2 research outputs found
1,3-<i>syn</i>-Diaxial Repulsion of Typical Protecting Groups Used in Carbohydrate Chemistry in 3‑<i>O</i>‑Substituted Derivatives of Isopropyl d‑Idopyranosides
The
strength of 1,3-<i>syn</i>-diaxial repulsion was
evaluated for main types of protecting groups (alkyl, silyl, and acyl)
usually used in carbohydrate chemistry. As molecular probes for this
study, derivatives of isopropyl 2-<i>O</i>-benzyl-4,6-<i>O</i>-benzylidene-α-d-idopyranoside bearing allyl,
acetyl, and <i>tert</i>-butyldiphenylsilyl (TBDPS) protecting
groups at O-3 were prepared from <i>p</i>-methoxyphenyl d-galactopyranoside. The equilibrium between <sup><i>O</i></sup><i>S</i><sub>2</sub> and <sup>4</sup><i>C</i><sub>1</sub> conformations in these compounds was investigated using <sup>3</sup><i>J</i><sub>H,H</sub> and <sup>3</sup><i>J</i><sub>C,H</sub> coupling constants that were determined from 1D <sup>1</sup>H NMR and 2D <i>J</i>-resolved HMBC spectra in various
solvents. The analysis of the corresponding coupling constants calculated
using DFT/B3LYP/pcJ-1 approximation applied to conformations optimized
at DFT/B3LYP/6-311++G** level supported the investigation. Proportions
of conformers in the equilibrium revealed the highest repulsion between
the 3-allyloxy group and the isopropoxy aglycon and its dependence
on the solvent polarity. Differences in the conformational behavior
of 3-<i>O</i>-allyl and 3-<i>O</i>-acetyl-α-d-idopyranoside derivatives complied with the notion that higher
electron density on O-3 increased 1,3-<i>syn</i>-diaxial
repulsion. 3-<i>O</i>-TBDPS derivative existed mainly in <sup>4</sup><i>C</i><sub>1</sub> conformation. The attenuation
of the 1,3-<i>syn</i>-diaxial repulsive interaction indicates
that TBDPS has stereoelectronic properties that may have significance
in context of fixing unnatural pyranoside conformation with the help
of silyl groups but have been disregarded until now
Combination of 3‑<i>O</i>‑Levulinoyl and 6‑<i>O</i>‑Trifluorobenzoyl Groups Ensures α‑Selectivity in Glucosylations: Synthesis of the Oligosaccharides Related to <i>Aspergillus fumigatus</i> α‑(1 → 3)‑d‑Glucan
Stereospecific
α-glucosylation of primary and secondary
OH-group
at carbohydrate acceptors is achieved using glucosyl N-phenyl-trifluoroacetimidate (PTFAI) donor protected with an electron-withdrawing
2,4,5-trifluorobenzoyl (TFB) group at O-6 and the participating levulinoyl
(Lev) group at O-3. New factors have been revealed that might explain
α-stereoselectivity in the case of TFB and pentafluorobenzoyl
(PFB) groups at O-6. They are of conformational nature and confirmed
by DFT calculations. The potential of this donor, as well as the orthogonality
of TFB and Lev protecting groups, is showcased by the synthesis of
α-(1 → 3)-linked pentaglucoside corresponding to Aspergillus fumigatus α-(1 → 3)-d-glucan and of its hexasaccharide derivative, bearing β-glucosamine
residue at the non-reducing end