Major product formed in the following synthetic sequence is: · ABC26OR0703
Subject: Organic Chemistry · Chapter: Reagents, Reaction Mechanisms & Named Reactions · Topic: Reagents, Reaction Mechanisms & Named Reactions – General · Exam: GATE 2000-2022 · Marks: 2 · Difficulty: Medium
Major product formed in the following synthetic sequence is: 
(a)
(b)
(c)
(d)
Answer
Answer (as printed): A
Explanation
Step (i): the vinyllithium adds 1,2 to the enone C=O, giving a tertiary alkoxide that carries the new 3-methylbut-1-en-2-yl group; the adjacent ring carbon carries the isopropenyl group, so a 3-oxido-1,5-diene is formed. Step (ii): KH / 18-crown-6 gives the 'naked' potassium alkoxide, which undergoes an anionic oxy-Cope [3,3] shift. The C1–C6 ring bond breaks, the two terminal CH₂ carbons join, and the six-membered ring expands into a ten-membered ring (germacrane skeleton). The enolate formed then tautomerises to the ketone. The C=O is flanked by the ring C=C(Me) (enone) on one side and by CH(iPr) on the other, and the second trisubstituted C=C(Me) lies across the ring. This is structure (a).
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