Control of regiochemistry in nitrone cycloadditions. Regioselectivity of the reactions of trisubstituted nitrones with electron-deficient and conjugated dipolarophiles
作者:Marina Burdisso、Remo Gandolfi、Paolo Grünanger
DOI:10.1016/s0040-4020(01)89503-x
日期:1989.1
temperatures. Under these latter conditions the 5-substituted isoxazolidine was clearly prevalent owing to its being sterically less congested than the 4-substituted derivative. Evaluation of relative cycloaddition rates for 1a showed that the activating effect of formyl (rel. rate = 1.5) and acetyl groups (1.00) is about five times higher than that of cyano (0.18) and methoxycarbonyl groups (0.22) whilst
三苯基硝酮1a和C,C-二苯基-N-甲基硝酮1b在室温下与带有电子吸引基团(即甲酰基,乙酰基,氰基,甲氧羰基,磺酰基和硝基)的单取代烯烃反应,得到两种区域异构体的动力学控制混合物。通常,由于对区域化学的电子控制,所以4-取代的异恶唑烷比5-区域异构体占优势(仅4-硝基异构体由硝基乙烯获得)。由于这些反应易于逆转,因此可以在较高温度下实现热力学控制。在这些后面的条件下,由于5-取代的异恶唑烷在空间上比4-取代的衍生物少拥塞,因此显然是普遍的。估算相对环加成率图1a显示甲酰基(相对比率= 1.5)和乙酰基(1.00)的活化作用比氰基(0.18)和甲氧羰基(0.22)的活化作用高约五倍,而硝基的作用显着高。 (1,155)尤其是与磺酰基(0.03)的效果相当低相比。在1a和1b与苯乙烯和1,1-二取代衍生物(例如,甲基丙烯醛)的反应中仅获得5-取代的衍生物,而在相同硝酮与单取代乙炔(例如,甲基)