All-trans-retinol (1 g) was vacuum dried and dissolved in anhydrous CH2Cl2 (10 ml) in an argon atmosphere. Diisopropylethylamine (2.65 ml, 21.5 mmol) was syringed in and the reaction mixture was cooled in an ice-bath. 2-cyanoethyl-N,N-diisopropylchlorophosphoramidate (2.5 g, 2.45 ml, 10.5 mmol) was slowly added by syringe under argon atmosphere. The reaction mixture was stirred for 30 min. at which time TLC (CH2Cl2:CH3OH:Et3N, 90:10:0.1) indicated complete conversion of the alcohol to its phosphoramidite. The reaction mixture was added to 100 ml of saturated NaHCO3 followed by washing the reaction flask with (2×25 ml) CH2Cl2. The CH2Cl2 layer was separated and washed with 100 ml of saturated NaCl solution and dried over anhydrous MgSO4 and evaporated into a yellow foam. The amidite was used directly in the DNA synthesizer without further purification since decomposition of this amidite was noted upon silica column purification.
