Cholestane, 3,4-dibromo

An illustration of the difference in reactivity of a-and / -halides is provided by the ready elimination of 1,4a-dibromo-5a-cholestan-3-one to 4a-bromo-5a-cholest-l-en-3-one in collidine at room temperature. Calcium carbonate in refluxing DMA is necessary to complete the dehydrobromination to the l,4-dien-3-one. ... 

The addition proceeds most smoothly with highly functionalized (more polar) steroids as seen in examples by Bernstein and others. The polar reaction conditions pose solubility problems for lipophilic androstane, cholestane and pregnane derivatives. Improved yields can be obtained in some cases by using dimethyl sulfoxide or t-butanol " as solvents and by using sodium A-bromobenzenesulfonamide or l,3-dibromo-5,5-dimethyl hydantoin (available from Arapahoe Chemicals) as a source of positive bromine. The addition of bromo acetate and bromo formate to steroid olefins has been studied to a limited extent. ... 

The submitters prepared the dibromide derivative, 3a,4fi-dibromo-5j3-cholestane, m.p. 98-99°. The melting point of the dibromide is reported as 98-100°. The mass spectrum of the dibromide exhibits three molecular ions at m/e (relative intensity, assignment) 532 (25%, C27H46 Br Br), 530 (50%, C27H46" Br Br), 528 (25%, C27H46 Br Br). 

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. 

Reduction of 5,6/3-dibromo-5a-cholestan-3-one (128) by sodium borohydride in ethanol is subject to steric effects, giving cholest-5-en-3a-ol (130) as the main product. The 3a-hydroxy-5a,6/3-dibromide (129) is thought to be formed first and to suffer reductive elimination of the bromo-substituents with participation by the 3a-hydroxy-group.135 This convenient route to the 5-en-3a-oI is competitive in yields with some older routes. 

The transient appearance of molecular bromine during the reaction was confirmed when it was intercepted by added 2-naphthol [2y]. A. solution of 2,2-dibromo-3a-cholestan-3-one... 

Cholestane, 5a,6j8-dibromo-, mutarota-tion of, 13 Chromatography in enzyme purification, 283 molecular-weight determination of enzymes, 287... 

The intramolecular rearrangement of 5,6)3-dibromo-5a-cholestan-3y5-yl benzoate to give the more stable 5, 6a-dibromo-isomer is catalysed by added mercuric bromide. The mechanism is unknown. 

The synthesis of novel 4a-substituted sterols was undertaken in the laboratory of C.H. Robinson. These compounds are potential inhibitors of sterol 4-demethylation. To prepare the desired 4-allenyl-5a-cholestan-3p-ol, the exocyclic olefin precursor was first reacted with bromoform/potassium f-butoxide to afford the geminal dibromo-substituted cyclopropane derivative. Next, methyllithium was used to bring about the rearrangement to afford the allene, and finally acidic conditions were applied for the removal of the THP protecting group. 

Ozone on silica gel introduces a 25-hydroxy-substituent into suitable compounds with a cholestane side-chain. By using the la,3/8-diacetoxy-6/8,7a-dibromo-derivative (383), prepared from the known 6-ene, the 25-hydroxylated compound was obtained as the only product (11% conversion). Trifluoroacetylation followed by dehydrobromination afforded the 5,7-diene (385), which was transformed into la,25-dihydroxy-vitamin D3 by the usual method. 

Syntheses of 5j8-cholest-l-ene and 5/3-cholest-2-ene have been reported from 2/3-bromo-5/S-cholestan-3-one, which was prepared from the 2/3,4/3-dibromo-ketone by reaction with chromous acetate. A series of 5/3-substituted-5a-hydroxy-5a-cholest-2-enes was prepared from 5a,6a-epoxy-cholestanes. Acid-catalysed dehydrobromination of the 7,11,22,23-tetra-bromide (254) gave the aromatic dibromide (255). ... 

S 25S)-16) -Acetoxy-22,26-acetimino-5a-cholestan-3-one treated at 10-15° with bromine in glacial acetic acid containing HBr (22S 22S) -2a,4a-dibromo-16) -acetoxy-22,26-acetimino-5 -cholestan-3-one (Y 81%) added to acetone containing Nal and iodacetone (prepared by pretreating the solvent with some... 

Dibromo- and 1,1,-diiodoethane, 1, l-dibromo-2,2-dimethylpropane and benzylidene bromide under several reaction conditions did not show any evidence of the formation of a geminal dimetallic species, although in all cases the starting material disappeared. Trying to capture this species immediately after its formation by an in situ reaction (as this procedure was systematically named) with cholestan-5a-3-one, resulted only in traces of the ethylenic compound being isolated. The bulk of the reaction mixture consisted of three products ... 

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