Synthesis of single- and double-13C-labeled cholesterol oleate.

Cholesterol oleate with the 13C-label in oleic acid at the carbonyl and/or in the sterol ring at position 4 was synthesized by two methods: (1) cholesterol was condensed with oleic anhydride, prepared from [1-13C] oleic acid, in the presence of dimethylaminopyridine (DMAP) in anhydrous chloroform at room temperature for 4--5 h; (2) cholesterol or 13C-enriched cholesterol at position 4 were reacted with 90% [1-13C]-oleic acid in the presence of dicyclohexylcarbodiimide (DCC) and DMAP at room temperature in anhydrous chloroform for 1.25 h. The single-13C and double-13C-labeled cholesterol oleate were obtained in 90% yields after purification by silicic acid column chromatography. Their purity was assessed by thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC) and 13C-NMR spectroscopy. Tritium-labeled cholesterol oleate was also synthesized by method 1 using the fatty acid anhydride.