DIAD (3.72 ml, 18.9 mmol) was added to a solution of methyl (2S,4S)-1-tert-butoxycarbonyl-4-hydroxy-2-pyrrolidinylcarboxylate (4.22 g, 17.2 mmol), 2-naphthol (2.73 g, 18.9 mmol) and triphenylphosphine (4.96 g, 18.9 mmol) in THF (80 ml) under stirring at room temperature in a nitrogen gas stream. The mixture was stirred overnight at room temperature. The residue, which had been obtained by concentrating the reaction mixture under reduced pressure, was purified by chromatography on a silica gel column (silica gel: 600 g), whereby from chloroform/ethyl acetate (10/1) eluate fractions, the title compound (5.37 g) was obtained as an amorphous substance (this compound was provided for the subsequent reaction without further purification).
