Rates of methanolysis reactions of D-xylose, D-arabinose, D-lyxose, and D-ribose have been determined. It was found that methanolysis of a pentose proceeds to equilibrium through four distinguishable, competing reactions: (1) pentose → furanosides; (2) anomerization of furanosides; (3) furanosides → pyranosides; (4) anomerization of pyranosides. The glycoside compositions at equilibrium are interpreted in terms of stabilities of each of the four glycosides from each sugar as influenced by steric and ionic effects; a system of conformational analysis of furanoside rings is presented. The free energies of reaction in anomerization of pyranosides were in excellent agreement with values calculated from previously reported interaction energies in the pyranoid ring. The relative rates of the reactions were consistent with the view that non-bonded interactions in the methyl glycosides are relieved in the transition states for their interconversions.
甲醇解反应的速率确定了D-木糖、D-阿拉伯糖、D-来苏糖和D-核糖的速率。发现戊糖的甲醇解反应通过四个可区分的、竞争性反应达到平衡:(1)戊糖→呋喃糖苷;(2)呋喃糖苷的异构化;(3)呋喃糖苷→吡喃糖苷;(4)吡喃糖苷的异构化。平衡时的糖苷组成是根据每种糖的四种糖苷的稳定性来解释的,这种稳定性受到立体和离子效应的影响;提出了呋喃糖苷环的构象分析方法。吡喃糖苷异构化反应的自由能变化与先前报道的吡喃环相互作用能计算出的值非常一致。这些反应的相对速率与这种观点相符,即在甲基糖苷的过渡态中,非键合相互作用得到了缓解。