A 100-ml., Claisen distillation flask with two 14/20 standard taper joints and a thermometer-inlet is equipped with a gas-inlet adapter, a receiver, a thermometer, and a magnetic stirring bar. A 40-ml. calibration mark is made on the flask, and 970 mg. (2.50 mmoles) of cholest-4-en-3β-ol (Note 1) is introduced. Triethyl orthoacetate is then distilled under argon into the flask to the 40-ml. mark (Note 2). The mixture is stirred, effecting solution while the flask is purged with argon, then the top joint is sealed with a thermometer (Figure 1). The stirred solution is heated under a positive pressure of argon so that the vapor reflux level is just below the side arm of the flask; the temperature on the lower thermometer is 142–147°; the upper thermometer temperature is kept between 25 and 70° (Note 3). After 8 days of reflux, during which time a small amount of the volatile material distills into the receiver, the reaction flask is cooled, and all the volatile materials are removed at reduced pressure (Note 4). The residue (1.3 g. of a pale yellow oil) is chromatographed on 120 g. of silica gel with 10% diethyl ether in petroleum ether as the eluant (Note 5). The side products eluted with the first 240 ml. of the solvent are discarded; further elution with 120 ml. of the solvent affords 690 mg. of ethyl 5β-cholest-3-ene-5-acetate as a clear, colorless oil. Trituration of this product with acetone produces 560–690 mg. (49–60%) of the ester as white plates, m.p. 89–92.5°.