Enzymatic kinetic resolution of racemic mixtures has multiple applications in the preparation of single enantiomers from racemates. When kinetic resolution is coupled with an in situ racemization, chemical or enzymatic, the yield limitation can be overcome leading to a much more efficient process: a deracemization based on a dynamic kinetic resolution (DKR). Requisites for a successful DKR are an enzyme selective for one form of the racemic mixture, a racemizing system acting on the substrate but not on the product, and a rate of racemization higher than the enzymatic reaction. These conditions require the design of suitable substrates. Since in thioesters the acidity of the hydrogen in α-position is higher in comparison to the corresponding oxo-esters, amides or acids, we attempted to submit a number of N-protected-α-aminoacid thioesters to enzymatic hydrolysis (subtilisin) in the presence of a base-catalyzed continuous substrate racemization. In a preliminary work, complete deracemization of a number of α-aryl-substrates has been effected, giving quantitative yields of L-amino acids in enantiomerically pure form. The application of this technology to non-aromatic substrates required a careful design of the reaction conditions, in terms of solvent, racemizing agent, temperature, and enzyme form. In particular, suitable racemization conditions have been studied also with the aid of computational means. A set of conditions appropriate for a one-pot DKR was found, and several aromatic as well as aliphatic N-Boc-amino acids have been obtained with high conversion and in enantiopure form.

Deracemization of non-natural Amino Acids based on DKR of the corresponding N-protected Thioesters

TESSARO, DAVIDE;CERIOLI, LORENZO;D'ARRIGO, PAOLA;SERVI, STEFANO;
2011

Abstract

Enzymatic kinetic resolution of racemic mixtures has multiple applications in the preparation of single enantiomers from racemates. When kinetic resolution is coupled with an in situ racemization, chemical or enzymatic, the yield limitation can be overcome leading to a much more efficient process: a deracemization based on a dynamic kinetic resolution (DKR). Requisites for a successful DKR are an enzyme selective for one form of the racemic mixture, a racemizing system acting on the substrate but not on the product, and a rate of racemization higher than the enzymatic reaction. These conditions require the design of suitable substrates. Since in thioesters the acidity of the hydrogen in α-position is higher in comparison to the corresponding oxo-esters, amides or acids, we attempted to submit a number of N-protected-α-aminoacid thioesters to enzymatic hydrolysis (subtilisin) in the presence of a base-catalyzed continuous substrate racemization. In a preliminary work, complete deracemization of a number of α-aryl-substrates has been effected, giving quantitative yields of L-amino acids in enantiomerically pure form. The application of this technology to non-aromatic substrates required a careful design of the reaction conditions, in terms of solvent, racemizing agent, temperature, and enzyme form. In particular, suitable racemization conditions have been studied also with the aid of computational means. A set of conditions appropriate for a one-pot DKR was found, and several aromatic as well as aliphatic N-Boc-amino acids have been obtained with high conversion and in enantiopure form.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11311/762886
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