Synthesis of (2S,4S)- and (2S,4R)-5-fluoroleucine and (2S,4S)-[5,5-2H2]-5-fluoroleucine

Abstract

Syntheses of (2S,4S)- and (2S,4R)-5-fluoroleucine, and, and of (2S,4S)-[5,5-(2)H(2)]-5-fluoroleucine, have been completed. The methodology allows these compounds to be prepared in sufficient quantities for incorporation by solid-state protein synthesis into strategic sites in proteins for folding studies. X-ray structures of the epimers and have been obtained and show the presence of conformational isomerism. The torsion angles between the F-C bond and the main chain are compared with values found in a mutant of the protein ubiquitin in which (2S,4S)-5-fluoroleucine replaces leucine residues 50 and 67 in the native protein.

Synthesis and Preliminary Evaluation of 5-[18F]fluoroleucine

“…Displacement of the mesylate function in the precursor with fluorine using tetrabutyl ammonium fluoride (55%), and removal of all protecting groups by treatment with TFA (96%) gave the final product 5-fluoroleucine. NMR data of this compound and the intermediates were consistent with their structure and the data reported [12]. However, unlike in this reported procedure, we did not use 2-mesitylene sulfonic acid in the conversion of 7 to 8.…”

“…This compound and the mesylate precursor 7 were synthesized essentially following a procedure reported for synthesis of (2S,4S)-and (2S,4R)-5-fluoroleucine and (2S,4S)- [5,5-2 H2]-5-fluoroleucine [12]. 18 F activity trapped in a QMA cartridge (Waters, Milford, MA) was eluted with a mixture of Kryptofix (28 mg) and potassium carbonate (2.4 mg) in 0.75 ml of 95% acetonitrile, and dried by azeotroping with acetonitrile three times.…”

Solid state 19F NMR parameters of fluorine-labeled amino acids. Part II: Aliphatic substituents

“…The polycrystalline 19 F-labeled amino acids were used as supplied by the following commercial sources: 3F-Ala was purchased from Bachem (Bubendorf, Switzerland), 3,3,3F 3 -Ala from Chempur (Karlsruhe, Germany), 3F-Val and 5,5,5F 3 -Leu from Lancaster (now Alfa Aesar, Karlsruhe, Germany), 4CF 3 -Phe and 4CF 3 -Phg from ABCR (Karlsruhe, Germany). The leucine derivatives 5F-(2S,4S)-leucine (5F-Leu(2S,4S)), 5F-(2S,4R)-leucine (5F-Leu(2S,4R)), Fmoc-5F-(2S,4S)-leucine (Fmoc-5F-Leu) were synthesized by D. Young and J.-D. Charrier (University of Sussex, UK) as previously described [9,10], as was 5F-5,5D 2 -(2S,4S)-leucine (5F-5,5D 2 -Leu), while 5,5,5D 3 -L-leucine (5,5,5D 3 -Leu) was supplied from Cambridge Isotopes (Andover, MA, USA). 3-(Trifluoromethyl)bicyclopent[1,1,1]-1-yl-glycine (CF 3 -Bpg) was synthesized as described [7,11], and 3CF 3 -Phe was a gift from T. Asakura (Tokyo University, Japan).…”

Synthesis of Isotopically Labeled α‐Amino Acids

“…The research groups of Young and Nishiyama have used pyroglutamate analogs as precursors for the synthesis of a range of isotopically labeled a-amino acids such as leucine, isoleucine, valine, and proline [49][50][51][52][53][54][55][56][57]. Their strategy utilizes the pyroglutamate ring to introduce additional functionality, followed by facile ring-opening 480j 11 Synthesis of Isotopically Labeled a-Amino Acids reactions for the synthesis of linear amino acids.…”

“…Their strategy utilizes the pyroglutamate ring to introduce additional functionality, followed by facile ring-opening 480j 11 Synthesis of Isotopically Labeled a-Amino Acids reactions for the synthesis of linear amino acids. For example, Young and coworkers used pyroglutamate analog 39 to good effect for the preparation of (2S,3R)-[3 0 ,3 0 ,3 0 -2 H 3 ] valine 43 (Scheme 11.12) [51]. Reaction of 39 with a copper Grignard reagent allowed the stereoselective incorporation of a CD 3 group on the least hindered face of the ring.…”