We utilized the indium-mediated allylation reaction for the synthesis of carbohydrate structures containing the 3-deoxy-2-ulose motif, a barely investigated compound class. The stereoselective outcome can be controlled by the presence or absence of a chelating group in α-position to the carbonyl function. By introduction of an UV-active allyl building block, we enabled epimer separation by HPLC towards the synthesis of 3-deoxy-d-glycero-d-galacto-2-nonulose, the carboxylreduced analogue of widely distributed 3-deoxy-d-glycero-d-galacto-nonulosonic acid (Kdn). Ozonolysis of the introduced 2-C-methylidenepropan-1-ol motif provided the desired 3-deoxy-2-uloses.
The indium-mediated allylation reaction has been applied to melibiose, a disaccharidic substrate. This elongation methodology allows for a short, efficient and diastereoselective approach towards complex glycosylated carbohydrate structures. The stereochemical outcome of the key intermediates, allylated disaccharides, has been determined by X-ray analysis. Ozonolysis of the introduced double bond yielded the unprotected elongated disaccharides in the equilibrium of the pyranoid as well as furanoid isomers in both anomeric forms, respectively. Per-O-acetylation has been performed to facilitate separation of the isomeric mixture for structural identification. The main product revealed to adopt a β-pyranoid form of the elongated unit at the reducing end of the disaccharide.
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