The intramolecular Diels-Alder reaction of furanyl amides derived from 2-furylacetic acid has been examined. Substrates containing either an imide or tertiary amide linkage between the furan and the dienophile underwent smooth cycloaddition upon thermolysis. By varying the reaction conditions, either the primary cycloadduct or the ring-opened and acetylated product could be isolated in excellent yield. The stereochemical outcome of the IMDAF cycloaddition has the side arm of the tethered alkenyl group oriented syn with respect to the oxygen bridge. Semi-empirical AM1 calculations show that the exo-cycloadduct is 11 kcal lower in energy than the corresponding endo adduct and, presumably, some of this energy difference is reflected in the transition state for the cycloaddition. The IMDAF reaction of N-allyl-[2-(3,4-dimethoxyphenethyl)]-2-furanyl-2-yl-acetamide proceeded in 90% yield upon heating in xylene. The 4+2-cycloadduct undergoes ring-opening on treatment with base and the resulting alcohol was converted into the corresponding benzyl ether. Raney nickel reduction followed by Bischler-Napieralski cyclization furnished the tetracyclic skeleton of the berberine alkaloids.Key words: intramolecular, cycloaddition, Diels-Alder, furanylamide, heterocycle.
从2-呋喃乙酸衍生的呋喃基酰胺的分子内Diels-Alder反应已经得到研究。包含呋喃和双烯体之间的酰亚胺或三级酰胺键的底物在热解时发生平滑的环加成反应。通过改变反应条件,可以在良好的收率下分离出主要的环加成产物或环开放和乙酰化产物。IMDAF环加成的立体化学结果表明,锚定烯基基团的侧臂与氧桥相关联。半经验AM1计算表明,外向环加成物的能量比相应的内向加成物低11千卡,并且可能这些能量差异的一部分反映在环加成的过渡态中。N-烯丙基-[2-(3,4-二甲氧基苯乙基)]-2-呋喃基-2-基乙酰胺的IMDAF反应在二甲苯中加热后以90%的收率进行。4+2-环加成物在碱处理时发生环开放,生成的醇转化为相应的苄醚。随后进行雷尼镍还原和Bischler-Napieralski环化反应,形成小檗碱生物碱的四环骨架。关键词:分子内、环加成、Diels-Alder、呋喃酰胺、杂环。