The synthesis of lactam analogues of fentanyl

Abstract

Fentanyl, sufentanil and alfentanil are clinically widely used anaesthetics and are structurally related to drugs with entirely different pharmacological activity such as droperidol, loperamide and lorcainide, etc. Therefore, in order to test their pharmacological activity, lactam analogues of fentanyl, a novel class of compounds, have been synthesized. In the first step, various primary amines have been selectively added to 1 equiv. of α,β-unsaturated esters, to afford the β-amino esters. N-Acylation of these intermediates with dimethyl malonate yields the amido esters, which have been further subjected to Dieckmann-type cyclization, to produce the corresponding 3-methoxycarbonylpiperidine-2,4-diones. The cyclization has been effected under phase-transfer conditions, utilizing potassium carbonate as base and 18-crown-6 as catalyst. This eliminates the need for strong and hazardous bases such as molten sodium or NaH. In the next step, acid hydrolysis and decarboxylation furnish the substituted piperidine-2,4-diones in good yields, as pure products. Alkylation of the N-phenethylpiperidine-2,4-dione with methyl iodide and potassium carbonate in DMSO gives the 3,3-dimethyl derivative. The alkylation procedure is also applicable to other alkylating agents. Reductive amination of the prepared piperidine-2,4-diones with aniline and NaBH₃CN in buffered methanol gives the corresponding pure 4-anilino-2-piperidones. The lactam function can be readily reduced (NaBH₄–BF₃·Et₂O), as exemplified with the 3,3-dimethyl derivative, thus providing access to additional fentanyl analogues, not readily accessible by other routes. The synthesis is completed by N-acylation of the anilines with propionyl chloride using triethylamine as base. The prepared 4-propionanilido-2-piperidones and 4-propionanilidopiperidines are expected to provide useful structure-activity relationship data in the pharmacological studies.