3-Aminodeoxypyranoses in Glycosylation: Diversity-Oriented Synthesis and Assembly in Oligosaccharides

Ferrier/Aza-Wacker/Epoxidation/Glycosylation (FAWEG) Sequence to Access 1,2-Trans 3-Amino-3-deoxyglycosides

3-Amino-3-deoxyglycosides constitute an essential class of nitrogen-containing sugars. Among them, many important 3-amino-3-deoxyglycosides possess a 1,2-trans relationship. In view of their numerous biological applications, the synthesis of 3-amino-3-deoxyglycosyl donors giving rise to a 1,2-trans glycosidic linkage is thus an important challenge. Even though glycals are highly polyvalent donors, the synthesis and reactivity of 3-amino-3-deoxyglycals have been little studied. In this work, we describe a new sequence, involving a Ferrier rearrangement and subsequent aza-Wacker cyclization that allows the rapid synthesis of orthogonally protected 3-amino-3-deoxyglycals. Finally a 3-amino-3-deoxygalactal derivative was submitted for the first time to an epoxidation/glycosylation with high yield and great diastereoselectivity, highlighting FAWEG (Ferrier/Aza-Wacker/Epoxidation/Glycosylation) as a new approach to access 1,2-trans 3-amino-3-deoxyglycosides.

Automated, Multistep Continuous-Flow Synthesis of 2,6-Dideoxy and 3-Amino-2,3,6-trideoxy Monosaccharide Building Blocks

An automated continuous flow system capable of producing protected deoxy-sugar donors from commercial material is described. Four 2,6-dideoxy and two 3-amino-2,3,6-trideoxy sugars with orthogonal protecting groups were synthesized in 11-32 % overall yields in 74-131.5 minutes of total reaction time. Several of the reactions were able to be concatenated into a continuous process, avoiding the need for chromatographic purification of intermediates. The modular nature of the experimental setup allowed for reaction streams to be split into different lines for the parallel synthesis of multiple donors. Further, the continuous flow processes were fully automated and described through the design of an open-source Python-controlled automation platform.

From Glycals to Nitrogen Heterocycles and Carbocycles via "Cleavage-Intramolecular Recombination Strategy"

Glycals (carbohydrate enol-ethers) have enjoyed profound applications in organic synthesis for more than a century. They not only serve as versatile glycosyl donors or as substrates for Ferrier rearrangement, but also find extensive synthetic applications especially as a "chiral pool" for accomplishing the synthesis of a variety of natural and biologically important compounds. As cyclic enol ethers, they demonstrate high reactivity and are among the most and variously transformable monosaccharide derivatives. The uniqueness of the reactivity of glycals is that they can be synthetically tuned to get a library of derivatives through stereo- and regioselective introduction of a variety of functional groups at C1, C2, C3 as well as C4 carbons of the sugar. We have developed a practical approach for stereoselective mono- and diamination of glycals and over the years utilized these scaffolds for the synthesis of a variety of biologically important nitrogen heterocycles and carbocycles through a "Diversity Oriented Approach". Our synthetic strategy in this direction mainly relied on the cleavage of ring O-C bond of the sugar followed by an "intramolecular recombination" reaction. Utilizing this strategy, we have accomplished the synthesis of several biologically important natural products, their analogues and related unnatural derivatives. Examples of such compounds reported from our group include polyhydroxypyrrolidines, DMDP, anisomycin, steviamine, pochonicine, conduramines, bulgecinine, aminocyclitols, azepanes, 4-hydroxy-D-proline, azanucleosides and their analogues. A personal account highlighting these syntheses is presented here.

Reactivity-Stereoselectivity Mapping for the Assembly of Mycobacterium marinum Lipooligosaccharides

The assembly of complex bacterial glycans presenting rare structural motifs and cis-glycosidic linkages is significantly obstructed by the lack of knowledge of the reactivity of the constituting building blocks and the stereoselectivity of the reactions in which they partake. We here report a strategy to map the reactivity of carbohydrate building blocks and apply it to understand the reactivity of the bacterial sugar, caryophyllose, a rare C12-monosaccharide, containing a characteristic tetrasubstituted stereocenter. We mapped reactivity-stereoselectivity relationships for caryophyllose donor and acceptor glycosides by a systematic series of glycosylations in combination with the detection and characterization of different reactive intermediates using experimental and computational techniques. The insights garnered from these studies enabled the rational design of building blocks with the required properties to assemble mycobacterial lipooligosaccharide fragments of M. marinum.