Pterocarpanes

Benzofurobenzopyrans, 4, 995-1000 6H-Benzofuro[3,2-c][l]benzopyrans, 6a,l la-dihydro-— see Pterocarpans Benzofurofurans mass spectrometry, 4, 585 Benzofuro[2,3-d]pyridazines synthesis, 4, 985... 

Wamke, M.M. et al.. Use of native and derivatized cyclodextrin based and macrocychc glycopeptide based chiral stationary phases for the enantioseparation of pterocarpans by HPLC, J. Liq. Chrom. Rel. TechnoL, 28, 823, 2005. 

The lithiation of phenols protected as acetals—methoxymethyl acetals like 140 in particular—is especially valuable the second oxygen supplies a powerful coordination component to their directing effect (Scheme 69) The regioselective lithiation of 141 was used in the synthesis of the pterocarpans 4 -deoxycabenegrins A-I. 

Kiss, L. et al., Chiroptical properties and synthesis of enantiopure cis and trans pterocarpan skeleton. Chirality, 15, 558, 2003. 

Pterocarpan 6a-hydroxylase (3,9-dihydroxypterocarpan 6a-hydroxylase) P6aH (DH6aH) 1.14.13.28 CytP450 (CYP93A) 202... 

FIGURE 3.5 Biosynthetic route to isoflavonoids (and some derivatives) from the 5-deoxyflavanone liquiritigenin. A possible route to the retrochalcone echinatin is also shown. Unlabelled arrows indicate biosynthetic steps for which the enzyme(s) have not been characterized. Enzyme abbreviations are defined in the text and in Table 3.1, except for P2CP, pterocarpan 2-C-prenyltransferase P4CP, pterocarpan 4-C-prenyltransferase. 

The 2 -hydroxyisoflavones are reduced to the corresponding isoflavanones by a NADPH-dependent isoflavone reductase (IFR). The isoflavanones are the final isoflavonoid intermediates of pterocarpan biosynthesis. Variant IFR activities between species are thought to contribute to the stereochemistry of the pterocarpans produced, in particular, (-l-)-maackiain in P. sativum, (—)-maackiain in C. arietinum, (—)-3,9-dihydroxypterocarpan in G. max, and (—)-medicarpin in M. sativa. The (—) indicates 6aRllai stereochemistry. 

Based on analysis of enzyme preparations, the conversion of isoflavanones to pterocarpans was thought initially to be catalyzed by a single NADPH-dependent enzyme, termed the pterocarpan synthase (PTS). However, it was subsequently shown that in M. sativa the conversion of vestitone to medicarpin involves two enzymes, VR and 7,2 -dihydroxy-4 -methoxyisoflavanol (DMI) dehydratase (DMID). The reaction series from vestitone to the pterocarpan is thought to proceed by the VR-catalyzed reduction of vestitone to DMI, followed by the loss of water and formation of the C-O-C bridge between the heterocycle and the B-ring, catalyzed by DMID. 

In some species, such as C. arietinum and M. saliva, the products of VR and DMID (maackiain and medicarpin) are the main pterocarpan phytoalexins. They are typically glucosylated and malonylated and stored in the vacuole. In species such as G. max, P. sativum, and P. vulgaris, the pterocarpans are further converted by a series of reactions to species-specific compounds. For G. max and P. sativum, the initial reaction is a hydro-xylation catalyzed by P6aH. 

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. 

Modification of isoflavonoid biosynthesis may have a wide range of applications for improving not only plant defense characters but also the health benefits of food for humans. With the exception of IFD (which may not be required in vivo), cDNA clones are available for all of the enzymes needed for the production of the isoflavonoid vestitone. Furthermore, as VR cDNAs have been cloned, only clones for the DMID are lacking for the biosynthetic branch of the antifungal pterocarpans. To date, experiments have focused on 2HIS, but there has also been success using IFR, I2 H, and I70MT. 

Tiemann, K. et al., Pterocarpan phytoalexin biosynthesis in elicitor-challenged chickpea (Cicer arietinum L.) cell cultures. Purification, characterization and cDNA cloning of NADPHnsofiavone oxidoreductase. Eur. J. Biochem., 200, 751, 1991. 

Welle, R. and Grisebach, H., Induction of phytoalexin synthesis in soybean enzymatic cyclization of prenylated pterocarpans to glyceollin isomers. Arch. Biochem. Biophys., 263, 191, 1988. 

FIGURE 8.6 Structure of the phytoalexin isoflavonoid pterocarpans, maackianin, and pisatin from garden pea, and the isoflavones daidzein and genistein from soybean. 

---