In the reactions above, taken from Tetrahedron Lett. 1982, 23 (23), 2355–2358, the choice of protecting group on the α-hydroxyl group has a large influence on the diastereoselectivity of reduction.
Why does the benzyloxymethyl protecting group lead to the formation of the anti diol (consistent wiht the Cram chelate model), whereas the tert-butyldiphenylsilyl protecting group gives the syn diol (consistent with the Felkin–Anh model)?
Answer
In order for the Cram chelate product to predominate after the addition of hydride donor to a chiral carbonyl compound, which contains a heteroatom in the alpha position, this heteroatom and part of the reagent must be able to form a five-membered ring chelate. If this is not possible, one observes Felkin-Anh proudct.
In the mentioned reaction, when protecting group is not bulky it form five-membered ring chelate easily with Li+ after addition of Hydride to carbonyl resulted in cram chelate product as major product. However, when bulky protecting group is present it doesn't five-membered ring, consequently resulted in Felkin-Anh product as major product.
No comments:
Post a Comment