Condensation of 4-amino-1-benzylpiperidine (I) with benzoyl isocyanate gave the benzoyl urea (II), which was subsequently hydrolyzed to urea (III) with NaOH in aqueous methanol. Then, condensation of (III) with 2-hydroxypropiophenone (IV) in the presence of trifluoroacetic acid afforded a mixture of minor amounts of the required imidazolone (V) along with the major undesired regioisomer (VI).
Aminopiperidine (I) was alkylated with 2-bromopropiophenone (VII), and the resulting unstable aminoketone (VIII) was treated with benzoyl isocyanate to produce the benzoyl urea (IX). Cyclization of (IX), followed by benzoyl group cleavage upon treatment with methanolic NaOMe gave the hydroxy imidazolinone (X). This was dehydrated to the imidazolone (V) by means of trifluoroacetic acid. The intermediate (V) was debenzylated to (XII) by reaction with 1-chloroethyl chloroformate, followed by treatment of the resulting (1-chloroethyl) carbamate (XI) with boiling MeOH. The debenzylated piperidine (XII) was finally alkylated with alpha-bromo-m-tolunitrile (XIII) to give the target m-cyanobenzyl derivative.