Inhibitors of sterol synthesis. Chemical synthesis, structure, and biological activities of (25R)-3β,26-dihydroxy-5α-cholest-8(14)-en-15-one, a metabolite of 3β-hydroxy-5α-cholest-8(14)-en-15-one

3β-Hydroxy-5α-cholest-8(14)-en-15-one (I) is a potent inhibitor of sterol synthesis with significant hypocholesterolemic activity. (25R)-3β,26-Dihydroxy-5α-cholest-8(14)-en-15-one (II) has been shown to be a major metabolite of I after incubation with rat liver mitochondria. Described herein is the chemical synthesis of II from diosgenin. As part of this synthesis, improved conditions are described for the conversion of diosgenin to (25R)-26-hydroxycholesterol. Benzoylation of the latter compound gave (25R)-cholest-5-ene-3β,26-diol 3β,26-dibenzoate which, upon allylic bromination followed by dehydrobromination, gave (25R)-cholesta-5,7-diene-3β,26-diol 3β,26-dibenzoate. Hydrogenation-isomerization of the Δ^5,7-3β,26-dibenzoate to (25R)-5α-cholest-8(14)-ene-3β,26-diol 3β,26-bis(cyclohexanecarboxylate) followed by controlled oxidation with CrO₃-dimethylpyrazole gave (25R)-3β,26-dihydroxy-5α-cholest-8(14)-en-15-one 3β,26-bis(cyclohexanecarboxylate). Acid hydrolysis of the Δ(8(14))-15-ketosteryl diester gave II. ¹³C NMR assignments are given for all synthetic intermediates and several major reaction byproducts. The structure of II was unequivocally established by X-ray crystal analysis. II was found to be highly active in the suppression of the levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase in cultured mammalian cells and to inhibit oleoyl coenzyme A-dependent esterification of cholesterol in jejunal microsomes.