A mixture of 2.4 g of cis-4-(4-bromophenoxy)-3-phenylpiperidine, 7.2 ml of 100% formic acid and 0.81 ml of 37% aqueous formaldehyde solution is refluxed for one hour under nitrogen, cooled and made basic with 35 ml of 10% aqueous sodium hydroxide solution. The mixture is extracted with ether (2×50 ml), the ether extracts are washed with saturated sodium chloride solution, dried overnight over anhydrous sodium sulfate, filtered and concentrated in vacuo to afford a yellow oil. The oil is dissolved in 50 ml of ether and treated dropwise with stirring with a solution of 0.89 g of fumaric acid in a mixture of 75 ml of ether and 15 ml of absolute ethanol. The colorless, fine crystals which precipitate are filtered, washed with ether and dried to afford solid product, mp 172° C. dec.
