Complex sentence: Pethidine intermediate A is a 4-phenylpiperidine derivative that is a precursor to the opioid analgesic drug pethidine (meperidine) scheduled by UN Single Convention on Narcotic Drugs.
Split into simpler sentences: Pethidine intermediate A is a 4-phenylpiperidine derivative that is a precursor to the opioid analgesic drug pethidine (meperidine). It is scheduled by UN Single Convention on Narcotic Drugs.