Step 1: Understand opioid agonist vs. antagonist.
Opioid analgesics (e.g., morphine, codeine) are agonists at the mu-opioid receptor. Opioid antagonists (e.g., naloxone, naltrexone) block those receptors. The difference in pharmacological activity comes down to a specific structural change in the molecule.
Step 2: What changes agonist to antagonist?
The nitrogen substituent on the piperidine nitrogen of the morphine scaffold is the critical determinant. In morphine and most agonists, the nitrogen carries a simple N-methyl group. Replacing N-methyl with a larger allyl group (N-allyl, which is N-CH2-CH=CH2) converts the agonist to an antagonist.
Step 3: Evaluate each option.
Addition of methyl at C3 (Option 1): C3 has a phenolic -OH in morphine; methylating it gives codeine, which is still an agonist. Reduction of the 7,8-double bond (Option 3): This gives dihydromorphine, still an agonist. Addition of phenolic hydroxyl (Option 4): Already present at C3; does not convert to an antagonist.
Step 4: Why N-allyl works.
The allyl group is bulkier than the N-methyl group. This steric bulk prevents the molecule from producing the conformational change needed for receptor activation, turning it into a competitive antagonist. This is seen in naloxone (N-allyl noroxymorphone) and naltrexone (N-cyclopropylmethyl).
Step 5: Confirm the answer.
Replacement of N-methyl with N-allyl is the classic and well-established structural modification that converts opioid agonists to antagonists.
Answer: Option (2) — Replacement of N-methyl with N-allyl