2,6-dimethylpyridine-3-carboxaldehyde (Aurora Feinchemie GmbH) (2.00 g, 16.1 mmol) and (D)-alloisoleucine methyl ester hydrochloride (2.93 g, 16.1 mmol) in methanol (50 mL) and 2,2,2-trifluoroethanol (50 mL) were treated with triethylamine (2.24 mL, 16.1 mmol) and the mixture was stirred under nitrogen at room temperature for 20 h. (2R)-2,3-dihydro-1H-inden-2-yl({[(phenylmethyl)oxy]carbonyl}amino)ethanoic acid (5.24 g, 16.1 mmol) and 2-benzyloxyphenylisonitrile (3.37 g, 16.1 mmol) were added and the mixture was stirred at room temperature under nitrogen for 4 days. The mixture was concentrated under reduced pressure then partitioned between ethyl acetate (150 mL) and water (150 mL) plus saturated aqueous sodium hydrogen carbonate (6 mL). The aqueous phase was back-extracted with ethyl acetate (50 mL) and the combined organic extracts were washed successively with semi-saturated aqueous solutions of sodium hydrogen carbonate, ammonium chloride and sodium chloride (100 mL each), dried over anhydrous magnesium sulphate and evaporated under reduced pressure to give the crude product (12.01 g). This was purified on a Redisep silica column (330 g) eluted with 20-50% ethyl acetate in cyclohexane to afford 7.46 g of the title compound as a pair of diastereomers.
