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REACTIONS OF TRIAZINES AND TETRAZINES WITH DIENOPHILES (REVIEW) A. M. Prokhorov1 and D. N. Kozhevnikov1,2* Information on the development of a valuable synthetic methodology based on inverse electron-demand Diels–Alder reaction in the series of π-deficient azadienes (triazines and tetrazines) is summarized and classified. By this method it is possible to obtain mono-, di-, and triazines containing functional substituents both from the initial azine and from the dienophile. Such reactions are often the simplest and even the only possible method for the synthesis of substances with properties suitable for practical applications. Keywords: azadienes, pyridazines, pyridines, pyrimidines, tetrazines, triazines, Diels–Alder reaction. Azines can enter into inverse electron-demand Diels–Alder reactions. Here π-deficient triazines and tetrazines are used as dienes, while alkenes or alkynes with electron-donating substituents are used as dienophiles. Cycloaddition initiates a cascade of reactions including elimination of a nitrogen molecule (path 1) or nitrile (path 2) from the cycloadduct, as a result of a retro-Diels–Alder reaction and subsequent aromatization of the intermediate product by one of the possible ways, e.g., by the elimination of an amine N H N Z
Y
D
N
Path 1
Z Y
X
H
H
N
–N N
X
D Z
Y
–HD
X
Z
Y
X
D R H
H X Y
R
N Z
N
D
Path 2
Z Y
N N X
D
X – R C N Y
D Z
N
X –HD
Y
Z
N
H D = NR2, OR; X, Y, Z = N, CH
_______ *To whom correspondence should be addressed, e-mail: [email protected]. 1
Ural Federal University named after the First President of Russia B. N. Yeltsin, 19 Mira St., Yekaterinburg 620002, Russia; e-mail: [email protected]. 2 I. Ya. Postovskii Institute of Organic Synthesis, Ural Branch, Russian Academy of Sciences, 20 S. Kovaleskoi St. / 22 Akademicheskaya St., Yekaterinburg 620990, Russia. _________________________________________________________________________________________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1237-1261, August, 2012. Original article submitted May 21, 2012. 0009-3122/12/4808-1153©2012 Springer Science+Business Media New York
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molecule. The final product of such a transformation is pyridazine, pyrimidine, or pyridine. Fundamental papers in this area are listed as references in the paper by Boger [1]. The numerous examples of the reaction published before 1996 can be seen in the reviews [2-4]. Many new interesting examples of the use of the Diels–Alder reaction in the series of triazines and tetrazines have appeared in the last 15 years. The present review is devoted to generalization of these reactions. The reactions are classified according to the type of azadiene. When applying the Diels–Alder reaction in the series of triazines and tetrazines, it is necessary to resolve problems involving increasing the reactivity and regioselectivity of cycloaddition. It is clear that the energy and structure of the frontier orbitals of both reagents in this case play a crucial role. Unfortunately, the only attempt [1] at a th
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