Lappaconitine-Catalyzed Asymmetric α-Hydroxylation of β-Keto Esters: A Brønsted Base Organocatalyst Developed from Terpenoid Alkaloids

Abstract

Discovering organocatalysts with novel framework from terpenoid alkaloids is presented in this paper. Lappaconitine was found to enantioselectively catalyze α-hydroxylation of β-keto ­esters using tert-butyl hydroperoxide as the oxidant in chloroform to afford the corresponding products in high yields and good enantioselectivity (up to 85% ee).

Key words

lappaconitine - asymmetric α-hydroxylation - β-keto ­esters - organocatalyst - terpenoid alkaloid

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12

General Procedure for Lappaconitine-Catalyzed Asymmetric α-Hydroxylation of β-Keto Esters
A mixture of β-keto ester (1 mmol), tert-butyl hydroperoxide (5 mmol), lappaconitine (0.1 mmol) in CHCl₃ (6 mL) was stirred for 72 h at 15 ˚C. The reaction was monitored by TLC. After the starting material vanished, the mixture was washed with 10 wt% Na₂SO₃ and extracted with CH₂Cl₂. The organic layer was then dried over anhyd Na₂SO₄, filtered, and evaporated. The residue was purified by flash chromatography (n-hexane-EtOAc = 3:1) to give the product. The ee value was determined by chiral HPLC on CHIRALCEL OD-H or AD-H column (hexane-i-PrOH = 90:10, flow rate 1.0 mL/min, 254 nm, unless noted). See Supporting Information for data of all compounds.