Cholane-24-ol (1.033 g., 3 mmoles) (Note
1) is dissolved by gentle warming in
10 ml. of anhydrous benzene (Note
2) in a
50-ml. flask, and
10 ml. of rigorously dried dimethyl sulfoxide (Note
3) is added. To the clear solution are added
0.24 ml. (3.0 mmoles) of anhydrous pyridine (Note
4),
0.12 ml. (1.5 mmoles) of distilled trifluoroacetic acid, and
1.85 g. (9 mmoles) of dicyclohexylcarbodiimide (Note
5), in that order. The flask is tightly stoppered and left at room temperature for 18 hours (Note
6).
Benzene (30 ml.) is then added, and the crystalline
dicyclohexylurea is removed by filtration (Note
7) and washed with
benzene. The combined filtrates and washings are extracted three times with 50-ml. portions of water (Note
8) to remove the
dimethyl sulfoxide. The organic layer is dried with
sodium sulfate and evaporated to dryness under reduced pressure. There is obtained
2.12 g. of syrup which partially crystallizes. Thin-layer chromatography of this material (Note
9) shows a very intense spot of
cholane-24-al, traces of starting material, and two compounds near the solvent front as well as excess carbodiimide (Note
9).
The crude product is dissolved in
benzene-hexane (1:1) and applied to a
column containing 125 g. of silicic acid (Note
10). Elution with the same solvent gives traces (less than 5 mg. each) of the two fast-moving components in fractions 2 and 4 (125-ml. fractions) and chromatographically pure
cholane-24-al in fractions 5–8 (Note
11). Evaporation of the pooled fractions yields
870 mg. (
84%) of the pure crystalline aldehyde, m.p.
102–104°. Recrystallization from
5 ml. of acetone raises the melting point to
103–104° (Note
12).