The dialdehyde shown is treated with dilute base and heated. Which structure is the intramolecular aldol condensation product?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

The drawn compound is hexanedial. The enolate at C2 attacks C6, forming a ring of C2-C6 (five atoms). Dehydration gives cyclopent-1-ene-1-carbaldehyde, conjugated with the C=O. A 6-membered product would need heptanedial.

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