Q103 · CSIR NET Chemistry, June 2014
Paper: CSIR NET June 2014 · Subject: Organic Chemistry · Chapter: Reagents, Reaction Mechanisms & Named Reactions · Topic: Reagents, Reaction Mechanisms & Named Reactions – General · Marks: 2 · Difficulty: Medium
The major product formed in the following is: 
(a)
(c)
(d)
Answer
Answer: A ✓ checked by 4AB · confidence high
Key A confirmed. The old explanation's '6-endo/5-exo' first step is wrong; the cascade is 5-exo-trig then 5-exo-dig.
Explanation
Bu₃SnH/AIBN abstracts Br to give the primary radical. It cyclises 5-exo-trig onto the nearer (CH) carbon of the ring alkene, forming a cyclopentane and leaving a tertiary radical on the methyl-bearing carbon. That radical closes 5-exo-dig onto the alkyne, forming the second cyclopentane, and the resulting vinyl radical takes H from Bu₃SnH. The product is the 5-6-5 tricycle with an angular methyl and an exocyclic CH₂ — option (a).
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