Anthraquinones are the main active ingredients in herbal remedies commonly used to relieve constipation. They are widely distributed in plants, especially in the families Leguminosae, Liliaceae, Polygonaceae and Rhamnaceae. Anthraquinones are usually present in living plants as glycosides, and several groups are distinguished according to the degree of oxidation of the nucleus and whether one or two units form the core of the molecule.
Anthraquinone components include anthraquinones and their products with different degrees of reduction. According to the parent nucleus, they can be divided into mono-anthracenes and di-anthracenes, and according to the degree of oxidation, they can be further divided into oxidized anthraquinones, anthraquinones, anthraquinones and anthraquinone dimers.
1. Anthraquinone and its glycosides
Natural anthraquinones are most commonly known as 9,10-anthraquinone, whose C-9 and C-10 are the highest oxidized state and are more stable.
(1) Rhubarbin type anthraquinone has its hydroxyl group distributed on both sides of the benzene ring, and most of the compounds are yellow in color. Many Chinese medicines such as rhubarb and thuja have laxative effect of the active ingredients belong to this kind of compounds.
(2) Alizarin type anthraquinones have their hydroxyl groups distributed on one side of the benzene ring and are orange-yellow to orange-red in color, with fewer varieties.
2. Oxidized anthraquinones
Anthraquinone can be reduced by zinc powder in alkaline solution to form anthraquinone oxide and its reciprocal isomer anthraquinone. Both anthraquinone oxide and anthraquinone are unstable, anthraquinone oxide is easily oxidized to anthrone or anthraquinone, and anthraquinone is easily oxidized to anthraquinone, so they are less present in plants.
3. Anthracenols or anthraquinones
When anthraquinone is reduced in acidic solution, anthracenol and its mutant isomer anthrone are produced. Anthraquinones are contained in fresh rhubarb, but anthraquinones are not detectable when stored for more than 2 years. If the hydroxyl group at the meo position of anthraquinol derivative is condensed with sugar to form a glycoside, it is more stable in nature, and only after hydrolysis to remove sugar, it is easily converted to anthraquinones by oxidation.
4. C-glycosyl anthracenes
These anthracene derivatives are directly linked to glycosides through carbon-carbon bonds with sugar as side chain.
1. Dithranolone derivatives
Dithranolones are compounds formed by combining two molecules of anthracenes with each other after removing one molecule of hydrogen. The upper and lower rings have the same structure and are symmetrical, and can be divided into the forms of intermediate linkage and alpha linkage. The dianthrone mostly exists in the form of glycoside, and if catalytic hydrogenation reduces the principle to produce dimeric anthrone, oxidation with FeCl3 will produce dimeric anthraquinone.
Anthraquinones can be formed by the dehydrocondensation of anthraquinones or the oxidation of dithranolones. The two anthraquinone rings in natural dianthraquinones are identical and symmetrical, and due to the mutual exclusion of spatial sites, the two anthraquinone rings are arranged in opposite directions, such as sambucus bisquinone.
The dianthrone is further oxidized by removing one molecule of hydrogen, and the two rings are connected by double bonds, which is called dehydrodianthrone.
4. Sunshine anthrone
Dehydrodanthenones are further oxidized and the α and α' positions are linked to form a six-membered ring to form sunshine anthrone compounds.
5. Intermediate benzodioxyanthrone
The structure of these compounds has the highest degree of oxidation among natural anthracene derivatives and is one of the highly dense polycyclic systems among natural products.