Chain-ring-chain tautomerism in 2-aryl-substituted hexahydropyrimidines and 1H-2,3-dihydroperimidines. Does it appear?

Syntheses, Structures, and Biological Activity of NiII, PdII, and PtII Complexes with New Tetradentate Benzamidine Ligands

“…Similar ring opening reactions are observed when tetrahydroquinazolines of the type 3 are treated with an additional equivalent of aldehyde or ketone to give the corresponding bis-imine products. [17] The spectroscopic data support the formation of H 2 L R . The IR spectra of the compounds exhibit bands expected for O-H and N-H vibrations in the region above 3200 cm -1 and very strong C=N absorptions around 1620 cm -1 .…”

“…The benzamidine unit does not bind to the aromatic amine, but to the benzylamine and the azomethine bond is formed with the nitrogen atom of the aromatic amine. The C8-N7 bond length of 1.284(2) Å is clearly within the range of carbon-nitrogen double bonds, [4,17] which additionally confirms the formation of an azomethine group with the aromatic Morph . [14] Hydrogen atoms bonded on carbon atoms are omitted for clarity.…”

A comparison of the electron ionization and electrospray behaviour of some N,N′‐disubstituted hexahydropyrimidines

“…In organic synthesis, the hexahydropyrimidine nucleus has been employed as a protecting group in selective acylations and additions of 1,3‐diamines, due to its easy cleavage in mild acidic medium,11, 12 while some derivatives are synthetic intermediates for antineoplastic drugs and structural units in new trypanothione reductase inhibitors 13, 14. Some recent reports have been devoted to the study of ring‐chain tautomerism in hexahydropyrimidines,15–17 their benzo‐fused derivatives (1,2,3,4‐tetrahydroquinazolines),18 and related compounds 19–21…”

“…discussed the fragmentation of some tricyclic hexahydropyrimidines (tetraponerines) isolated from natural sources 9. More recently, mass spectrometry was employed to assess the presence of chain tautomers in N‐unsubstituted hexahydropyrimidines and related compounds 17, 21. To our knowledge, data concerning electrospray ionization (ESI) mass spectra of hexahydropyrimidines have not previously been reported.…”

Efficient Solvent‐Free Syntheses of [2]‐ and [4]Rotaxanes

“…[8] We chose 1,8-diaminonaphthalene (3) as a suitable diamine for the reaction with a threadlike moiety terminated with a formyl group because of its steric bulk and the stability of the resulting dihydropyrimidine stopper units. [9] Previously, we reported that the oxygen-deficient macrocycle 1 forms a complex with a dibenzylammonium (DBA) ion in CD 3 CN (K a = 200 m À1 ). [10] Thus, as the first step toward preparing a [2]rotaxane under solvent-free conditions from such components, we concentrated an equimolar mixture of the macrocycle 1 and the dialdehyde 2-H·PF 6 in CH 3 CN under reduced pressure to afford a white solid, which we assumed to contain predominantly the [2] (Figure 1).…”