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All total 12 Articles be foundMemory of chirality of tertiary aromatic amides: A simple and efficient method for the enantioselective synthesis of quaternary α-amino acids
Branca, Mathieu,Pena, Sebastien,Guillot, Regis,Gori, Didier,Alezra, Valerie,Kouklovsky, Cyrille
scheme or table, p. 10711 - 10718 (2009/12/04)
A new methodology for the asymmetric synthesis of quaternary R-substituted amino acids using memory of chirality has been developed. The strategy utilizes the dynamic axial chirality of tertiary aromatic amides to memorize the initial chirality of an α-amino acid during an enolization step. Starting from five different L-amino acids, the corresponding oxazolidin-5-ones containing a tertiary aromatic amide group have been synthesized in one step and then alkylated with various electrophiles, with good yields and enantioselectivities (up to 96% and up to >99% after recrystallization). One-step deprotection affords enantioenriched or enantiopure quaternary α-amino acids. We describe here the optimization process, the results obtained in each series and a plausible explanation, based on NMR studies, DFT calculations and crystallographic structures. The methodology presented herein constitutes an efficient synthesis of enantiopure quaternary R-amino acids (three steps only) starting from tertiary L-amino acids, without any external source of chirality.
Tertiary aromatic amide for memory of chirality: Access to enantioenriched α-substituted valine
Branca, Mathieu,Gori, Didier,Guillot, Regis,Alezra, Valerie,Kouklovsky, Cyrille
, p. 5864 - 5865 (2008/09/20)
A new methodology for the asymmetric synthesis of quaternary α-substituted amino acids using memory of chirality has been developed. This strategy employs dynamic axial chirality of tertiary aromatic amides to memorize the initial chirality of an α-amino acid during the enolization step. Starting from l-valine, an oxazolidin-5-one containing a tertiary aromatic amide was synthesized in one step and then alkylated with various electrophiles with good yield and enantioselectivity (up to 96%). Quaternary products can be obtained enantiomerically pure by recrystallization. One-step deprotection affords enantioenriched (S)-α-methyl valine (ee = 94%) or enantiopure (S)-α-isopropyl aspartic acid (ee >99%) in only three steps starting from L-valine. Copyright
L-phenylalanine cyclohexylamide: A simple and convenient auxiliary for the synthesis of optically pure α,α-disubstituted (R)- and (S)-amino acids
Obrecht,Bohdal,Broger,Bur,Lehmann,Ruffieux,Schonholzer,Spiegler,Muller
, p. 563 - 580 (2007/10/02)
This work describes L-phenylalanine cyclohexylamide (5c) as a simple, cheap, and powerful chiral auxiliary for the synthesis of a series of optically pure α,α-disubstituted (R)- and (S)-amino acids of type 1, such as (R)- and (S)-2-methyl-phenylalanine (1a), (R)- and (S)-2-methyl-2-phenylglycine (1b), and (R)- and (S)-2-methylvaline (1c). These amino acids were efficiently transformed into the suitably protected and activated amino-acid building blocks (R)- and (S)-12b and (R)- and (S)-12c which are ready for incorporation into peptides by solution or solid-phase techniques. Based on the crystal structures of 6b, 6c, and 7a belonging to the diastereoisomeric peptides series 6 and 7, the absolute configurations of each member of the series were determined. β-Turn geometries of type II' and I were observed for 6b and 7a, respectively, whereas 6c crystallized in an extended conformation. The impacts of side-chain variation on conformation and crystal packing of these triamides are discussed.
