Segregation and identification of oligomers in the synthesis of kukurbiturils

Bicyclic ureas, especially 2,4,6,8-tetraazabicyclo[3.3.0.]octan-3,7-dione (glycoluril) and its derivatives, have a special place in chemistry of heterocyclic compounds. The geometrical features of glycoluryls, and in particular the framework structure, essentially determined the possibility of synthesizing and studying on their basis supramolecular polycondensed compounds such as cucurbit[n]urils, which are used in the synthetic, medical, and material sectors. Cucurbit[n]uryls are macrocyclic molecules consisting of glycoluryl monomers linked by methylene bridges (-CH₂-). Oxygen atoms are located along and tilted inward, forming a partially closed cavity. These systems are usually synthesized by a polycondensation reaction between glycoluryl and formaldehyde in acidic solutions. As a rule, for multifunctional monomers such as glycoluryl, stepwise polymerization is typical with the production of acyclic oligomers, which leads to a decrease in the yield of the target cucurbit[n]uryl. In this case, the key factor affecting the yield of cucurbit[n]uryl and its structural parameters is the ratio of the starting reagents. The identification of cucurbit[n]urils is complicated by extremely difficult solubility. The formation of a macrocyclic structure is confirmed on the basis of X-ray diffraction analysis and IR spectroscopy. In this work, we studied the condensation reactions of glycoluril with formaldehyde in the ratios 1: 1, 1: 1.3, 1: 4 in an acidic medium, where dimeric and trimeric oligomers, the precursors in the synthesis of cucurbi[n]urils, were isolated and identified. The structures of the isolated compounds are proved on the basis of IR, NMR spectroscopy. The oligomeric nature is evidenced by the signals of the terminal NH- and OH-groups. In the process of research, a method for the selective production of oligomers by regulating the amount of formaldehyde was developed. The conditions of the condensation process were also found under which the final stage of cyclization to cucurbit[n]urils is not achieved, where the formation of a methylolderivative inhibits further condensation, since the OHCH₂-groups "stop” the further process. This structure of the substance explains that during a long reaction and high temperature insoluble precipitates do not form, indicating the formation of macromolecular systems.

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