Sonication-assisted oligomannoside synthesis

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

Synthesis and photophysical properties of a bichromophoric system hosting a disaccharide spacer

The synthesis of an efficient energy donor-acceptor system is reported, together with its photophysical properties. The bichromophoric species has been conceived to show potentialities for biological applications since a biocompatible disaccharide spacer, constituted of d-galactose and d-glucose derivatives, was used in compound 12 to connect two BODIPY units with different absorption/emission properties. The luminescence spectrum in acetonitrile of 12 shows an intense fluorescence band with a maximum at about 770 nm that is almost identical to that of the lowest-energy BODIPY, regardless of the excitation wavelength used. The quantum yield is 0.2 with an excited state lifetime of 2.5 ns. Excitation and ultrafast transient absorption spectroscopy demonstrates that a very efficient energy transfer takes place in 12 from the highest-energy lying BODIPY subunit to the lowest-energy emissive BODIPY moiety, with a time constant of about 31 ps. Noteworthily, the emission of 12 falls in the near infrared window, suitable for potential biological applications.

Orthogonal cleavage of the 2-naphthylmethyl group in the presence of the p-methoxy phenyl-protected anomeric position and its use in carbohydrate synthesis

Orthogonal removal of naphthylmethyl (NAP) and anomeric O-p-methoxyphenyl (PMP) ethers using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and cerium(iv) ammonium nitrate, respectively, is described.

β-Mannosylation with 4,6-benzylidene protected mannosyl donors without preactivation

Mannosylations with benzylidene protected mannosyl donors were found to be β-selective even when no preactivation was performed. It was also found that the kinetic β-product in some cases anomerizes fast to the thermodynamically favored α-anomer under typical reaction conditions.

The characteristic structure of anti-HIV actinohivin in complex with three HMTG D1 chains of HIV-gp120

The anti-HIV lectin actinohivin (AH) specifically interacts with HMTG (high-mannose-type glycan), which is attached to the glycoprotein gp120 of HIV-1 in a process in which the three branched mannotriose chains (D1, D2, and D3) of HMTG exhibit different binding affinities, it being estimated that that of D1 is the strongest, that of D3 is weaker, and that of D2 is undetectable. These properties have been ascribed to the stereochemical differences in linkages between the second and the third mannose residues of the three chains. In order to clarify the interaction geometry between AH and the major target D1, an X-ray determination of the crystal structure of AH in complex with D1-which is α(1,2)mannotriose composed of three mannose (Man) residues linked together only by α(1,2) bonding-has been performed. In each of the three D1-binding pockets of AH, two Man residues of D1 are accommodated at zones 1 and 2 in the pocket, in the same way as those found in the α(1,2)mannobiose-bound AH crystals. However, an OMIT map shows poor densities at both ends of the two residues. This suggests the existence of positional disorder of D1 in the pocket: the two zones are each occupied by two Man residues in two different modes, with mode A involving the Man1 and Man2 residues and mode B the Man2 and Man3 residues. In each mode, D1 is stabilized by adopting a double-bracket-shaped conformation through CH⋅⋅⋅O interactions. In mode B, however, the Man1 residue, which is the most sensitive residue to AH binding, protrudes wholly into the solvent region without contacts with AH. In mode A, in contrast, the Man3 residue interacts with the essential hydrophobic amino acid residues (Tyr and Leu conserved between the three pockets) of AH. Therefore, mode A is likely to be the one that occurs when whole HMTG is bound. In this mode, the two hydroxy groups (O3 and O4) of the Man2 residue are anchored in zone 2 by four hydrogen bonds with Asp, Asn, and Tyr residues of AH. In addition, it has been found that an isolated water molecule buried in the hydrophobic long loop bridges between Asp of AH and the hydroxy group of Man2 through hydrogen bonds. The most interesting feature is found in the interaction of the Man1 and Man3 residues with AH. All eight hydroxy groups of the two residues are completely exposed in the solvent region, whereas their hydrophobic parts make contacts with a Leu residue and two Tyr residues so that the shape of D1 and the surface of AH fit well over a wide area. These structural characteristics are potentially useful for development of AH to produce more effective antiretroviral drugs to suppress the infectious expansion of HIV/AIDS and to help expedite an end to the HIV/AIDS pandemic in the near future.

Development of αGlcN(1↔1)αMan-based lipid A mimetics as a novel class of potent Toll-like receptor 4 agonists

The endotoxic portion of lipopolysaccharide (LPS), a glycophospholipid Lipid A, initiates the activation of the Toll-like Receptor 4 (TLR4)–myeloid differentiation factor 2 (MD-2) complex, which results in pro-inflammatory immune signaling. To unveil the structural requirements for TLR4·MD-2-specific ligands, we have developed conformationally restricted Lipid A mimetics wherein the flexible βGlcN(1→6)GlcN backbone of Lipid A is exchanged for a rigid trehalose-like αGlcN(1↔1)αMan scaffold resembling the molecular shape of TLR4·MD-2-bound E. coli Lipid A disclosed in the X-ray structure. A convergent synthetic route toward orthogonally protected αGlcN(1↔1)αMan disaccharide has been elaborated. The α,α-(1↔1) linkage was attained by the glycosylation of 2-N-carbamate-protected α-GlcN-lactol with N-phenyl-trifluoroacetimidate of 2-O-methylated mannose. Regioselective acylation with (R)-3-acyloxyacyl fatty acids and successive phosphorylation followed by global deprotection afforded bis- and monophosphorylated hexaacylated Lipid A mimetics. αGlcN(1↔1)αMan-based Lipid A mimetics (α,α-GM-LAM) induced potent activation of NF-κB signaling in hTLR4/hMD-2/CD14-transfected HEK293 cells and robust LPS-like cytokines expression in macrophages and dendritic cells. Thus, restricting the conformational flexibility of Lipid A by fixing the molecular shape of its carbohydrate backbone in the “agonistic” conformation attained by a rigid αGlcN(1↔1)αMan scaffold represents an efficient approach toward powerful and adjustable TLR4 activation.

β-Selective mannosylation with a 4,6-silylene-tethered thiomannosyl donor

Mannosylations using the new conformationally restricted donor phenyl 2,3-di-O-benzyl-4,6-O-(di-tert-butylsilylene)-1-thio-α-D-mannopyranoside (6) have been found to be β-selective with a variety of activation conditions. The simplest activation conditions were NIS/TfOH, in which case it is proposed that the β-mannoside is formed from β-selective glycosylation of the oxocarbenium ion 25 in a B(2,5) conformation.

Alternative energy input: mechanochemical, microwave and ultrasound-assisted organic synthesis

Microwave, ultrasound, sunlight and mechanochemical mixing can be used to augment conventional laboratory techniques. By applying these alternative means of activation, a number of chemical transformations have been achieved thereby improving many existing protocols with superior results when compared to reactions performed under traditional conditions. The purpose of this critical review is to highlight the advances in this general area by presenting such newer applications in organic synthesis (175 references).

Synthesis and structural verification of the xylomannan antifreeze substance from the freeze-tolerant Alaskan beetle Upis ceramboides

Tetra-, hexa-, and octasaccharide subunits of the [→4)-β-D-Manp-(1→4)-β-D-Xylp-(1→](n) xylomannan motif proposed as the structure of a novel nonprotein, thermal hysteresis-producing antifreeze substance from the freeze-tolerant Alaskan beetle Upis ceramboides have been accessed by total chemical synthesis. Comparison of their NMR spectral data with data of the isolate lends strong support to the proposed structure. Synthetic tetrasaccharides representing various linkage isomers considered (α- rather than β-manno, and linkage through mannose O3 rather than O4) show more significant chemical shift differences with the isolate and are therefore excluded from further consideration.

Pre-activation based stereoselective glycosylations: Stereochemical control by additives and solvent

Stereochemical control is an important issue in carbohydrate synthesis. Glycosyl donors with participating acyl protective groups on 2-O have been shown to give 1,2-trans glycosides reliably under the pre-activation based reaction condition. In this work, the effects of additives and reaction solvents on stereoselectivity were examined using donors without participating protective groups on 2-O. While several triflate salt additives did not have major effects, the amount of AgOTf was found to significantly impact the reaction outcome. Excess AgOTf led to lower stereochemical control presumably due to its coordination with the glycosyl triflate intermediate and a more S(N)1 like reaction pathway. In contrast, the stereoselectivity could be directed by reaction solvents, with diethyl ether favoring the formation of α glycosides and dichloromethane leading to β isomers. The trend of stereochemical dependence on reaction solvent was applicable to a variety of building blocks including the selective formation of β-mannosides.