2-Oxobutanoic Acid → 2-oxo-butyric acid (1R,2S,5R)-5-methyl-2-(1-methyl-1-phenyl-ethyl)-cyclohexyl ester

A solution of 2.28 g 2-oxobutyric acid (2, 22.11 mmol, 1.3 eq), 4.04 g (−)-8-phenylmenthol (18, 17.02 mmol, 1.0 eq) and 169.9 mg para-toluenesulfonic acid monohydrate (0.884 mmol, 0.52 eq) in 48 mL benzene was heated to reflux for 20 h 35 min. After cooling down to room temperature, the solution was washed twice with 50 mL saturated aqueous sodium hydrogencarbonate solution, and subsequently with 50 mL water and 50 mL brine. The organic phase was dried over 5 g sodium sulfate and was filtered. The filter cake was washed with 10 mL benzene. The organic phase was evaporated to dryness in a rotary evaporator (40° C., 20 mbar) yielding the title compound (4.98 g, 92% by weight) as colorless solid.