Step 1: The core structure of penicillins.
All penicillins share a common bicyclic nucleus: a thiazolidine ring fused to a beta-lactam ring. The beta-lactam ring is essential for antibacterial activity.
Step 2: The key acid derivative.
The basic building block from which all penicillins are chemically synthesised is 6-Amino Penicillanic Acid (6-APA). This is obtained by enzymatic or chemical removal of the acyl side chain from natural penicillins (like penicillin G).
Step 3: Why 6-APA is the answer.
6-APA has a free amino group at position 6 of the penicillanic acid nucleus. Different acyl groups are attached to this amino group to produce semi-synthetic penicillins like ampicillin, amoxicillin, and cloxacillin.
Step 4: Eliminate wrong options.
7-Aminocephalosporanic acid (7-ACA) is the parent nucleus of cephalosporins, not penicillins. 6-Nitro penicillanic acid does not exist as a pharmaceutical precursor. 7-Amino penicillanic acid does not exist (penicillins use position 6, not 7).
Step 5: Confirm.
Penicillins are derivatives of 6-Amino Penicillanic Acid (6-APA).
Answer: Option (3) — 6-Amino penicillanic acid