The reverse anomeric effect: further observations on N-glycosylimidazoles

A True Reverse Anomeric Effect Does Exist After All: A Hydrogen Bonding Stereocontrolling Effect in 2-Iminoaldoses

The reverse anomeric effect is usually associated with the equatorial preference of nitrogen substituents at the anomeric center. Once postulated as another anomeric effect with explanations ranging from electrostatic interactions to delocalization effects, it is now firmly considered to be essentially steric in nature. Through an extensive research on aryl imines from 2-amino-2-deoxyaldoses, spanning nearly two decades, we realized that such substances often show an anomalous anomeric behavior that cannot easily be rationalized on the basis of purely steric grounds. The apparent preference, or stabilization, of the β-anomer takes place to an extent that not only neutralizes but also overcomes the normal anomeric effect. Calculations indicate that there is no stereoelectronic effect opposing the anomeric effect, resulting from the repulsion between electron lone pairs on the imine nitrogen and the endocyclic oxygen. Such data and compelling structural evidence unravel why the exoanomeric effect is largely inhibited. We are now confident, as witnessed by 2-iminoaldoses, that elimination of the exo-anomeric effect in the α-anomer is due to the formation of an intramolecular hydrogen bond between the anomeric hydroxyl and the iminic nitrogen, thereby accounting for a true electronic effect. In addition, discrete solvation may help justify the observed preference for the β-anomer.

On the anomeric preference of the isothiocyanato group

Anomeric effect of the isothiocyanato group has been quantified for the first time in xylopyranose triacetates; both anomers being synthesized.

Mechanisms of Stereodirecting Participation and Ester Migration from Near and Far in Glycosylation and Related Reactions

This review is the counterpart of a 2018 Chemical Reviews article (Adero, P. O.; Amarasekara, H.; Wen, P.; Bohé, L.; Crich, D. Chem. Rev. 2018, 118, 8242-8284) that examined the mechanisms of chemical glycosylation in the absence of stereodirecting participation. Attention is now turned to a critical review of the evidence in support of stereodirecting participation in glycosylation reactions by esters from either the vicinal or more remote positions. As participation by esters is often accompanied by ester migration, the mechanism(s) of migration are also reviewed. Esters are central to the entire review, which accordingly opens with an overview of their structure and their influence on the conformations of six-membered rings. Next the structure and relative energetics of dioxacarbeniun ions are covered with emphasis on the influence of ring size. The existing kinetic evidence for participation is then presented followed by an overview of the various intermediates either isolated or characterized spectroscopically. The evidence supporting participation from remote or distal positions is critically examined, and alternative hypotheses for the stereodirecting effect of such esters are presented. The mechanisms of ester migration are first examined from the perspective of glycosylation reactions and then more broadly in the context of partially acylated polyols.

The Problem of Origins and Origins of the Problem: Influence of Language on Studies Concerning the Anomeric Effect

Cause and effect: In science, language is often subordinated to empirical data, but words can influence our ability to understand and communicate science. This Essay argues that imprecise language has confounded studies that probe the interactions underlying the anomeric effect and related phenomena.

Stereoselective glycosylations of 2-azido-2-deoxy-glucosides using intermediate sulfonium ions

TMSOTf-promoted glycosylations of 2-azido-2-deoxy-glucosyl trichloroacetimidates provide excellent alpha-anomeric selectivities when performed at a relatively high reaction temperature in the presence of PhSEt or thiophene. NMR and computation studies have shown that these glycosylations proceed through an equatorial anomeric sulfonium ion, which upon displacement by a sugar alcohol provides an axial glycoside. Computational studies have indicated that steric factors determine the selective formation of the beta-anomeric sulfonium ion.

Absence of Reverse Anomeric Effect in Furanosides

A series of conformationally restricted N-"furanosides" has been synthesized, where the carbons of the tetrahydrofuran ring are kept in one plane by a rigid norbornane skeleton, permitting only the ring oxygen to move above or below the tetrahydrofuran ring plane. This causes the substituents of the anomeric carbon to occupy a pseudoaxial or a pseudoequatorial position. On protonation of these "norbornane-furanosides" with trifluoromethanesulfonic acid, all three compounds exhibited decreasing coupling constants for the anomeric proton, indicating a shift toward the pseudoaxial conformation. The coupling constant measurements were supported by volume integration of NOESY cross-peaks, which also showed a change toward the pseudoaxial conformation upon protonation of the nitrogen. These results provide no evidence for the so-called reverse anomeric effect; on the contrary they are in full agreement with a small normal anomeric effect.

New class of quaternary ammonium salts, derivatives of methyl d-glucopyranosides

Reactions of two aromatic and two aliphatic amines with methyl 6-O-p-toluenesulfonyl-alpha-D-glucopyranoside or methyl 6-O-p-toluenesulfonyl-beta-D-glucopyranoside were performed on a micro-scale. The synthesis and preparative isolation methods have been developed for quaternary N-(methyl 2,3,4-tri-O-acetyl-6-deoxy-alpha- and -beta-D-glucopyranoside-6-yl)ammonium salts derived from three amines: trimethylamine, 2-methylpyridine, and pyridine. The reaction products were examined with 1H, 13C NMR spectroscopy. N-(Methyl 2,3,4-tri-O-acetyl-6-deoxy-beta-d-glucopyranoside-6-yl)trimethylammonium tosylate was additionally analyzed with X-ray crystallography.

Conformational preferences in glycosylamines. implications for the exo-anomeric effect

The conformational preferences about the C-N bond in N-(4-methoxyphenyl)-2,3,4,6-tetra-O-acetyl-alpha (1) and beta-D-glucopyranosylamine (2), in the solid state and in solution, have been investigated. The crystal structure of the axially substituted alpha anomer (1) indicates a conformational preference about the C-1-N bond in which nN-->sigma*C-O exo-anomeric interactions may be expressed, although this conformational preference is not displayed in solution. The solution conformation relieves steric interactions that result from expression of the exo-anomeric effect in the solid-state conformation. The conformational preference in the equatorially substituted beta anomer (2) both in solution and in the solid state is similar and permits expression of nN-->sigma*C-O exo-anomeric interactions. The structural data for 1 and 2 indicate significant differences in O-5-C-1-N-1 bond angles but insignificant differences in each of the O-5-C-1 or C-1-N-1 bond lengths. The J(C-1-H-1 coupling constants in 1 and 2 indicate a greater coupling constant for the alpha anomer that is consistent with a dominant nO-->sigma*C-H orbital interaction in the beta anomer that weakens the C-1-H-1 bond.

Is there a generalized reverse anomeric Effect? substituent and solvent effects on the configurational equilibria of neutral and protonated N-arylglucopyranosylamines and N-aryl-5-thioglucopyranosylamines

The effects of substitution and solvent on the configurational equilibria of neutral and protonated N-(4-Y-substituted-phenyl) peracetylated 5-thioglucopyranosylamines (Y = OMe, H, CF(3), NO(2)) 1-4 and N-(4-Y-substituted-phenyl) peracetylated glucopyranosylamines (Y = OMe, H, NO(2)) 9-11 are described. The configurational equilibria were determined by direct integration of the resonances of the individual isomers in the (1)H NMR spectra after equilibration of both alpha- and beta-isomers. The equilibrations of the neutral compounds 1-4 in CD(3)OD, CD(3)NO(2), and (CD(3))(2)CO were achieved by HgCl(2) catalysis and those of the neutral compounds 9-11 in CD(2)Cl(2) and CD(3)OD by triflic acid catalysis. The equilibrations of the protonated compounds in both the sulfur series (solvents, CD(3)OD, CD(3)NO(2), (CD(3))(2)CO, CDCl(3), and CD(2)Cl(2)) and oxygen series (solvents, CD(2)Cl(2) and CD(3)OD) were achieved with triflic acid. The substituent and solvent effects on the equilibria are discussed in terms of steric and electrostatic effects and orbital interactions associated with the endo-anomeric effect. A generalized reverse anomeric effect does not exist in neutral or protonated N-aryl-5-thioglucopyranosylamines and N-arylglucopyranosylamines. The anomeric effect ranges from 0.85 kcal mol(-)(1) in 2 to 1.54 kcal mol(-)(1) in 10. The compounds 1-4 and 9-11 show an enhanced endo-anomeric effect upon protonation, ranging from 1.73 kcal mol(-)(1) in 2 to 2.57 kcal mol(-)(1) in 10. We estimate the increase in the anomeric effect upon protonation of 10 to be approximately 1.0 kcal mol(-)(1). However, this effect is offset by steric effects due to the associated counterion which we estimate to be approximately 1.2 kcal mol(-)(1). The values of K(eq)(axial-equatorial) in protonated 1-4 increase in the order OMe < H < CF(3) < NO(2), in agreement with the dominance of steric effects (due to the counterion) over the endo-anomeric effect. The values of K(eq)(axial-equatorial) in protonated 9-11 show the trend OMe > H < NO(2) that is explained by the balance of the endo-anomeric effect and steric effects in the individual compounds. The trends in the values of the C(1)-H(1) coupling constants for 1-4 and the corresponding deacetylated compounds 5-8 as a function of substituent and alpha- or beta-configuration are discussed in terms of the Perlin effect and the interplay of the endo- and exo-anomeric effects.

Synthesis and structural studies of asparagine-modified 2-deoxy-alpha-N-glycopeptides associated with the renin-angiotensin system

Following addition of N-iodosuccinimide to glycals, reductive hydrogenolysis and ring opening gave 2-deoxy-alpha-N-glycopeptides carrying a deaminated asparagine unit. This reaction could be performed employing glucal, galactal, L-rhamnal, L-fucal and lactal to give the corresponding glycoconjugate building blocks 11, 12, 17, 22, 27 and 32. Further NIS-mediated glycosylation of the rhamno derivative 21 led to simple trisaccharide peptide adducts 45. Peptide synthesis of the gluco building unit with different preassembled oligopeptides afforded glycoconjugates 36, 39, 41 and 42 assumed to be of interest as potential inhibitors of the renin-angiotensin system.