Ruthenium‐Catalysed Epimerisation of Carbohydrate Alcohols as a Method to Determine the Equilibria for Epimer Interconversion in Hexopyranosides

Abstract

Aromaticity lost: In the presence of [{Ir(cod)Cl}2] and a binol‐derived phosphoramidite ligand, spirocyclohexadienone derivatives were obtained with up to 97 % ee through iridium‐catalyzed intramolecular asymmetric allylic dearomatization of phenols (see scheme; cod=cycloocta‐1,5‐diene).

Iridium-catalysed condensation of alcohols and amines as a method for aminosugar synthesis

“…Here again the alkylation reaction with cyclohexanol (2a) proceeded efficiently to give the secondary amine products for all of the different configurations tested. Products due to redox epimerisation 16,17 or amination of the unprotected secondary alcohol groups were not detected. Benzyl alcohol (2b) could also be used as an alkylating reagent, affording b-Glc-7b in excellent yield.…”

Epimerization of Glycal Derivatives by a Cyclopentadienylruthenium Catalyst: Application to Metalloenzymatic DYKAT

“…The epimerization of glycals 4 and 8 with cyclopentadienylrutheium catalyst 1, studied by NMR spectroscopic methods, was found to be significantly more efficient than that of saturated carbohydrates. [15] The epimerization rates varied with the substitution pattern of the glycals; the 4,6-di-O-benzylidene-protected compounds (4 and 5) reacted faster than the 4-Obenzyl-6-deoxy compounds (8 and 9). Thanks to the efficacy of the epimerization, which occurred at ambient temperatures, the combination of the epimerization protocol with an enzymatic resolution was possible, resulting in a DYKAT.…”

“…The epimerization of glycals 4 and 8 with cyclopentadienylrutheium catalyst 1 , studied by NMR spectroscopic methods, was found to be significantly more efficient than that of saturated carbohydrates 15. The epimerization rates varied with the substitution pattern of the glycals; the 4,6‐di‐ O ‐benzylidene‐protected compounds ( 4 and 5 ) reacted faster than the 4‐ O ‐benzyl‐6‐deoxy compounds ( 8 and 9 ).…”

“…[13,14] Transition metal-catalyzed epimerization of secondary alcohols in carbohydrates, utilizing catalyst 1, has been studied previously. [15] By subjecting partially protected glucose, mannose, and allose derivatives to activated 1 in p-xylene at 120 8C, it was possible to determine the thermodynamic equilibrium between the non-anomeric epimers of the carbohydrates. Typically, catalyst 1 is able to epimerize secondary alcohols at ambient temperature at a rapid rate; however in the case of these protected carbohydrates, where the secondary alcohol is in close proximity to several electron-withdrawing groups the oxidation step of the racemization is much harder to achieve.…”

“…Transition metal‐catalyzed epimerization of secondary alcohols in carbohydrates, utilizing catalyst 1 , has been studied previously 15. By subjecting partially protected glucose, mannose, and allose derivatives to activated 1 in p ‐xylene at 120 °C, it was possible to determine the thermodynamic equilibrium between the non‐anomeric epimers of the carbohydrates.…”