Anomalous enantioselectivity in the sharpless asymmetric dihydroxylation reaction of 24-nor-5beta-cholest-23-ene-3alpha,7alpha,12alpha-triol: synthesis of substrates for studies of cholesterol side-chain oxidation.
Recently we described a block in bile acid synthesis in cerebrotendinous xanthomatosis (CTX), a lipid storage disease related to an inborn error of bile acid metabolism. In this disease a defect in hepatic microsomal (24S) hydroxylation blocks the transformation of 5beta-cholestane-3alpha,7alpha,12alpha,25-tetrol into (24S) 5beta-cholestane-3alpha,7alpha,12alpha,24,25- pentol and cholic acid. Mitochondrial cholesterol 27-hydroxylation has also been reported to be abnormal in CTX subjects, but the relative importance of the enzymatic defect in this alternative microsomal pathway (namely, the 24S hydroxylation of 5beta-cholestane-3alpha,7alpha, 12alpha,25-tetrol relative to the abnormality in mitochondrial 27-hydroxylase) has not been established in CTX. To delineate the sequence of side-chain hydroxylations and the enzymatic block in bile acid synthesis, we synthesized the (23R and 23S) 24-nor-5beta-cholestane-3alpha,7alpha, 12alpha,23,25-pentols utilizing a modified Sharpless asymmetric dihydroxylation reaction on 24-nor-5beta-cholest-23-ene-3alpha, 7alpha, 12alpha-triol, a C26 analog of the naturally occurring C27 bile alcohol, 5beta-cholest-24-ene-3alpha,7alpha,12alpha-triol . Stereospecific conversion of the unsaturated 24-nor triol to the corresponding chiral compounds (23R and 23S), 24-nor-5beta-cholestane-3alpha,7alpha,12alpha,23 ,25-pentols, was quantitative. However, conversion of the unsaturated 24-nor triol to the chiral nor-pentols had absolute stereochemistry opposite to the products predicted by the Sharpless steric model. The absolute configurations and enantiomeric excess of the C26 nor-pentols and the C27 pentols (synthesized from 5beta-cholest-24-ene-3alpha,7alpha,12alpha-triol for comparison) were confirmed by nuclear magnetic resonance and lanthanide-induced circular dichroism Cotton effect measurements. These results may contribute to a better understanding of the role of the 24S-hydroxylation vs. 27-hydroxylation step in cholic acid biosynthesis.
Ertel
N H
NH
Robert Wood Johnson Medical School, New Brunswick, New Jersey, USA.
Dayal
B
B
Rao
K
K
Salen
G
G
D002784:Cholesterol / Q000737:chemistry*; D002942:Circular Dichroism; D006888:Hydroxycholesterols / Q000737:chemistry*; D006900:Hydroxylation; D009682:Magnetic Resonance Spectroscopy; D010084:Oxidation-Reduction; D016339:Spectrometry, Mass, Fast Atom Bombardment; D013237:Stereoisomerism
C121470:24-norcholest-23-ene-3,7,12-triol; D006888:Hydroxycholesterols; D002784:Cholesterol