2a-Bromo-5a-cholestan-3-one

The presence of an a-bromo substituent may cause anomalies. With NaBH4, 2a-bromo-5a-cholestan-3-one gives a mixture of epimers, in which the 3p-o predominates. 4 -Bromo-17)5-hydroxy-5)5-androstan-3-one acetate gives 25% of the 315,4 -bromohydrin and 34% of the 3a,4)5-compound. Reduction of 7a-bromo-3)5,5a-diacetoxycholestan-6-one gives exclusively 7a-bromocholestane-3)5,5a,6a-triol 3,5-diacetate,whereas reduc-... 

To a solution of 2a-bromo-5a-cholestan-3-one (7.1 g, 15.2 mmol) in 175 ml dry acetone is added dropwise a solution of potassium ethyl xanthate (2.6 g, 16.2 mmol) in 90 ml acetone. The reaction mixture is stirred at 20° for 12 hr and then evaporated to dryness under vacuum. The resulting solid is treated with 100 ml hexane to dissolve the organic material and the inorganic salts are removed by filtration. The hexane filtrate is concentrated under vacuum and the resulting yellow solid ca. 7.5 g) is crystallized from chloroform-ethanol to give the xanthate (137) as white needles, ca. 5 g mp 114-115°. 

The methyl ester (100, R = CH3), derived from this A-nor acid by treatment with diazomethane, is different from the ester (102) obtained either by Favorskii rearrangement of 2a-bromo-5a-cholestan-3-one (101) or by the action of cyanogen azide on 3-methoxy-5a-cholest-2-ene (103) followed by hydrolysis on alumina. The ketene intermediate involved in photolysis of (99) is expected to be hydrated from the less hindered a-side of the molecule to give the 2j -carboxylic acid. The reactions which afford (102) would be expected to afford the 2a-epimer. These configurational assignments are confirmed by deuteriochloroform-benzene solvent shifts in the NMR spectra of esters (100) and (102). ... 

The tetrazole analogue (341) of 7-oxocholest-5-en-3/3-yl acetate has been prepared 82 by reaction of the afi-unsaturated ketone with hydrazoic acid and boron trifluoride etherate hydrolysis of the 3)3-acetate and oxidation of the resultant alcohol furnished 7a-aza-B-homocholest-4-eno[7a,7-d]tetrazol-3-one. Cycloaddition of ethylenediamine to 2a-bromo-5a-cholestan-3-one has furnished a simple one-step route to 5 -cholestane[2,3-e]dihydro-2,3-pyrazine (342).183 Analogous cycloaddition has been achieved using 16a-bromoandrostan-17-ones. 

The reaction of 2a-bromo-5a-cholestan-3-one with zinc and allyl bromide in benzene-DMSO failed to produce the expected 2a-allyl derivative, giving instead a complex mixture having the A-nor allyl ketone (170) as its major component. The isolation of a 3-allyl-3-hydroxy-derivative (171) in low yield suggests that reaction occurs first at C-3, with subsequent semipinacolic contraction of ring... 

Chloroketones are not susceptible to halogen migration this is emphasised by the specific rearrangement [3s] of 2"bromo-2"Chloro 5a"Cholestan-3 One (4) with HBr (but not with HCl) to give 2a"Chloro-4a-bromo-5a-cholestan"3"One (5),... 

Reactions involving Enols or Enolic Derivatives.—Polyhalogenation studies on 5/8-cholestan-2-one show that chlorination occurs in the order l/8,3a,3/8, followed in the case of chlorination by the highly hindered la-position. Attempted bromination beyond the 1/8,3,3-tribromide gave only l,l,3-tribromo-5/8-cholest-3-en-2-one. ° Acid-catalysed bromination of la-methyl and la-phenyl-5a-cholestan-3-ones occurred normally to give 2a-bromo-derivatives. The bromination of 5/3-cholestan-3-one at the 4/8-position is also unaffected by the presence of a la-methyl... 

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