In analogy with Example 5d), 5.0 g (16.34 mmol) of 5(S)-[1(S)-(Boc-amino)-2-phenylethyl]dihydrofuran-2-(3H)-one, dissolved in 24 ml of abs. THF and 2.7 ml of DMPU, are deprotonated, at -75° C., with 32.7 ml of a 1 M solution of lithium bis(trimethylsilyl)amide in THF, and alkylated, at -75° C., with 9.67 g (24.5 mmol) of p-(2,6-dichlorobenzylsulfonyl)benzyl bromide (Maybridge; Tintagel/UK) in 50 ml of abs. THF. Protonation, at -75° C., with 6.1 ml (81.7 mmol) of propionic acid and 6.1 ml of water, extraction and column chromatography (SiO2, hexane/ethyl acetate, 2:1) of the crude product, and crystallization from DIPE, affords the title compound: TLC Rf (D)=0.30; tRet (II)=17.3 min.
