We report an improved synthesis of the (22R)- and (22S)-epimers of 3α,7α,12α,22-tetrahydroxy-5β-cholan-24-oic acid and 3α,7α,22-trihydroxy-5β-cholan-24-oic acid from cholic acid (CA) and chenodeoxycholic acid (CDCA), respectively. The principal reactions involved were as follows: (1) oxidative decarboxylation of the bile acid peracetates with lead tetraacetate, and (2) subsequent Reformatsky reaction of the 23,24-dinor-22-aldehydes with ethyl bromoacetate in the presence of activated Zn as a catalyst with the reaction temperature maintained precisely at 75 °C. The absolute configuration of the chiral center at C-22 of each epimer was established by single-crystal X-ray diffraction data using its ethyl ester-peracetate derivative. The (1)H- and (13)C-NMR spectra that permit the (22R)- and (22S)-epimers to be distinguished are reported as well as the specific (1)H shift effects induced by C(5)D(5)N. Bile acids having hydroxyl groups at C-22 are present in a variety of animal biles, previously have been difficult to identify, and are known to have distinctive physicochemical and biological properties.

Two pairs of C-24 epimeric (24R)-/(24S)-24-hydroxy-24-methyl-5α-cholestan-3β-yl acetates and (24R)-/(24S)-25-hydroxy-24-methyl-5α-cholestan-3β-yl acetates as well as some related 24-ethyl oxysterol analogs were stereoselectively synthesized directly from the respective parent 24-alkyl sterols by a remote O-insertion reaction with 2,6-dichloropyridine N-oxide (DCP) in the presence of a catalytic amount of (5,10,15,20-tetramesitylporphrinate) ruthenium(II) carbonyl complex [Ru(TMP)CO] and HBr. ¹H- and ¹³C-NMR signals serving to differentiate each of the two epimeric pairs were interpreted. The C-24 alkyl oxysterols epimeric at C-24 were found to be effectively characterized by the aromatic solvent-induced shift (ASIS) by C₅D₅N, particularly for the difference in the ¹³C resonances in the substituted cholestane side chain. A method for differentiating the ¹H and ¹³C signal assignment of the terminal 26-/27-CH₃ in the iso-octane side chain was also discussed on the basis of a combined use of the preferred conformational analysis and HMQC and HMBC techniques. The present method may be useful for determining the stereochemical configuration at C-24 of this type of 24-alkyl oxysterols.