Unexpected fluorous solvent effect on oxidation of 1-thioglycosides

Acid Catalyzed Stereocontrolled Ferrier-Type Glycosylation Assisted by Perfluorinated Solvent

Described herein is the first application of perfluorinated solvent in the stereoselective formation of O-/S-glycosidic linkages that occurs via a Ferrier rearrangement of acetylated glycals. In this system, the weak interactions between perfluoro-n-hexane and substrates could augment the reactivity and stereocontrol. The initiation of transformation requires only an extremely low loading of resin-H+ and the mild conditions enable the accommodation of a broad spectrum of glycal donors and acceptors. The 'green' feature of this chemistry is demonstrated by low toxicity and easy recovery of the medium, as well as operational simplicity in product isolation.

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.

Selective oxidation of glycosyl sulfides to sulfoxides using magnesium monoperoxyphthalate and microwave irradiation

A protocol that uses moist magnesium monoperoxyphthalate (MMPP) as an oxidant under microwave irradiation rapidly yields a variety of glycosyl sulfoxides from corresponding sulfides in high yields with high selectivity.

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.