Synthetic approaches towards novel 3-pyrazolidinone derivatives functionalized at positions N(1) and/or C(5) were studied. 5-Aminoalkyl-3-pyrazolidinones were prepared in four steps from N-protected glycines via Masamune-Claisen homologation, reduction, O-mesylation, and cyclisation with a hydrazine derivative. The free amines were prepared by acidolytic deprotection. Title compound was also prepared by 'ring switching' transformation of N-Boc-pyrrolin-2(5H)-one with hydrazine hydrate. Hydrogenolytic deprotection of 5-(N-alkyl-N-Cbz-aminomethyl)pyrazolidine-3-ones followed by cyclisation with 1,1'-carbonyldiimidazole (CDI) gave two novel representatives of perhydroimidazo [1,5-b] pyrazole, which is an almost unexplored heterocyclic system. Amidation of 3-oxopyrazolidine-5-carboxylic acid gave the corresponding carboxamides in moderate yields. Diastereomeric non-racemic carboxamides obtained from (S)-AlaOMe and (S)-ProOMe were separated by MPLC.