The development of switchable chemo-, regio-, or diastereodivergent reactions, in which two or more structurally and stereogenically different types of chiral products could be produced efficiently from an identical set of starting materials under readily tunable catalytic conditions, is one of the ultimate goals in asymmetric catalysis. This tutorial review will focus on the application of a variety of chiral organic catalysts, including Lewis bases, and Bronsted bases and acids, in a diverse range of stereoselective reactions in a switchable manner. Meanwhile, a few examples of divergent reactions through synergistic activation of organocatalysis and metal catalysis will also be discussed.

Switchable divergent asymmetric synthesis via organocatalysis

The catalytic enantioselective assembly of spirocyclic molecules featuring a spiro quaternary carbon stereocenter is currently of great interest because such privileged 3D structures are widely present in natural products that exhibit a broad spectrum of biological and pharmacological activities. This review summarizes the advances based on six major synthetic strategies and showcases the reaction mechanisms in detail. The advantages and limitations of each synthetic strategy are presented, and the remaining synthetic opportunities are outlined.

Catalytic Enantioselective Construction of Spiro Quaternary Carbon Stereocenters

A highly diastereo- and enantioselective methodology for the asymmetric synthesis of vicinal diaxial styrenes and multiaxis system was achieved by organocatalysis. Various vicinal diaxial styrenes and multiaxis systems were obtained in excellent enantioselective manners. The mechanism studies revealed that a new tetra-substituted vinylidene ortho-quinone methide (VQM) intermediate was likely involved and accounted for the excellent enantioselectivity.

Enantioselective Construction of Vicinal Diaxial Styrenes and Multiaxis System via Organocatalysis.

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an em...

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The concept of pericyclic reactions and the explanation of their specificity through orbital symmetries introduced a new way of understanding reactions and looking for new ones. One of the 1965 Woodward-Hoffmann communications described "the (as yet unobserved) symmetry-allowed 6 + 4 combination", the prediction of a new field of "higher-order" cycloadditions, involving more than six electrons. Later these authors predicted exo-stereoselectivity for the [6 + 4]-cycloaddition. Chemists rushed to test this prediction (for the most part successfully). For more than half a century, chemists have hunted for additional higher-order cycloadditions. The application of catalysis within organic chemistry allows the accomplishment of previously unattainable reactions, including higher-order cycloadditions. The many examples of [8 + 2], [6 + 4], and cycloadditions of even higher electron-counts discovered since the Woodward-Hoffmann rules were introduced illustrate the difficulty in predicting which of these transformations will occur when two highly unsaturated molecules react. Periselectivity has been a challenge, and the development of enantioselective variants has been elusive. While progress was made, the rise of organocatalysis in asymmetric synthesis has led to a surge of interest in stereoselective versions of higher-order cycloadditions. Through organocatalytic activation of conjugated cyclic polyenes and heteroaromatic compounds, asymmetric [8 + 2]-, [6 + 4]-, and [10 + 4]-cycloadditions have been realized by our groups. In this century, [6 + 4]-cycloadditions have been found also to occur in enzyme-catalyzed reactions for the biosynthesis of spinosyn A, heronamide, and streptoseomycin natural products. A whole new class of enzymes, the pericyclases that catalyze pericyclic reactions, has been discovered. A remarkable aspect of these recent developments is the cross-disciplinary research involved: from organic synthesis to computational studies integrated with experimental studies of reaction mechanisms, intermediates, and dynamics, to understanding mechanisms of enzyme catalysis and engineering of enzymes. This Account describes how our groups have been involved in the expansion of the higher-order cycloaddition frontiers. We describe both the history and recent progress in higher-order cycloadditions, and how these advances have been made by our collaborative experimental and computational studies. Progress in asymmetric organocatalysis, incorporating enantioselective higher-order cycloadditions in organic synthesis, and the stereoselective synthesis of important scaffolds will be highlighted. Experimental progress and computational modeling with density functional theory (DFT) has identified ambimodal cycloaddition pathways and led to the realization that multiple products of pericyclic reactions are linked by common transition states. Molecular dynamic simulations have provided fundamental understanding of factors controlling periselectivity and have led to discoveries of a group of enzymes, the pericyclases, which catalyze pericyclic reactions such as [6 + 4]-cycloadditions.

Expanding the Frontiers of Higher-Order Cycloadditions.

Catalytic Regio- and Enantioselective [6+2], [4+2], and [2+2] Cycloadditions

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Catalytic Regio- and Enantioselective [6+2], [4+2], and [2+2] Cycloadditions