A Free-Energy Relationship between the Rate of Acidic Hydrolysis of Glycosides and the pKa of Isofagomines

Abstract

Eine neue Antwort auf eine hundert Jahre alte Frage – warum Galactoside schneller hydrolysiert werden als Glucoside – wird hier gegeben. Dieses unterschiedliche Verhalten wurde lange darauf zurückgeführt, dass axiale OH‐Gruppen Konformationsänderungen auf dem Weg zum Übergangszustand erleichtern. Doch die hier vorgestellten Ergebnisse sprechen dafür, dass Unterschiede in der elektronenziehenden Wirkung (Pfeile) äquatorialer (A) und axialer OH‐Gruppen (B) entscheidend sind.

Superarmed and Superdisarmed Building Blocks in Expeditious Oligosaccharide Synthesis

“…It appears that there is a certain inconsistency between this result and the comprehensive programmable approach which showed benzylidene derivatives to be more reactive than their disarmed counterparts. Although a direct investigation of these two findings is not yet available, studies reported by Bols [63] offer the explanation that the disparity could be simply explained by the benzylidene orientation; axial –galactose (Wong et al) vs equatorial – glucose (Demchenko et al).…”

“…A model study of the relative pK a values for protonated heterocyclic amines showed that equatorial substituents are significantly more deactivating than their axial counterparts (compare 67 and 68 in Fig. 5) [63]. Further revealed by these findings was that a perturbation of the equilibrium between ring conformations may also occur upon protonation of the heterocyclic amine [68].…”

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

“…This reflects the higher ability of the electron withdrawing equatorial substituent to destabilize a positive charge at the anomeric position, as extensively investigated by the Bols group. 431,432 Scheme 128. Formation of 1,3-difluorinated 2-azidohexopyranoses based on a DAST-induced 1®6 migration process.…”

Automated Chemical Oligosaccharide Synthesis: Novel Approach to Traditional Challenges

“…In addition, over the recent years there has been a noticeable shift in focus of the mechanistic glycosylation chemistry field toward studying stereoelectronics and conformation of the starting material and key reaction intermediates. ,,,,,,,,,,,− While the stereoelectronic and conformational effects on reactivity have been studied extensively, the impact of these effects on stereoselectivity remains elusive. Although some model studies helped to establish general trends, ,,− ,, practical application of the conformational factors to stereocontrol of glycosylation is still limited.…”