Inhibitors of sterol synthesis. Chemical synthesis of 7α-ethyl and 16α-ethyl derivatives of Δ 8(14)-15-oxygenated sterols and their effects on 3-hydroxy-3-methylglutaryl coenzyme A reductase in CHO-K1 cells

Abstract

The enolate of 3β-hydroxy-5α-cholest-8(14)-en-15-one ( II), formed upon treatment of II with potassium tert-butoxide in tert-butanol, was alkylated with ethyl iodide. In addition to the major products, 3β-hydroxy-14α-ethyl-5α-cholest-7-en-15-one and its 3β-ethyl ether, small amounts of 3β-hydroxy-7α-ethyl-5α-cholest-8(14)-en-15-one ( V), 3β-hydroxy-16α-ethyl-5α-cholest-8(14)-en-15-one ( VI) and the 3β-ethyl ether of VI were isolated. When the enolate of II was formed by treatment with lithium diisopropylamide in tetrahydrofuran, the same alkylation furnished VI as the major product. Reduction of VI with lithium aluminum hydride gave 16α-ethyl-5α-cholest-8(14)-ene-3β,15α-diol ( IX) and its 15β epimer X, which were separated by column chromatography. Full 1H and 13C nuclear magnetic resonance (NMR) assignments, augmented by nuclear Overhauser effect difference spectra for VI, established the stereochemistry of these diols at C-15 and C-16. The NMR results indicate that the 16α-ethyl group affects the side-chain conformation. The effects of II, V, VI, IX and X on the levels of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase activity were studied in CHO-K1 cells. With the exception of IX, each of the compounds reduced the levels of HMG-CoA reductase activity. The order of potency with respect to suppression of the elevated levels of HMG-CoA reductase activity induced by transfer of the cells to lipid-deficient medium, was II > V > VI > X.