Synthesis and Crystal Structure of the Azoxydichinyl Helicene, Pyrido[3,2-f]quinolino[6,5-c]cinnoline 5-Oxide Monohydrate

Crystal structure of 3,13-dimethoxy-5,6,10,11-tetrahydrofuro[3,4-i][5]helicene-7,9-dione

In the crystal, the mol­ecules form a layered structure parallel to (10) via C—H⋯O hydrogen-bonding inter­actions. Adjacent layers are also linked by C—H⋯O hydrogen bonds, forming a three-dimensional structure.

The title compound, C₂₆H₂₀O₅, crystallizes with two independent molecules (A and B) in the asymmetric unit, which differ primarily in the location of the –OCH₃ groups. In the crystal, the mol­ecules form a layered structure parallel to (10-1) by C—H⋯O hydrogen-bonded networks. Adjacent layers are also linked by further C—H⋯O hydrogen bonds, forming a three-dimensional structure.

3,12-Dimeth-oxy-5,6,9,10-tetra-hydro-[5]helicene-7,8-dicarbo-nitrile

The complete molecule of the title compound, C₂₆H₂₀N₂O₂, is generated by a crystallographic twofold axis. The torsion angle between the terminal and central benzene rings is −32.5 (2)°. The torsion angle along the inner helical rim of the molecule is −18.8 (2)° with each other. The C⋯C distance between the terminal rings is 3.016 (2) Å. In the crystal, weak C—H⋯N hydrogen bonds are observed.

Hypoxia-selective, enzymatic conversion of 6-nitroquinoline into a fluorescent helicene: pyrido[3,2-f]quinolino[6,5-c]cinnoline 3-oxide

Regions of low oxygen concentration (hypoxia) occur in both normal human physiology and under pathophysiological conditions. Fluorescent probes for the direct imaging of cellular hypoxia could be useful tools that complement radiochemical imaging and immunohistochemical staining methods. In this work, we set out to characterize the hypoxia-selective enzymatic metabolism of a simple nitroaryl probe, 6-nitroquinoline (1). We envisioned that this compound might undergo hypoxia-selective, bioreductive conversion to the fluorescent product, 6-aminoquinoline (2). The probe 1 was, indeed, converted to a fluorescent product selectively under hypoxic conditions by the one-electron reducing enzyme NADPH:cytochrome P450 reductase. However, inspection of the fluorescence spectrum and LC-MS analysis of the reaction mixture revealed that the expected product 2 was not formed. Rather, the 63-fold increase in fluorescence emission at 445 nm resulting from the hypoxic metabolism of 1 was due to formation of the azoxy-helicene product, pyrido[3,2-f]quinolino[6,5-c]cinnoline 3-oxide (4). The generation of 4 involves an unusual biaryl bond formation under reductive conditions. The mechanism of this process remains uncertain but could proceed via combination of a nitroaryl radical anion with a neutral nitrosoaryl radical, followed by tautomerization and intramolecular condensation between the resulting hydroxylamine and nitroso functional groups. Bioreductive metabolism of nitroaryl compounds represents a promising strategy for the selective delivery of cytotoxic agents and fluorescent markers to hypoxic tissue, but the results described here provide an important glimpse of the chemical complexity that can be associated with the enzymatic one-electron reduction of nitroaryl compounds.