C-Prenylation

In addition to the bitter acids and essential oils, the flowers of hops offer a rich array of polyphenolic compounds, primarily chalcones and their accompanying flavanones, many of which are prenylated derivatives (Stevens et al., 1997,1999a, b). The most prominent flavonoid in all plants studied was xanthohumol [342] (3 -prenyl-6 -0-methylchalconaringenin chalconaringenin is 2, 4, 6, 4-tetrahydroxychalcone) (see Fig. 4.11 for structures 342-346). Several additional chalcones—variously adorned with 0-methyl and/or C-prenyl functions—were also encountered, along with their respective flavanones. Three new compounds were described in the Stevens et al. 

Glomset, J. A., Gebb, M. H. and Farnsworth, C. C. Prenyl proteins in eukaryotic cells a new type of membrane anchor. Trends Biochem. Sci. 15 139-142,1990. 

Fig. 5. Schematic representation of lipid anchors. The arrow heads and tails represent the N termini and the C termini of mature proteins, respectively, (a) Palmitoylation, (b) Ahnyristoylation, (c) Prenylation, (d) GPI anchor... 

The formation of phytoalexins such as glyceollins and phaseollins requires C-prenylation by a range of pterocarpan prenyltransferase (PTP) activities, with dimethylallyl pyrophosphate (DMAPP) as the prenyl donor. For glyceollins and phaseollins, prenylation occurs at position C-2 or C-4 of glycinol or C-10 of 3,9-dihydroxypterocarpan. ° ° However, there are differing activities in other species. For example, in Lupinus albus (white lupin) a prenyltransferase acting at the C-6, -8, and -3 positions of isoflavones has been identified.PTPs have also been characterized in detail for the formation of prenylated flavanones in Sophora flavescens (see, e.g., Ref. 207). However, no cDNA clones for flavonoid-related prenyltransferases have been published to date. 

OH-, OMe-groups C-methyl methylenedioxy groups C-prenylation (9-prenylation (dihydro)furanosubstitution pyranosubstitution complex cyclosubstitution aromatic substitution esterification chlorination. These residues may also occur combined in one flavonoid structure. 

Data on this type of flavonols are summarized in Table 12.4. In contrast to the corresponding flavones, the number and complexity of derivatives is smaller. This concerns particularly the formation of furano-, pyrano- and other cyclic flavonols. There is a remarkable number of 0-prenylated flavonols known to date, contrasting to only very few flavones exhibiting this substitution pattern (see Table 12.3). Similar trends have been earlier documented in the review of Barron and Ibrahim. The occurrence of a series of glycosides based on C-prenylated structures is considerable. This substitution trend concerns also some of the dihydroflavonols, thus indicating specific enzyme activities probably dependent on the presence of a 3-OH group. 

Species of the genus Vellozia have been extensively studied for their flavonoid complement in relation to chemosystematics. In addition to a series of C-methylflavonols and two C-prenylated flavonols, derivatives of vellokaempferol and velloquercetin are accumulated in whole plants, leaves, and leaf exudates. The basic structure of these compounds is characterized by 7,6-isopropenylfurano substitution, based upon kaempferol, quercetin, and their 0-methyl ethers. In addition, 8-C-methyl derivatives of these compounds were also identified from leaves of V. stipitata " So far, species of this genus are the only reported sources of these compounds, which in parts have been proved to be accumulated externally. ° Structures are exemplified by Figure 12.10. 

Kaouadji, M. et al., 6-Hydroxygalangin and C-prenylated kaempferol derivatives from Platanus acerifolia Buds., Phytochemistry, 31, 2131, 1992. 

C-Prenyl-5,7,4 -trihydroxy-3 -methoxyflavone (8-C-prenylchrysoeriol) 7-Glucosyl(l — 3)-a-L-arabinopyranoside Erythrina indica seeds Leguminosae 164... 

In the O-prenylation of hydroxychromones or hydroxyflavonoids, C-prenylation often proceeds simultaneously but when 5,7-dihydroxy-2-methylchromone (539) is treated with... 

Identification and quantitative analyses of 25 compounds in steam distillates of burley tobacco stalk were accomplished. Compounds included twelve C,-C. compounds that were probable fatty acid oxidation products and 13 compounds >C that varied in origin. The latter included oxidation products of fatty acids, a C. prenyl pyrophosphate metabolite, and biodegradation products of carotenoids and chlorophyll. About 1/3 of the distillate mass was accounted for. Burley tobacco stalk headspace volatiles were also studied. When compared to the steam distillate, the headspace contained greater concentrations of sesquiterpenoids but lower concentrations of C, and C aldehydes and alcohols. Volatiles in steam distillates of tobacco stalk were not quantitatively different in a fungal resistant and a fungal sensitive variety of tobacco. Yield comparisons were made of headspace volatiles from tobacco and wheat. 

Three methods of C prenylation are known. They are as follows ... 

Fig. 11 C prenylation with prenyl bromide in the presence of strong base [58]...

It is also noteworthy that some of the phloroglucinols isolated from Hypericum species, are characterized by O-prenylation, which, in contrast to C-prenylation, is quite rare for natural phenolic compounds. 

Vismiaphenone C Prenyl HO J OMe pren> O OH 380 Gamma pseudogunifera. Garcinia myrtifolia [49, 50]... 

Guttiferone C Prenyl Oil Prenyl. / prenyl no, 0 °H 670 Symphonia globulifera [881... 

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