Condensation of 2-amino-2-deoxysugars with isothiocyanates. Synthesis of cis-1,2-fused glycopyrano heterocycles

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.

Electronic effects in tautomeric equilibria: the case of chiral imines from d-glucamine and 2-hydroxyacetophenones

A one-pot procedure for preparing a series of chiral imines by direct condensation of d-glucamine with 2-hydroxyacetophenones is described. Under conventional acetylation an unexpected mixture of two different peracetylated molecules is obtained, one with an open enamine structure, and the other incorporating an N-acetyl-1,3-oxazolidine into the acyclic skeleton. Surprisingly, both molecules coexist within the crystal's unit cell, as inferred from single-crystal X-ray analysis of a 5-bromo-substituted aryl derivative. Moreover, the 1,3-oxazolidine ring exists as rotational conformers (E,Z) owing to the restricted rotation around the N-acetyl bond. The equilibrium involving imine and enamine structures has been assessed in detail, providing in addition linear free-energy relationships between the tautomerization constants (K_T) and the electronic effect of the substituents.

Basic study for acyl chitosan isothiocyanates synthesis by model experiments using glucosamine derivatives

Affecting factors to the acyl chitosan isothiocyanate synthesis by N-phenylthiocarbamoylation and the following acylation was investigated using octyl 2-amino-2-deoxy-β-D-glucopyranoside as a model compound. It was found from the acetylation of N-phenylthiocarbamoyl glucosamine derivative with acetic anhydride/pyridine that the glucosamine isothiocyanate was formed via N,N-(acetyl)phenylthiocarbamoyl glucosamine derivative and the conversion of N,N-(acetyl)phenylthiocarbamoyl glucosamine derivative to the glucosamine isothiocyanate proceeded mainly by thermal degradation of N,N-(acetyl)phenylthiocarbamoyl groups. The reaction temperature was an important factor to the isothiocyanate synthesis. On the other hand, it was found from the acetylation of N-ethylthiocarbamoyl derivative (or N-allylthiocarbamoyl derivative) with acetic anhydride/pyridine that the glucosamine isothiocyanate was also prepared from N-ethylthiocarbamoyl glucoamine derivative (or N-allylthiocarbamoyl glucosamine derivative), but that N-phenylthiocarbamoylation was more preferable to the isothiocyanate synthesis than N-ethylthiocarbamoylation and N-allylthiocarbamoylation. Then, acylation products of N-phenylthiocarbamoyl chitosan (N,N-(hexanoyl)phenylthiocarbamoyl chitosan hexanoate or hexanoyl chitosan isothiocynate) could be controlled by the reaction temperature and reaction system.

Synthesis and biological evaluation of ureido and thioureido derivatives of 2-amino-2-deoxy-D-glucose and related aminoalcohols as N-acetyl-beta-D-hexosaminidase inhibitors

Ureido and thioureido derivatives of 2-acetamido-2-deoxy-beta-D-glucose, 1-amino-1-deoxy-D-glucitol and 2-(2-aminoethoxy)ethanol were prepared as N-acetyl-beta-D-hexosaminidase (NAHase) inhibitors and were evaluated on Trichomonas vaginalis NAHase. Although none showed complete inhibition of the enzyme at 100 microM, 1-amino-1-deoxy-D-glucitol derivatives acted as competitive inhibitors of the NAHase of T. vaginalis.