Bifunctional Amine-Squaramides: Powerful Hydrogen-Bonding Organocatalysts for Asymmetric Domino/Cascade Reactions

Asymmetric synthesis of polysubstituted chiral chromans via an organocatalytic oxa-Michael-nitro-Michael domino reaction

A catalytic asymmetric method for the synthesis of polysubstituted chromans via an oxa-Michael-nitro-Michael reaction has been developed. The squaramide-catalyzed domino reaction of 2-hydroxynitrostyrenes with trans-β-nitroolefins produced chiral chromans with excellent enantioselectivities (up to 99% ee), diastereoselectivities (up to >20 : 1 dr), and moderate to good yields (up to 82%).

Asymmetric Conjugate Additions of Carbonyl Compounds to Nitroalkenes under Solvent-Free Conditions Using Fluorous Diaminomethylenemalononitrile Organocatalyst

The novel fluorous organocatalyst bearing a diaminomethylenemalononitrile motif is prepared. The fluorous organocatalyst efficiently promotes asymmetric conjugate additions of ketones to nitroalkenes and results in high yields of these addition products with excellent enantioselectivities under solvent-free conditions.

Organocatalytic Michael/cyclization cascade reactions of 3-isothiocyanato oxindoles with 3-trifluoroethylidene oxindoles: an approach for the synthesis of 3′-trifluoromethyl substituted 3,2′-pyrrolidinyl-bispirooxindoles

A series of 3′-trifluoromethyl substituted 3,2′-pyrrolidinyl-bispirooxindoles were constructed via an asymmetric Michael/cyclization cascade reaction.

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.

Electron-deficient pyridinium salts/thiourea cooperative catalyzed O-glycosylation via activation of O-glycosyl trichloroacetimidate donors

The glycosylation of O-glycosyl trichloroacetimidate donors using a synergistic catalytic system of electron-deficient pyridinium salts/aryl thiourea derivatives at room temperature is demonstrated. The acidity of the adduct formed by the 1,2-addition of alcohol to the electron-deficient pyridinium salt is increased in the presence of an aryl thiourea derivative as an hydrogen-bonding cocatalyst. This transformation occurs under mild reaction conditions with a wide range of O-glycosyl trichloroacetimidate donors and glycosyl acceptors to afford the corresponding O-glycosides in moderate to good yields with predictable selectivity. In addition, the optimized method is also utilized for the regioselective O-glycosylation by using a partially protected acceptor.

A complementary pair of enantioselective switchable organocatalysts

A pair of enantioselective switchable bifunctional catalysts are shown to promote a range of conjugate addition reactions in up to 95 : 5 e.r. and 95% conversion. Each catalyst can be switched OFF using conditions that switch the other catalyst ON. Catalyst ON : OFF ratios of up to 98 : 2 and 1 : 99 were achieved, with a ratio of reaction rates of up to 16 : 1 between the ON and OFF states, maintained over complete ON-OFF-ON and OFF-ON-OFF cycles. However, simultaneous operation of the catalyst pair in the same reaction vessel, which in principle could allow product handedness to be switched by simple E-Z isomerisation of the catalyst pair, was unsuccessful. In this first generation complementary pair of enantioselective switchable organocatalysts, the OFF state of one catalyst inhibits the ON state of the other.

Supported and Unsupported Chiral Squaramides as Organocatalysts for Stereoselective Michael Additions: Synthesis of Enantiopure Chromenes and Spirochromanes

Novel supported chiral bifunctional squaramides have been easily prepared starting from diamines derived from natural amino acids and commercially available aminoalkyl polystyrene resins. These squaramides behave as excellent stereoselective recoverable organocatalysts in different Michael additions, in neat conditions at room temperature. The reaction on 2-(2-nitrovinyl) phenol as electrophile lead, in excellent yields and enantioselectivities, to intermediates that can be easily transformed into 4H-chromenes, and spirochromanones.

Switchable Stereoselectivity in Bromoaminocyclization of Olefins: Using Brønsted Acids of Anionic Chiral Cobalt(III) Complexes

Brønsted acids of anionic chiral Co^III complexes act as bifunctional phase-transfer catalysts to shuttle the substrates across the solvent interface and control stereoselectivity. The diastereomeric chiral Co^III -templated Brønsted acids, with the same chiral ligands, enabled a switch in the enantioselective bromoaminocyclization of olefins to afford the two enantiomers of 2-substituted pyrrolidines with high enantioselectivities (up to 99:1 e.r.).

One-pot asymmetric synthesis of a spiro[dihydrofurocoumarin/pyrazolone] scaffold by a Michael addition/I2-mediated cyclization sequence

An asymmetric formal one-pot reaction of 4-hydroxycoumarins with unsaturated pyrazolones has been developed by merging a chiral bifunctional organocatalyst with molecular iodine, which furnished a series of optically active spiro[dihydrofurocoumarin/pyrazolone] heterocycles with spiro quaternary stereogenic centers in moderate to excellent yields (up to 99%) with excellent diastereoselectivities (up to >99 : 1 dr) and good to excellent enantioselectivities (up to 99% ee). The application in the gram-scale synthesis of chiral spiro[dihydrofurocoumarin/pyrazolone] compounds was also successfully realized.

Development of Chiral, Bifunctional Thiosquaramides: Enantioselective Michael Additions of Barbituric Acids to Nitroalkenes

We report a general method for the synthesis of chiral thiosquaramides, a class of bifunctional catalysts not previously described in the literature. Thiosquaramides are found to be more acidic and significantly more soluble in nonpolar solvents than their oxosquaramide counterparts, and they are excellent catalysts for the unreported, enantioselective conjugate addition reaction of the barbituric acid pharmacaphore to nitroalkenes, delivering the chiral barbiturate derivatives in high yields and high enantioselectivities, even with catalyst loadings as low as 0.05 mol%.