Hemiterpenoids

Hemiterpenes, by definition, consist of only one isopentane unit and are, thus, the simplest members of terpenoids. The basic building blocks of isoprenoids, namely IPP and DMAPP, must be universally present in green plants, being the progenitors of the phytyl side chain of chlorophyll, which is essential for biological activity (Sect. 8.1.4). These do not appear to be accumulated by plants (3), though the related prenyl alcohol (3,3-dimethylallyl alcohol, 9) and its isomer isoprene alcohol (10) have been found to occur in a few essential oils (229). Isoprene (11) itself is an important emittant of pine forests (165). 

Metabolic activity of DMAPP and IPP sometimes results in prenylation of plant phenolics leading to prenyl ethers, several of which hare been isolated from woody tissues. For example, imperatorin (12), a prenylated coumarin, is present in the roots of Imperatoria ostruthium (228). From African sneezewood Ptaerox-ylon obliquum, prenyletin (13) was obtained along with several other coumarins (125). Another example of a naturally occurring prenyl ether is the isoflavone, maxima-substance C (14) from roots of Tephrosia maxima (229). Several other ex- 

Many natural products contain an isolated isoprene unit derived from mevalonic acid hence, these compounds may be considered as hemiterpenoid derivatives. In most cases the immediate precursor is probably dimethyl allyl pyrophosphate 

Many secondary metabolites contain a hemiterpenoid unit. Hamada and Chubachi have examined the biosynthesis of rotenone (10). They found that the 

Tryptophan, alanine, and mevalonic acid have been established in earlier 

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Hemiterpenoids, 24 483 Hemizygous transgenics, 12 456 Hemocyanin, copper containing, 7 776 Hemodiafiltration (HDF), 26 832-833 Hemodialysis, 16 20 26 813-835 cellulosic fibers in, 16 18-19 dialyzer design and performance,... 

Non-hemiterpenoid Quinolines.—A re-examination of the alkaloids of Haplophyllum dubium resulted in the isolation of two members of the 2-aryl-4-quinolone group, i.e. graveoline (2 R = Me) and the new alkaloid norgraveoline (2 R = H).5 A new bacterial pseudan (3) has been obtained from Pseudomonas aeruginosa.8... 

Figure 6.10 De novo biosynthesis of isoprenoid pheromone components by bark and ambrosia beetles through the mevalonate biosynthetic pathway. The end products are hemiterpenoid and monoterpenoid pheromone products common throughout the Scolytidae and Platypodidae (Figure 6.9A). The biosynthesis is regulated by juvenile hormone III (JH III), which is a sesquiterpenoid product of the same pathway. The stereochemistry of JH III is indicated as described in Schooley and Baker (1985). Although insects do not biosynthesize sterols de novo, they do produce a variety of derivatives of isopentenyl diphosphate, geranyl diphosphate, and farnesyl diphosphate. Figure adapted from Seybold and Tittiger (2003).

Non-hemiterpenoid Quinolines.—New sources of the simple quinolines 4-methoxy-l-methyl-2-quinolone and its 8-methoxy-derivative (folimine) have been reported the former was isolated from Myrtopsis sellingii9 and from Zanthoxylum cuspidatum,16 and folimine was shown to be a constituent of Haplophyllum perforatum.5 The latter species also contains foliosidine (9), previously isolated from H. foliosum. The micro-organism Pseudomonas aertiginosa has been shown to contain 2-(hept-l-enyl)-4-quinolone (12).10 The structure of the alkaloid was established by n.m.r. and mass spectroscopy and by its synthesis from aniline and the j3-keto-ester Me(CH2)4CH=CHC0CH2C02Me. 

Carbon-13 N.M.R. Spectroscopy.—A study of the, 3C n.m.r. spectra of twenty-five hemiterpenoid quinoline alkaloids and related prenylquinolines, including C-, 0-, and A-prenyl-quinoline and -quinolone derivatives, hydroxyisopropyldihydro-furoquinolinones, hydroxydimethyldihydropyranoquinolinones, and furoquino-lines, has been carried out 6 only isolated examples were reported previously.7,8... 

Hemiterpenoids are produced from the isoprenyl diphosphate DMAPP. All other terpenoids are produced from DMAPP and IPP via longer-chain prenyl diphosphate intermediates formed by prenyl transferases. Prenyl transferases (20) catalyze the formation of geranyl diphosphate (GPP), famesyl diphosphate (FPP), and geranylgeranyl diphosphate (GGPP) from one molecule of DMAPP and one, two, or three molecules of IPP, respectively (Fig. 1). Isoprenyl diphosphates are the substrates for all TPS, which lead to the hemiterpenoids, monoterpenoids, sesquiterpenoids, and diterpenoids, which will be highlighted with selected examples in the following sections. 

Several reviews have appeared, on such topics as steroid and terpenoid biogenesis, hemiterpenoid alkaloids,indole alkaloids (monoterpenoids), ophiobolins (sesterterpenoids), sterols in Digitalis, oestrogens, plant tri-terpenoids, and carotenoids. ... 

Prenylquinoline Alkaloids.—A large number of alkaloids have already been separated from Ptelea trifoliata, including the hemiterpenoids with terminal double bonds that are characteristic of this species (cf. Vols. 1—3, 5, and 6). Two groups have now continued the investigation of the tertiary bases and quaternary salts of P. trifoliata. 

Two useful supplementary volumes " " update the corresponding chapters in the second edition of Rodd on acyclic and monocyclic monoterpenoids/ on bicyclic monoterpenoids,and on the biogenesis of mevalonate, hemiterpenoids, and monoterpenoids.A useful textbook on natural plant constituents includes some biochemistry and chemistry of monoterpenoids. ... 

C]Mevalonolactone is synthesized in two steps from [2- C]acetic acid and 4-benzyloxybutan-2-one although a longer route is required for [3,4- C2]mevalonolactone. The structure of the hemiterpenoid mustelan (8) from the anal glands of the mink (Mustela vison) and the polecat (Mustela putorius) has been... 

Terpenoid Synthesis from Isoprene.—Co-oxidation of thiophenol and isoprene with oxygen yields the synthons (9) and (10) in useful yields and the isoprene epoxide (11) is a useful hemiterpenoid synthon with carbanions. ... 

Ergot Alkaloids.—The hemiterpenoid unit of the ergot alkaloids is interesting in that, as expected, only one carbon atom is labelled by [2- C]mevalonic acid [marked by in formulae (14)—(17)] but the stereochemistry of this atom changes. Extensive studies by Floss etalf indicate that chanoclavine I (14) is converted into elymoclavine (17) via the aldehyde (15) and possibly via agroclavine (16) (however, see ref. 36a). This process involves two isomerisations about the isolated double bond. In dimethyl allyl pyrophosphate (5) the trans-methyl is labelled by... 

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