Benzylidenation of D-arabinose diethyl and dipropyl dithioacetals with α,α-dimethoxytoluene in the presence of p-toluenesulfonic acid has been studied in detail. Under kinetic control the two terminal dioxolan -type 4,5-O-(R)- and 4,5-O-(S)-benzylidene diastereomers are formed first which are in equilibrium with each other In the thermodynamic phase of the reaction the corresponding dioxan -type 3,5-O-(R)- benzylidene isomer is formed too, but all three monobenzylidene isomers are gradually converted into the four possible dioxolan -type 2,3 : 4,5-di-O benzylidene diastereomers . The dioxan -type 2,4:3,5-di-O-benzylidene isomer was present only in trace amounts. When benzaldehyde was used as reagent in the presence of hydrochloric acid or zinc chloride only the 2,3: 4,5-di-O-benzylidene diastereomers were formed. Partial hydrolysis of the dibenzylidene derivatives yielded the corresponding 2,3-O-benzylidene diastereomers. Structures, including the chirality of the benzylidene groups, were determined by n.m.r. spectroscopy. A mechanism suggested for the reaction was partially supported by equilibration studies.
我们详细研究了在对甲苯磺酸存在下,D-阿拉伯糖二乙基和二丙基二硫代乙醛与 α,α-二甲氧基甲苯的亚苄基化反应。在动力学控制下,首先形成了两种末端二氧戊环型 4,5-O-(R)- 和 4,5-O-(S)- 亚苄基非对映异构体,这两种非对映异构体处于相互平衡状态。在反应的热力学阶段,也形成了相应的二氧戊环型 3,5-O-(R)- 亚苄基异构体,但所有三种单亚苄基异构体都会逐渐转化为四种可能的二氧戊环型 2,3 :4,5-二-O-亚苄基非对映异构体。二氧戊环型 2,4:3,5-二-O-亚苄基异构体仅有微量存在。在盐酸或氯化锌存在下使用苯甲醛作为试剂时,只有 2,3,4,5-二-O-亚苄基异构体存在:4,5-二-O-亚苄基非对映异构体。部分水解二亚苄基衍生物可得到相应的 2,3-O-亚苄基非对映异构体。通过 n.m.r.光谱测定了其结构,包括亚苄基的手性。平衡研究部分支持了该反应的机理。