Preparation of BAL-phosphoramidite (2) and SC-silane (3). Reagents and conditions: (i) 2,2-diethyl-1,3-propanediol, Et2O·BF3, CH2Cl2, room temperature, 15 min; (ii) 2-cyanoethyldiisopropylchlorophosphoramidite, DIEA, CH2Cl2, room temperature, 1 h; (iii) hydrazine, CH3CN, room temperature, 1 h.