7- Ketolithocholic acid

F7. Fromm, H., Sarva, R. P., and Bazzoli, F., Formation of ursodeoxycholic acid from chenodeoxycholic acid in the human colon studies of the role of 7-ketolithocholic acid as an intermediate. /. Lipid Res. 24, 841-853 (1983). 

Ursodeoxycholic acid was detected very early by Hammarsten in polar bear bile (85). It was isolated in crystalline form by Shoda (58) and characterized later by Iwasaki (84). The acid, which is the 7/3-epimer of chenodeoxycholic acid, may be prepared in good yield from 7-ketolithocholic acid by reduction with sodium in propanol according to Kanazawa et al. (86). Ursodeoxycholic acid was originally considered to be a unique constituent of bear bile but has since been detected as a minor constituent in the bile of several mammals including man (2). It is also present in human feces (52). 

Ketolithocholic acid was isolated from guinea pig bile by Imai (146) and from the bile of the coypu by Brigl and Benedict (147). The acid from the coypu was characterized by Kazuno and Takuma (148) and by Haslewood in 1954 (149). 7-Ketolithocholic acid has been found in chicken bile and in the bile of a number of mammalian species (2). It also occurs in human feces (50, 52). The acid may be prepared by partial oxidation of chenodeoxycholic acid (150). 

A somewhat better yield of 7-ketoIithocholic acid can be obtained by an alternative procedure, in which the 3a-hydroxyl group of chenodeoxy-cholic acid is protected from chromic oxidation by the formation of succinic ester. The procedure of Heusser and Wuthier (53) for the preparation of 3a-succinic ester of methyl cholate requires the use of succinic anhydride in pyridine. The use of pyridine is obnoxious in many respects, and it is omitted in the following modified procedure for the preparation of 7-ketolithocholic acid. 

Chenodeoxycholic acid (15 g) is refluxed with succinic anhydride (4 g) in benzene (270 ml) overnight, and the solvent is then evaporated. Without isolation of the succinate (XXX), the residue (containing succinic anhydride) is dissolved in acetic acid (200 ml) and oxidized at room temperature with a solution of CrOj (3.5 g) in 80% acetic acid (5 ml). After 3 hr, the mixture is diluted with water, and the green precipitate formed is collected by filtration, and then boiled in 3% KOH for 15 min to hydrolyze the succinic ester. The green flocculent precipitate of CrgOj HgO is removed by filtration and the filtrate containing the potassium salt of 7-ketolithocholic acid is acidified with acetic acid. 7-Ketolithocholic acid (XXIX) precipitates as crystals, and is recrystallized from acetic acid and water, or from acetone and water yield, 65-70%. 

Ketolithocholic acid (10 g) is methylated by dissolving in methanol (200 ml) containing cone. HCl (12 ml). After being kept at room temperature overnight, the mixture is diluted with water and extracted with ether. The semicrystalline residue from the ether extract is subjected to acetylation with acetic anhydride (10 ml) in pyridine (10 ml) at room temperature overnight. The methyl ester acetate (XXXI) crystallizes upon addition of ice water to the reaction mixture, and is recrystallized from acetone and water m.p. 140-142 °C yield, almost quantitative. 

The secondary bile acids result from the activity of anaerobic intestinal microorganisms in the ileum, caecum and colon, (s. fig. 3.3) Deconjugation, with the subsequent release of free bile acids, is a prerequisite for these reactions. This is followed by 7a-dehydroxylation of cholic acid and chenodeoxycholic acid to yield deoxy-cholic acid and lithocholic acid, respectively. 7a-de-hydrogenation and oxidation of chenodeoxycholic acid also yield ketolithocholic acid ... 

The tertiary bile acids are formed in the liver as well as in the gut. (s. fig. 3.3) Intestinally absorbed lithocholic acid is enzymatically converted to sulpholitho-cholic acid in the liver. Ketolithocholic acid is transformed to (hypercholeretic) ursodeoxycholic acid in both the intestine and the liver. When passing through the canaliculi, UDC is partly reabsorbed by epithelial cells and returned to the liver via the blood circulation (= cholehepatic shunt). (41) The latter is chemically and structurally identical to chenodeoxycholic acid, of which it is deemed to be the 7P-epimer ... 

Synthesis of Cortisone from 11-Ketolithocholic Acid Figs, 8 and 4)... 

This acid was isolated as a trace component of the mixed acids obtained from the bile of pig bladders (95). It has not been detected elsewhere as a naturally occurring compound. The acid may be prepared by NaBH4 reduction of 6-ketolithocholic acid (96, 97). 

This keto acid was first identified in beef bile by Wieland and Kishi (36). It had been obtained earlier as a molecular complex with chenodeoxycholic acid (152). 12-Ketolithocholic acid has been identified as a fecal acid in several species (4). The acid may be prepared by direct oxidation of deoxy-cholic acid (153) but better overall yields are obtained by protection of the hydroxyl group at carbon 3 (154). 

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