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JEE Advanced levelSingle correct optionDifficulty 3 of 3co010

Methyl 4-oxopentanoate, $CH X_{3} COCH X_{2} CH X_{2} COOCH X_{3}$ , is treated with (i) ethylene glycol, $H X^{+}$ , (ii) $LiAlH X_{4}$ then $H X_{2} O$ , (iii) aqueous $H X_{3} O X^{+}$ . The final product is

A4-oxopentanoic acid
Bpentane-1,4-diol
Cmethyl 4-hydroxypentanoate
D5-hydroxypentan-2-one

Answer: D

The ketone is protected as its cyclic acetal, which is inert to hydride.

LiAlH X_{4}

reduces only the ester to

CH X_{2} OH

. Aqueous acid removes the acetal to restore the ketone, giving

CH X_{3} COCH X_{2} CH X_{2} CH X_{2} OH

; without the protection, both groups would be reduced.

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