Center for Analysis and Synthesis, Lund University, PO Box 124, SE-221 00 Lund, Sweden.
Carbohydr Res. 2011 Sep 6;346(12):1358-70. doi: 10.1016/j.carres.2011.03.032. Epub 2011 Apr 1.
The use of benzylidene acetals as protecting groups in carbohydrate chemistry is utterly important. The main advantage of benzylidene acetal is the ability for regioselective openings. 4,6-benzylidene acetal can be opened selectively under reductive conditions to yield either free 4-OH or 6-OH. There are a plethora of methods available for regioselective openings, but only a few of these are widely used. In recent years, the mechanism has been investigated for borane mediated openings and it seems likely that the regioselectivity is determined by borane, rather than Lewis acid. When borane is activated by Lewis acids, borane is the most electrophilic species that consequently coordinates to the most nucleophilic oxygen of the acetals, usually O-6. This results in the formation of 6-O-benzyl ethers. If borane is not activated, Lewis acid is the most electrophilic species that thus adds to O-6 and hence generates the 4-O-benzyl ether.
苄叉缩醛在碳水化合物化学中作为保护基团的用途是非常重要的。苄叉缩醛的主要优点是具有区域选择性开环的能力。4,6-苄叉缩醛可以在还原条件下选择性地开环，得到游离的 4-OH 或 6-OH。有许多方法可用于区域选择性开环，但只有少数几种方法被广泛使用。近年来，硼烷介导开环的机制已被研究，似乎区域选择性是由硼烷决定的，而不是路易斯酸。当硼烷被路易斯酸活化时，硼烷是最具亲电性的物种，因此与缩醛中最具亲核性的氧原子（通常为 O-6）配位。这导致 6-O-苄基醚的形成。如果硼烷未被活化，则路易斯酸是最具亲电性的物种，因此会与 O-6 加成，从而生成 4-O-苄基醚。
