(R)-2-[2-(tert-Butoxy)-2-oxoethyl]-5-phenylpentanoic acid cyclohexylamine salt (691 g, 1.77 mol) and ethyl acetate (7.0 liters) were added to an aqueous solution of citric acid (10%, 6.3 liters) and the organic phase was separated, washed with water (7.0 liters) and concentrated in vacuo to a yellow oil. A solution of the oil and 5% rhodium on carbon (51.6 g) in methanol (7.0 liters) was stirred at 20-25° C., under hydrogen (10 atmospheres, 150 p.s.i.) for 48 hours and then filtered through celite. To the filtrate was added cyclohexylamine (202 ml, 1.77 mol) and the methanol solution was stripped and replaced with methylethyl ketone by distillation at atmospheric pressure, to a volume of 5.5 liters. The mixture was allowed to cool to 20-25° C. where it was stirred for 48 hours and then filtered. The residue was washed with methylethyl ketone (2×500 ml) and then dried in vacuo at 45° C. for 4 hours to leave the title compound as a colourless solid (495 g, 71% yield, 99% pure by HPLC).
