Stereoelectronic effects at oxygen. A very large effect on the hydrolysis of a conformationally locked acetal: implications for β-glycosidase mechanisms
作者:Andrew J. Briggs、Christopher M. Evans、Robert Glenn、Anthony J. Kirby
DOI:10.1039/p29830001637
日期:——
Acetal [6; R = 2,4-(NO2)2C6H3] is rigidly fixed in the equatorial conformation. As a result the lone-pair electrons on the ring oxygen cannot assist C–OR cleavage, and the compound is hydrolysed ca. 1013 times more slowly than a comparable axial tetrahydropyranyl acetal, allowing an estimate of 19 kcal mol–1 for the stereoelectronic barrier to the cleavage of an equatorial tetrahydropyranyl acetal
乙缩醛[6; R = 2,4-(NO 2)2 C 6 H 3 ]牢固地固定在赤道构象中。结果,环氧上的孤对电子无法协助C-OR裂解,并且该化合物被水解。比同等轴向四氢吡喃基乙缩醛慢10 13倍,从而估计在其基态椅构象中立体电子屏障裂解赤道四氢吡喃基乙缩醛或β-葡萄糖苷的速率约为19 kcal mol –1。因此,构象变化是菲利普斯及其同事针对溶菌酶反应最初提出的那种变化,因此被证明是必不可少的。 初步切割任何β-糖苷。