Sesquiterpenes pentalenene

This [3 + 2]cyclopentene annelation has been used to obtain the sesquiterpene pentalenene (6) from the enone 2.2 Reaction of 2 with the dienolate 1 provides the... 

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... 

This annelation provides a key step in a synthesis of the sesquiterpene pentalenene (2, equation I). ... 

The MVA pathway was accepted as the unique biosynthetic pathway for the formation of aU isoprenoids in aU living organisms. Discrepancies with this general assertion appeared, however, as early as the 1950s (1, 2). For instance, -labeled MVA was not incorporated into chloroplast isoprenoids (e.g., carotenoids 25 and phytol 24 from chlorophylls Fig. 6), whereas it was well incorporated into phytosterols 27 synthesized in the cytoplasm. Unexpected labeling patterns were found in the prenyl chain of ubiquinone 22 in Escherichia coli at incorporation of C-labeled acetate. Finally, the labeling pattern in an isoprene unit from the sesquiterpenic pentalenene 21 series from a Streptomyces species at incorporation of uniformly... 

Pauson-Khand reaction. In the presence of tetramethylthiourea the Pauson-Khand reaction suceeds with enynes in which the double bond is present in a silylcyclopropene unit. It enables a synthesis of the angular triquinane sesquiterpene (-)-pentalenene. ... 

Pentalenene synthase fi-om Streptomyces sp. was cloned and overexpressed in Escherichia coli getting pentalenene, a known sesquiterpene site-directed mutants yielded other known sesquiterpenes, A -protoilludene and germacrene A (Seemann 1999). 

Intramolecular tandem Michael reaction.1 Angular triquinane sesquiterpenes such as pentalenene (5) have been synthesized from the bis(enone) 1 by an ntramolecular tandem Michael reaction as the key step for conversion to the rncvclic system. Lithium amides, even bulky ones, are not useful for this step, nor > dimethyl-t-butylsilyl triflate. Annelation can be achieved by reaction of 1 with c lSi(CH3)3, zinc chloride, and N(C2H5), at 160° in either toluene or CH2C12. Use... 

In addition to ( )-valerane, a wide variety of other sesquiterpenes, including ( )-ishwarane, ( )-ish-warone, ° copaene, ylangene, ( )-seychellene ( )-sativene, ( )-longifoline, " ( )-copa-camphene," ( )-damsin," ( )-A < >-capnellene, ( )-pentalenene, (-)-P-vetivone" and ( )-P-eudesmol have been synthesized by pathways involving cycloalkylation of saturated ketone enolates. 

Many synthetic endeavors have been undertaken to produce some of the tricyclic sesquiterpenes, and some biosynthetic efforts, such as that shown in Scheme 11.59, for pentalenene are under way, to attempt to find the path from farnesyl diphosphate to them. 

This a-domain is common to all class I terpene synthases and is the only domain in bacterial and fungal class I enzymes such as the pentalenene synthase from Streptomyces ey oliatus or the trichodiene synthase from Fusarium sporotri-chioides (Fig. 87.14a) [191,194]. Plant hemi-, mono-, and sesquiterpene synthases exhibit a second helical p-domain that resembles a barrel structure. An example of this class is given by the (+)-5-cadinene synthase from G. arboreum (Fig. 87.14b) [195]. In plant diterpene cyclases and in the exceptional case of the ( )-a-bisabolene synthase from A. grandis (Fig. 87.14c), a third helical y-domain with a barrel-like structure is present [197, 200, 202]. The p- and y-domains in the plant enzymes are nonfunctional but are required for correct enzyme folding. 

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