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GATE 2025 CY – Question 44

Organic Chemistry · Organic Synthesis: Synthesis, reactions, mechanisms and selectivity involving the following classes of compounds – alkenes, alkynes, arenes, alcohols, phenols, aldehydes, ketones, carboxylic acids, esters, nitriles, halides, nitro compounds, amines and amides. Uses of Mg, Li, Cu, B, Zn, P, S, Sn and Si based reagents in organic synthesis. Carbon-carbon bond formation through coupling reactions – Heck, Suzuki, Stille, Sonogashira, Negishi, Kumada, Hiyama, Tsuji-Trost reactions; olefin metathesis, McMurry coupling and Buchwald-Hartwig amination reactions. Baylis-Hillman, Henry, Ritter, Sakurai, Tebbe olefination, Pauson-Khand and Nazarov cyclization reactions. Concepts of multistep synthesis – retrosynthetic analysis, strategic disconnections, synthons and synthetic equivalents. Atom economy and green chemistry, Umpolung reactivity – formyl and acyl anion equivalents. Selectivity in organic synthesis – chemo-, regio- and stereoselectivity. Protection and deprotection of functional groups. Concepts of asymmetric synthesis – resolution (including enzymatic), desymmetrization and use of chiral auxiliaries, organocatalysis. Carbon-carbon and carbonheteroatom bond forming reactions through enolates (including boron enolates), enamines and silyl enol ethers. Stereoselective addition to C=O groups (Cram, Prelog and Felkin-Anh models). Asymmetric aldol reactions – Evans reaction and proline catalyzed reaction. · 2 marks · Multiple choice

In the following asymmetric transformation, the key aldol reaction involves the attack of

Chiral oxazolidinone-directed aldol reaction followed by auxiliary cleavage.
  1. Si face of enolate on to the Re face of aldehyde
  2. Si face of enolate on to the Si face of aldehyde
  3. Re face of enolate on to the Re face of aldehyde
  4. Re face of enolate on to the Si face of aldehyde

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Correct answer: (D) Re face of enolate on to the Si face of aldehyde

Explanation

The chiral auxiliary controls the boron Z-enolate face in a chairlike Zimmerman–Traxler transition state. Assigning priorities at the enolate and benzaldehyde carbon gives Re-enolate attack on the Si-aldehyde face for the drawn syn product.

Official GATE 2025 answer key: https://gate2026.iitg.ac.in/doc/download/2025_Key/CY_Keys.pdf