An organocatalytic strategy for the stereoselective synthesis of C-galactosides with fluorine at the pseudoanomeric carbon

Pseudo-glycoconjugates with a C-glycoside linkage

Work by the author and colleagues has been focused on the development of pseudo-glycans (pseudo-glycoconjugates), in which the O-glycosidic linkage of the natural-type glycan structure is replaced by a C-glycosidic linkage. These analogs are not degraded by cellular glycoside hydrolases and are thus expected to be useful molecular tools that may maintain the original biological activity for a long period in the cell. However, their biological potential is not yet well understood because only a few pseudo glycans have so far been synthesized. This article aims to provide a bird's-eye view of our recent studies on the creation of C-glycoside analogs of ganglioside GM3 based on the CHF-sialoside linkage, and summarizes the chemical insights acquired during our stereoselective synthesis of the C-sialoside bond, ultimately leading to pseudo-GM3. Conformational analysis of the synthesized CHF-sialoside disaccharides confirmed that the anticipated conformational control by F-atom introduction was successful, and furthermore, enhanced the biological activity. In order to improve access to C-glycoside analogs based on pseudo-GM3, it is still important to streamline the synthesis process. With this in mind, we designed and developed a direct C-glycosylation method using atom-transfer radical coupling, and employed it in syntheses of pseudo-isomaltose and pseudo-KRN7000.

De Novo Synthesis of Orthogonally-Protected C2-Fluoro Digitoxoses and Cymaroses: Development and Application for the Synthesis of Fluorinated Digoxin

Inspired by Roush's pioneering work on rare sugars, we have developed a scalable, stereoselective, de novo synthesis of orthogonally protected C2-fluoro digitoxose and cymarose, utilizing Sharpless kinetic resolution and organocatalytic fluorination as key steps. The utility of this strategy is demonstrated by the synthesis of a fluorinated analogue of digoxin, which indicates the fluorine on the sugar ring may have a significant impact on biological activity.

Copper Catalyzed Decarboxylative Functionalization of Ketoacids

Selective copper catalyzed activation of ketoacids and notably bio-sourced 1,3-acetonedicarboxylic acid, represents an attractive strategy to solve key synthetic challenges. Condensation with aldehydes under exceedingly mild conditions can create more rapidly known natural products scaffolds such as 1,3 polyols. In this account, the recent progress in this field, notably through multicatalytic combination with organocatalysis is described. In addition to the rapid preparation of natural product fragments, cascade incorporation of fluorine also provided new type of synthetic analogues of improved properties in a broad range of applications.

Regioselective Synthesis of Difluorinated C-Furanosides Involving a Debenzylative Cycloetherification

A highly regioselective synthesis of valuable gem-difluorinated C-furanosides from unprotected aldoses via a debenzylative cycloetherification (DBCE) reaction induced by diethylaminosulfur trifluoride is descibed. The scope and limitations of this DBCE reaction are described using a series of commercially available pentoses and hexoses to afford, without selective protection/deprotection sequences, the corresponding gem-difluorinated C-furanosides in moderate to good yields.

The Crotylation Way to Glycosphingolipids: Synthesis of Analogues of KRN7000

A synthesis of glycosphingolipids that centers on the reaction of O- and C-glycosyl crotylstannanes and relatively simple lipid aldehydes is described. The modularity of this strategy and versatility of the crotylation products make this an attractive approach to diverse, highly substituted libraries. The methodology is applied to analogues of the potent imunostimulatory glycolipid KRN7000, including O-, methylene-, and fluoromethine-linked isosteres with diastereomeric ceramide segments and 2-amido substitutes.

Diastereoselective Synthesis and Conformational Analysis of (2R)- and (2S)-Fluorostatines: An Approach Based on Organocatalytic Fluorination of a Chiral Aldehyde

Stereoselectively fluorinated analogues of the amino acid statine have been efficiently synthesized. The key step is an organocatalytic electrophilic fluorination of a chiral β-oxygenated aldehyde, which provided a test of both diastereoselectivity and chemoselectivity. The target statine analogues were found to adopt unique conformations influenced by the fluorine gauche effect, rendering them potentially valuable building blocks for incorporation into bioactive peptides.