Sulfur atom configuration of sulfinyl galactofuranosides determines different reactivities in glycosylation reactions

Facile Diastereoseparation of Glycosyl Sulfoxides by Chiral Stationary Phase

Separation of the diastereomers of glycosyl sulfoxides differing in the sulfur chirality has been difficult. This article presents a fast and scalable method for their diastereoseparation using a chiral stationary phase. The usefulness of this method was demonstrated in a 500-mg scale separation within 20 min, and in the separation of trisaccharyl sulfoxide diastereomers. Chirality 28:534-539, 2016. © 2016 Wiley Periodicals, Inc.

Studies on the diastereoselective oxidation of 1-thio-β-D-glucopyranosides: synthesis of the usually less favoured R(S) sulfoxide as a single diastereoisomer

A detailed study on the diastereoselective oxidation of 1-thio-β-D-glucopyranosides is reported. It has been shown that the sense and the degree of stereochemical outcome of the oxidation are highly dependent on the substituent of the sulfur and on the protective group of the C2-OH. In the case of thioglycosides with a bulky aglycone, the mesylation of C2-OH has a significant effect on the stereochemical outcome of the oxidation, affording the usually less favoured RS sulfoxide as a single diastereoisomer. The absolute configuration of the final sulfinyl glycosides was ascertained by NMR analysis and corroborated by X-ray crystallography.

In a glycosylation reaction how does a hydroxylic nucleophile find the activated anomeric carbon?

The mechanism by which nucleophilic hydroxyls are attracted to activated glycopyranosyl donors is not known. Besides the intrinsic attraction of oxygen centred negative dipoles towards the developing electron deficiency at the anomeric carbon only a few suggestions have been given in the literature. By studying the effect on Density Functional Theory (DFT) modelled glycosylation reactions on the presence of polar additives as tested with acetonitrile two possible effects have been identified. One was noted in a previous publication (Carbohydr. Res.2012, 356, 180-190) and two further examples discovered here that suggest that a lone pair of a nucleophile approaching a donor with a β-leaving group from the α-face can act as the antiperiplanar lone pair that assists leaving group departure. This interaction starts at just under a nucleophile C-1 separation of 3Å and has an incipient bond angle of O-5-C-1-Nuc(O or N) of very close to 90° which can be at C-1 with the p-type orbital at C-1-O-5 of the incipient oxacarbenium ion, that is, the LUMO of the activated donor. The 2nd interaction is less well studied and is suggested to be a similar bonding interaction which moves β-face nucleophiles to O-Nuc-C-1-leaving groups angles close to 180°.

Configurations and conformations of glycosyl sulfoxides

X-ray crystallographic studies of two axial glycosyl sulfoxides having RS configurations (derivatives of phenyl 2-azido-2-deoxy-1-thio-α-d-galactopyranoside S-oxide) show that they adopt anti conformations in the solid state, in contrast to previous observations and assumptions. Density functional theory (DFT) calculations at the B3lYP6–311G+(d,p)/6–31G(d) level confirm that anti conformations of both phenyl and methyl RS glycosyl sulfoxides of 2-azido-2-deoxy-α-d-pyranosides are more stable than exo-anomeric conformations in the gas phase. 1D NOE measurements indicate that the more polar exo-anomeric conformers are only populated to a slight extent in solution. The anti conformations are distorted so that the glycosyl substituents are closer to being eclipsed with H1. This distortion allows S n → σ* overlap if the sulfur lone pair is a p-type lone pair. Evidence for this overlap comes from short C1–S bond distances, as short as the comparable bond distances in the X-ray crystal structure and in the results from DFT calculations for the SS glycoside, which does adopt the expected exo-anomeric conformation, both in the solid state and in solution, and has normal n → σ* overlap. For 2-deoxy derivatives not bearing a 2-azido group, gas-phase DFT calculations at the same level indicate that the anti- and exo-anomeric conformers have comparable stabilities. Comparison of the results of the two series shows that electronegative substituents in equatorial orientations at C2 destabilize conformations with parallel S–O arrangements, the conformation favored by having an endocyclic C–O dipole antiparallel to the S–O dipole, by about 2.5 kcal mol–1 (1 cal = 4.184 J). An equatorial glycosyl sulfoxide, (SS) phenyl 3,4,6-tri-O-acetyl-2-deoxy-2-phthalimido-1-thio-β-d-glucopyranoside S-oxide, also adopts an anti conformation in the solid state as shown by X-ray diffraction. It also adopts this conformation in solution, in contrast to studies of other equatorial glycosyl sulfoxides.

Highly diastereoselective oxidation of 2-amino-2-deoxy-1-thio-beta-D-glucopyranosides: synthesis of imino sulfinylglycosides

A synthetic route to imino thioglycosides and to imino sulfinylglycosides has been developed. A detailed study on the diastereoselective oxidation of 2-amino-2-deoxy-1-thio-beta-D-glucopyranosides is reported. It has been shown that the stereochemical outcome of the oxidation is highly dependent on the protective group of the amine function. While the oxidation of iminothioglycosides is slightly diastereoselective (up to 30% de in favor of the R(S) sulfoxide), a single isomer is obtained in the case of tetrachlorophthalimido derivatives. The absolute configuration of the sulfinyl glycoside was ascertained by NMR analysis using our recent model on the basis of the exo-anomeric effect corroborated by X-ray crystallography.