Higher-Order Cyclopropenimine Superbases: Direct Neutral Brønsted Base Catalyzed Michael Reactions with α-Aryl Esters

The synthesis and characterization of six new classes of higher-order superbases, including five that incorporate cyclopropenimine functionality, has been achieved. We propose a nomenclature that designates these as the CG2, GC2, PC3, PC1, C3, and GP2 classes of superbases. The pK(BH+) values were measured to be between 29.0 and 35.6 in acetonitrile. Linear correlations of ten superbase basicities vs that of their substituents demonstrated the insulating effect of the cyclopropenimine core. The molecular structures of several of these materials were obtained by single-crystal X-ray analysis, revealing interesting aspects of conformational bias and noncovalent organization. The types of superbasic cores and substituents were each reliably shown to affect selectivity for deprotonation over alkylation. Higher-order cyclopropenimine and guanidine superbase stability to hydrolysis was found to correlate to basicity. Finally, a GC2 base was found to catalyze conjugate additions of α-aryl ester pronucleophiles, representing the first report of a neutral Brønsted base to catalyze such reactions.

Chiral bifunctional guanidine-catalyzed enantioselective Aza-Henry reaction of isatin-derived ketimines

An efficient asymmetric aza-Henry reaction of isatin-derived ketimines has been achieved by using a chiral guanidine-amide organocatalyst. A series of 3-substituted 3-amino-2-oxindoles was obtained with excellent results (up to 99% yield, 94% ee). Other functionalized derivatives were also conveniently transformed. This metal-free system was convenient, practical, and insensitive to air and moisture. On the basis of the crystal structure of the catalyst and NMR spectra analysis, a bifunctional catalytic model was suggested to explain the origin of the asymmetric process.

Controllable synthesis of SnO2@C yolk-shell nanospheres as a high-performance anode material for lithium ion batteries

In this work, we report a facile synthesis of uniform SnO2@C yolk-shell nanospheres as high-performance anode materials for lithium ion batteries (LIBs). The yolk-shell structured SnO2@C nanospheres were fabricated through a two-step sol-gel coating process by using tetraethyl orthosilicate (TEOS) and resorcinol-formaldehyde (RF) as precursors, where the silica interlayer not only acts as a template to produce the void space, but also promotes the coating of the RF layer. The synthesis is easy to operate and allows tailoring the carbon shell thickness and void space size. The resultant SnO2@C yolk-shell nanospheres possess a hollow highly crystalline SnO2 core (280-380 nm), tailored carbon shell thickness (15-25 nm) and a large void space size (100-160 nm), a high surface area (∼205 m(2) g(-1)), a large pore volume (∼0.25 cm(3) g(-1)), as well as a high SnO2 content (77 wt%). When evaluated as an anode of LIBs, the materials manifest superior electrochemical performance with a high lithium storage capability (2190 mA h g(-1) in initial discharge capacity; >950 mA h g(-1) in the first 10 cycles), a good cycling performance and an excellent rate capability.

Direct chromatographic enantioresolution of fully constrained β-amino acids: exploring the use of high-molecular weight chiral selectors

To the best of our knowledge enantioselective chromatographic protocols on β-amino acids with polysaccharide-based chiral stationary phases (CSPs) have not yet appeared in the literature. Therefore, the primary objective of this work was the development of chromatographic methods based on the use of an amylose derivative CSP (Lux Amylose-2), enabling the direct normal-phase (NP) enantioresolution of four fully constrained β-amino acids. Also, the results obtained with the glycopeptide-type Chirobiotic T column employed in the usual polar-ionic (PI) mode of elution are compared with those achieved with the polysaccharide-based phase. The Lux Amylose-2 column, in combination with alkyl sulfonic acid containing NP eluent systems, prevailed over the Chirobiotic T one, when used under the PI mode of elution, and hence can be considered as the elective choice for the enantioseparation of this class of rigid β-amino acids. Moreover, the extraordinarily high α (up to 4.60) and R S (up to 10.60) values provided by the polysaccharidic polymer, especially when used with camphor sulfonic acid containing eluent systems, make it also suitable for preparative-scale enantioisolations.

Facile creation of 3-substituted-3-hydroxy-2-oxindoles by arginine-catalyzed aldol reactions of α,β-unsaturated ketones with isatins

An efficient approach for the synthesis of 3-substituted-3-hydroxy-2-oxindoles has been achieved via an aldol reaction of α,β-unsaturated ketones and isatins using arginine as an organocatalyst. A range of 3-substituted-3-hydroxy-2-oxindoles were obtained in moderate to high (up to 99%) yields. These 3-substituted-3-hydroxy-2-oxindoles with an additional enone moiety provide an opportunity for further elaboration of the products and for potentially interesting biological activities. In addition, the formation of 3-substituted-3-hydroxy-2-oxindole 3a was confirmed by X-ray crystallography. The possible reaction mechanism reveals that the reaction proceeds via a double action process.

Novel synthesis of substituted furo[3,2-c]chromen-4-ones via four-component reaction from substituted nitrostyrenes, aromatic aldehydes, coumarins, and ammonium acetate

An efficient and direct synthesis of 2-arylideneamino-3-aryl-4H-furo[3,2-c]chromen-4-ones has been developed via four-component reaction from substituted nitrostyrenes, aromatic aldehydes, coumarins, and ammonium acetate under very mild conditions, which involves sequentially a Michael addition, an aza-nucleophilic addition, of the imine to the double bond, an intermolecular nucleophilic addition and a dehydration reaction. The resulting biologically intriguing structures could have broad applications in related biomedical-program structures.

Synthesis of polysubstituted 2-piperidinones via a Michael addition/nitro-Mannich/lactamization cascade

An efficient and practical method has been developed for the diversity-oriented synthesis of polysubstituted 2-piperidinones via four-component reaction between substituted nitrostyrenes, aromatic aldehydes, ammonium acetate, and dialkyl malonates for the generation of a wide range of structurally interesting and pharmacologically significant compounds. It is worth mentioning that in the course of this reaction, the formation of products was highly stereoselective. Two differently stereochemical classes of polysubstituted 2-piperidinones depended on the substitutent position of aromatic aldehyde.

A Chiral Amino-Naphthalene-Derived Prolinamide Catalyst for the Enantioselective Michael Addition of Ketones to Nitroolefins

An enantioselective Michael addition of ketones to nitroolefins has been accomplished using a novel chiral amino-naphthalene-derived prolinamides catalyst 1. The desired Michael adducts were obtained in high yields (up to 93%) as well as good diastereoselectivities (>99:1) and enantioselectivities (48%-99% ee).

Observation of neighboring ortho-hydroxyl group participation in organocatalytic asymmetric sequential Michael-lactonization reactions: synthesis of highly substituted chiral spirodihydrocoumarins

A general approach to asymmetric synthesis of highly substituted spirodihydrocoumarins with a quaternary stereocenter was achieved through neighboring ortho-hydroxyl group induced sequential Michael-lactonization reactions on 2-(2-nitrovinyl)phenols with alkyl cyclopentanone-2-carboxylates in the presence of a catalytic amount of quinine-NH-thiourea followed by p-TSA.

Arginine mimetics using α-guanidino acids: introduction of functional groups and stereochemistry adjacent to recognition guanidiniums in peptides

Arginine residues are broadly employed for specific biomolecular recognition, including in protein-protein, protein-DNA, and protein-RNA interactions. Arginine recognition commonly exploits the potential for bidentate electrostatic and hydrogen-bonding interactions. However, in arginine residues, the guanidinium functional group is located at the terminus of a flexible hydrocarbon side chain, which lacks the functionality to contribute to specific arginine-mediated recognition and may entropically disfavor binding. In order to enhance the potential for specificity and affinity in arginine-mediated molecular recognition, we have developed an approach to the synthesis of peptides that incorporates an α-guanidino acid as a novel arginine mimetic. α-Guanidino acids, derived from α-amino acids, with guanidinylation of the amino group, were incorporated stereospecifically into peptides on solid phase via coupling of an Fmoc amino acid to diaminopropionic acid (Dap), Fmoc deprotection, guanidinylation of the amine on solid phase, and deprotection, generating a peptide containing an α-functionalized arginine mimetic. This approach was examined by incorporating arginine mimetics into ligands for the Src, Grb, and Crk SH3 domains at the site of the key recognition arginine. Protein binding was examined for peptides containing guanidino acids derived from Gly, L-Val, L-Phe, L-Trp, D-Val, D-Phe, and D-Trp. We demonstrate that paralogue specificity and target site affinity may be modulated with the use of α-guanidino acid-derived arginine mimetics, generating peptides that exhibit enhanced Src specificity by selection against Grb and peptides that reverse the specificity of the native peptide ligand, with enhancements in Src target specificity of up to 15-fold (1.6 kcal mol(-1)).