Chiral pyrazolones with a spirocyclic centre at the C4-position are widely found in a large family of medically relevant compounds. In recent years, organocatalysis, particularly that performed with quiral N-heterocyclic carbenes (NHCs), has allowed the enantioselective synthesis of these spirocyclic compounds despite its inherent difficulty. In this work, we describe the fully diastereo- and highly enantioselective synthesis of novel spirocyclic pyrazolone γ-butyrolactones via NHC-catalysed [3 + 2] annulation reaction of enals and 1H-pyrazol-4,5-diones. To understand the catalytic mechanism and origin of stereoselectivity, electronic structure calculations were carried out. After considering various pathways, we concluded that the stereoselectivity-determining step is the formation of the lactone that proceeds after addition of the NHC derived homoenolate to the electrophilic carbonyl group of pyrazolin-4,5-dione. Our calculations predict that the free energy barrier is lower for the (RS) product, which is also the main product experimentally obtained.