1: pyridine; cinchonine / tetrahydrofuran / 3.5 h / -25 - 0 °C 2: calcium chloride; sodium tetrahydroborate / ethanol; tetrahydrofuran / 6 h / -10 - 5 °C 3: calcium chloride; sodium tetrahydroborate / ethanol; tetrahydrofuran / 21.5 h / -10 - 7 °C 4: N-ethyl-N,N-diisopropylamine; methanesulfonyl chloride; sodium carbonate / tetrahydrofuran / 8 h / 25 °C View Scheme