Bozell J J, Miller D, Hames B R, Loveless C
National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, USA.
J Org Chem. 2001 May 4;66(9):3084-9. doi: 10.1021/jo005736t.
Cyclic ortho esters undergo stereoselective and regioselective reaction with phenols when treated with BF(3) x OEt(2) at low temperatures. Attack of the phenol on the ortho ester occurs at an open carbon para to electron-donating groups on the phenol ("C-addition") or at the phenolic hydroxyl group ("O-addition") depending on the nature of the cation formed from reaction of the ortho ester and BF(3) x OEt(2). Products resulting from O-addition undergo reversion to a mixture of starting phenol, C-addition product, and O-addition product if treated with BF(3) x OEt(2) at room temperature, but C-addition products are stable under the same conditions. X-ray structural analysis of the C-addition compound indicates that its stereochemistry is opposite to that observed in reaction of similar ortho esters with chloride from TMSCl. However, the stereochemistry of the reaction can be rationalized by the ability of the ortho ester to isomerize via an intermediate benzylic cation and examination of the preferred trajectory of attack of the nucleophile on the intermediate oxonium ion.
环状原酸酯在低温下用BF(3)·OEt(2)处理时，会与酚发生立体选择性和区域选择性反应。酚对原酸酯的进攻发生在酚上给电子基团对位的一个开链碳上（“C-加成”）或酚羟基上（“O-加成”），这取决于由原酸酯与BF(3)·OEt(2)反应形成的阳离子的性质。如果在室温下用BF(3)·OEt(2)处理，由O-加成得到的产物会逆转为起始酚、C-加成产物和O-加成产物的混合物，但C-加成产物在相同条件下是稳定的。C-加成化合物的X射线结构分析表明，其立体化学与类似原酸酯与TMSCl的氯反应中观察到的相反。然而，该反应的立体化学可以通过原酸酯经由中间体苄基阳离子异构化的能力以及对亲核试剂进攻中间体氧鎓离子的优选轨迹的考察来解释。
