Synthesis of Fused Cyclic Systems Containing Medium-Sized Rings through Tandem ROM-RCM of Norbornene Derivatives Embedded in a Carbohydrate Template
作者:Chanchal K. Malik、Ram Naresh Yadav、Michael G. B. Drew、Subrata Ghosh
DOI:10.1021/jo802077t
日期:2009.3.6
under Lewis acid catalysis producing the diastereoisomers 11a−d exclusively. The reversal of facial selectivity from an uncatalyzed reaction to a catalyzed one is unprecedented. The observed stereochemical dichotomy is attributed to rotation of the enone moiety along the σ bond linking the sugar moiety during formation of the chelate 13. This makes the Re-face of the enone moiety in 4a−d unhindered
已经开发了合成包含中等大小环(7-9)的稠合环状系统的通用方法。关键步骤涉及与具有环戊二烯的呋喃糖连接的亲二烯体4a - d的非对映体选择性Diels-Alder反应,以及所得降冰片烯衍生物10a - d和11a - d的开环(ROM)-闭环置换(RCM)。在没有催化剂的情况下,亲二烯体4a - d与环戊二烯的狄尔斯-阿尔德反应是通过向不受阻碍的Si中添加二烯而产生的主要产物10a - d-脸。这些亲二烯体的Diels-Alder反应最有趣和新的方面是Re- face的可及性,该表面在路易斯酸催化下被烯基链封闭,仅产生非对映异构体11a - d。从未催化反应到催化反应的面部选择性逆转是史无前例的。观察到的立体化学二分法归因于在螯合物13形成期间烯酮部分沿着连接糖部分的σ键旋转。这使得再在烯酮部分的-面4A - ð畅通无阻。碳环类似物的狄尔斯-阿尔德反应15在路易斯酸催化下,产生了加合物16