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

The (25R)- and (25S)-epimers of C27 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 C24allo-cholic acid performate with NaBH4/triethylamine/ethyl chloroformate, (2) iodination of the resulting 3,7,12-triformyloxy-5α-cholan-24-ol with I2/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 (25R)/(25S)-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 (25R)- and (25S)-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 C18 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.

► The (25R)- and (25S)-epimers of unconjugated 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.
► Each epimer of the three variants of unconjugated and conjugated C27 5α-bile acids was efficiently separated by RP-HPLC and their structural characteristics, were revealed by the NMR.
► These C27 5α-bile acids are likely key intermediates of bile acid biosynthesis from cholesterol.