Peter J. Duggan

Selective fructose transport through supported liquid membranes containing diboronic acid or conjugated monoboronic acid-quaternary ammonium carriers

Abstract The design and preparation of two new classes of boronic acid carriers is described along with an evaluation of their abilities to extract and transport the commercially important sugars, fructose, glucose, and sucrose through polymer supported liquid membranes. Transport fluxes with diboronic acid carriers are lower than those observed with monoboronic acids. However, fructose selectivity is improved when the diboronic linker group allows the formation of a 1:1 macrocyclic complex. A conjugated monoboronic acid-quaternary ammonium carrier facilitates fructose transport about twenty times better than an analogous monoboronic acid/quaternary ammonium mixture. The rate-determining step for the transport is diffusion through the membrane.

Competitive transport of reducing sugars through a lipophilic membrane facilitated by aryl boron acids

The extraction and competitive transport of fructose, glucose and sucrose through dichloroethane facilitated by combinations of aryl boron acids and tetraalkyl ammonium salts is described. Although extraction abilities of the boron acids are comparable, there are distinct differences in their sugar transport properties. The aqueous solubility of the aryl boron acid is apparently a critical controlling factor of sugar flux. A combination of PBA and aliquat® 336 in the membrane effectively separates fructose from an equimolar mixture of fructose, glucose and sucrose, despite extraction experiments displaying comparable extraction of glucose and fructose.

Mechanism of facilitated saccharide transport through plasticized cellulose triacetate membranes

Abstract Mechanistic insight is gained for saccharide transport through plasticized cellulose triacetate (CTA) membranes containing lipophilic ion-pair transport carriers. The molecular structures of the different membrane components are systematically varied and diagnostic transport characteristics such as saccharide–carrier diffusion constant and saccharide extraction constant are determined. The observed percolation thresholds support a jumping mechanism, however, the diffusion constants are found to decrease as the size of the saccharide, carrier cation, and carrier anion increase, indicating that the rate-limiting step in the transport process involves diffusion of a complex comprised of all three components. The data is reconciled in terms of mobile-site jumping mechanism where the saccharide is relayed along a sequence of ion-pair carriers that are locally mobile. In an attempt to improve saccharide selectivity, calix-[4]-arene dicarboxylates were evaluated as potential ditopic transport carriers. This produced no major change in saccharide extraction constants.

Nucleotide carrier mixture with transport selectivity for ribonucleoside-5′-phosphates

A carrier mixture of lipophilic boronic acid and bis(quaternary ammonium cation) greatly facilitates the transport of ribonucleoside monophosphates through liquid organic membranes, at neutral pH. Transport selectivity was determined for the isomers of GMP. The observed order of non-competitive transport rates was 5′-GMP > 2′-GMP > 3′-GMP.

Fructose-Permeable Liquid Membranes Containing Boronic Acid Carriers

Liquid membranes that contain boronic acids have potential application in environmentally benign industrial D-fructose production. This review describes our efforts to develop boronic acid carriers that promote high fluxes, that are resistant to leaching, and that are highly selective for fructose over other sugars. Considerable progress has been made with multidentate boronic acid carriers. Initial attempts to transport D-fructose as macrocyclic β-D-fructopyranose diesters appeared to suffer from competitive transport of macrocyclic α-D-glucofuranose diesters, and did not lead to high D-fructose selectivity. On the other hand, carriers that can bind multiple equivalents of D-fructose as tridentate β-D-fructofuranose esters have been much more effective. Unfortunately, D-fructose selectivity is still apparently limited by a competitive and boronic acid-independent non-selective ‘mobile fixed site relay’ transport process. However, further progress will be possible through improved carrier design, careful investigation of the various stages of the transport process, and the use of more industrially useful membrane configurations.

The quasi-homo-anomeric interaction in substituted tetrahydropyranyl radicals: Diastereoselectivity

Abstract The isomer distribution of products from the reaction of a range of simple cyclohexyl and tetrahydropyranyl radicals ( 3 – 7 ) with Bu 3 SnD and with allyltributyltin, has been determined in order to gauge the influence of the classical anomeric effect and the quasi-homo-anomeric effect on stereoselectivity. In these unencumbered radicals, both anomeric effects caused a strong preference for trans deuterated products, but no significant stereoselectivity was observed for the allylation reactions. The difference between the two types of reaction suggests that deuteration is mainly under kinetic control whereas allylation has a greater tendency for thermodynamic control.

ω-Conotoxin GVIA Mimetics that Bind and Inhibit Neuronal Cav2.2 Ion Channels

The neuronal voltage-gated N-type calcium channel (Cav2.2) is a validated target for the treatment of neuropathic pain. A small library of anthranilamide-derived ω-Conotoxin GVIA mimetics bearing the diphenylmethylpiperazine moiety were prepared and tested using three experimental measures of calcium channel blockade. These consisted of a 125I-ω-conotoxin GVIA displacement assay, a fluorescence-based calcium response assay with SH-SY5Y neuroblastoma cells, and a whole-cell patch clamp electrophysiology assay with HEK293 cells stably expressing human Cav2.2 channels. A subset of compounds were active in all three assays. This is the first time that compounds designed to be mimics of ω-conotoxin GVIA and found to be active in the 125I-ω-conotoxin GVIA displacement assay have also been shown to block functional ion channels in a dose-dependent manner.

Enhanced Anti-Fungal Activity of the Organo-Soluble Borate Ester, Tetra-n-butylammonium Bis(ortho-hydroxymethylphenolato)borate

The borate ester, tetra-n-butylammonium bis(ortho-hydroxymethylphenolato)borate, NBu4[B(o-hmp)2], was synthesized and characterized by NMR spectroscopy (1H, 13C, 11B), ES-MS, and X-ray crystallography. The anti-fungal activity of this compound, as well as its sodium salt, the parent phenol, tetra-n-butylammonium bromide, and boric acid were evaluated against two wood decay fungi. The tetraalkylammonium borate NBu4[B(o-hmp)2] shows the highest activity out of the compounds examined. This finding suggests that the formation of lipophilic borate esters is a promising approach for the development of leach-resistant, borate-based wood preservatives.

Boron acids as protective agents and catalysts in synthesis

Covering: 1978–2000. Previous review: R. J. Ferrier, Adv. Carbohydr. Chem. Biochem., 1978, 35, 31–80.

The mechanism of the β-acyloxyalkyl radical rearrangement. Part 2: β-acyloxytetrahydropyranyl radicals

The rate of rearrangement of the 3-butanoyloxytetrahydropyran-2-yl radical (4) to give the product 5 of 1,2-migration of the acyloxy group has been determined by competition against the reaction of 4 with tributylstannane. The rate constant is larger than that for rearrangement of the acylic radical 18 but less than that for the substituted cholestanyl radical 19. Experiments with 17O- and 18O-labelled substrates indicate that the rearrangement of 4 proceeds with ca. 33% transposition of the ether and carbonyl oxygen atoms, while there is also a small amount of scrambling in the product 12 formed by direct reduction of 4. The isotope labelling studies and the Arrhenius parameters (log [A/s–1]= 12.7, Eact= 58 kJ mol–1) are consistent with the view that the reaction proceeds, at least in part, via dissociation into a tight anion radical-cation pair 22. A pathway via the three-membered transition structure 21 might also be involved.