Organocatalytic domino/cascade reactions provide a convenient method for the construction of complex molecular structures bearing multiple stereocenters in a highly stereoselective fashion. Among various organocatalysts, bifunctional squaramides have emerged as powerful hydrogen‐bonding catalysts for promoting a wide array of useful asymmetric organocascade sequences. Higher activity and ease of preparation make these catalysts excellent alternatives for thiourea‐based organocatalysts. This review highlights the advances in the rapidly growing field of amine‐squaramide‐catalyzed domino/cascade reactions and their mode of action.magnified image
Due to the competing 1,4‐addition reactions and the distance from the chirality information, the construction of a remote stereogenic center via 1,6‐addition reactions is often regarded as a great challenge in asymmetric synthesis. Recently, the use of organocatalysts has provided new, excellent alternatives to activate the remote δ‐position and facilitate the enantioselective 1,6‐addition reactions. In this regard, different new activation modes have been devised and new types of substrates have been employed. This review describes all the recent developments in this area.magnified image
N-Heterocyclic olefins (NHOs), the alkylidene derivatives of N-heterocyclic carbenes (NHCs), have recently emerged as a new family of promising organocatalysts with strong nucleophilicity and Brønsted basicity. The development of a novel method is shown using NHOs as efficient promoters for the direct dehydrogenative silylation of alcohols or hydrosilylation of carbonyl compounds. Preliminary results of the first NHO-promoted asymmetric synthesis are also discussed.
A new series of N-Boc ketimines derived from pyrazolin-5-ones have been used as electrophiles in asymmetric Mannich reactions with pyrazolones. The amino-bis-pyrazolone products are obtained in excellent yields and stereoselectivities by employing a very low loading of 1 mol % of a bifunctional squaramide organocatalyst. Depending on the substitution at position 4 of the pyrazolones, the new protocol allows for the generation of one or two tetrasubstituted stereocenters, including a one-pot version combing the Mannich reaction with a base-mediated halogenation.
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