An Asymmetric Organocatalytic Quadruple Domino Reaction Employing a Vinylogous Friedel-Crafts/Michael/Michael/Aldol Condensation Sequence

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

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 emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Achieving Molecular Complexity via Stereoselective Multiple Domino Reactions Promoted by a Secondary Amine Organocatalyst

In the last two decades, organocatalysis has emerged as an intensively investigated and rapidly growing area of research facilitating many known and many new transformations to provide efficient novel entries to complex molecules of high stereochemical purity. The organocatalysts have not only shown their efficiency for catalyzing the reactions in which one bond is formed, but they have also been effectively exploited in various versions of one-pot reactions. Domino reactions are one of the most important classes of one-pot reactions, where the target structure can be obtained in one pot without changing any reaction conditions while each reaction occurs as a consequence of the intermediates generated in previous steps. Owing to the synthetic importance and operational advantages associated with the use of organocatalysts and the development of domino reactions, various asymmetric transformations leading to a complex structure of choice have been explored. The early era of organocatalysis exhibits a limited growth in the development of asymmetric domino reactions with special emphasis on two reactions occurring one after the other. In 2006, our group made a step forward to develop more complex domino reactions catalyzed by a secondary amine organocatalyst, wherein three reactions take place in one pot to provide cyclohexene carbaldehydes bearing four stereogenic centers with excellent stereocontrol. This triggered our interest to develop new organocatalytic domino sequences, especially for multiple domino reactions. After our seminal contribution, domino reactions catalyzed by secondary amine organocatalysts not only became more popular, but they also could be catalyzed by other classes of organocatalysts, such as bifunctional hydrogen bonding catalysts, chiral Brønsted acids, and N-heterocyclic carbenes. The mode of activation in this triple domino reaction relied on the sequential generation of enamine and iminium intermediates using a proline-based chiral secondary amine organocatalyst. By employing this strategy, we have developed several triple domino reactions leading to the formation of carbo- and heterocyclic structures bearing multiple stereogenic centers with excellent levels of stereoselectivities. The applications of the secondary amine organocatalysts have been further extended to more complex quadruple domino sequences. Moreover, these multiple domino sequences have been combined successfully with other transformations in one pot to create densely functionalized polycyclic compounds. This Account gives an overview of our research in the area of organocatalytic asymmetric multiple domino reactions with special emphasis on the secondary amine catalyzed triple and quadruple domino reactions via a sequential generation of enamine and iminium intermediates. The multiple cascade reactions assisted by di- and tri-iminium and -enamine species as well as other types of organocatalysts have been excluded from the scope of this Account.