The paper discusses the results of X-ray diffraction studies of 2,4,6,8-tetraazabicyclo[3.3.0.]octane-3,7-dione (glycoluril) and its derivatives. Geometrical characteristics of glycoluril derivative molecules are analyzed depending on the type of the substituent and the formation of hydrogen bonds in crystals.

A new method has been developed for the synthesis of 2,4,6,8-tetramethyl-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-dione in 62% yield using etidronic acid [1-hydroxyethane-1,1-diylbis(phosphonic acid), HEDP] as a “green” catalyst. A plausible stepwise mechanism of the reaction has been proposed and partially confirmed by NMR data and studying model processes. The effects of the reaction medium and HEDP on particular species involved in the reaction, namely N,N′-dimethylurea, 4,5-dihydroxy-1,3-dimethylimidazolidin-2-one, and 2,4,6,8-tetramethyl-2,4,6,8-tetraazabicyclo[3.3.0]octane-3,7-dione, have been studied by ¹H and ¹³C NMR spectroscopy in D₂O, neutral and acidified solutions (acetic acid-d₄), and solutions containing 1 equiv of HEDP.

Bicyclic bisureas, especially 2,4,6,8-tetraazabicyclo[3.3.0.]octan-3,7-dione (glycoluril), have a special place in chemistry of heterocyclic compounds. The carbamide fragment in glycoluril structure determines the properties of the molecule, which are due to the presence of two reaction centers (NH-groups and C=O-groups) in the molecule. In this work, we analyzed the proton and carbon chemical shifts of glycoluril and its derivatives (86 compounds) in the NMR spectra to reveal the effect of the donor-acceptor substituents on the changes in the electron density in the bicyclic framework from the position of symmetry and asymmetry. A general analysis of the 1H and 13C NMR spectra of glycolurils makes it possible to accurately determine the spatial configurations of molecular symmetry, in the presence of which (σ1 and / or σ2) the enantiotopic hydrogen and carbon atoms of the bicyclic framework are manifested by equivalent signals. Also, according to the 1H and 13C chemical shifts in the NMR spectra, glycolurils with electron-acceptor substituents can be clearly distinguished by the shielding of carbon atoms of C=O-groups, and with electron-donating substituents by the deshielding of CH-CH-carbons, due to the rearrangement of electron density and the occurrence of local paramagnetic contributions owing to anisotropy.

The effect of phosphorus-containing catalysts, namely phosphorous acid, phosphoric acid, diethyl hydrogen phosphite, and 1-hydroxyethane-1,1-diyldiphosphonic acid (HEDP), in the synthesis of tetraacetylglycoluril by acylation of glycoluril with acetic anhydride at 140°C has been studied. The use of 4 equiv of phosphorous or phosphoric acid with respect to glycoluril has been found to be the most appropriate for achieving the maximum yield (95–98%), and specific effect of HEDP leads to N-acetylation–deacetylation of substrates. The positive effect of phosphorus acids on the formation of tetraacetylglycoluril may be related to not only their catalytic action but also the ability to solubilize the substrate.