
TY  - JOUR
A1  - Collins, DJ 
T1  - Synthesis and Rearrangement Reactions of 4&#945;-Bromo-5&#945;-cholestane-3&#945;,5-diol 3-Acetate
JF  - Australian Journal of Chemistry
JO  - Aust. J. Chem.
Y1  - 1963
VL  - 16
IS  - 4
SP  - 658
EP  - 671
KW  - 
UR  - https://doi.org/10.1071/CH9630658 
N2  - The structure of the bromohydrin 4&#945;-bromo-5&#946;-cholestane-3&#946;,5-diol 3-acetate (V), obtained by the addition of hypobromous acid to cholest-4-en-3&#946;-ol3-aoetate, has been proved conclusively by an unequivocal synthesis from cholest-3-ene-5&#946;-ol.   An equilibrium mixture (approx. 1 : 2) of (V) and the isomeric bromohydrin 5-bromo-5&#945;-cholestane-3&#946;,4&#946;-diol3-acetate (IV; R = Ac) [prepared1 by the addition of HBr to 4&#946;,5-epoxy-5&#946;-cholestan-3&#946;-ol 3-acetate (I; R = Ac)] resulted when either pure isomer was refluxed with a trace of mineral acid in an inert solvent.   The 4&#946;-hydroxy-5&#945;-bromo derivative (IV; R = Ac) can be acetylated or oxidized without rearrangement. Interestingly, under mild conditions of acetylation or oxidation, the isomeric bromohydrin (V) (having a tertiary 5&#946;-hydroxyl) reacted with rearrangement to give 5-bromo-5&#945;-cholestane-3&#946;,4&#946;-diol 3,4-diacetate (II; R1 = R2 = Ac) or 5-bromo-5&#945;-cholestan-4&#946;-ol-3-one 4-acetate (VI), respectively.   The acetylation of (V) to (II; R1 = R2 = Ac) does not proceed via equilibration of (V) with (IV; R = Ac) and acetylation of the latter. Rather, it has been shown that through participation of the 5&#946;-hydroxyl, migration of the 3-acetate to the 4-position occurs with concomitant shift of the 4&#945;-bromine to the 5&#945;-position, displacement of the 5&#946;-hydroxyl, and acetylation at C3. Evidence for this mechanism follows from the fact that treatment of (V) with benzoyl chloride in pyridine afforded a benzoate different from that (II; R1 = Ac, R2 = Bz) obtainable from (IV; R = Ac), and identical with the 3-benzoate 4-acetate (II ; R1 = Bz, R2 = Ac) which was synthesized from 4&#946;,5-epoxy-5&#946;-cholestan-3&#946;-ol 3-benzoate (I; R = Bz) via 5-bromo-5a-cholestane-3&#946;,4&#946;-diol 3-benzoate (IV; R = Bz). The proposed mechanism is supported by other evidence.   It is suggested that the oxidative rearrangement of (V) to (VI) proceeds by a mechanism similar to that which operates in the acylation-rearrangement reaction, differing only in oxidation rather than acylation at C3. 
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