Polyacetylenes biosynthesis

The ease with which thiophenes are formed in the reaction of acetylenic epoxides " and of polyacetylenes with hydrogen sulfide is of great interest in connection with the biosynthesis of the naturally occurring thiophenes (cf. Section VIH,A) and also of preparative importance. 2-Methyl-l,2-oxido-5-hexene-3-yne (56) in water containing barium hydroxide reacts with HzS at 50°C to give 4-... 

The involvement of desaturase-type enzymes in the biosynthesis of some important lipidic antifeedants now has been firmly established (Fig. Ic) (7). An instructive example of this phenomenon is the production of ricinoleate (a purgative) by a desaturase homolog found in the castor plant. Here, a subtle variation in mechanistic pathway is responsible for the introduction of a C-12 hydroxyl group rather than a 12,13-double bond (7). Another interesting case features the putative dehydrogenation of a cyclopropyl fatty acid to produce sterculate, a potent inhibitor of the mammalian but not the plant A desaturase (8). Finally, the biosynthesis of polyacetylenes with antifungal properties such as falcarindiol is thought to involve a sequence of unique desaturase-mediated oxidations (9). 

The biosynthesis of polyacetylenes and prostaglandins The formation of unsaturated fatty acids, e.g. oleic acid (7.2), which are also primary metabolites, may occur by at least two routes, one aerobic the other anaerobic. Essentially though, both involve desaturation of a fully saturated fatty acid [2]. Polyacetylenes, e.g. crepenynic acid 1.16), which are secondary metabolites, also... 

Table 5.1). Some of these acetylenes have also been isolated from Solanaceous food plants such as tomatoes and aubergines, where they appear to be phytoalexins (see Section 5.2.2.1). The biosynthesis of polyacetylenes of the falcarinol-type follows the normal biosynthetic pathway for aliphatic Ciy-acetylenes (Bohlmann et al. 1973 Hansen and BoU 1986), with dehydrogenation of oleic acid leading to the Cig-acetylenes crep-enynic acid and dehydrocrepenynic acid, which is then transformed to C17-acetylenes by -oxidation (Figure 5.2). Further oxidation and dehydrogenation leads to acetylenes of the falcarinol-type as outlined in Figure 5.2. 

Bu Lock, J.D. and Smalley, H.M. (1962) The biosynthesis of polyacetylenes. Part V. The role of malonate derivatives, and the common origin of fatty acids, polyacetylenes, and acetate-derived phenols. /. Chem. Soc.y 4662-4664. 

Hansen, L. and BoU, P. M. (1986) Polyacetylenes in Araliaceae their chemistry, biosynthesis and biological significance. Phytochemistry, 25, 285-293. 

Jones, E. R. H., G. Lowe, and P. V. R. Shannon Natural acetylenes XXI. The biosynthesis and transformation of some polyacetylenic metabolites of Merulius lacrymans Fr. and Clitocyhe rhizophora Velen. J. Chem. Soc. 1966b, 144. 

Bu Lock, J.D. Biosynthesis of polyacetylenes in fungi, p. 141. In J. F. Snell, Biosynthesis of antibiotics. New York Academic Press 1967. 

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