NAD/NADH models with axial/central chiralities: superiority of the quinoline ring system

Redox- and hypoxia-responsive MRI contrast agents

The development of responsive or "smart" magnetic resonance imaging (MRI) contrast agents that can report specific biomarker or biological events has been the focus of MRI contrast agent research over the past 20 years. Among various biological hallmarks of interest, tissue redox and hypoxia are particularly important owing to their roles in disease states and metabolic consequences. Herein we review the development of redox-/hypoxia-sensitive T1 shortening and paramagnetic chemical exchange saturation transfer (PARACEST) MRI contrast agents. Traditionally, the relaxivity of redox-sensitive Gd(3+) -based complexes is modulated through changes in the ligand structure or molecular rotation, while PARACEST sensors exploit the sensitivity of the metal-bound water exchange rate to electronic effects of the ligand-pendant arms and alterations in the coordination geometry. Newer designs involve complexes of redox-active metal ions in which the oxidation states have different magnetic properties. The challenges of translating redox- and hypoxia-sensitive agents in vivo are also addressed.

The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction

In biological reduction processes the dihydronicotinamides NAD(P)H often transfer hydride to an unsaturated substrate bound within an enzyme active site. In many cases, metal ions in the active site bind, polarize and thereby activate the substrate to direct attack by hydride from NAD(P)H cofactor. This review looks more widely at the metal coordination chemistry of organic donors of hydride ion--organo-hydrides--such as dihydronicotinamides, other dihydropyridines including Hantzsch's ester and dihydroacridine derivatives, those derived from five-membered heterocycles including the benzimidazolines and benzoxazolines, and all-aliphatic hydride donors such as hexadiene and hexadienyl anion derivatives. The hydride donor properties--hydricities--of organo-hydrides and how these are affected by metal ions are discussed. The coordination chemistry of organo-hydrides is critically surveyed and the use of metal-organo-hydride systems in electrochemically-, photochemically- and chemically-driven reductions of unsaturated organic and inorganic (e.g. carbon dioxide) substrates is highlighted. The sustainable electrocatalytic, photochemical or chemical regeneration of organo-hydrides such as NAD(P)H, including for driving enzyme-catalysed reactions, is summarised and opportunities for development are indicated. Finally, new prospects are identified for metal-organo-hydride systems as catalysts for organic transformations involving 'hydride-borrowing' and for sustainable multi-electron reductions of unsaturated organic and inorganic substrates directly driven by electricity or light or by renewable reductants such as formate/formic acid.

Deracemization of axially chiral nicotinamides by dynamic salt formation with enantiopure dibenzoyltartaric acid (DBTA)

Dynamic atroposelective resolution of chiral salts derived from oily racemic nicotinamides and enantiopure dibenzoyltartaric acid (DBTA) was achieved by crystallization. The absolute structures of the axial chiral nicotinamides were determined by X-ray structural analysis. The chirality could be controlled by the selection of enantiopure DBTA as a chiral auxiliary. The axial chirality generated by dynamic salt formation was retained for a long period after dissolving the chiral salt in solution even after removal of the chiral acid. The rate of racemization of nicotinamides could be controlled based on the temperature and solvent properties, and that of the salts was prolonged compared to free nicotinamides, as the molecular structure of the pyridinium ion in the salts was different from that of acid-free nicotinamides.

Europium(III) DOTA-tetraamide complexes as redox-active MRI sensors

PARACEST redox sensors containing the NAD(+)/NADH mimic N-methylquinolinium moiety as a redox-active functional group have been designed and synthesized. The Eu(3+) complex with two quinolinium moieties was nearly completely CEST-silent in the oxidized form but was "turned on" upon reduction with β-NADH. The CEST effect of the Eu(3+) complex containing only one quinolinium group was much less redox-responsive but showed an unexpected sensitivity to pH in the physiologically relevant pH range.

Consideration of molecular arrangements in regio- and enantioselective reduction of an NAD model compound controlled by carbonyl oxygen orientation

The regio- and enantioselectivity of the reduction of an NAD model compound having axial chirality with respect to the C(3)(quinolinium)-C(carbonyl) bond, 3-piperidinylcarbonyl-1,2,4-trimethylquinolinium ion (1), by using several reducing agents is described. Reaction of 1 with sodium hydrosulfite affords the 1,4-reduced product, 3-piperidinylcarbonyl-1,2,4-trimethyl-1,4-dihydroquinoline (), with low enantioselectivity, whereas sodium borohydride promotes 1,2-reduction, affording 3-piperidinylcarbonyl-1,2,4-trimethyl-1,2-dihydroquinoline () as the sole product in a moderate enantioselectivity. When 1 was reduced by the chiral NADH model compound, 2,4-dimethyl-3-(N-alpha-methylbenzylcarbamoyl)-1-propyl-1,4-dihydropyridine (Me(2)PNPH (4)), the regioselectivity and enantioselectivity of the reaction were significantly altered by the stereochemistry of 1 and 4. An achiral NADH model compound, 1-propyl-1,4-dihydronicotinamide (PNAH (5)) exhibited both high regio- and enantioselectivities. The product selectivity reflects the change in molecular arrangement in the transition state of the reaction and reveals the relative importance of the parameters governing the molecular arrangement in the reaction.

A reductase-mimicking thiourea organocatalyst incorporating a covalently bound NADH analogue: efficient 1,2-diketone reduction with in situ prosthetic group generation and recycling

A new class of bifunctional organocatalyst promotes the chemoselective reduction of diketone electrophiles at catalytic loadings in the presence of an inorganic co-reductant.

Carbonyl orientation determines regio- and enantioselectivity in 1,2-/1,4-reduction of an NAD model compound

An optically active, axially chiral NAD model compound(1) with a quinoline ring system was reduced by the chiral NADH model compound (4), affording a mixture of 1,2- and 1,4-dihydroquinolines. The carbonyl orientation governs the molecular arrangement in the transition state of the reaction and determines the regio- and enantioselectivity of the product.

Studies on the 4-benzoylpyridine-3-carboxamide entity as a fragment model of the Isoniazid-NAD adduct

An ortho-metallation-electrophilic substitution sequence was employed as a key step to build the 4-benzoylpyridine framework. It was found that 4-benzoylpyridine-3-carboxamide and an N-pyridyl alkylated derivative both exist in a unique cyclized hemiamidal structure, not in the usually expected keto-amide open form. These structures represent fragment models of the Isoniazid-NAD adducts involved in the mechanism of action of the antituberculous drug Isoniazid.

AM1 CI and ZINDO/S study of quaternary salts of diazaphenanthrenes with haloalkanes

For quaternary salts of diazaphenanthrenes 1-3 with methyl iodide 4-6, diiodomethane 7-9 and 1,2-dibromoethane 10-12 UV spectral values have been calculated by AM1 CI, and in the case of 4-6 also with ZINDO/S method. Correlations of experimental and calculated wavenumber values of considered compounds show good compatibility. For geometry optimisation of 4-9 the AM1 CI method has been used.