Inner Workings of a Cinchona Alkaloid Catalyzed Oxa-Michael Cyclization: Evidence for a Concerted Hydrogen-Bond-Network Mechanism
作者:Lukas Hintermann、Jens Ackerstaff、Florian Boeck
DOI:10.1002/chem.201203505
日期:2013.2.11
specificity. A concerted hydrogen‐bond network mechanism is proposed, in which the alkaloid hydroxy group acts as a general acid in the protonation of the α‐carbanionic center of the product enolate. The importance of concerted hydrogen‐bond network mechanisms in organocatalytic reactions is discussed. The relative stereochemistry of protonation is proposed as analytical tool for detecting concerted addition
金鸡纳生物碱催化的4-(2-羟基苯基)-2- butenoates的氧杂迈克尔环化苯并-2,3-二氢呋喃-2-基乙酸酯和在高达99%的收率和91%的相关底物 EE(EE =对映异构体过量的)。催化剂和底物变异研究揭示了生物碱羟基在反应机理中的重要作用,但在双功能催化的Hiemstra-Wynberg机理假设的底物羰基的氢键活化意义上却没有。在C-2的基板节目氘标记,添加RO的 H至所述链烯酸酯与发生顺非对映选择性≥99:1,表明基于机制的特异性。提出了协同的氢键网络机制,其中生物碱羟基在产物烯醇化物的α-碳负离子中心的质子化中充当一般酸。讨论了协同氢键网络机制在有机催化反应中的重要性。质子化的相对立体化学被认为是检测一致的加成机理的分析工具,与离子的1,4-加成相反。