Discussion
The major product T formed in the following reaction is
- A.
- B.
- C.
- D.
Answer: A
T is (A), the alcohol at C3 next to the methoxy group, with Br kept at C5.
At -100 °C, nBuLi performs a fast bromine-lithium exchange, not a deprotonation. Of the two bromines, the one at C3 is exchanged selectively because the adjacent 2-methoxy oxygen coordinates the lithium and stabilises the 3-pyridyllithium (an ortho-directing effect). Adding benzaldehyde then traps that organolithium as the secondary alcohol ArCH(OH)Ph at C3.
(B) puts the new group at C5, the bromine without a neighbouring donor. (C) and (D) keep both bromines and put the CH(OH)Ph group next to nitrogen, which would need deprotonation of a ring C-H rather than halogen exchange; (C) also loses the methoxy group.
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