Direct asymmetric aldol reaction co-catalyzed by l-proline and isothiouronium salts

Organo-catalysis as emerging tools in organic synthesis: aldol and Michael reactions

Organocatalysis has occupied sustainable position in organic synthesis as a powerful tool for the synthesis of enantiomeric-rich compounds with multiple stereogenic centers. Among the various organic molecules for organocatalysis, the formation of carbon–carbon is viewed as a challenging issue in organic synthesis. The asymmetric aldol and Michael addition reactions are the most significant methods for C–C bond forming reactions. These protocols deliver a valuable path to access chiral molecules, which are useful synthetic hybrids in biologically potent candidates and desirable versatile pharmaceutical intermediates. This work highlighted the impact of organocatalytic aldol and Michael addition reactions in abundant solvent media. It focused on the crucial methods to construct valuable molecules with high enantio- and diastereo-selectivity.

Probing the Synergistic Catalytic Model: A Rationally Designed Urea-Tagged Proline Catalyst for the Direct Asymmetric Aldol Reaction

A urea tag was incorporated at the C-4 position of proline, cis to its COOH group, in order to explore the prospect of a synergistic effect between the two functional groups in the transition state of the enamine route to the asymmetric aldol reaction. The catalyst proved to be an excellent performer, delivering aldols in high yields and with excellent enantio- and diastereoselectivities using just 2 mol % loading in the presence of water; it also exhibited good levels of recyclability under aqueous conditions. The favorable results reveal the interesting possibility of an intramolecular host-guest interaction between the urea and the amino acid moieties, exerting a beneficial effect on catalysis. The concept could certainly offer a new direction toward more efficient catalyst design.

Towards the development of continuous, organocatalytic, and stereoselective reactions in deep eutectic solvents

Different deep eutectic solvent (DES) mixtures were studied as reaction media for the continuous synthesis of enantiomerically enriched products by testing different experimental set-ups. L-Proline-catalysed cross-aldol reactions were efficiently performed in continuo, with high yield (99%), anti-stereoselectivity, and enantioselectivity (up to 97% ee). Moreover, using two different DES mixtures, the diastereoselectivity of the process could be tuned, thereby leading to the formation, under different experimental conditions, to both the syn- and the anti-isomer with very high enantioselectivity. The excess of cyclohexanone was recovered and reused, and the reaction could be run and the product isolated without the use of any organic solvent by a proper choice of DES components. The dramatic influence of the reaction media on the reaction rate and stereoselectivity of the process suggests that the intimate architecture of DESs deeply influences the reactivity of different species involved in the catalytic cycle.