Diazines pyrimidines

Intennoleciilar Diels-Alder Reactions of Pyrimidines (1 -Dlazines) 

The addition of strong electron-withdrawing substituents to the pyrimidine nucleus increases the facility and rate with which the system participates in inverse electron demand [4 + 2] cycloadditions with electron-rich 

Complementary substitution of the pyrimidine nucleus with two (or three) strong electron-donating groups at C-2 and C-4 (and C-6) is sufficient to permit the 47t participation of the pyrimidine in apparent normal (HOMO-diene Controlled) Diels-Alder reactions with dimethyl acetylenedicarboxy-late [Eq. (19) and Table 10-V]. 

The intramolecular Diels-Alder reactions of simple, 4-hydroxy- or 4-alkyl-6-oxopyrimidines ° bearing olefinic, acetylenic, or nitrile dienophiles have been investigated (Table 10-VI). In many instances, the initial intramolecular pyrimidine Diels-Alder products are thermally stable due to the formation of a bicyclic amide. The elimination of cyanic acid (HNCO) apparently requires thermal tautomerization of the amide to the hydroxyimine and subsequent retro-Diels-Alder reaction. The application of these observations in the total synthesis of ( )-acetinidine has been detailed [Eqs. (20) and (21).  

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Pyrimidine (1,3-diazine) and pyrazine (1,4-diazine) exhibit weak fluorescences73,74 in solutions or as vapors at room temperature, and strong phosphorescences 76-79 in dilute solid solutions at low temperatures (77 or 90°K). The phosphorescent quantum yields have never been accurately measured in these solid solutions. In the vapor phase or in ordinary solutions, at room temperature, these two compounds do not phosphoresce. Radiationless deactivation processes must be considered again and a deactivation through an isomer cannot be excluded. 

This section deals with molecules derived from the diazines C4H4N2 pyrazine (1,4-diazine 7), pyrimidine (1,3-diazine 8) and pyridazine (1,2-diazine 9). 

In this section we shall present and compare different computational strategies one can adopt to simulate the effect of the environment on spectroscopic properties of solvated systems. In particular, as a representative example, we shall summarize the results of two studies [30,31] we have published in the last years on the environment effects on the nitrogen nuclear shieldings of a specific class of molecular systems containing sp2-type nitrogens three diazines, also known as pyridazine (1,2-diazine), pyrimidine (1,3-diazine) and pyrazine (1,4-diazine). 

Regiospecific inverse electron demand Diels-Alder reactions of enamines with 1,3-diazines or 1,2,3- and 1,2,4-triazines (see Section IH.D.l), which on elimination of HCN or Nj, respectively, produce a pyridine ring, can be used with 1,3,5-triazines and 1,2,4,5-tetrazines as a useful method for the synthesis of pyrimidines (1,3-diazines) and pyridazines (1,2-diazines). Examples of the use of this methodology are the preparation of the pyrimidine substituted benzomorphane 356 (equation 77) and the pyridazine 359 (equation 78), intermediate in the total synthesis of cis- and trans-trikentrin... 

Pyrimidine 1,3-diazine, a heterocyclic compound, consisting of a six-membered ring with 2 nitrogen atoms (Fig.l), M, 80.1, m.p. 20-22°C, b.p. 124°C. The P. ring system is present in many natural compounds, e. g. antibiotics (nucleoside antibiotics), pterins, purines and vitamins, it is especially important in the pyrimidine bases. Cytosine (see). Uracil (see) and Thymine (see), which are constituents of nucleic acids. Pyrimidine itself does not occur naturally. Pyrimidine analogs (see) can also be incorporated into nucleic acids. 

Diazines are aromatic six-membered heterocycles that contain two sp -hybrid-ized nitrogen atoms in the ring. The three diazine isomers are pyridazine (1,2-dia-zine), pyrimidine (1,3-diazine) and pyrazine (1,4-diazine). The most important naturally occurring diazines are the pyrimidine bases uracil, thymine, and cytosine, which comprise the fundamental nucleoside building blocks in deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Pyrazines occur frequently as... 

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