Direct Experimental Characterization of a Bridged Bicyclic Glycosyl Dioxacarbenium Ion by ¹ H and ¹³ C NMR Spectroscopy: Importance of Conformation on Participation by Distal Esters.

ABSTRACT: Low-temperature NMR studies with a 4-C-methyl-4-O-benzoyl galactopyranosyl donor enable the observation and characterization of a bridged bicyclic dioxacarbenium ion arising from participation by a distal ester. Variable-temperature NMR studies reveal this bridged ion to decompose at temperatures above ≈-30 °C. In the absence of the methyl group, the formation of a bicyclic ion is not observed. It is concluded that participation by typical secondary distal esters in glycosylation reactions is disfavored in the ground state conformation of the ester from which it is stereoelectronically impossible. Methylation converts the secondary ester to a conformationally more labile tertiary ester, removes this barrier, and renders participation more favorable. Nevertheless, the minor changes in selec

SUBMITTER: Upadhyaya K 

PROVIDER: S-EPMC8595841 | biostudies-literature | 2021 Nov

REPOSITORIES: biostudies-literature