Triterpenoid Saponins

The pentacyclic triterpenoid skeletons exemplified by lupeol, a-amyrin, and P-amyrin (Figure 5.60) are frequently encountered in the form of triterpenoid saponin structures. Saponins are

H^ squalene cyclization is initiated by protonation of double bond to give tertiary cation

H^ squalene cyclization is initiated by protonation of double bond to give tertiary cation further carbocation-mediated cyclizations: rings C and D are initially formed 5-membered via Markovnikov addition; they are then expanded to 6-membered rings via W—M rearrangements "OH2

hopan-22-ol further carbocation-mediated cyclizations: rings C and D are initially formed 5-membered via Markovnikov addition; they are then expanded to 6-membered rings via W—M rearrangements "OH2

hopan-22-ol

tetrahymanol

Figure 5.61

tetrahymanol

Figure 5.61

glycosides which, even at low concentrations, produce a frothing in aqueous solution, because they have surfactant and soaplike properties. The name comes from the Latin sapo, soap, and plant materials containing saponins were originally used for cleansing clothes, e.g. soapwort (Saponaria officinalis; Caryophyllaceae) and quillaia or soapbark (Quillaja saponaria; Rosaceae). These materials also cause haemolysis, lysing red blood cells by increasing the permeability of the plasma membrane, and thus they are highly toxic when injected into the blood stream. Some saponin-containing plant extracts have been used as arrow poisons. However, saponins are relatively harmless when taken orally, and some of our valuable food materials, e.g. beans, lentils, soybeans, spinach, and oats, contain significant amounts. Sarsaparilla (see page 242) is rich in steroidal saponins but is widely used in the manufacture of nonalcoholic drinks. Toxicity is minimized during ingestion by low absorption, and by hydrolysis. Acid-catalysed hydrolysis of saponins liberates sugar(s) and an aglycone (sapogenin), which can be either triterpenoid or steroidal (see page 237) in nature. Some plants may contain exceptionally high amounts of saponins, e.g. about 10% in quillaia bark.

Triterpenoid saponins are rare in monocotyledons, but abundant in many dicotyledonous families. Medicinally useful examples are mainly based on the P-amyrin subgroup (Figure 5.62), and many of these possess carboxylic acid groups derived by oxidation of methyl groups, those at positions 4 (C-23), 17 (C-28), and 20 (C-30) on the aglycone ring system being subject to such oxidation. In some structures, less oxidized formyl (-CHO) or hydroxymethyl (-CH2OH) groups may also be encountered. Positions 11 and 16 may also be oxygenated. Sugar residues are usually attached to the 3-hydroxyl, with one to six monosaccharide units, the most common being glucose, galactose, rhamnose, and arabinose, with uronic acid units (glucuronic acid and galacturonic acid) also featuring (see page 467). Thus, quillaia* bark contains a saponin mixture with quillaic acid (Figure 5.62) as the principal aglycone, and the medicinally valuable root of liquorice* (Gly-cyrrhiza glabra; Leguminosae/Fabaceae) contains glycyrrhizin, a mixture of potassium and calcium salts of glycyrrhizic acid (Figure 5.63), which is composed of the aglycone glycyrrhetic acid and two glucuronic acid units. Seeds of horsechestnut (Aesculus hippocastrum; Hippocas-tanaceae), sometimes used in herbal preparations as an anti-inflammatory and anti-bruising remedy, contain a complex mixture of saponins termed aescin, based on the polyhydroxylated aglycones protoaescigenin and barringtogenol

Pentacyclic Triterpenoid Ring

24 23

pentacyclic triterpenoid skeleton

24 23

pentacyclic triterpenoid skeleton

Aglycon Bond
O
Biosynthesis Glycyrrhizin

H pCO2H "OH

quillaic acid potential sites for oxidation (P-amyrin type)

H pCO2H "OH

quillaic acid

D-glucuronic acid

CO2H HO O\

D-glucuronic acid

Figure 5.62

CO2H

potential sites for oxidation (P-amyrin type)

OH OH

"oh

R = OH, protoaescigenin R = H, barringtogenol

OH OH

"oh

R = OH, protoaescigenin R = H, barringtogenol

Protoaescigenin

carbenoxolone sodium glycyrrhetic acid carbenoxolone sodium glycyrrhizic acid

GlcO

liquiritin

Figure 5.63

isoliquiritin

liquiritin

isoliquiritin

Figure 5.63

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