A carefully dried 5-l. round-bottomed flask equipped with a reflux condenser carrying a calcium chloride tube is charged with 100 g. of cholesterol (Note 1), 750 ml. of acetone (Note 2), and 1 l. of benzene (Note 3). A boiling tube is introduced to prevent bumping (Note 4), and the mixture is heated to boiling in an oil bath which is maintained at 75–85° during the reaction. A solution of 80 g. of aluminum tert-butoxide (p. 48) in 500 ml. of dry benzene is added in one portion to the boiling solution. The mixture turns cloudy and in 10–15 minutes develops a yellow color. Gentle boiling is continued at a bath temperature of 75–85° for a total of 8 hours. The mixture is then cooled, treated with 200 ml. of water and then 500 ml. of 10% sulfuric acid, shaken vigorously, and transferred to a 5-l. separatory funnel. The mixture is diluted with 1.5 l. of water and shaken for several minutes, after which the yellow aqueous layer is drawn off into a second separatory funnel and shaken out with a small amount of benzene (Note 5). The combined benzene extracts are washed thoroughly with water and dried by filtration through a layer of sodium sulfate; the solvent is evaporated, the last traces being removed by heating the residue at 60° at the water pump vacuum. The oily yellow residue solidifies when it is cooled in an ice-salt bath and scratched. For crystallization the material is dissolved in a mixture of 70 ml. of acetone and 100 ml. of methanol; the solution is allowed to cool very slowly and is seeded, for otherwise the product tends to separate as an oil. After the bulk of the material has crystallized, the mixture is allowed to stand for 1 day at 0°; the product is then collected, washed with 100 ml. of ice-cold methanol, and dried in vacuum at room temperature. The yield of almost colorless cholestenone, m.p. 77–79°, is 70–81 g. (70–81%). Recrystallization by the same method gives material melting at 78.5–80.5° with 90% recovery (Note 6) and (Note 7).