The sodium salt of L-phenylalanine (I) was condensed with pivalaldehyde (II) in boiling pentane to afford imine (III). Subsequent reaction of (III) with benzyl chloroformate generated oxazolidinone (IV) as a 9:1 mixture of diastereoisomers, which were separated chromatographically. The required cis compound was treated with allyl bromide (V) in the presence of potassium hexamethyldisilazide in THF at -78 C to provide (VI) as the major isomer. Aldehyde (VII) was then obtained by ozonolysis of (VI) at -78 C. Reductive amination of (VII) with L-homophenylalanine methyl ester (VIII) using NaBH3CN yielded the corresponding amine (IX) which, upon heating in toluene in the presence of 1-hydroxybenzotriazole, afforded pyrrolidinone (X). Reduction of the ester function of (X) with Ca(BH4)2 gave alcohol (XI), which was finally oxidized under Swern conditions to the target aldehyde.

The sodium salt of L-phenylalanine (I) was condensed with pivalaldehyde (II) in boiling pentane to afford imine (III). Subsequent reaction of (III) with benzyl chloroformate generated oxazolidinone (IV) as a 9:1 mixture of diastereoisomers, which were separated chromatographically. The required cis compound was treated with allyl bromide (V) in the presence of potassium hexamethyldisilazide in THF at -78 C to provide (VI) as the major isomer. Aldehyde (VII) was then obtained by ozonolysis of (VI) at -78 C. Reductive amination of (VII) with L-homophenylalanine methyl ester (VIII) using NaBH3CN yielded the corresponding amine (IX) which, upon heating in toluene in the presence of 1-hydroxybenzotriazole, afforded pyrrolidinone (X). The carbobenzoxy group of (X) was then removed by hydrogenolysis over Pd/C, and the resulting amine was acylated with benzenesulfonyl chloride to produce sulfonamide (XI). Finally, the target aldehyde was obtained by the sequence of ester reduction of (XI) with Ca(BH4)2, followed by Swern oxidation of the resulting alcohol.