Theoretical Gibbs free energies have been determined for a set of cycloaddition products of allene with CO2, CS2 or OCS molecules, and of CH2CX with CH2CY molecule (X, Y=O, S). Each of the cycloaddition reactions leads to one or two isomeric products with four-membered heterocyclic ring. Theoretical thermochemical data suggest that the isomers in which sulphur atom is build into the four-membered ring can be easily formed in different reactions whereas formation of those with oxygen atom in the ring strongly depends on the considered reaction. Analysis of the theoretical 1H and 13C chemical shifts forms a basis for the detection of the isomer traces in reaction mixtures as well as for the elucidation of future experimental spectra.