Dihydropyrimidines, stabilities

A similar effect is produced by cocrystallization with protic solvents capable of forming a hydrogen bond-stabilized environment. Thus, dihydropyrimidine 47 (R = R = aryl,R = R = COOR, R = H) cocrystallizes with water (1 1) exclusively as the 1,6 tautomer (98T9837). 2,4,6,6-Tetraphenyldihydropyrimidine 47 (R = R = R = R = Ph, R = H) exists as the 1,6 tautomer in its solvate with... 

This subsection examines the hydrolytic stability of cyclic structures containing a ureido link. Schematically, ring closure can be achieved by N-alkylation or by /V-acylation of the second N-atom of the ureido moiety. The former results in the formation of, e.g., hydantoins and dihydropyrimidines. The latter ring closure leads to, e.g., barbituric acids. Taken together, cyclic ureides can also be regarded as ring structures that contain an imido function with an adjacent N-atom. We begin our discussion with the five-membered hydantoins, to continue with six-membered structures, namely dihydropyrimidines, barbituric acids, and xanthines. 

There are five dihydropyrimidines (455)-(459). Most of those known have either the 1,2- or the tautomeric 1,4- or 1,6-dihydro structures. Gaussian 70 ab initio calculations of the energy of unsubstituted dihydropyrimidines yielded the following order of stability (457) > (456) > (455) > (458) > (459). The results agree with the experimentally observed behavior of these compounds... 

Gaussian 70 ab initio calculations of the energy of unsubstituted dihydropyrimidines (9a-e)21 yielded the following order of stability 9c > 9b > 9a > 9d > 9e. These results agree with the experimentally observed behavior of... 

Similarly, introduction of electron-donating substituents such as NR2, SR, or OR in the a or y position of usually unstable cyclic imines E, F, and G enhances chemical stability and enables these compounds to be easily isolated in each DHA family, for instance, 3,4-dihydropyridines (18a), 2,5-dihydro-pyrimidines (18b), 4,5-dihydropyrimidines (18c), and 4,5-dihydropyridazines (18d). 

The dihydropyrimidines 104 are the only known isolated compounds with 2,5-dihydro structures that can undergo oxidation to the corresponding pyrimidines. The existence of this structure may probably be explained by stabilization by the two alkoxy groups in positions 4 and 6. 

For these measurements, 6-methyl-2,4-diphenyl-1,4-dihydropyrimidine (MDHP) was used because of its relative stability and the near unity of its tautomeric equilibrium constant in aprotic solvents. 

Dihydro-1,3,5-triazines are of fundamental interest because of their ability to undergo amidinic tautomerism. Furthermore, as these are nitrogen-containing analogues of dihydropyrimidines (methylene at position 5 replaced by N) it would certainly be interesting to compare the effects of nitrogen substitution on the structual stability and tautomeric behavior of these compounds. 

In recent years, metal-free stabilizer systems have been developed because of environmental concerns of traditional lead and tin stabilizers. Chemically regarded nitrogen-based molecules are used, for example, P-aminocrotonates, dihydropyrimidines [15], trialkanolamines, as well as their reaction products and salts, for example, the perchlorate salt [16], etc. (Fig. 11.11). Owing to their good compatibility with other stabilizers, organic-based stabilizers are predestinated to be used in PVC recycling in case restabilization is needed [17]. 

Biginelli reaction (combining ArCHO, MeCOCH2COEt, and urea) has been catalysed by a combination of a chiral bifunctional primary amine-pyridine and HCl in dioxane/CHCl3 to form dihydropyrimidines with up to 99% ee Lewis-acid-promoted formations of dihydropyrimidinones via Biginelli reactions catalysed by imidazolium-based ionic liquids have been found by NMR, ESI-MS, and theoretical studies to proceed via stabilized charged intermediates. ... 

---