Conformations in solution of the stereoisomeric, peracetylated aldohexose dimethyl acetals and diethyl dithioacetals

Add time:07/23/2019    Source:sciencedirect.com

The conformations in solution of acyclic carbohydrate derivatives having four contiguous asymmetric centers in all eight diastercoisomeric forms have been studied by 1H-n.m.r. spectroscopy. The 250-MHz, 1H-n.m.r. spectra for solutions in chloroform-d of eight penta-O-acetylaldohexose dimethyl acetals, and the corresponding diethyl dithioacetals, furnished a complete set of chemical shifts and proton-proton spin-couplings that are interpreted in terms of conformational compositions at room temperature. The galacto and manno derivatives adopt planar, extended conformations, whereas the other six stereoisomers all adopt one or more non-extended (“sickle”) conformations. The results are interpreted on the basis of the avoidance of parallel 1,3-interactions of substituents. The conformational assignments are correlated with observations made previously for aldopentose analogs. An assessment is made of the extent to which valid conformational predictions may be advanced for four-center, and longer, asymmetrically substituted chains, based on observations made for shorter-chain analogs.

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