Résumé
The deprotonation of differently substituted propargyl xylosides with s-BuLi/TMEDA followed by protonation with t-butanol at-115 °C provided a range of new axially chiral 1,3-disubstituted alkoxyallenes in a diastereoselective way. Numerous reaction parameters such as solvent, temperature or protonating agent were examined as well as protecting groups on the xyloside moiety and the influence of the substituents on the alkyne part. The configuration of the main dia-stereoisomer of 3-methyl-1-xyloside-allene was determined for the first time by single crystal X-ray diffraction analysis and nOe NMR experiments. Furthermore, DFT calculations on the propargyl/allenyl lithium intermediates formed in the course of the deprotonation reaction provided new structural insights of these complexes. The subsequent protonation process with alcohols was investigated by theoretical surface exploration, revealing the importance of the approach of the alcohol towards the lithium compounds on the reaction outcome.