Discussion
The major products P and Q in the following reaction sequence are
- A.
- B.
- C.
- D.
Answer: B
The answer is (B): P has the new stereocentre with the butenyl chain on a hashed bond, and Q is (S)-cyclopent-2-en-1-ylmethanol with CH2OH on a hashed bond.
LDA forms the Z-lithium enolate of the N-acyloxazolidinone, chelated between the enolate oxygen and the ring carbonyl. The isopropyl group of the (S)-auxiliary blocks one face, so 4-bromobut-1-ene alkylates the opposite face (Evans alkylation). With the CIP priorities in the order C(O)N, then vinyl, then butenyl, then H, the new centre in P is S, as drawn in (A) and (B).
Grubbs-II then joins the vinyl and butenyl alkenes by ring-closing metathesis, releasing ethylene and forming a cyclopentene, and LiBH4 cleaves the auxiliary to the primary alcohol. The stereocentre is untouched, and the priorities keep the same order (CH2OH, then the ring CH=, then the ring CH2, then H), so Q is also S, the hashed CH2OH of (B).
(A) has the right P but draws Q as the R alcohol. (C) and (D) show the alkylation on the face shielded by the isopropyl group (P is R); (C) also pairs it with an S alcohol that it cannot give.
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