General procedure for the synthesis of 2,3-O-isopropylidene-D-sitosterol from dimethyl (4R,5R)-2,2-dimethyl-1,3-dioxolane-4,5-dicarboxylate: an anhydrous tetrahydrofuran (120 ml) solution of lithium aluminium hydride (10 g, 267 mmol) was stirred and refluxed under nitrogen protection for 30 min. Subsequently, a solution of anhydrous tetrahydrofuran (200 ml) of 2,3-O-isopropylidene-L-tartaric acid dimethyl ester (53 g, 243 mmol) was slowly added dropwise with stirring, and heat generated during the reaction induced a mild reflux. After continuing the reflux reaction for 3 h, the reaction solution was cooled to 0 °C. Water (8.4 ml), 4M sodium hydroxide solution (8.4 ml) and water (32 ml) were added sequentially to the reaction solution. The resulting precipitate was removed by filtration and the filtrate was extracted with ether (using Soxhlet extractor). The organic phases were combined, dried and concentrated under reduced pressure to give 2,3-O-isopropylidene-L-sitosterol (35 g, 216 mmol, 89% yield). The product was identified by 1H NMR (CDCl3): δ 3.98 (m, 2H), 3.77 (m, 4H), 3.64 (s, 2H), 1.45 (s, 6H); 13C NMR (CDCl3) showed the characteristic peaks: δ 109.53, 78.24, 62.25, 27.28; low resolution electron bombardment mass spectrometry (LREIMS) revealed that molecular ion peak m/z 162.1 (M+). The crude product obtained can be directly used in the subsequent reaction.
