Discussion
Predict stereochemical outcome of the following reaction :
Answer: B
The new stereocentre has the R configuration (option B).
This is an Evans alkylation. n-BuLi at -75°C makes the Z-enolate of the N-acyl oxazolidinone, chelated through lithium between the enolate oxygen and the ring carbonyl. The bulky tert-butyl group on C-4 shields one face, so allyl chloride attacks from the other face and one diastereomer is formed in large excess. The drawn auxiliary has the (S) configuration at C-4, and with this (S) auxiliary the allylation gives the (R) centre at C-2 of the new 2-ethylpent-4-enoyl group (the same sense as the textbook propionyl case, where the (S)-4-benzyl auxiliary with allyl iodide gives the (R)-2-methylpent-4-enoyl product).
S would need attack on the shielded face. Equal amounts of both diastereomers would mean no facial control, which is the very reason for the auxiliary. The product does have a new chiral centre, since C-2 carries C(=O)N, allyl, ethyl and H.
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