Substituent effects in isoxazoles: identification of 4-substituted isoxazoles as Michael acceptors
作者:Connie K. Y. Lee、Christopher J. Easton、Mariana Gebara-Coghlan、Leo Radom、Anthony P. Scott、Gregory W. Simpson、Anthony C. Willis
DOI:10.1039/b207808b
日期:2002.12.6
Crystallographic and theoretical studies have been used to investigate substituent effects, which are manifest in electrochemical and yeast-catalysed reactions of 4- and 5-acyl-, alkoxycarbonyl-, cyano- and phenyl-substituted isoxazoles. The results show that isoxazoles substituted at the 4-position with π-electron-withdrawing substituents have enhanced C4–C5 bond polarity and are structurally similar to Michael acceptors. As a consequence there is elongation and weakening of their N–O bonds. By contrast, their 5-substituted regioisomers and isoxazoles substituted at C4 with conjugating, but not π-electron-withdrawing, substituents have diminished C4–C5 bond polarity. This results in the selective electrochemical and yeast-catalysed reduction of 4-substituted isoxazoles, as well as their hydrogenolytic ring cleavage and conjugate reduction with sodium borohydride.
我们利用晶体学和理论研究来探讨取代基效应,这些效应体现在 4-和 5-酰基、烷氧羰基、氰基和苯基取代的异噁唑的电化学反应和酵母催化反应中。结果表明,在 4 位上被π-电子吸收取代基取代的异噁唑具有更强的 C4-C5 键极性,在结构上类似于迈克尔受体。因此,它们的 N-O 键会变长和变弱。相比之下,它们的 5-取代基异构体和在 C4 处被共轭取代(而不是被π-电子吸收取代)的异噁唑的 C4-C5 键极性则会减弱。这导致 4-取代的异噁唑在电化学和酵母催化下发生选择性还原,以及与硼氢化钠一起发生氢解环裂解和共轭还原。