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Reduction of the Ester 397 to the Alcohol 398 It is done using the method described in §7.0.2.a using 397 either as a single enantiomer, a mixture of two diastereoisomers or a 1/1/1/1 mixture of 4 stereoisomers. For a 1/1 diastereoisomeric mixture of tert-butyl (2S)-2-[4-(hydroxymethyl)-2-oxo-1-pyrrolidinyl]butanoate 398: GC/MS: 257 M+ Oxydation to the Aldehyde 396 In a three necked flask, under argon, a solution of tert-butyl (2S)-2-[4-(hydroxymethyl)-2-oxo-1-pyrrolidinyl]butanoate 398 (4.0 g, 0.016 mol.) in CH2Cl2 (8 ml) is added to a suspension of CrO3 (6.2 g, 0.062 mol.) in pyridine (11.3 ml)/CH2Cl2 (80 ml) stirred at room temperature. The temperature increases to 30° C. and the suspension is stirred for 0.2 h. The suspension is filtered through celite and the filtrate is washed successively with HCl 1N, brine, dried over magnesium sulfate and concentrated in vacuo to afford the crude aldehyde which is purified by column chromatography on silicagel (hexane/acetone 70/30 (v/v)) to afford 2.03 g of 1-[(1S)-1-(tertbutoxycarbonyl)propyl]-5-oxo-3-pyrrolidinecarboxaldehyde 396 (41%). Alternatively, the reaction can also be conducted with racemic ester to afford the racemic aldehyde with a similar yield. Analysis of a 1/1 mixture of 1-[(1S)-1-(tertbutoxycarbonyl)propyl]-5-oxo-3-pyrrolidinecarboxaldehyde 396: 1H NMR (250 MHz, (CDCl3): 0.91 (t, 3H), 1.44 (s, 9H), 1.55-1.77 (m, 1H), 1.90-2.15 (m, 1H), 2.63-2.82 (m, 2H), 3.47-3.61 (m, 1H), 3.65-3.79 (m, 1H), 3.83-3.94 (m, 1H of one of the diastereoisomers), 4.48-4.62 (m, 1H), 9.74 (s (broad), 1H). 6.2. Olefination of 1-[(1S)-1-(tertbutoxycarbonyl)propyl]-5-oxo-3-pyrrolidinecarboxaldehyde 396 6.2.1. Synthesis of Ethylenic Derivatives. Alternatively to §6.2.3., ethylenic derivatives can be obtained by Wittig olefination of the 1-[(1S)-1-(tertbutoxycarbonyl)propyl]-5-oxo-3-pyrrolidinecarboxaldehyde 396 and a phosphonium salt in the presence of a strong base. For example, (2S)-2-(2-oxo-4-vinyl-1-pyrrolidinyl)butanoic acid 2,2dimethyl)ethyl ester is obtained by reaction of the aldehyde 396 with Ph3PCH3Br and n-BuLi in THF. 6.2.2. By Olefination with Ph3P/CBr4 Alternatively to §6.2.3., halovinyl derivatives can be obtained by Wittig olefination of the 1-[(1S)-1-(tertbutoxycarbonyl)propyl]-5-oxo-3-pyrrolidinecarboxaldehyde 396 in the presence of a phosphine and an halogenomethane. For example, (2S)-2-(2-oxo-4-(2,2-dibromovinyl)-1-pyrrolidinyl)butanoic acid 2,2-(dimethyl)ethyl ester is obtained from aldehyde 396 and CBr4 in the presence of triphenylphosphine. 6.2.3. By olefination with (Me2N)3P/CF2Br2