An Improvement in the Bending Ability of a Hinged Trisaccharide with the Assistance of a Sugar--Sugar Interaction

Ring-System-Based Conformational Switches and their Applications in Sensing and Liposomal Drug Delivery

In the past decades, a great number of stimuli-responsive systems have been developed to be used as drug-delivery systems with high sensitivity and selectivity in targeted therapy. Despite promising results, the current stimuli-responsive systems suffer from the complexity of preparation, as most novel stimuli-responsive systems are based on polymers. Small molecules have often been neglected as candidates for application for stimuli-responsive systems. Recently, structures based on six-membered ring molecules or bicyclic molecules have been developed into conformational switches working through conformational interconversion. These single conformational switches have significantly reduced the complexity of material preparation compared to polymers or copolymers. In this review, we focus on ring-system-based conformational switches that are involved in sensors and smart drug-delivery systems. We hope that this review will shed light on ring-system-based single conformational switches for use in the development of stimuli-responsive systems.

1 Introduction2 Conformation Switches Based On Bispidine Derivatives3 Conformation Switches Based On Cycloalkanes4 Conformation Switches Based On Carbohydrates5 Conclusion

Conformationally Switchable Glycosyl Donors

Glycosyl donors functionalized with 2,2'-bipyridine moieties on the 3-OH and 6-OH or the 2-OH and 4-OH undergo a conformational change when forming 1:1 complexes with Zn²⁺ ions. The pyranoside ring of the zinc complexes adopted axial-rich skew boat conformations. The reactivities of the two glycosyl donors were investigated by performing a series of glycosylations in the presence or absence of Zn²⁺ ions. These glycosylations suggested a decrease in reactivity when binding Zn²⁺. The conformational effect of binding Zn²⁺ was therefore studied using a third glycosyl donor, unable to undergo conformational changes when binding Zn²⁺. From competition experiments, it was observed that the binding-induced conformational change increased the reactivity slightly compared to the glycosyl donor unable to undergo a conformational change.

A ring-flippable sugar as a stimuli-responsive component of liposomes

For the development of a liposome that takes in and out a drug in response to stimuli, 2,4-diaminoxylose (Xyl), which allows stimuli-responsive conformational switches between (4)C1 and (1)C4, was incorporated into a lipid structure: Xyl derivatives with C8 and C16 methylene chains at the 1,3-positions (C8Xyl and C16Xyl) were synthesized. (1)H NMR spectroscopy indicates that the addition of Zn(2+) and then H(+) induces conformational switches from the chair ((4)C1) to the reverse chair ((1)C4) and (1)C4-to-(4)C1, respectively, at Xyl; this leads to transformation of the lipids between linear and bent structures. Osmotic pressure and electron microscopy studies demonstrate that C8Xyl in water forms spherical solid aggregates (C8Xyl-Zn), which are converted into liposomes (C8Xyl+Zn) upon the addition of Zn(2+), and C16Xyl forms liposomes regardless of the presence of Zn(2+). The aggregates of C8Xyl±Zn incorporated a fluorophore and only C8Xyl+Zn released the content upon the addition of HCl. This study shows that Xyl could be a stimuli-responsive component of a liposome.

A novel proton-selective sensor based on a sugar with hinge flexibility

A hinge sugar, a 2,4-diamino-2,4-dideoxy-beta-D-xylopyranoside derivative, turns its four equatorial substituents into axial orientations through a (4)C(1)-to-(1)C(4) ring flip in response to chelation to a metal ion. This hinge-like motion enables two components attached at the 1- and 3-positions to switch between far and near states. In this study, we examined the effect of N-alkylation on the bendability of the hinge molecule and synthesized a 2,4-dipyrenylmethyl derivative as a proton-selective sensor. (1)H NMR studies showed that N-alkylations of the hinge sugar facilitated (1)C(4) formation in the presence of an acid, probably because the increased basicity of the amino group promoted the intramolecular hydrogen bond between the 2- and 4-amino substituents, whereas chelation to a metal ion was hampered by the increased bulkiness. In accordance with the above results, N,N'-dipyrenylmethyl hinge sugar 3 emitted excimer fluorescence (445 nm) owing to the pyrene stacking as a result of the (1)C(4) formation at lower concentrations of trifuluoroacetic acid (TFA), while no significant changes in fluorescence spectra were observed when metal ions were added. Increase of the monomer fluorescence (375 nm) at higher TFA concentrations was also observed. These observations indicate that 3 could be used as a proton-selective sensor that covers a wide range of proton concentrations through monitoring of the two fluorescence maxima.

Computational Study of the Conformational Space of Methyl 2,4-Diacetyl-β-d-xylopyranoside: 4C1 and 1C4 Chairs, Skew-Boats (2SO, 1S3), and B3,O Boat Forms

Ring and substituent rotamer conformations of methyl 2,4-diacetyl-beta-D-xylopyranoside, for which experimental results are controversial, were studied in the gas phase and in solvents of different polarity (CCl4, CHCl3, DMSO, and H2O) by B3LYP density functional theory. The 1C4 chair is the most stable ring form in the gas phase, followed by 4C1 and 2S0. Solvents of increasing polarity shift the equilibrium toward the 4C1 chair. Homodesmotic reaction energies show that the 1C4 and 2SO forms are stabilized by hydrogen bonding and anomeric effects and that steric repulsion is smallest in the 4C1 chair and largest in skew-boats.

Exploitation of sugar ring flipping for a hinge-type tether assisting a [2 + 2] cycloaddition

Methyl 3-O-p-methoxybenzyl-beta-D-xylopyranoside (2) was exploited as a novel hinge-type tether for the [2 + 2] cycloaddition of cinnamate. The major ring conformation occupied by the 2,4-dicinnamate derivative of 2 was 4C1, which extends two cinnamates along a diequatorial orientation. However, 3-O-deprotected dicinnamate 5, when in a non-polar solvent, favours the 1C4 conformation, which assists the approach of two cinnamates with the 1,3-diaxial scaffold. Photoirradiation of compound at 313 nm in CHCl3 afforded the intramolecular cycloaddition of cinnamates to give methyl beta-, delta-, and xi-truxinates in a 86 : 8 : 6 ratio after transesterification with methanol. The regio- and stereoselectivities are comparable to those reported by others for tethered cinnamates. The per-deuterated dicinnamate derivative of 5 facilitated the conformation analyses of the pyranoside rings by 1H NMR, indicating that all the products of photoirradiation had 1C4-fixed pyranosides. Excellent beta-selectivity was achieved when m-bromocinnamate was subjected to hinge-tethered [2 + 2] cycloaddition.