Chirality-switchable, 4-aminopyridine-based, pseudo-enantiomeric helicenes can catalyze enantiodivergent Steglich rearrangement in up to 91% ee (R) and 94% ee (S), respectively.
Sampling the conformational space of mono-saccharides with first-principle methods is important as a database of local minima provides a solid base for interpreting experimental measurements such as Infrared photo-dissociation (IRPD)...
Understanding of the mechanism in collision-induced dissociation (CID) of mono-saccharides with density functional theory (DFT) is a challenging task because of many possible reaction paths originated from its high structural...
2,3]-Wittig rearrangements of sugar-derived dihydropyran allyl propargyl ethers located at the 2-or 4-position have been studied as useful means for extending the carbon chains of the 4-or 2-position with chirality transfer. The stereochemical course of these reactions depends on the following factors: (1) deprotonation of pro-R or pro-S-H, (2) equilibration of the lithiated stereogenic carbanion, (3) conformational inversion during the rearrangement, and (4) concerted [2,3]-or [1,2]-Wittig rearrangement. In some cases, a stepwise mechanism that involves the allyl-C−O bond cleavage is shared as the first step by both the [2,3]-and [1,2]-Wittig rearrangements. The stereochemical courses of the rearrangements are compared among the lithiated reactants to determine the reaction pathways. These mechanisms in the polyoxygenated dihydropyran ring system were further supported by DFT calculations.
The Nicholas reaction is a proven and versatile tool in glycoside chemistry. Its application is less clear-cut, if the intermediate Nicholas cation has two reactive sides that enable different reactions. Density functional theory (DFT) calculations are used to rationalize the observed difference in product yields for the Nicholas epimerization and substitution obtained from experiments with very similar glycosides. Substituents, which can increase the rigidity of the backbone, should be avoided in epimerization reactions as they can slow down reorientation of the intramolecular ion pair so that the substitution reaction dominates.
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