Assignment First

美国代课:无锐化不对称环氧化合物的重要性

无锐化不对称环氧化的重要性有很多原因。第一个原因是环氧化合物很容易转化为醚、二醇和氨基醇。由于手性环氧化合物在天然产物的形成中起着重要的作用,因此这种环氧化反应为手性环氧化合物的形成提供了灵活性。第二个原因是无锐化不对称环氧化反应能够与许多一级和二级烯丙基醇反应。第三个原因是无锐化不对称环氧化产物的对映体含量经常超过90%。第四个原因是,用于无锐化不对称环氧化的反应物在商业上是可用的,而且价格低廉。

美国代课:无锐化不对称环氧化合物的重要性

碳水化合物化学的一个特征是反常效应。写一个简短的解释来解释这是什么(长度不要超过1页)。

最初的定义是键合在C-1上的极性基团(葡萄糖基衍生物的异位碳)占据轴向位置的热力学偏好。

为了使异常稳定完全影响构象,它必须比所有空间因子的和更稳定。

在上面的例子中,环己烷环上的甲氧基(顶部)更倾向于靠近赤道的位置。然而,在四氢吡喃环(底部),甲氧基更喜欢轴向位置。这是因为在环己烷环中,Y=碳,这不是一个杂原子,所以没有观察到异常效应,而立体结构占据了观察到的取代基位置。在四氢吡喃环中,Y=氧,这是一个杂原子,所以反常效应有助于稳定所观察到的取代基位置。在这两种情况下,X= OMe。

美国代课:无锐化不对称环氧化合物的重要性

The importance of Sharpless asymmetric epoxidation is due to many reasons. First reason, epoxides are easily converted in to ethers, diols, and amino alcohols. Since, in the formation of natural products, chiral epoxides play important role and hence this epoxidation provides the flexibility in formation of chiral epoxides. Second reason is that the Sharpless asymmetric epoxidation are able to react with many primary and secondary allylic alcohols. Third reason is that Sharpless asymmetric epoxidation based products have frequently enantiomeric excesses above 90%. Fourth reason is that, the reactants for the Sharpless asymmetric epoxidation are commercially available and cheap.

美国代课:无锐化不对称环氧化合物的重要性

A characteristic feature of carbohydrate chemistry is something termed the anomeric effect. Write a short explanation explaining what this (do not exceed 1 page in length).
Originally defined as the thermodynamic preference for polar groups bonded to C-1 (the anomeric carbon of a glycopyranosyl derivative) to take up an axial position.
For the anomeric stabilization to affect the conformation at all it must be more stabilizing then the sum of all of the steric factors.
In the above case, the methoxy group on the cyclohexane ring (top) prefers the equatorial position. However, in the tetrahydropyran ring (bottom), the methoxy group prefers the axial position. This is because in the cyclohexane ring, Y= carbon, which is not a heteroatom, so the anomeric effect is not observed and sterics dominates the observed substituent position. In the tetrahydropyran ring, Y= oxygen, which is a heteroatom, so the anomeric effect contributes and stabilizes the observed substituent position. In both cases, X= OMe.