Question

The product X in the following reaction sequence is 

• A. (A)
• B. (B)
• C. (C)
• D. (D)

Hint:

In organic reactions, always consider the reactivity of each step — reduction, dihydroxylation, and dehydration — in determining the final product.

Answer 1

The Correct Option is D

Step 1: $\text{TiCl}_3$, $\text{Zn-Cu}$ (Pinacol Coupling)

This is a reductive coupling reaction. Two molecules of cyclopentanone dimerize at the carbonyl carbons.

Product: $\text{1,1'-Bicyclopentane-1,1'-diol}$ (a bicyclic pinacol).

Step 2: Cold alkaline $\text{KMnO}_4$

While cold $\text{KMnO}_4$ is a mild oxidant, its primary role is often ignored in this specific sequence when followed by acid. The overall transformation is the Pinacol-Pinacolone Rearrangement.

Step 3: $\text{H}_2\text{SO}_4$ (Pinacol-Pinacolone Rearrangement)

The diol (pinacol) undergoes acid-catalyzed rearrangement.

One $\text{OH}$ group is lost as $\text{H}_2\text{O}$ to form a carbocation, followed by a 1,2-alkyl shift (the $\text{C}-\text{C}$ bond linking the two rings migrates).

This migration causes a ring expansion of one 5-membered ring to a 6-membered ring.

Final Product X: A spiroketone: Spiro[5.4]decan-1-one, where a 6-membered ring and a 5-membered ring share a single carbon atom (the ketone carbon).

Answer: (D).