Discussion
Which of the following alcohols is NOT suitable substrate for sharpless asymmetric epoxidation reaction ?
- A.
- B.
- C.
- D.
Answer: C
The alcohol that is not a suitable substrate is option C.
Sharpless asymmetric epoxidation works on allylic alcohols: the hydroxyl group binds titanium, which delivers the oxygen to the neighbouring double bond. In C the double bond is the ring-fusion double bond and the hydroxyl-bearing carbon is separated from it by one more carbon, so the OH is homoallylic and cannot direct the epoxidation. Options A and B are cinnamyl alcohols (Z and E), and D is a cyclic allylic alcohol with the double bond next to the carbinol carbon, so all three are allylic alcohols.
The Z isomer in A reacts less well than the E isomer in B, but it is still an allylic alcohol and so a substrate.
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