Structure and Stereochemistry of the Higher Bile Acid Isolated from Turtle Bile : (22S, 25R)-3α, 12α, 15α, 22-Tetrahydroxy-5β-cholestan-26-oic Acid

The bile of the turtle or the tortoise is known to contain a number of chemically and biologically interesting bile acids and bile alcohols. More than 50 years ago, two bile acid lactones called tetrahydroxysterocholanic lactone (TSL) from the bile of Amyda (A.) japonica (turtle) and tetrahydroxyisosterocholanic lactone (TISL) from the bile of Emys orbicularis (tortoise) were isolated by Suganami and Yamasaki, 1) and by Kim, 2) respectively, and were assigned the same chemical formula, C 28 H 46 O 5 . Later, in 1965, Amimoto et al. 3) showed in the study of the bile of the turtle (A. japonica) that the two lactones mostly exist in unconjugated acid forms, namely tetrahydroxysterocholanic acid (TSA) and tetrahydroxyisosterocholanic acid (TISA), respectively, in the bile and that the largest portion of them could be lactonized to TSL and TISL, respectively, by treatment with hydrochloric acid (HCl) during the course of the extraction. Amimoto et al. 3) also suggested that the chemical constitution of TSL is 3a,7a,12a,22z-tetrahydroxycoprostanic lactone, and that TISL is not a stereoisomer of TSL, but a position isomer in which the position of one hydroxyl group is different. Twenty years thereafter the full structure and configuration of TSL was determined as (22S,25R)-3a,7a,12a-trihydroxy-5bcholestano-26,22-lactone on the basis of 1 H-NMR data by Fujimoto et al. 4) As for TISL or TISA, however, their structures have not yet been fully elucidated in spite of numerous studies. The present paper describes the isolation of TISA, the preparation of its ethyl ester and the establishment of the complete chemical structure of the ethyl ester of TISA by means of X-ray crystallographic analysis.