Selective preparation of tetrasubstituted fluoroalkenes by fluorine-directed oxetane ring-opening reactions

The selective ring-opening reaction of fluoroalkylidene-oxetanes was directed by the presence of the fluorine atom, enabling a two-step access to tetrasubstituted fluoroalkenes with excellent geometry control. Despite its small van der Waals radii electronic, rather than steric influences of the fluorine atom governed the ring-opening reaction with bromide ions, even in the presence of bulky substituents.

Review of Mutarotase in ‘Metabolic Subculture’ and Analytical Biochemistry: Prelude to 19F NMR Studies of its Substrate Specificity and Mechanism

This is the first paper in a sequential pair devoted to the enzyme mutarotase (aldose 1-epimerase; EC 5.1.3.3). Here, the broader context of the physiological role of mutarotase, among those enzymes considered to be part of ‘metabolic structure’, is reviewed. We also summarise the current knowledge about the molecular mechanism and substrate specificity of the enzyme, which is considered in the context of the binding of fluorinated glucose analogues to the enzyme’s active site. This was done as a prelude to our experimental studies of the anomerisation of fluorinated sugars by mutarotase that are described in the following paper.

Anomerisation of Fluorinated Sugars by Mutarotase Studied Using 19F NMR Two-Dimensional Exchange Spectroscopy

Five 19F-substituted glucose analogues were used to probe the activity and mechanism of the enzyme mutarotase by using magnetisation-exchange NMR spectroscopy. The sugars (2-fluoro-2-deoxy-d-glucose, FDG2; 3-fluoro-3-deoxy-d-glucose, FDG3; 4-fluoro-4-deoxy-d-glucose, FDG4; 2,3-difluoro-2,3-dideoxy-d-glucose, FDG23; and 2,2,3,3-tetrafluoro-2,3-dideoxy-d-glucose (2,3-dideoxy-2,2,3,3-tetrafluoro-d-erythro-hexopyranose), FDG2233) showed separate 19F NMR spectroscopic resonances from their respective α- and β-anomers, thus allowing two-dimensional exchange spectroscopy measurements of the anomeric interconversion at equilibrium, on the time scale of a few seconds. Mutarotase catalysed the rapid exchange between the anomers of FDG4, but not the other four sugars. This finding, combined with previous work identifying the mechanism of the anomerisation by mutarotase, suggests that the rotation around the C1–C2 bond of the pyranose ring is the rate-limiting reaction step. In addition to d-glucose itself, it was shown that all other fluorinated sugars inhibited the FDG4 anomerisation, with the tetrafluorinated FDG2233 being the most potent inhibitor. Inhibition of mutarotase by F-sugars paves the way for the development of novel fluorinated compounds that are able to affect the activity of this enzyme invitro and invivo.

Transmembrane Exchange of Fluorosugars: Characterization of Red Cell GLUT1 Kinetics Using 19F NMR

We have developed a new approach, to our knowledge, to quantify the equilibrium exchange kinetics of carrier-mediated transmembrane transport of fluorinated substrates. The method is based on adapted kinetic theory that describes the concentration dependence of the transmembrane exchange rates of two competing, simultaneously transported species. Using the new approach, we quantified the kinetics of membrane transport of both anomers of three monofluorinated glucose analogs in human erythrocytes (red blood cells) using ¹⁹F NMR exchange spectroscopy. An inosine-based glucose-free medium was shown to promote survival and stable metabolism of red blood cells over the duration of the experiments (several hours). Earlier NMR studies only yielded the apparent rate constants and transmembrane fluxes of the anomeric species, whereas we could categorize the two anomers in terms of the catalytic activity (specificity constants) of the glucose transport protein GLUT1 toward them. Differences in the membrane permeability of the three glucose analogs were qualitatively interpreted in terms of local perturbations in the bonding of substrates to key amino acid residues in the active site of GLUT1. The methodology of this work will be applicable to studies of other carrier-mediated membrane transport processes, especially those with competition between simultaneously transported species. The GLUT1-specific results can be applied to the design of probes of glucose transport or inhibitors of glucose metabolism in cells, including those exhibiting the Warburg effect.