A novel analogue of fentanyl, 3-carbomethoxyfentanyl, or "iso-carfentanil", was synthesized by a simple and efficient route. In the first step phenethylamine was condensed with two equivalents of methyl acrylate to afford the amino-diester Ib in quantitative yield. Dieckmann cyclization of this intermediate yielded 3-carbomethoxy N-phenethyl-4-piperidone 2 in ca. 80% yield, after mild hydrolysis. Condensation of this beta-keto ester with aniline in acetic acid gave the stable enamine 3 (70% yield) which was then reduced with NaBH3CN in methanol at pH approximate to 5, to yield 4-anilino-3-carbomethoxy-N-phenethyl piperidine, quantitatively. This intermediate was obtained as a 50:50 mixture of the (+/-) cis and (+/-) trans isomers, 4a and 4b, respectively. After the mixture of diastereoisomers was separated on a neutral aluminium oxide column, the pure 4a and 4b isomers were acylated with propionyl chloride, thus completing the synthesis of 3-carbomethoxy fentanyl 5a and 5b. The relative stereochemistry was H-1-NMR spectroscopy. These compounds present regioisomers of determined by carfentanil, one of the most potent narcotic analgesic known to date. Preliminary pharmacological evaluation (tail-withdrawal test in rats) revealed substantially reduced potency of both diastereoisomers, the (+/-) trans 5b in particular, compared to carfentanil. The computational (molecular mechanics) search of the low energy regions of the conformational space of the cis 5a and trans 5b isomers revealed the difference in their conformational mobility. Besides being more conformationaly flexible, the trans isomer has unfavorable orientation of the 4-N-phenylpropanamide group compared to the other active analogs of fentanyl. This is believed to be the reason of its reduced potency relative to fentanyl
