, Scheme 50), and ipsdienol (266 a, Scheme 52). c -Verbenol was first synthesized by Silverstein et al. (51, 136) by stereospecific reduction of verbenone (233) with sodium borohydride. Mori (47) synthesized Irani-verbenol (232), a Dendroctonus spp. attractant, by lead tetraacetate oxidation of a-pinene (230) followed by hydrolysis of the resultant acetate (Scheme 46). Repeated recrystallization of the 3 3-acetoxyetienic acid ester afforded optically pure lra 5-verbenol. Oxidation (48) of the optically pure irawi-epimer to verbenone and reduction with lithium aluminium hydride yielded optically pure cM-verbenol. 

Ips calligraphus responds to ipsdienol and cw-verbenol (323). (8)-cis-Verbenol (234) is the active enantiomer and the response is not inhibited by (7 )-c -verbenol when present at a 1 1 ratio but is inhibited when the JR-enantiomer is present at a 10-fold higher amount (324). I. typo-graphus also responds to (5)-cw-verbenol (325). The lack of information... 

Hughes and Renwick (395) have recently conducted additional exposure experiments with I. paraconfusus males. In this species it was found that no appreciable synthesis of cw-verbenol occurred in the absence of exogenous (- )-a-pinene. Therefore, unlike D. frontalis (376), newly emerged I. paraconfusus males do not appear to have an endogenous precursor of cw-verbenol. 

Miller, D. R. and J. P. LaFontaine, cw-Verbenol An aggregation pheromone for the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera Scolytidae). J. Entomol. Soc. Brit. Columbia 88 34-38, 1991. 

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