Electronic Resource
Springer
Amino acids
11 (1996), S. 409-424
ISSN:
1438-2199
Keywords:
Amino acids
;
Fluorinated amino acids
;
Alkylation
;
Glycine esters
;
1-Bromo-2-fluoroalkanes
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary Methods for the synthesis of racemic and optically active title compounds are presented. Key step of these four-step procedures is the alkylation with 1-bromo-2-fluoroalkanes of glycine-ester-derived imines in anhydrous medium using lithium diisopropylamide as a base at low temperature or phase transfer catalyzed alkylation with 50% NaOH and triethylbenzylammoniumchloride as the phase transfer catalyst, respectively. Subsequent three-step deprotection gave the free acids in 13–33% overall yield. Deracemization ofγ-fluoro-α-aminobutyric acid methyl and ethyl esters withα-chymotrypsin was shown to give the (−)-enantiomers of the esters and (+)-γ-fluoro-α-aminobutyric acid in 〉98% ee, while from thetert-butylester the opposite stereochemical result was observed giving the (−)-acid with 88% ee. Optically activeγ-fluoro-α-amino acids were synthesized alternatively by phase transfer catalysis with N-benzyl-cinchonium chloride or using an auxiliary-directed asymmetric alkylation of the imine derived from (R)-(+)-camphor or (R)-(+)-2-hydroxypinan-3-one. These processes gave different enantiomers ofγ-fluoro-α-aminobutyric acid via a monomeric lithium enolate in the first or a dimeric lithium enolate in the second case, respectively. The enantiomeric excess can be improved by lithium/magnesium exchange.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00807945
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