Enantioselective Diels-Alder Reactions with G-Quadruplex DNA-Based Catalysts

Chalcones, a Privileged Scaffold: Highly Versatile Molecules in [4+2] Cycloadditions

Chalcones are aromatic ketones found in nature as the central core of many biological compounds. They have a wide range of biological activity and are biogenetic precursors of other important molecules such as flavonoids. Their pharmacological relevance makes them a privileged scaffold, advantageous for seeking alternative therapies in medicinal chemistry. Due to their structural diversity and ease of synthesis, they are often employed as building blocks for chemical transformations. Chalcones have a carbonyl conjugated system with two electrophilic centers that are commonly used for nucleophilic additions, as described in numerous articles. They can also participate in Diels-Alder reactions, which are [4+2] cycloadditions between a diene and a dienophile. This microreview presents a chronological survey of studies on chalcones as dienes and dienophiles in Diels-Alder cycloadditions. Although these reactions occur in nature, isolation of chalcones from plants yields very small quantities. Contrarily, synthesis leads to large quantities at a low cost. Hence, novel methodologies have been developed for [4+2] cycloadditions, with chalcones serving as a 2π or 4π electron system.

Harnessing DNA as a Designable Scaffold for Asymmetric Catalysis: Recent Advances and Future Perspectives

Since the first report of DNAzyme by in vitro selection in 1994, catalytic DNA has investigated extensively, and their application has expanded continually in virtue of rapid advances in molecular biology and biotechnology. Nowadays, DNA is in the second prime time by way of DNA-based hybrid catalysts and DNA metalloenzymes in which helical chirality of DNA serves to asymmetric catalysis. DNA-based hybrid catalysts are attractive system to respond the demand of the times to pursuit green and sustainable society beyond traditional catalytic systems that value reaction efficiency. Herein, we highlight the recent advances and perspective of DNA-based hybrid catalysts with various aspects of DNA as a versatile scaffold for asymmetric synthesis. We hope that scientists in a variety of fields will be encouraged to join and promote remarkable evolution of this interesting research.

Switchable DNA-based Peroxidases Controlled by a Chaotropic Ion

Here we demonstrate a switchable DNA electron transfer catalyst, enabled by selective destabilization of secondary structure by a denaturant, perchlorate. The system is comprised of two strands, one of which can be selectively switched between a G-quadruplex and duplex or single-stranded conformations. In the G-quadruplex state, it binds hemin, enabling peroxidase activity. This switching ability arises from our finding that perchlorate, a chaotropic Hofmeister ion, selectively destabilizes duplex over G-quadruplex DNA. By varying perchlorate concentration, we show that the DNA structure can be switched between states that do and do not catalyze electron transfer catalysis. State switching can be achieved in three ways: thermally, by dilution, or by concentration.

Highly Efficient Cyclic Dinucleotide Based Artificial Metalloribozymes for Enantioselective Friedel-Crafts Reactions in Water

The diverse secondary structures of nucleic acids are emerging as attractive chiral scaffolds to construct artificial metalloenzymes (ArMs) for enantioselective catalysis. DNA-based ArMs containing duplex and G-quadruplex scaffolds have been widely investigated, yet RNA-based ArMs are scarce. Here we report that a cyclic dinucleotide of c-di-AMP and Cu²⁺ ions assemble into an artificial metalloribozyme (c-di-AMP⋅Cu²⁺ ) that enables catalysis of enantioselective Friedel-Crafts reactions in aqueous media with high reactivity and excellent enantioselectivity of up to 97 % ee. The assembly of c-di-AMP⋅Cu²⁺ gives rise to a 20-fold rate acceleration compared to Cu²⁺ ions. Based on various biophysical techniques and density function theory (DFT) calculations, a fine coordination structure of c-di-AMP⋅Cu²⁺ metalloribozyme is suggested in which two c-di-AMP form a dimer scaffold and the Cu²⁺ ion is located in the center of an adenine-adenine plane through binding to two N7 nitrogen atoms and one phosphate oxygen atom.

Enantioselective Catalysis by Using Short, Structurally Defined DNA Hairpins as Scaffold for Hybrid Catalysts

A new type of DNA metal complex hybrid catalyst, which is based on single-stranded DNA oligonucleotides, is described. It was shown that oligonucleotides as short as 14 nucleotides that fold into hairpin structures are suitable as nucleic acid components for DNA hybrid catalysts. With these catalysts, excellent enantioinduction in asymmetric Diels-Alder reactions with selectivity values as high as 96 % enantiomeric excess (ee) can be achieved. Molecular dynamics simulations indicate that a rather flexible loop combined with a rigid stem region provides DNA scaffolds with these high selectivity values.

Design of Surface-Active Artificial Enzyme Particles to Stabilize Pickering Emulsions for High-Performance Biphasic Biocatalysis

Surface-active artificial enzymes (SAEs) are designed and constructed by a general and novel strategy. These SAEs can simultaneously stabilize Pickering emulsions and catalyze biphasic biotransformation with superior enzymatic stability and good re-usability; for example, for the interfacial conversion of hydrophobic p-nitrophenyl butyrate into yellow water-soluble p-nitrophenolate catalyzed by esterase-mimic SAE.

Tuning the stereoselectivity of a DNA-catalyzed michael addition through covalent modification

Complexes of G-quadruplex DNA and Cu(II) ions have previously been applied as catalysts in asymmetric reactions, but the largely unspecific and noncovalent nature of the interaction has impeded understanding of the structural basis of catalysis. To better control the formation of a catalytically competent species, DNA quadruplexes were derivatized with linker-bpy-Cu(II) complexes in a site-specific manner and applied in asymmetric aqueous Michael additions. These modified quadruplexes exhibited high rate acceleration and stereoselectivity. Different factors were found to be important for the catalytic performance of the modified G-quadruplexes, among them, the position of modification, the topology of the quadruplex, the nature of the ligand, and the length of the linker between the ligand and DNA. Moving the same ligand by just two nucleotides inverted the stereochemical outcome: quadruplexes modified at position 10 formed the (-)-enantiomer with up to 92 % ee, while DNA derivatized at position 12 formed the (+)-enantiomer with up to 75 % ee. This stereopreference was maintained when applied to structurally different Michael acceptors. This work demonstrates a new and simple way to tune the stereoselectivity in DNA-based asymmetric catalysis.

Octahedral Chiral-at-Metal Iridium Catalysts: Versatile Chiral Lewis Acids for Asymmetric Conjugate Additions

Octahedral iridium(III) complexes containing two bidentate cyclometalating 5-tert-butyl-2-phenylbenzoxazole (IrO) or 5-tert-butyl-2-phenylbenzothiazole (IrS) ligands in addition to two labile acetonitrile ligands are demonstrated to constitute a highly versatile class of asymmetric Lewis acid catalysts. These complexes feature the metal center as the exclusive source of chirality and serve as effective asymmetric catalysts (0.5-5.0 mol % catalyst loading) for a variety of reactions with α,β-unsaturated carbonyl compounds, namely Friedel-Crafts alkylations (94-99% ee), Michael additions with CH-acidic compounds (81-97% ee), and a variety of cycloadditions (92-99% ee with high d.r.). Mechanistic investigations and crystal structures of an iridium-coordinated substrates and iridium-coordinated products are consistent with a mechanistic picture in which the α,β-unsaturated carbonyl compounds are activated by two-point binding (bidentate coordination) to the chiral Lewis acid.

Conformationally constrained cyclic peptides: powerful scaffolds for asymmetric catalysis

Cyclic peptides containing a disulfide bridge were identified as a simple and versatile coordination sphere for asymmetric catalysis. Upon complexation with Cu(2+) ions they catalyze Diels-Alder and Friedel-Crafts reactions with high enantioselectivities of up to 99% ee and 86% ee, respectively. Moreover, the peptides ligands were systematically optimized with the assistance of "Alanine Scanning". This biomolecular design could greatly expand the choice of peptide scaffolds for artificial metallopeptide catalysts.

Expanding the potential of G-quadruplex structures: formation of a heterochiral TBA analogue

In order to expand the potential applications of G-quadruplex structures, we explored the ability of heterochiral oligodeoxynucleotides based on the thrombin-binding aptamer (TBA) sequence to fold into similar complexes, with particular focus on their resistance in biological environments. A combination of CD and NMR techniques was used. Similarly to TBA, the ODN ggTTggtgtggTTgg (lower case letters indicate L residues) is able to fold into a chair-like antiparallel G-quadruplex structure, but has a slightly higher thermal stability. The discovery that heterochiral ODNs are able to form stable G-quadruplex structures opens up new possibilities for their development in several fields, as aptamers, sensors and, as recently shown, as catalysts for enantioselective reactions.