Cyclopentane Monoterpenes

Bornane monoterpenes are exemplified by camphene (2,2-dimethyl-3-methylene-bicyclo[2,2,1]heptane), a structure in which two fused cyclopentane rings share three Cs. We can simply represent the camphene skeleton as a cyclohexane with a methylene (—CH2—) cross-link (G6(-CH2—)). The keto derivative camphor (camphor smell), the ether eucalyptol (eucalyptus smell) and the simple bornene a-pinene (pine smell) are familiar examples. 

Additional chemical diversity of monoterpenes is apparent from the natural occurrence of their bicyclic analogs that bear cyclopropane (carane and thujane types), cyclobutane (pinane type), and cyclopentane (camphene/bornane, isoeamphane and fenchone types) rings (Figs. 6 and 7). The carane type of bicyclic monoterpenoids in plants is represented by (+)-3-carene... 

Iridoids are derivatives of monoterpenes and occur usually, but not invariably, as glycosides.100,101 Structurally, they are cyclopentano [r] pyran monoterpenoids and they provide a biogenetical and chemotaxonomical link between terpenes and alkaloids. The cleavage of the cyclopentane ring of iridoids produces secoiridoids.10... 

The close relationship between monoterpenes and insects is, of course, well known, and work has intensified on what attracts, say, bark beetles to appropriate trees. One of the most intriguing aspects is the potentiating effect ethanol has on the aggregation of certain species when in conjunction with the pheromone. This aspect has little to do with total synthesis, but the discovery of new pheromones and defense secretions certainly has. Many are described in this chapter, but one of the more unusual types is the new cyclopentane structure found by Eisner and Meinwald in pheromones of certain carrion beetles, the synthesis of which is described in the appropriate section (Section 6). °... 

If we look beyond the cleavage of the cyclopentane ring, there is an analogous alkaloid. Although not a simple monoterpene alkaloid, but rather a monoterpene indole alkaloid, deppeaninol (381) from Deppea blumena-... 

Iridoids are monoterpene compounds characterized by a cyclopenta [c] pyranoid skeleton, also known as the iridane skeleton (cis-2-oxabicyclo-[4, 3, 0]-nonane). These are also found to occur in a variety of animal species. In a broad sense, it is acceptable to include in this group the secoiridoids, which arise from the latter by cleavage of the 7, 8 bond of the cyclopentane ring. This group (of about 500 known structures) chiefly comprises iridoid glycosides (>300), secoiridoid glycosides (>100), and non glycosidic compounds (>100). 

In the end, it is necessary to cite, in this biogenetic discourse, the secoiridoid group which is derived from methylcyclopentanoid monoterpenes, even if it does not maintain the characteristic methylcyclopentanoid moiety. Secoiridoids arise, in fact, from the oxidative opening of the cyclopentane ring of both glycosidic or non-glycosidic iridoids by cleavage of the C-8/C-9 bond. 

The synthesis of the skeleton of these two monoterpenic alkaloids and the control of the relative stereochemistry at C4, CS, and C6 was aehieved using a Wolff rearrangement followed by a photoreductive cyclization of unsaturated Af-alkyl-2-oxo-cyclopentane carboxamides of type D (Scheme 40). 

Plant feeding insects are able to store host plant monoterpenes and use the compounds in their own defense. Pine sawfly larvae store mono- and diterpenes of host plant resin for defensive purposes (see diterpenes). Larvae of Chrysomelid leaf beetles feeding on Salix and Populus are able to store and sequester precursors of iridoids (cyclopentane monoterpenoids) derived from host plant. In addition to this sequestration iridoids may even have a dual origin as larvae of Plagiodera versicolora and Phratora laticollis have the potential to de novo produce iridoids from the precursors in their food [8]. 

The most important, indeed crucial, idea put forward on the biosynthesis of these alkaloids was that they are formed by fragmentation of a cyclopentane monoterpene [as (6.237)]. The pathway outlined in Scheme 6.43 indicates how the major skeletal types represented by ajmalicine (6.243) and akuammicine (6.242), catharan-thine (6.239), and vindoline (6.244) may be formed where one of... 

The appropriate incorporations of mevalonic acid (6.235) [as elsewhere in biosynthesis it is the (3-/ )-isomer which is utilized], and geraniol (6.236) and nerol (6.245) prove the essential correctness of the hypothesis the hydroxy-derivatives (6.246) and (6.247) were also utilized. Ensuing crucial experiments lay with identification of the cyclopentane monoterpene loganin (6.249) as a key intermediate in biosynthesis. Not only did (6.249) prove to be a specific alkaloid precursor, but its presence and biosynthesis from geraniol in Catharan-thus roseus, a plant used for most of the experiments, could be demonstrated [173, 174]. 

Iridoid monoterpenes called iridoids are a group derived from the skeleton of the hydrocarbon iridane (9-206). Their usual structure is represented by the formula 9-207. The opening of a cyclopentane ring and introduction of various functional groups (mainly by oxidation) yields secoiridoids (9-208), which include many bitter substances of plants. 

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