A5-Cholesten-3-one

Muzart and coworkers investigated the CrOs/TBHP catalyzed oxidation of various benzylic methylenes447 and the allylic oxidation of A4- and A5-cholesten-3-one to the corresponding ketones in CH2CI2424 as well as benzotrifluoride (BTF) as solvent (Scheme 130)407. It could be shown that BTF in most cases improved the results even though the reactions were carried out with less TBHP than in CH2CI2 (4 eq. compared to 7 eq.). Yields in CH2CI2 varied from 21 to 94% and those in BTF from 40 to 99%. 

This latter point was experimentally proved by converting A5-cholesten-3-one into the conjugated A4 isomer in the presence of CU/AI2O3. The subsequent hydrogen addition to the A4-3-one follows the stereochemistry expected in the presence of heterogeneous catalysts as CU/AI2O3 which produces an excess of the 5p isomer also with gaseous H2. 

D. A5-Cholesten-3-one. (Note 12.) The moist 5a,6/3-dibromo-cholestan-3-one from 150 g. of cholesterol is transferred to a 3-1. 

E. Ai-Cholesten-3-one. A mixture of 100 g. of A5-cholesten-3-one (0.26 mole), 10 g. (0.11 mole) of anhydrous oxalic acid (Note 14), and 800 ml. of 95% ethanol is heated on the steam bath until all the solid is dissolved (15 minutes) and for 10 minutes longer, and then is allowed to stand at room temperature. If crystallization has not started after a period of several hours, the solution is seeded or scratched. After crystallization has proceeded at room temperature and then at 0-4°, the large, colorless, prismatic needles that separate are collected by suction filtration yield in the first crop 88-92 g., m.p. 81-82°, [a]ff 92° chloroform... 

Inspection of Table 6.1 shows that the classical oxidation of sterols on the alcohol at the 3-position, using acetone as oxidant, works efficiently thanks to the migration of the alkene. Thus, the oxidation of cholesterol with acetone (E0 = 129 mV) must proceed via the thermodynamically disfavoured A5-cholesten-3-one (E0 = 153 mV) that evolves to the very stable A4-cholesten-3-one (E0 = 63 mV). In fact, acetone lacks oxidizing power for the obtention of many ketones as well as for the preparation of virtually all aldehydes. 

The Oppenauer oxidation of cholesterol to A4-cholesten-3-one of m.p. 77-79° in 70-93% yield has been reported in these volumes.10 Isomerization of A5-cholesten-3-one by a mineral acid or a base has been conducted satisfactorily only on a micro scale 8 the method of isomerization with oxalic acid has been reported.8... 

The annulation of steroid skeleton with N-vinylpyrrole fragment is accomplished via the reaction of A5-cholesten-3-one oxime with acetylene under pressure in the system KOH/DMSO (120°C, 30 min. Scheme 1.41) [213]. The pyrrolization... 

The progesterone molecule combines almost exactly the structural fragments of pregnenolone and A5-cholesten-3-one (except for the position of the C=C bond). However, the results of interaction between progesterone dioxime and acetylene in... 

The second step in the synthesis of bile acids, according to Hylemon et al. (1991), is the conversion of 7a-hydroxycholesterol to 7a-hydroxy-4-cholesten-3-one by NAD+-dependent 3/3-hydroxy-A5-C27-steroid oxidoreductase. This enzyme is located in the endoplasmic reticulum of liver, and its catalysis of the 3/3-hydroxy group also results in isomerization of the double bond from A5 to A4. 

Details of the reaction of 3/ -acetoxy-A5-cholestene-7-one (2, 427) have been published.2... 

In the latter, the stereospecific oxidation of the 3-hydroxyl group is rate-limiting because the subsequent isomerization at the A5 position proceeds very rapidly. The reaction product, 4-cholesten-3-one, inhibits competitively (Ki = 0.13 mmol/1). 

Chlorotrifluoromethylcarbene, 223 N-Chlorourethane, 60, 61 AM-Cholestadiene-3,6-dione, 60 5a-Cliolestane-3,6-dione, 60 5oCholestane-2a , 3a-oxide, 135 5CfrCholestane-2j3,3(3-oxide, 135 5(3-Cholestanone-3,6-dione, 60 A4-Cholestene-3,6-dione, 60, 65 Al-5/3-Cholestene-3,6-dione, 60 A4-Cholestene-3-one, 98 A4-Cholestene-4-one, 60 -Cholestene-a-epoxide, 116 A5-Cholestenone, 65 Cholesteryl acetate, 35, 171 Cholesteryl acetate dibromide, 40 Chromic acid, 54, 246 Chromic anhydride, 54-57, 150 Chromic anhydride-Acetic acid, 56 Chromic anhydride-Dimethylformamide, 56 Chromic anhydride-Pyridine complex,... 

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