Reductive condensation of N-Boc-4-piperidone (I) with 4-(benzyloxy)aniline (II) in the presence of NaBH(OAc)3 produced the secondary amine (III), which was further alkylated with 4-bromo-2-methyl-2-butene (IV) to afford the tertiary amine (V). Subsequent acid cleavage of the Boc protecting group of (V) furnished the intermediate piperidine (VI).
Piperazine-2-carboxylic acid (VIII) was prepared by catalytic hydrogenation of pyrazine (VII). Sequential protection of the N-4 of (VIII) with 2-(t-butoxycarbonyloxyimino)-2-phenylacetonitrile and the N-1 with benzyl chloroformate provided the diprotected piperazinecarboxylic acid (X), which was subsequently esterified with diazomethane to give the methyl ester (XI). The N-benzyloxycarbonyl group of (XI) was selectively deprotected by hydrogenation in the presence of Pd/C to yield the N-Boc-piperazine (XII). Reductive alkylation of (XII) with isovaleraldehyde (XIII) afforded the corresponding N-isoamyl piperazine (XIV). After acid removal of the N-Boc group of (XIV) to yield (XV), a second reductive alkylation with acetone (XVI) produced the dialkyl piperazine (XVII). Hydrolysis of the methyl ester group of (XVII) under acidic conditions gave acid (XVIII). This was finally coupled with the intermediate piperidine (VI) in the presence of HBTU to provide the title compound.