Preference for Axial <i>N</i>-Alkylation of Tetrahydro-1,3-oxazines and Hexahydropyrimidines: Manifestation of a Kinetic Anomeric Effect
作者:David A. Berges、Jianmei Fan、Sylvie Devinck、Kendall Mower
DOI:10.1021/jo991752i
日期:2000.2.1
α-anomer is ascribed principally to a kinetic anomeric effect that destabilizes the transition state for equatorial N-alkylation and formation of the β-anomer. The α-anomer is principally formed by axial N-alkylation. Reaction of meso-2,4-pentanediamine with 5-bromo-5-deoxy-d-xylose in aqueous solution gives octahydropyrido[1,2-a]pyrimidine-7,8,9-triols 16α and 17β, which cannot interconvert. In this
3-氨基-1-丙醇与 5-溴-5-脱氧-d-木糖在水溶液中反应得到异头六氢吡啶并[2,1-b][1,3]恶嗪-7,8,9-三醇2α和2β。当通过 1H NMR 监测该反应时,观察到 α-异头物的形成速度快 20 倍,但 β-异头物更稳定 (Kβ/α = 7.3)。评估了形成这些产物的反应途径,并确定非对映体四氢-1,3-恶嗪中间体的 N-烷基化是区分步骤。α-异头物的更快形成主要归因于动力学异头效应,该效应使赤道N-烷基化的过渡态和β-异头物的形成不稳定。α-异头物主要通过轴向N-烷基化形成。meso-2,4-pentanediamine 与 5-bromo-5-deoxy-d-xylose 在水溶液中反应得到八氢吡啶并[1, 2-a]pyrimidine-7,8,9-triols 16α 和 17β,它们不能相互转化。在这种情况下,16α 的形成速度是 17β 形成速度的两倍。这种费率差异归因于