Polyfonctional Oxaprolines: Asymmetric Synthesis via 1,3-Dipolar Cycloaddition, Evaluation as New Organocatalysts and as Precursors of Disubstituted and Constrained Aminoacids – OXAPROL

The major objective of this project was to develop access to novel, highly functionalized oxaproline-derived molecules. An efficient asymmetric synthesis of variously functionalized oxaprolines was designed, based on a dipolar cycloaddition reaction of nitrones with alkenes. The asymmetric induction in this reaction depends on the substituents on the alkenes. With activated alkenes, asymmetric catalysis methods have been implemented (metallocatalysis or organocatalysis). With electron-rich alkenes, catalytic methods are more speculative, and the use of nitrones equipped with a chiral auxiliary was chosen, which allowed the synthesis of enantiopure oxaprolines.
In a second part of the project, the obtained oxaprolines were chemically modified to adapt for applicative objectives. They were evaluated as organocatalysts, and used as precursors of cyclic quaternary amino acids. The latter induce conformational structuration when introduced in peptidic sequences. The development of selective chemical reactions has been crucial in this context. Finally, the incorporation of oxaproline in short oligopeptides was studied using peptide synthesis in solution and on solid support.

The asymmetric synthesis of oxaprolines has been implemented through 2 types of approaches. Major results were obtained through complementary enantioselective approaches with two types of catalysts (Mc Millan ‘s organocatalyst or Kundig’s Ru-based catalyst). An efficient diastereoselective approach gave access to the large-scale synthesis of enantiopure oxaproline derivatives and to build a highly functionalized ‘superaspartic’ platform. The latter is precursor of a variety of constrained unnatural amino acids which are able to induce ß-turn conformations when introduced within peptidic scaffolds. Finally, a novel deprotection method using MgI2, particularly mild and of general use, was discovered and developped during this work.

The demonstrated efficacy of oxaproline derived compounds as organocatalysts could be highly valued in reactions for which proline has a low catalytic activity. If we can generalize the ring opening of oxaproline targets, this will open a new access to a wide variety of non-natural amino acids and useful for their potential incorporation into peptides. The final compounds (cyclopeptides, amino acids constrained by a cycle,…) are of great interest for biological evaluations and understanding of biochemical mechanisms. The oligomerization of oxaproline and its insertion into peptides of interest is also highly valuable if a conformational signature is highlighted during the study performed in Montpellier. The replacement of natural proline by oxaproline in peptides of interest, structured in helicoidal PPII, could permit us to target protein-protein interactions in the Src kinase family, which are implicated in the chronic myeloid leukemia (partnership with the team of X. Morelli and Y. Collette CRCM).

Chem. Rev. 2016 : review of synthetic methods to access the heterocyclic core of oxaproline.
Tet. Asym. 2012, J. Org. Chem. 2014 : enantioselective synthesis of oxaprolines through organo- or metallocatalysis
Org. Lett. 2014, Eur. J. Org. Chem. 2014 et 2015 : synthesis of oxaprolines and DAAs through a diastereoselective route
Chem. Eur. J. 2015, Pept. Sc. 2016 : mild deprotection in peptidic and supported chemistry, based on a novel orthogonality concept
J. Pept. Sc. 2014: quaternary oxaprolines as intermediates for the synthesis of cyclic constrained DAAs inducing ?-turn conformations in peptidic scaffolds.

The OXAPROL project is submitted for the second time for the ANR support and has no link with any anterior ANR-supported project. Based on the collaboration of 3 teams (Le Mans, Grenoble, Montpellier) in its new form, this project deals with 3 main domains of research in current organic chemistry :
(i) the control of enantioselectivity in pericyclic reactions of high synthetic potential
(ii) the development of new organocatalysts
(iii) the asymmetric synthesis of non-natural « difficult to make » amino acids and some application of biological relevance.

The first goal is to extend the scope of enantioselective 1,3-DC reactions of nitrones to the case of ketonitrones that derive formally from alpha-amino acids (or from alpha-ketoesters, depending if the nitrone is compared to the corresponding amine or carbonyl compound)*. The rational is to exploit the remarkable property of these dipoles to display a stable E configuration in solution, in contrast to aldonitrones formally deriving from glycine (or glyoxylate)*, for which an equilibrium exists between the E and Z configuration in solution. Such outstanding configurational stability is favorable for achieving a diastereo- and enantiocontrol during the formation of isoxazolidine adducts by 1,3-DC reactions.

These heteroadducts, which contain a quaternary-center bearing two functionalized side-chains, have been recently reported by our group in racemic series but are undescribed in enantiopure form. They displays a high synthetic potential, by ability (i) to be chemoselectively N-deprotected (ii) to undergo N-O cleavage via N-transprotection.

The second goal of the project is to access via a « 1,3-DC/N-deprotection route » to an original family of enantiopure 5-oxaproline analogs and to evaluate these original N-alkoxy amino acids as enantioselective organocatalysts. Interestingly, although simple L-oxaproline is a known product, to-date no report has concerned its effective use in organocatalysis. Electronic and steric effects induced by the constrained oxa-proline framework on the catalytic activity and the selectivity will be evaluated on selected reactions and in a comparative way to standard organocatalysts. The possibility to use oxaprolines as building-blocks in more elaborated organocatalysts of optimal efficiency (such as oxa-analogues of proline-containing tripeptides) will be also explored.

The third objective of the project deals with the application of the preceeding strategies to the asymmetric synthesis of several classes of disubstituted amino acids (DAA) : alpha,alpha , beta,beta ( beta3,beta3-disubstituted homoglycines) and even gamma,gamma ( gamma4,gamma4-disubstituted bis-homoglycines,). For this purpose, different transformations of enantiopure 1,3-DC adducts involving N-O cleavage of the isoxazolidine ring will be extensively pursued. A practicle synthetic access to a large diversity of quaternary alpha and beta amino acids possessing two different functional side-chains could be thus opened, towards different synthetic developpements. Different axes will be investigated : (i) the synthesis of alpha or beta amino acids constrained by a functionalized cycle (ii) the formation of new alpha alpha , or beta,beta cyclodipeptides. As another axis of application of the methodologies developped in the Oxaprol project, oxaproline oligomers will be prepared and subjected to conformational study and to biological evaluation.
To end, the incorporation of DAA into oligopeptides, with the aim to generate some conformational organization based on H-bonds, DAA potentially acting as specific « conformational inducing AA » could be an important prospect of the present project in its last part and will be possibly investigated in the final phase of the project.