Synthetic 1,4-disubstituted 1,4-dihydro-5H-tetrazol-5-one derivatives of fentanyl: Alfentanil (R 39209), a potent, extremely short-acting narcotic analgesic

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Abstract
The synthesis of a series of N-1,4-disubstituted-1,4-dihydro-5H-tetrazol-5-one piperidinyl derivatives of fentanyl (10), carfentanil (11), and sufentanil (12) is described. The 1-substituted tetrazolinones 2 were essentially prepared via the addition reaction of aluminum azide to isocyanates or acid chlorides in tetrahydrofuran. Alkylation of 2 under neutral or weakly basic conditions afforded almost exclusively the 1,4-disubstituted tetrazolinone isomer 3. N-Alkylation of the piperidine derivatives 4 with 3 in dimethylformamide yielded 9a-v. The morphinomimetic activity in rats, after intravenous injection of the compounds, was evaluated in the tail withdrawal reflex test. The fentanyl analogues 9a-c (R4=H) are inactive at the measured dose of 2.5 or 10 mg/kg (iv). For the carfentanil analogues (R4=COOCH3) maximal narcotic activity is found when R1 represents a lower alkyl group (9d-f) or a thienylethyl group (9n). The sufentanil analogues (R4=CH2OCH3) show the same structure-activity relationship (SAR) profile as the carfentanil derivatives (R4=COOCH3). The structural requirements for optimal activity are in good agreement with earlier observations in the series of 10-12. From the series the ethyl tetrazolinone derivative 9r, alfentanil (R 39209), was selected for clinical investigation. As an analgesic in rats, 9r is 140 times more potent than pethidine 15 and 72 times more potent than morphine 14. Alfentanil reaches its peak effect within 1 min after injection, and its duration of action is very short; at 2 times its MED50, 9r has a duration of action of 11 min. This duration is 30 min for 10 and 90 min for 14. Compared to 10, alfentanil 9r is about 4 times faster but 3 times shorter acting. Structurally, 9r shows most resemblance to sufentanil 12, since it differs only by substitution of a 4-ethyltetrzolinone ring for the thiophene ring. The considerable differences in their pharmacological profiles were explained in terms of marked variations in physicochemical and, hence, pharmacokinetic properties.