In previous [1] attempts to synthesize a Schiff base from 3-benzoyl-4-hydroxy-1-methyl-4-phenylpiperidine (1) [2] and 1,2-diaminobenzene we identified an unusual route for their condensation under standard type conditions (refluxing in toluene in the presence of a catalytic amount of para-toluenesulfonic acid). In place of the expected imine the mono-and di-N-(benzoylethyl)-substituted ortho-phenylenediamines and benzo derivative of the macrocycle 1,4,8-triazacycloundecane were separated from the reaction mixture. The structure of the compounds obtained in this case pointed to a complex reaction cascade, of which the principal might be decyclization of piperidol 1, transamination of the decyclization products and intramolecular cyclocondensation of the novel Mannich bases. We now present the results of a systematic study of the reaction of piperidol 1 with para-, meta-, and ortho-substituted anilines and arylhydrazines. In all cases the reaction of two equivalents of the arylamines with one equivalent of the piperidol 1 under the conditions indicated above gave the expected N-monobenzoylethyl aniline derivatives 2a-u (in agreement with the results of [1]), the charcteristics of which are given in Table 1. For the unsubstituted aniline the yield of the aminopropanone 2a was 49%. Introduction of electron-donor substituents into the para-position of the aniline lowers the NH-acidity of the arylamines leading to a marked decrease in the yield of the corresponding aminopropanones 2b-e.
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