Chiral Catalyst-Directed Dynamic Kinetic Diastereoselective Acylation of Anomeric Hydroxyl Groups and a Controlled Reduction of the Glycosyl Ester Products

Abstract

A catalytic method is developed for the diastereoselective acylation of the anomeric hydroxyl group in diverse carbohydrates to form either α- or β-anomeric esters. While exclusive formation of the β-isomer was observed in most sugar substrates with one enantiomer of the chiral catalyst, moderate to high α-selectivity was obtained by using the other enantiomer of the chiral catalyst. The resulting α- and β-anomeric esters have very different reactivity toward a reduction reaction.

Phosphorylation Organocatalysts Highly Active by Design

“…Recently, n –cation interactions between an oxygen lone pair and a positively charged heterocyclic moiety of an organocatalytic intermediate were implicated by Tang and co-workers as the key selectivity-determining factor in site-selective acylation of carbohydrates and enantioselective dynamic kinetic acylation of 2-chromanols . Furthermore, a recent review demonstrated that this paradigm of directing n –cation noncovalent interactions was also responsible for inducing diastereoselectivity in dynamic kinetic acylation of anomeric hydroxyls in carbohydrates and their precursors, reported earlier by the same group …”

Site-Divergent Delivery of Terminal Propargyls to Carbohydrates by Synergistic Catalysis

“…Considering that the axial hydroxyls in a carbohydrate scaffold are in general less reactive than the equatorial ones, it is remarkable this reaction could override such reactivity bias and modify the axial 2-OH of 4 selectively. In 2017, the Tang group 47,50,51 reported a catalyst-controlled, site-divergent acylation of carbohydrates with benzotetramisole-based catalysts 52,53 (11 to 12 or 12 0 via 14; Figure 1D). It is proposed that the favorable interaction between the lone pair electrons on OR 0 of the nucleophile and the cationic center in the electrophile (cf.…”

Carbene‐Catalyzed Dynamic Kinetic Resolution and Asymmetric Acylation of Hydroxyphthalides and Related Natural Products

“…[13] Tang and co-workers reported the dynamic kinetic diastereoselective acylation of lactols through chiral isothiourea catalytic reactions. [14] We recently disclosed a carbene-catalyzed enantioselective modification of carboxylic acids for asymmetric access to optically enriched acetal products (Figure 1 b). [15] Here we disclose a new approach for enantiomeric access to acylated hydroxyphthalides by a carbene-catalyzed dynamic kinetic resolution (DKR) process (Figure 1 b).…”