Synthesis and Cleavage of the Cyclic Oxalates of 1,2-Glycols.

我们已经披露了草酰氯在三乙胺存在下与四氢呋喃中的无环1,2-二醇反应，产生不同类型的化合物作为主要产物，这取决于乙二醇的结构：未取代、单取代和红二取代乙二醇提供环草酸盐或聚合草酸盐，而三异构体和品纳醇提供环碳酸盐。我们发现唯一的例外是反式环己烷-1,2-二醇，它只提供环草酸盐。这个反应可能是通过船型的四面体中间体进行的，避免了桥头氢和羰基氧之间的空间干扰。用吡啶代替三乙胺生成环草酸酯，与品萘酚或三羧酸酯反应时，产物比例发生了戏剧性的逆转。1,1′-草酰二咪唑是环草酸盐排他性形成的较好选择。使用2,4,6-碰撞碱完全抑制了红-化合物聚合草酸盐的形成。得到的环草酸酯在pH为5的条件下进行酰基-烷氧基键的水解裂解，生成草酸单酯，速率比品纳酚环草酸酯快200-1000倍。x射线晶体分析表明，环草酸酯的羰基碳被四个甲基中的两个有效地阻断。

We have already disclosed that oxalyl chloride reacts with acyclic 1,2-glycols in tetrahydrofuran in the presence of triethylamine to afford a different type of compound as a major product, depending on the structure of the glycol : unsubstituted, monosubstituted, and erythro-disubstituted ethylene glycols provided the cyclic oxalates or polymeric oxalates, while the threoisomers and pinacol afforded the cyclic carbonates.1.The only exception we found was trans-cyclohexane-1,2-diol, which exclusively afforded the cyclic oxalates. This reaction probably proceeded through the tetrahedral intermediate with a boat form, avoiding steric interference between the bridge-head hydrogen and carbonyl oxygen.2.The dramatic reversal of the product ratio was realized in the reaction with pinacol or the threo-compounds by the use of pyridine instead of triethylamine to afford the cyclic oxalates. 1,1'-Oxalyldiimidazole was found to be a good choice for the exclusive formation of the cyclic oxalates. The formation of the polymeric oxalates from the erythro-compounds was completely suppressed by the use of 2,4,6-collidine.3.The cyclic oxalates thus obtained underwent hydrolytic cleavage of the acyl-alkoxy linkage to afford the monoester of oxalic acid at pH 5 at the rate of 200-1000 times faster than the cyclic oxalate of pinacol. The three-dimensional structure elucidated by X-ray crystal analysis shows that the carbonyl carbons of the cyclic oxalate of pinacol are effectively blocked by two of the four methyl groups.