Irregular Monoterpenes

The other major group of irregular monoterpenes is formed by non-head-to-tail fusion of isoprene units. Important members include artemisia ketone 64, santolinatriene 65, chrysanthemol 66, yomogi alcohol 67 and lavandulol 68 (Structure 4.15). Lavandulane-type compounds occur in the families Lamiaceae (Labiatae) and Apiaceae (Umbelliferae), while chrysanthemane, artemisane and santolinane types occur in the family Asteraceae (Compositae) [47,48]. 

The root of carrot Daucus carota) is eaten raw or cooked. The characteristic aroma and flavour of carrots are mainly due to volatile compounds, although non-volatile polyacetylenes and isocoumarins contribute significantly to the bitterness of carrots [1,2]. More than 90 volatile compounds have been identified from carrots (Table 7.9) [207-215]. The carrot volatiles consist mainly of terpenoids in terms of numbers and amounts and include monoterpenes, sesquiterpenes and irregular terpenes. Monoterpenes and sesquiterpenes account... 

The metabolism of 7-endo-fenchol to ci-fenchone in fennel has been studied in quite some detail by Croteau and co-workers (Croteau and Felton, 1980). Croteau et al. (1980a) later reported a soluble enzyme preparation from the leaves of fennel which catalysed the cation-dependent cyclization of both geranyl pyrophosphate and neryl pyrophosphate to the bicyclic rearranged monoterpene 1-enc/o-fenchol. Croteau et al. (1980b) found that (+)-(lS)-fenchone, an irregular bicyclic monoterpene ketone thought to be derived... 

The deprotonation by LDA of the mixture of lactones derived from ( R)-benzylglycerol followed by kinetic protonation (Scheme 75) provided a regio- and stereospecific access to a key intermediate in the synthesis of irregular monoterpenes. This regioselectivity has been observed for the protonation of many steroidal dienolates364,365. 

In this section we analyze information about metabolic cleavage or breakdown of cyclopropane rings in three instances the biosynthesis of irregular monoterpenes, the ringopening of cycloartenol (20) derivatives, and the metabolic opening of 1-aminocyclopropane-1-carboxylic acid (ACPC) (9) by two quite distinct fragmentation routes. We will not explicitly discuss the processing of presqualene pyrophosphate (77) and prephytoene pyrophosphate (89) to squalene (76) and phytoene (88) respectively, since those transformations have already been dealt with in Section II. 

A. Degradation of Cyclopropane Rings in the Biosynthesis of Irregular Monoterpenes... 

Chrysanthemic acid (126), artemesia ketone (127), santolina triene (128) and lavandulol (129) are representatives of irregular monoterpenes which, unlike other regular monoterpenes, are not derived directly from geranyl pyrophosphate (59). By contrast to the regular... 

Artemisia species are rich sources of irregular monoterpenes. Thus Artemisia vulgaris contains a number of lyratol (195) esters, santolina alcohol (196) acetate, and the more oxygenated santolina compounds 197 and the two alcohols 198. The (Z)-isomer of lyratol acetate was also found. Lyratol (195) and its acetate... 

A number of monoterpenes do not fit the isoprene mle and are called irregular monoterpenes. There are three major stmctural types of irregular monoterpenes the chrysan-themyl (89), artemisyl (90), and santolinyl (91) skeletons (Charlwood and Banthorpe, 1978 Charlwood and Chari-wood, 1991a). These compounds are found primarily in the Asteraceae (they are especially well known from the genus Artemisia), although a few are found in the Apiaceae and Lamiaceae (Poulter, 1990). As two skeletal types often occur in the same plant, it is reasonable to suspect that a common biosynthetic pathway exists and it is possible that the three types arise from a common cationic species (Fig. 19.21). 

Fig. 19.21 (a b). Proposed biosynthesis of irregular monoterpenes (modified from Epstein and Poulter, 1973 used with permission of the copyright owner, Elsevier Science Ltd., The Boulevard Langford Lane, Kidlington 0X5 1GB, UK). 

It is of interest that in some nonenzymatic studies, a small percentage of head-to-head condensation products are observed. Chrysanthemyl alcohol (92) is an analog of presqua-lene alcohol and prephytoene (see Chapters 23 and 26), but the fissions postulated to lead to the irregular monoterpenes have no counterparts for the higher classes (Charlwood and Chari wood, 1991a). 

In cell-free extracts of Artemisia annua and Santolina chamaecyparissus, chrysanthemyl alcohol and its pyrophosphate are incorporated into artemisia ketone and alcohol. Artemisia alcohol (95) is converted into artemisia ketone (94) and tranj-chrysanthemic acid in the preparation from S. chamaecyparissus (Banthorpe et al., 1977a). IPP and DMAPP are incorporated into irregular monoterpenes whereas geranyl and neryl-OPP are not. An enzymatic sulf-hydryl group is involved. 

Banthorpe, D. V., S. Doonan, and J. A. Gutowski, Biosynthesis of irregular monoterpenes in extracts from higher plants. Phytochemistry, 16, 85-92 (1977a). 

Epstein, W. W. and C. C. Poultbr, A survey of some irregular monoterpenes and their biogenetic analogies to presqualene alcohol, Phytochemistry, 12, 737-747 (1973). 

Ishigmo K, Nakajima M, Fukumoto H, Isoi, K. A xanthone substituted with an irregular monoterpene in cell suspension cultures of Hypericum patulum. Phytochemistry 39(4), 903-905,1995. 

Epstein, W. W., and C. D. Poulter A Survey of Some Irregular Monoterpenes and Their Biogenetic Analogies to Presqualene Alcohol. Review Article. Phytochemistry 12, 737 (1973). 

It is now fairly well-established that GPP (8) is the biosynthetic precursor of almost all monoterpenoids, except for a small group of so-called irregular types. Acyclic monoterpenes, having 2,6-dimethyloctane framework, arise directly... 

IPP is the near-universal prenyl acceptor in the IPPS enzymes ( irregular monoterpene synthases are exceptions (vide infra)). In the protein prenyltransferases, IPP is absent, and a cysteine sulphur atom from the target protein acts as the electron-rich prenyl acceptor. 

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