Chemical synthesis of the (25 R)- and (25 S)-epimers of 3α,7α,12α-trihydroxy-5α-cholestan-27-oic acid as well as their corresponding glycine and taurine conjugates

Abstract

The (25 R)- and (25 S)-epimers of C 27 3α,7α,12α-trihydroxy-5α-cholestan-27-oic acid as well as their corresponding N-acylamidate conjugates with glycine or taurine were prepared starting from cholic acid in 14 steps. The principal reactions involved were (1) reduction of a key intermediary C 24 allo-cholic acid performate with NaBH 4/triethylamine/ethyl chloroformate, (2) iodination of the resulting 3,7,12-triformyloxy-5α-cholan-24-ol with I 2/triphenylphosphine; (3) nucleophilic substitution of the iodo derivative with diethylmethyl malonate/NaH; and (4) hydrolysis of the resulting 3,7,12-triformyloxy-25-methyl-26,27-diethyl ester with KOH, followed by decarboxylation of the geminal dicarboxylic acid with LiCl. N-Acylamidation of the resulting (25 R)/(25 S)-3α,7α,12α-trihydroxy-5α-cholestan-27-oic acid mixture with glycine or taurine afforded the corresponding epimeric mixtures of the glycine and taurine conjugates. The (25 R)- and (25 S)-epimers of the three variants of unconjugated and conjugated 3α,7α,12α-trihydroxy-5α-cholestan-27-oic acid were efficiently separated by HPLC on a reversed-phase C 18 column and their structural characteristics, particularly the chiral center at C-25, delineated using 1H and 13C NMR. These synthetic compounds should be useful as authentic reference standards for establishing their presence in bile as well as being useful in studies on the biosynthesis of allo-bile acids from cholesterol.