Fully deprotected beta-(1 -> 2)-mannotetraose forms a contorted alpha-helix in solution: convergent synthesis and conformational characterization by NMR and DFT

beta-(1 -> 2)-linked oligomannosides, found in the cell wall of Candida albicans, are promising structures for the development of C. albicans vaccines. Considerable effort in recent years has been devoted to the synthesis of these carbohydrate structures. As a result, several successful synthetic methodologies based on linear approaches have emerged. Here, we demonstrate that a fully deprotected beta-(1 -> 2)-linked mannotetrasaccharide can also be conveniently constructed by the convergent direct coupling of two appropriately protected disaccharides. This improved approach offers several advantages over previously published methods. The number of steps needed to reach larger oligosaccharides is decreased, while high selectivity is retained when the crucial glycosylation step is performed with two disaccharides, herein providing beta-(1 -> 2)-mannotetraose in 4.2% overall yield over 16 steps starting from D-mannose. Additionally, the complete structural characterization of the products by NMR spectroscopy is reported. In the conformational study of the final product, 2D-NOESY was used in combination with spectral simulations performed using PERCH software. The experimental results obtained confirm the contorted alpha-helical structure predicted earlier for these oligosaccharides in solution. As a culmination of the conformational study, a model was constructed by molecular modeling using DFT, and the minimum energy conformation was found to be in full agreement with the experimental results. This is the first study to date where the conformation of a fully deprotected mannotetraose has been reported.