Isoflavonoids and Neoflavonoids

Recent reviews of the isoflavonoids include those of Wong (in (15)), Dewick (in (14)), and Ingham (21). According to the latter two reviews, the number of 

Other related isoflavonoid types are known that may co-occur with one or more isoflavones in legume heartwoods. Related pterocarpans and isoflavans, for example, accompany isoflavones in the wood of various Dalbergia and Machaerium species (see Sect. 7.S.3.3). Again, the isoflavone quinone bowdichione (42) and the pterocarpan homopterocarpin, (46) occur with the isoflavones genistein (34) and calycosin (3 -hydroformononetin) (111) in the heartwood of Bowdichia nitida, 

Bowdichione (42) is colored, unlike most isoflavonoids, which are colorless. It is also unique in that it remains the only quinonoid derivative in this series. More numerous are the isoflavanones, of which 39 structures are known. Only some of these occur in woody tissues, such as dihydrodaidzein (43) in Pericopsis montana and dalbergioidin (44) in Lespedeza cyrtobotrya. Isoflavanones have an optically active center at C-3, as do the pterocarpens (e.g. 45-48). Racemization may occur during isolation so that optically inactive forms are sometimes described in the literature. In fact, the absolute configurations of most isoflavanones and pterocarpans have still to be determined. 

Although there are optically active centers at both the 6 a and 11a positions in pterocarpans, considerations of steric hindrance mean that only two of the four possible configurations are likely in nature, namely the 6aR llaR and the 6aS llaS structures. Note that the pterocarpan numbering differs from that in the isoflavones or isoflavanones. 

There are two series of pterocarpans in legume woods, those without and those with a hydroxyl at C-6a. Respective examples are medicarpin (45) from the 

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Closely related both structurally and biogenetically to the flavonoids are two groups of natural products—the isoflavonoids and neoflavonoids. In contrast to the flavonoids however, these groups are of a more limited taxonomic distribution in higher plants and are found primarily in the Leguminosae. Isoflavonoids have in common the 1,2-diphenyl propane skeleton (11) and include many classes of natural product such as the isoflavones, isoflava-nones, rotenoids, pterocarpans and coumestans. The term neo-flavonoid was introduced by Ollis and his associates to cover the third group of compounds which possess the 1,1-diarylpropane skeleton (12) and includes such compounds as the 4-phenyl-coumarins and the various dalbergiones. 

The structures of many of the naturally occurring flavonoids, isoflavonoids and neoflavonoids contain Cj units which are apparently derived from mevalonate the precursor of the isoprenoid units of terpenes and steroids. The biosynthetic processes in which the additional units are incorporated into the structures of these compounds presumably involved the intervention of yy-dimethyl-allylpyrophosphate or isopentenylpyrophosphate as intermediates . An additional, and rather unusual structural variation, which has recently been encountered is that of the flavonolignan class in which a flavonoid (taxifolin) is apparently condensed with a pbenylpropanoid intermediate (coniferyl alcohol) . Typical examples of the various classes and the considerable structural diversity which is to be found within them are shown in the accompanying formulae (13-33). 

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