A condensation between mesylamide and glyoxal under highly acidic conditions was studied. A series of new compounds whose structures contain a moiety of oxaazatetracyclo[5.5.0.03,11.05,9]dodecane were synthesized. The synthesis processes were optimized and main factors affecting the assembling of cage products were revealed. Assumptions are made regarding the formation mechanism of these chemical entities. For the first time, compounds have been obtained that include moieties of 2,6,8-trioxa-4,10,12-triazatetracyclo[5.5.0.03,11.05,9]dodecane and 2,4,6-trioxa-8,10,12-triazatetracyclo[5.5.0.03,11.05,9]dodecane. An X-ray diffraction analysis of the new compounds was performed.