Pyrimidine dihydro-, tautomerism

Surprisingly, Kashima et al. (83TL209) reported the formation of individual 1,4-dihydro- and 1,6-dihydropyrimidines on desulfurization of the corresponding pyrimidine-2-thiones with Raney Ni and claimed that no tautomerization occurs under the reaction conditions (heating under reflux in MeOH). 

Oxo-4-amino-l,2,5,6-tetrahydro-lf/-pyrimidine. A dihydropyrimidine, not a tetrahydropyrimidine (two exo- and endoeyclic double bonds) 4-Amino-5,6-dihydro-2(l//)-pyr>midone (Chem. Abstr.) assigns the correct oxidation state, tautomeric form and location of the proton the danger is that the latter is not always known, however. 

Pyrimidine-4,6(lH,3H)-dione, 5-ethyl-2,5-dihydro-2,5-diphenyl-synthesis, 3, 108 Pyrimidine-2,4( 1 H,3H)-diones thiation, 3, 89 Pyrimidine-4,5-diones purine synthesis from, 5, 582 Pyrimidine-4,6-diones tautomerism, 2, 27... 

The structure of the reaction product of 2-aminopyridine and diethyl malonate, described by Chichibabin as 2,4-dioxo-3,4-dihydro-2//-pyrido-[l,2-<7]pyrimidine,96 was first questioned by Snyder and Robison253 on the basis of the high melting point and poor solubility of the compound. They suggested the tautomeric 2-hydroxy-4-oxo-4H-pyrido[l,2-a]pyrimidine structure. The problem was solved by Katritzky and Waring273 who compared the UV spectrum of the product with that of fixed tautomers and found that the product may best be described as anhydro- 2-hydroxy-4-oxo-4/f-pyrido[l,2- ]pyrimidinium)hydroxide (63). Because of the chemical behavior of these compounds, however, the contribution of other mesomeric forms to the structure has also been considered.122 Thus, PPP-SCF quantum chemical calculations suggest that 1,4-dipolar cycloadditions to the C-3 and C-9a atoms are to be expected.352 This type of reaction does in fact occur (see Section III,C,10). Katritzky and Waring273 estimated the ratio of the mesomeric betaine (63 R = H) and the 2-hydroxy-4-oxo tautomers to be about 20 1. 

It has been shown [248] that unsubstituted dihydropyrimidine 427 existed in tautomeric form D. But introduction of phenyl substituents at positions 2 and 4 leads to a convergence of energies of the tautomeric forms and 4,6-diphenyl-l,2(2,5)-dihydropyrimidine is observed in solutions of CDC13 as a mixture of D and E in a ratio of 2 1 [248]. Pyrimidines 427 containing electron-donors at positions 4 and 6 exist in the dihydro form E [424]. 

Increasing the electron-donor properties of the R, substituent in the case of dihydro derivatives of 1,2,4-triazolo[l, 5-a]pyrimidines [X, Z are N, Y is C(R)], tetrazolo[l, 5-a]pyrimidines (X, Y, Z are N), and pyrimidofl, 2-a]benzimidazoles (X, Y are o-C() I4, Z is N) tends to stabilize the B form. This can be explained by the conjugation of Rj with the electron-withdrawing azomethine group and the azole moiety. Reduction of the electron-accepting properties of the azole fragment (imidazopyrimidines and pyrazolopyrimidines) decreases the influence of the Ri substituent on tautomeric composition. 

Strong heating of 2-substituted 5-propargylsulfanyl-3-aryl-3//-pyrimidin-4-ones, and their 5-allylsulfanyl derivatives, has been shown to lead to 2-substituted 3-aryl-6-methyl-3//-thieno[3,2-d]pyrimidin-4-ones and their 6,7-dihydro derivatives. The proposed mechanism in both cases is a [3,3]-sigmatropic thia-Claisen rearrangement followed by tautomerization and a 5-exodig or 5-exo-trig cyclization.38... 

Valence tautomerization of pyrimidine-alkyne photoadducts is mediated by treatment with trimethylsilyl chloride/hexamethyldisilazane, followed by silica gel, affording diazocin-2,4-dione derivatives (8) in good yield (vide supra) <82TL4207,83JOC2337). Oxidative cleavage of a bicyclic diaza compound by rose Bengal-sensitized addition of oxygen affords the benzo-3,4-dihydro-1,3-diazocin-2,6(17/,5//)-dione derivative (38) (Equation (15)) <87H(26)2477>. 

X-Ray analyses and solid-state IR spectra were recorded for a number of 1,4-and 1,6-dihydropyrimidines, demonstrating the dependency of the tautomeric composition in the crystal on the substitution in the pyrimidine ring and on the ability of these compounds to form intermolecular hydrogen bonds. Thus, 2,4,6-trisubstituted dihydropyrimidines exist in the solid state exclusively as 1,4-dihydro tautomers independently of the nature of the substituents (alkyl, aryl, heteroaryl, acyl, alkoxycarbonyl, etc.) [82H(19)493 86H(24)233 86T6429 90JMC1510]. Introduction of another alkyl substituent into the 4 position shifts the equilibrium toward the 1,6-dihydro isomer. For example, 2-phenyl-... 

Acylation and metallation studies on monomers, dimers, trimers, and tetramers containing linked thiophen, pyridine, pyrimidine, furan, benzofuran, benzothiophen, and indole moieties have been published/ Deuteriation, halogenation, and diazo-coupling reactions of 2-oxo- and 2-thioxo-l,2-dihy-dropyrimidinium salts have been studied and compared with results for 2,2-dialkyl-1,2-dihydropyrimidinium and 2,3-dihydro-1,4-diazepinium salts in order to demonstrate the effect of an adjacent 0x0- or thioxo-group on the properties of a 1,5-diazopentadienium system/ Vilsmeier formylation of, and tautomerism in, 2-hydroxypyrazolo[5,l-h]quinazolone and l-phenylpyrazolo[5,l-A]-quinazoline-2,9-dione have been studied/ The pyrazolo[3,4-c]pyrazole (71) has been methylated and acetylated, the major products being (72)/ ... 

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