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- Title
A Study on the Diastereoselective Synthesis of α-Fluorinated β<sup>3</sup>-Amino Acids by α-Fluorination.
- Authors
Peddie, Victoria; Abell, Andrew D.
- Abstract
The treatment of a β3-amino acid methyl ester with 2.2 equiv. of lithium diisopropylamide (LDA), followed by reaction with 5 equiv. of N-fluorobenzenesulfonimide (NFSI) at −78° for 2.5 h and then 2 h at 0°, gives syn-fluorination with high diastereoisomeric excess (de). The de and yield in these reactions are somewhat influenced by both the size of the amino acid side chain and the nature of the amine protecting group. In particular, fluorination of N-Boc-protected β3-homophenylalanine, β3-homoleucine, β3-homovaline, and β3-homoalanine methyl esters, 5 and 9- 11, respectively, all proceeded with high de (>86% of the syn-isomer). However, fluorination of N-Boc-protected β3-homophenylglycine methyl ester ( 16) occurred with a significantly reduced de. The use of a Cbz or Bz amine-protecting group (see 3 and 15) did not improve the de of fluorination. However, an N-Ac protecting group (see 17) gave a reduced de of 26%. Thus, a large N-protecting group should be employed in order to maximize selectivity for the syn-isomer in these fluorination reactions.
- Subjects
STEREOSELECTIVE reactions; STEREOCHEMISTRY; LITHIUM; FLUORINATION; ALANINE; AMINO acids
- Publication
Helvetica Chimica Acta, 2012, Vol 95, Issue 12, p2460
- ISSN
0018-019X
- Publication type
Article
- DOI
10.1002/hlca.201200520