Organic Syntheses, Coll. Vol. 4, p.195 (1963); Vol. 35, p.43 (1955).
A. Cholesterol dibromide
. In a 4-l. beaker 150 g. (0.39 mole) of commercial cholesterol (Note 1)
is dissolved in 1 l. of absolute ether
by warming on the steam bath
and stirring with a stout glass rod
; the solution is then cooled to 25°. A second solution is prepared by adding 5 g. of powdered anhydrous sodium acetate (0.06 mole) (Note 2)
to 600 ml. of acetic acid
, stirring the mixture and breaking up the lumps with a flat stirring rod
; 68 g. (0.4 mole) of bromine
is then added and the solution is poured with stirring into the cholesterol
solution. The solution turns yellow and promptly sets to a stiff paste of dibromide. The mixture is cooled in an ice bath
to 20°, and then the product is collected on a 16-cm. Büchner funnel (Note 3)
. The cake is pressed down and washed with acetic acid
until the filtrate is completely colorless; 500 ml. is usually sufficient. A second crop of satisfactory dibromide is obtained by adding 800 ml. of water to the combined filtrate and washings, collecting the precipitate, and washing it with acetic acid
until colorless. Dibromide moist with acetic acid
is satisfactory for most transformations; dry dibromide, even when highly purified by repeated crystallization, begins to decompose (darkens) within a few weeks. When material prepared as described is dried to constant weight at room temperature, it is obtained as the 1:1 dibromide/acetic acid complex. Yields obtained in the first and second crops, respectively, are: 171–186 g.
and 13–25 g.
, total yield 197–199 g.
) (Note 1)
B. Cholesterol from the dibromide.
The acetic acid-moist filter cake of dibromide from 150 g. of cholesterol
is transferred (Note 3)
to a 3-l. round-bottomed flask
and covered with 1.2 l. of U.S.P. ether
, and the suspension is stirred mechanically above a bucket of ice and water that can be raised as required (Note 4)
, (Note 5)
. Forty grams (0.61 g. atom) of fresh zinc dust
is added in the course of 5 minutes. The first 5- to 10-g. portion is added without cooling; when the reaction has started, as evidenced by solution of part of the dibromide and by ebullition, the cooling bath
is raised during the remainder of the addition. At the end, the ice bath is lowered, and the mixture, which soon sets to a paste of white solid (Note 6)
, is stirred for 15 minutes longer. Then 50 ml. of water is added to dissolve the white solid, and the ethereal solution is decanted into a separatory funnel
and washed with 400 ml. of water containing 25 ml. of 36% hydrochloric acid
. After three more washings with 400-ml. portions of water, the solution is shaken thoroughly with 300 ml. of water and 150 ml. of 25% sodium hydroxide
solution, and the ethereal solution is tested with moist blue litmus paper to make sure that all the acetic acid
is removed (Note 7)
. The solution is then dried over magnesium sulfate
and evaporated to a volume of 600 ml., methanol (600 ml.)
is added, and the solution is evaporated to the point where crystallization just begins (about 1 l.). After standing at room temperature and then at 0–4°, the main crop of cholesterol
is collected and dried at room temperature; yield 108–110 g.
, m.p. 149.5–150° (Note 8)
. A second crop of 8.4–10.4 g.
, m.p. 148–149°
, is obtained after evaporation of the mother liquor to a volume of 250 ml. (Note 9)
; total yield 117–120 g.
from commercial cholesterol
) (Note 10)
. The moist dibromide from 150 g. of cholesterol
(part A) is suspended in 2 l. of acetic acid
in a 5-l. round-bottomed flask
equipped with a stirrer and mounted over a bucket of ice and water that later can be raised to immerse the flask (Note 5)
. The suspension is stirred at room temperature (25–30°), and a solution, preheated to 90°, of 80 g. (2 oxygen equivalents) of sodium dichromate dihydrate
in 2 l. of acetic acid
is poured in through a funnel (Note 3)
. The mixture reaches a temperature of 55–58° during the oxidation, and all the solid dissolves in 3–5 minutes. After another 2 minutes the ice bucket is raised until the flask is immersed; stirring is then stopped, and the mixture is allowed to stand in the ice bath without disturbance for 10 minutes to allow the dibromoketone to separate in easily filterable crystals. With stirring resumed, the temperature is brought to 25° and then, after addition of 400 ml. of water, to 15°. The product is collected on a 21-cm. Büchner funnel
, and the filter cake is drained until the flow of filtrate amounts to no more than 25 drops per minute. The suction is released, the walls of the funnel are washed down with methanol
, and 200 ml. of methanol
is added. After a few minutes' standing, suction is applied and the crystals are drained thoroughly of solvent before they are washed in the same way with 200 ml. of fresh methanol
. The last drops of filtrate should be completely colorless. Dried to constant weight at room temperature in a dark cupboard, the dibromoketone consists of shiny white crystals, m.p. 73–75°
(dec.), [α]D25 −47° chloroform
= 2.11) (Note 11)
; yield about 171 g.
in the oxidation or 81% from cholesterol
. (Note 12)
The moist 5α,6β-dibromocholestan-3-one from 150 g. of cholesterol
is transferred to a 3-l. round-bottomed flask
and covered with 2 l. of U.S.P. ether
and 25 ml. of acetic acid
. The suspension is stirred mechanically, and ice bath is raised into position (Note 5)
, and the temperature is brought to 15°. The ice bath is then lowered, and 5 g. of fresh zinc dust
is added. As soon as the exothermic reaction of debromination sets in, the temperature is controlled to 15–20° by cooling during the addition (in about 5 minutes) of 35 g. more zinc dust
. The ice bath is then lowered, and the ethereal solution containing suspended zinc
dust is stirred for 10 minutes longer. With continued stirring, 40 ml. of pyridine
is added; this precipitates a white zinc salt (Note 13)
. The mixture is filtered through a Büchner funnel, and the filter cake is washed well with ether
. The colorless filtrate is washed with three 600-ml. portions of water and then shaken thoroughly with 600 ml. of 5% aqueous sodium bicarbonate
solution until free from acetic acid
as indicated by testing the ethereal solution with moist blue litmus paper. The solution is dried over magnesium sulfate
and evaporated to a volume of about 1 l.; 500 ml. of methanol
is added, and the evaporation is continued until the volume is approximately 1.2 l. Crystallization is allowed to proceed at room temperature, then at 0–4°, and the large colorless prisms are collected by suction filtration; yield in the first crop 87–94 g.
, melting point in the range 124–129°
(camphorlike odor), [α]D25 −2.5° chloroform
= 2.03), no selective absorption at 242 mμ. Concentration of the mother liquor gives a second crop of 12–19 g.
melting in the range 117–125°
and suitable for conversion to Δ4-cholesten-3-one
; total yield 106–108 g.
. A mixture of 100 g. of Δ5-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°
, [α]D25 92° chloroform
= 2.01); λmax.ethanol
242 mμ (ε = 17,000). A second crop (5.0–7.5 g.
, melting in the range 78–82°
) is obtained after concentration of the mother liquor to a volume of about 100 ml., and a third crop (3–4 g.
, low melting) by dilution with water. Recrystallization of these crops from 95% ethanol
gives a total of 6.8–8.1 g.
of satisfactory material, m.p. 81–82°
; total yield 96–98 g.
over-all yield from cholesterol
of high quality and of recent production was employed. Cholesterol
undergoes slow autoxidation in the solid state, and samples that have been in storage for a few years give lower yields of dibromide. The checkers used U.S.P.
material, m.p. 149–150°
, as supplied by Wilson Company, Chicago, Illinois.
The yield of dibromide dropped from 84% to 73%
when no buffering sodium acetate
was used. No improvement resulted from doubling the quantity of sodium acetate
specified. Buffered bromination in a stirred suspension of acetic acid
) at 20° raised the yield to 89%
, but this material on debromination afforded sterol of low melting point (145–147°
) containing halogen (Beilstein test).
The operation should be done in a hood
, and the hands should be protected with Neoprene gloves.
If dry dibromide is used, 25 ml. of acetic acid
is added to the suspension.
The ice bucket is conveniently mounted on an automobile jack.
The solid appears to be a complex of cholesterol
and a zinc salt.
The melting-point determination should be done in an evacuated capillary tube
. In an open tube autoxidation occurs readily enough to lower the melting point when the bath is heated very slowly.
The residual mother liquor contains about 4 g. of material containing bromine
not removed by repetition of the treatment with zinc
Purification of cholesterol
through the dibromide completely eliminates cholestanol
, and lathosterol (Δ7-cholestenol)
. The first crop of material from methanol
is also free from cerebrosterol (24-hydroxycholesterol)
, a product of autoxidation present in cholesterol
that has been stored in the crystalline state for a few years with access to air. When material of highest purity is desired, only first-crop dibromide should be employed, since debromination of second-crop material gives sterol melting at 146–147°
and giving a positive Beilstein test.
sometimes begins to decompose (turns purplish) after standing in the dark for a few hours; it rapidly darkens when dried at 70° or when exposed to bright sunlight. Hence it is advisable to use the material moist with methanol
directly after preparation.
For success in the preparation of this labile non-conjugated ketone in high yield and purity, the intermediates, cholesterol dibromide
, should be processed further in the solvent-moist state as soon as prepared. The three reactions can be completed easily in one day.
If the bulk of the ionic zinc
is not precipitated at this point it will cause troublesome emulsions when the solution is washed with water.
When isomerization of 100-g. batches of non-conjugated ketone was effected in ethanol
under catalysis by either hydrochloric acid
or sodium hydroxide
(followed by neutralization of the yellow enolate solution with acetic acid
), a permanent yellow coloration developed, the first-crop material was yellowish and melted at 78–80°
, and the second-crop material was very impure.
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