Lignans structures

In 1933, Erdtman suggested [20] that the basic lignan structure involved the coupling of two phenylpropanoid monomer units. Isotope tracer experiments on lignans were later described [2,21,22]. Significant advances have however been made in the chemistry and biosynthesis of lignans by Lewis... 

As interesting combination of flavonoid and lignan structures is found in a group of compounds called flavonolignans. They arise by oxidative coupling processes between a flavonoid and a phenylpropanoid, usually coniferyl alcohol. Silybin, silychristin and silydianin are well-known examples that are collectively termed silymarins , and are isolated from Silybum mari-anum [36] as flavonolignans. 

MeOH). The uv and IR spectra suggested the lignan structure. Acetylation of 41 gave a heptaacetate (41a), whose H-NMR spectra showed the presence of seven alcoholic acetyl groups (6 1.97, 2.03,... 

Table 7.3.4. Lignan structures classified according to skeletal types and subtypes...

Izuka, Y, Namiki, M and Yamashita, K (1996) Effect of lignan structure of sesame seed lignans and dietary alpha-tocopherol on alpha-tocopherol level in rats. Nippon Eiyo Shokuryo Gakkaishi (J. Jpn. Soc. Nutr. FoodScL), 49, 149-155. 

The structure of carpanone (1) was revealed by Australian scientists in 1969.1 Carpanone, a hexacyclic molecule and host of five contiguous stereogenic centers, is a lignan found in the bark of the car-pano tree. 

Lundquist, K. Stomberg, R. On the occurrence of structural elements of the lignan type (P-P structures) in lignins. The crystal structures of (+ )-pinoresinol and ( )-trans-3,4-divanillyltetrahydrofuran. Holzforschung 1988, 42, 375-384. 

Fig. 2.86. Chemical structures of the iosflavones daidzein, genistein, O-desmethylangolensin, equol, and the lignans enterolactone and enterodiol. Reprinted with permission from Z. Kuklenyik el al. [209].

Using this system, (Z)-hinokiresinol isolated from cultured cells of A. officinalis was determined to be the optically pure (75 )-isomer, while ( )-hinokiresinol isolated from cultured cells of C. japonica had 83.3% e.e. in favor of the (7S)-enantiomer (Table 12.1). The enzymatically formed (Z)-hinokiresinol obtained following incubation of p-coumaryl p-coumarate with a mixture of equal amounts of recZHRSa and recZHRSf) was found to be the optically pure (75)-isomer, which is identical to that isolated from A. officinalis cells (Table 12.1). A similar result was obtained with the crude plant protein from A. officinalis cultured cells, where the formed (Z)-hinokiresinol was almost optically pure, 97.2% e.e. in favor of the (75)-isomer (Table 12.1). In sharp contrast, when each subunit protein, recZHRSa or recZHRSP, was individually incubated with p-coumaryl p-coumarate, ( )-hinokiresinol was formed (Table 12.1). The enantiomeric compositions of ( )-hinokiresinol thus formed were 20.6% e.e. (with recZHRSa) and 9.0% e.e. (with recZHRSP) in favor of the (7S)-enantiomer (Table 12.1). Taken together, these results clearly indicate that the subunit composition of ZHRS controls not only cis/trans selectivity but also enantioselectivity in hinokiresinol formation (Fig. 12.3). This provides a novel example of enantiomeric control in the biosynthesis of natural products. Although the mechanism for the cis/trans selective and enantioselective reaction remains to be elucidated, for example by x-ray crystallography, the enantioselective mechanism totally differs from the enantioselectivity in biosynthesis of lignans, another class of phenylpropanoid compounds closely related to norlignans in terms of structure and biosynthesis. 

Youn B, Moinuddin SGA, Davin LB et al (2005) Crystal structures of apo-form and binary/temary complexes of Podophyllum secoisolariciresinol dehydrogenase, an enzyme involved in formation of health-protecting and plant defense lignans. J Biol Chem 280 12917-12926... 

Lignans are secondary plant metabolites possessing a variety of biological activities [1,2]. They are dimers of phenylpropenes, which are by definition connected between C(8) and C(8 ) [3]. Lignans are of great structural variety due to numerous potential oxidation states at the C(7)/C(7 ) and C(9)/C(9 ) positions, and to the possibility of aryl-aryl bond formation [1-3]. 

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