Synthesis and Characterization of Novel Methyl (3)5-(-Boc-piperidinyl)-1-pyrazole-4-carboxylates.

Series of methyl 3- and 5-(-Boc-piperidinyl)-1-pyrazole-4-carboxylates were developed and regioselectively synthesized as novel heterocyclic amino acids in their -Boc protected ester form for achiral and chiral building blocks. In the first stage of the synthesis, piperidine-4-carboxylic and ()- and ()-piperidine-3-carboxylic acids were converted to the corresponding β-keto esters, which were then treated with ,-dimethylformamide dimethyl acetal. The subsequent reaction of β-enamine diketones with various -mono-substituted hydrazines afforded the target 5-(-Boc-piperidinyl)-1-pyrazole-4-carboxylates as major products, and tautomeric NH-pyrazoles prepared from hydrazine hydrate were further -alkylated with alkyl halides to give 3-(-Boc-piperidinyl)-1-pyrazole-4-carboxylates. The structures of the novel heterocyclic compounds were confirmed by H-, C-, and N-NMR spectroscopy and HRMS investigation.