Sir: Bestatin is a specific inhibitor of aminopeptidase B and leucine aminopeptidase.l) It was isolated from a culture filtrate of Streptomyces olivoreticuli. Its structure was elucidated as [(2S,3 R)-3-amino-2-hydroxy -4phenylbutanoyl] L-leucine.2.3> (2S,3R)-3-Amino-2-hydroxy-4phenylbutanoic acid [abbreviated as (2S,3R)AHPAJ, an acid hydrolysis product of bestatin, is a new naturally-occuring amino acid. In this communication, the chemical synthesis of bestatin by the scheme shown in Fig. 1, is reported. To an ethyl acetate solution of benzyloxycarbonyl-D-phenylalanine kept at 0°C was added one equivalent of dicyclohexylcarbodiimide (DCC). Thirty minutes after addition, one equivalent of pyrazole was added. The reaction mixture was stirred for 16 hours at 0°C. After removal of resulting dicyclohexylurea by filtration, the filtrate was evaporated under reduced pressure to yield crude pyrazolide of benzyloxycarbonyl-D-phenylalanine (1). It was crystallized from ethyl acetate, mp 108-109°C (83//0 yield). To a tetrahydrofuran (THF) solution of 1 kept at -15--20'C was added 2 eq. of LiAIH4 in THE intermittently over a period of 30 minutes. The reaction mixture was stirred for 30 minutes at the same temperature. After decomposition of the excess reagent by addition of 2 N HCI, the solvent was evaporated and the residue was extracted with ethyl acetate. The extract was washed with water and then dried. The dried material, benzyloxycarbonyl D phenylalaninal (2), was used in the following reaction without further purification. An aqueous suspension of 2 was treated with 2 eq. of NaHSO; at 60°C for 2 hours to form the adduct (3). It was extracted from the clear reaction mixture with ethyl acetate and crystalline powder of 3 was obtained after evaporation of the solvent in 74° yield starting from the pyrazolide. To an ice-cold aqueous solution of 3 was added I eq. of NaCN over a period of l hour to form the cyanohydrin (4). It was extracted with ethyl acetate in 94'/0 yield. Refiux of 4 in 6 N HCI gave AHPA in 79 yield. As expected, AHPA thus obtained was a mixture of (2S,3R)and (2R,3R)-isomers. (2S, 3R)-AHPA was separated from its diastereoisomer by Dowex 50 chromatography using linear gradient elution between 0.36m pyridineacetate (pH 3.52) and 0.48 M pyridine-acetate (pH 3.80) buffer. (2S,3R)-AHPA was eluted later than the diastereoisomer. The synthetic (2S,3R)-AHPA was identical with natural material, [a]n'+27.7° (c 1.0, 1 N HCI) [Lit.°1 [a] 27.9° (c 0.717, 1 N HC])]. On a cellulose thinlayer chromatogram using butanol saturated with
[1]
T. Aoyagi,et al.
Bestatin, an inhibitor of aminopeptidase B, produced by actinomycetes.
,
1976,
The Journal of antibiotics.
[2]
T. Aoyagi,et al.
X-ray structure determination of (2S, 3R)-3-amino-2-hydroxy-4-phenylbutanoic acid, a new amino acid component of bestatin.
,
1976,
The Journal of antibiotics.
[3]
T. Aoyagi,et al.
The structure of bestatin.
,
1976,
The Journal of antibiotics.