A Mechanistic Probe into 1,2-cis Glycoside Formation Catalyzed by Phenanthroline and Further Expansion of Scope.

Abstract

Phenanthroline, a rigid and planar compound with two fused pyridine rings, has been used as a powerful ligand for metals and a binding agent for DNA/RNA. We discovered that phenanthroline could be used as a nucleophilic catalyst to efficiently access high yielding and diastereoselective α-1,2-cis glycosides through the coupling of hydroxyl acceptors with α-glycosyl bromide donors. We have conducted an extensive investigation into the reaction mechanism, wherein the two glycosyl phenanthrolinium ion intermediates, a 4C1 chair-liked β-conformer and a B2,5 boat-like α-conformer, have been detected in a ratio of 2:1 (β:α) using variable temperature NMR experiments. Furthermore, NMR studies illustrate that a hydrogen bonding is formed between the second nitrogen atom of phenanthroline and the C1-anomeric hydrogen of sugar moiety to stabilize the phenanthrolinium ion intermediates. To obtain high α-1,2-cis stereoselectivity, a Curtin-Hammett scenario was proposed wherein interconversion of the 4C1 chair-like β-conformer and B2,5 boat-like α-conformer is more rapid than nucleophilic addition. Hydroxyl attack takes place from the α-face of the more reactive 4C1 β-phenanthrolinium intermediate to give an α-anomeric product. The utility of the phenanthroline catalysis is expanded to sterically hindered hydroxyl nucleophiles and chemoselective coupling of an alkyl hydroxyl group in the presence of a free C1-hemiacetal. In addition, the phenanthroline-based catalyst has a pronounced effect on site-selective couplings of triol motifs and orthogonally activates the anomeric bromide leaving group over the anomeric fluoride and sulfide counterparts.