A series of new 1,3,4-thiadiazines with cycloalkylamino (cyclopropylamino, cyclobutylamino, cyclopentylamino, cyclohexylamino) groups were synthesized via cyclocondensation of α-haloacetophenones with thiosemicarbazides containing 4-cycloalkyl groups. Five of the synthesized compounds showed the capability to inhibit nonenzymatic glycosylation of proteins in vitro in a model system. The obtained test results allowed compounds containing cyclopropylamino residues (LT-1a and LT-1d) to be recommended for further in vivo testing.