合成路线1

The preparation of the precursor amino acids is shown in Schemes 1-4. Ozonolysis of (R)-citronellol (I), followed by Wittig reaction of the resulting aldehyde (II) with methylene triphenylphosphorane gave heptenol (III). Subsequent hydrogenation of (III) over Pd/C provided the saturated alcohol (IV), which was oxidized to the corresponding aldehyde (V) by treatment with Dess-Martin reagent. Asymmetric Strecker reaction in (V) utilizing (R)-phenylglycinol (VI) and trimethylsilyl cyanide afforded a diastereomeric mixture of aminonitriles (VII). After oxidative cleavage of (VII) with lead tetraacetate, the free (2S)-amine (VIII) was separated from its minor diastereomer by column chromatography. Acid hydrolysis of the nitrile group of (VIII) gave 5-propyl-L-leucine (IX), which was protected as the tert-butyl carbamate (X) with Boc2O.