A general six-step approach to alkyl 2-oxo-2,3,6,7-tetrahydro-1H-1,3-diazepine-5-carboxylates and 5-acyl-2,3,6,7-tetrahydro-1H-1,3-diazepin-2-ones based on the nucleophile-mediated ring expansion reaction of 5-functionalized 4-mesyloxymethyl-1,2,3,4-tetrahydropyrimidin-2-ones has been developed. Synthesis of the latter involved nucleophilic substitution of tosyl group in readily available N-[(2-benzoyloxy-1-tosyl)ethyl]urea with sodium enolates of β-oxoesters or 1,3-diketones, followed by dehydration or heterocyclization-dehydration of resulting products, removal of benzoyl protection, and conversion of hydroxymethyl group into mesyloxymethyl group. Conformations of the obtained tetrahydro-1H-1,3-diazepin-2-ones in solid state and solutions were established using X-ray diffraction and NMR spectroscopy. A plausible mechanism of tetrahydropyrimidine ring expansion based on DFT calculation at B3LYP/6-31+G(d,p) level and NMR monitoring experiments was discussed. The ring contraction reaction of methoxy- or phenylthio-diazepinones under acidic conditions resulted in the corresponding 3-functionalized 1-carbamoyl-1H-pyrroles.