Question

For the following reaction, the possible product(s) is/are 

 

• A.
• B.
• C.
• D.

Hint:

Multi-step reactions involving chiral centers often lead to mixtures of diastereomers. Consider the stereochemical outcome of each step, including nucleophilic additions to carbonyls and catalytic hydrogenations of alkenes, which can exhibit syn addition and facial selectivity influenced by existing chiral centers.

Answer 1

The Correct Option is A, C

The reaction proceeds in three steps:

Step 1: Grignard Addition
Methylmagnesium bromide (MeMgBr) is a strong nucleophile that attacks the carbonyl group of the enone. The nucleophilic attack can occur from either face of the molecule, resulting in a mixture of diastereomeric allylic alcohols after acidic workup. The methyl group adds to the carbon of the carbonyl group, generating a new chiral center.

The existing stereocenter remains unchanged, but the new chiral center (at the carbon bearing the hydroxyl group) can be either R or S depending on the face of attack.

Step 2: Oxidation with PCC
Pyridinium chlorochromate (PCC) is a mild oxidizing agent that converts the allylic alcohol to an α,β-unsaturated ketone (an enone). This step does not alter the stereochemistry at the adjacent carbon atom (bearing the methyl and hydrogen).

Step 3: Catalytic Hydrogenation with H₂, Pd/C
Hydrogenation with Pd/C reduces the double bond of the enone. The addition of hydrogen occurs syn (from the same face), which can happen from either the top or bottom face of the ring system, giving rise to different stereoisomers.

The stereochemical outcome of this step depends on steric hindrance—substituents such as methyl or ethyl groups may influence which face the hydrogenation occurs from. Nevertheless, both diastereomers are possible due to limited stereoselectivity.

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Summary:

• The Grignard addition creates a new chiral center — both configurations (R/S) are possible.
• The oxidation step with PCC retains the configuration at the chiral center.
• The hydrogenation step introduces a second new stereocenter — hydrogen can add from either face, resulting in two diastereomers.

Correct interpretation: Options (A) and (C) are valid stereoisomeric products resulting from the reaction sequence. Each represents a different diastereomer formed due to variations in Grignard attack and hydrogenation facial selectivity.

Options (B) and (D) show either incorrect connectivity or stereochemistry inconsistent with the mechanistic steps and are therefore incorrect.