Which of the following pinacol - pinacolone type of reaction will involve ring contraction ?

Options

A.
B.
C.
D.

Explanation

To determine which pinacol-pinacolone type of reaction involves ring contraction, we need to analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding Pinacol-Pinacolone Rearrangement**:
- The pinacol-pinacolone rearrangement typically involves the conversion of a pinacol (a compound with two hydroxyl groups on adjacent carbons) into a pinacolone (a ketone) through a series of acid-catalyzed steps.

2. **Analyzing the Options**:
- We have four options to consider. Each option involves different compounds and conditions.

3. **Option 1**:
- The first option involves a compound with two hydroxyl groups on adjacent carbons. Upon treatment with H+, the compound undergoes protonation, leading to the departure of water and rearrangement.
- This results in a six-membered ring formation, indicating ring expansion, not contraction.
- **Conclusion**: This option does not involve ring contraction.

4. **Option 2**:
- The second option involves diazotization where NH2 is converted to N2+. The N2+ is a good leaving group, and upon removal, the remaining structure undergoes rearrangement.
- This leads to the formation of a four-membered ring from a three-membered ring, indicating ring expansion.
- **Conclusion**: This option also does not involve ring contraction.

5. **Option 3**:
- The third option involves an epoxide structure that, upon treatment with H2O, forms OH2+. The positive charge on the oxygen allows for rearrangement.
- The rearrangement leads to a contraction from a six-membered ring to a five-membered ring due to the movement of electrons and the breaking of bonds.
- **Conclusion**: This option involves ring contraction.

6. **Option 4**:
- The fourth option is similar to the first option, where a compound undergoes rearrangement leading to a four-membered ring formation.
- This indicates ring expansion, similar to the first option.
- **Conclusion**: This option does not involve ring contraction.

### Final Answer:
The correct option that involves ring contraction during a pinacol-pinacolone type of reaction is **Option 3**.

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