Pharmacognosy with Basics of Plant Biochemistry - Kovalyov V. M. 2004

Special Part
Phenolic Compounds
Lignans

Lignans are dimers of phenylpropane derivatives (C₆—C₃)₂, the fragments of which are linked by a C—C bond between the central carbons of the side chains:

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The term "lignan" was introduced in 1936. These compounds were first isolated from wood (from the Latin lignum meaning wood, tree), which is how they got their name.

Classification

True lignans form several types. The classification is based on the presence of substituents in the aromatic ring, the degree of saturation of the branched carbon chain, and the oxidation state at C_α and C_γ:

1) diarylbutane type;

2) dihydronaphthalene type;

3) dioxabicyclooctane, or sesamin, type;

4) diaryloctane type;

5) tetrahydronaphthalene type;

6) diaryltetrahydrofuran type:

Nordihydroguaiaretic acid

Schisandrin

Podophyllotoxin

Lariciresinol

Neolignans consist of two C₆—C₃ fragments linked in a "HEAD-to-tail" manner; a double bond is frequently present at the C_β—C_γ position:

Lignoids refer to diverse groups of Phenolic Compounds containing additional C₆—C₃ lignan fragments, such as flavonolignans, xantholignans, and coumarinolignans.

Types of flavonolignans

Biosynthesis and Distribution

Little is known about The biosynthesis of lignans. Although related to lignins, lignans are not formed via free-radical coupling like lignins, owing to the presence of chiral centers in their molecules. Their biosynthesis is believed to involve the reductive Condensation of phenylpropane units (C₆—C₃) bearing unsaturated side chains.

Scheme of Lignan Biosynthesis

(* chiral center)

Lignans are widely distributed in such plant families as Pinaceae, Berberidaceae, Araliaceae, Schizandraceae, Crassulaceae, and Piperaceae, where they occur as constituents of resinous exudates in trees and shrubs.

Physicochemical Properties

Lignans are crystalline substances readily soluble in fatty oils, Essential Oils, and resins, as well as in chloroform, benzene, and diethyl ether. They exhibit typical phenol color reactions, such as reactions with iron salts, diazo Reagents, and alkalis.

Under UV light, lignans fluoresce with a blue or yellow color.

Lignans are isolated using ethyl ether, petroleum ether, benzene, and chloroform, followed by extract fractionation via Column Chromatography on silica gel and aluminum oxide.  

Biological Activity and Application

In recent years, these biologically active compounds have been extensively investigated. They exhibit valuable pharmacological properties, including antitumor (podophyllotoxin), stimulating and adaptogenic (schisandrin and syringaresinol derivatives), antihemorrhagic (sesamin), and antimicrobial (arctiin) activities. Furthermore, the flavonolignans from milk thistle demonstrate hepatoprotective effects.

Information regarding medicinal products derived from lignan-containing raw Materials is provided in Table 8 of the Appendices.