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The major product formed in the following conversion is _____. [a cyclopentanone with a $-CH_2-CO-CH_3$ side chain] $\xrightarrow{(i)\ NaBH_4\ (ii)\ H_2SO_4,\Delta}$ Major product
A
[add image: cyclopentene ring with a -CH=CH-CH3 (propenyl) side chain]
B
[add image: cyclopentene ring with an allyl-type side chain, alternate regiochemistry]
C
[add image: cyclopentanone ring retained with -CH=CH-CH3 side chain]
D
[add image: alternate structure with side chain and ring double bond]
Detailed Solution
$NaBH_4$ selectively reduces the more reactive ketone in the side chain to a secondary alcohol without touching the ring, giving a diol-like intermediate with $-CH_2-CH(OH)-CH_3$ on the side chain and the ring ketone reduced too, giving $-OH$ on the ring as well. On treatment with $H_2SO_4/\Delta$, acid-catalysed dehydration (E1) occurs at both positions, ultimately giving the major product with a cyclopentene ring conjugated to a $-CH=CH-CH_3$ side chain (the more substituted, conjugated/stable alkene being favoured, per Zaitsev's rule).