GATE 2019 CY – Question 45

Organic Chemistry · Reduction: Catalytic homogeneous and heterogeneous hydrogenation. Metal based reductions using Li/Na in liquid ammonia, magnesium, zinc, titanium, and samarium. Hydride transfer reagents: NaBH4, L-selectride, K-selectride, Luche reduction, LiAlH4 and DIBAL-H. · 2 marks · Multiple choice

Identify the major product after the illustrated chiral carboxylic acid–ester is treated with B₂H₆ followed by acid.

Structure A
Structure B
Structure C
Structure D

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Correct answer: (B) Structure B

Explanation

Borane preferentially reduces the free carboxylic acid to a primary alcohol, leaving the methyl ester available for cyclization. Acid workup enables intramolecular transesterification to a six-membered lactone. The original stereocentre is retained, and placing the carbonyl on the ester-derived side gives B. A interchanges which chain end was reduced; C and D remove the remaining ester carbonyl.

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