By Michael addition of the active methylene groups in pyridinium salts onto suitable acceptor compounds, a-pyridones, substituted pyridines, particularly pyridinecarboxylic acids and pyridylpyridines, including the minor alkaloid of tobacco nicotelline and annelized pyridines, can be prepared by a simple procedure and generally in good yieldr. From the Michael adducts, polycyclic aromatic hydrocarbons, e. g. substituted fluoranthenes and "bisfluoranthenes" can be prepared; internal Michael addition leads to pyrroiinopyridinium salts. For example, Michael addition of pyridinium salts onto quinones gives phenacyl substituted quinones, from which benzofurans and cinnolines can be readily ob tained.Because of their structure and the manner in which they react, phenacylpyridinium betaines have often been compared to @-diketones [2]. This analogy has also become apparent from the Michael addition of these pyridinium betaines onto the geminal double bonds of phenyl isocyanate [3] and phenyl isothiocyanate [3] or onto quinones [4]. Heffe, our coworker at that time, later established that phenacylpyridinium bromide ( l ) , for example, readily adds onto the double bond of diaroylethylenes to give an isolable intermediate capable of splitting off pyridine [5]. Subsequently, Thesing and coworkers [6] at Darmstadt and Krohnke and Zecher at Giessen were able to carry out abundant syntheses using the Michael addition of pyridinium salts. In view of the analogy between phenacylpyridinium salts and (3-diketones, the Michael addition of the former onto quinones was also investigated.
Cleavage of N-phenylpyridinium or N-vinylpyridinium salts with secondary aliphatic amines leads to aromatic amino compounds or enamines which are often unobtainable by other routes. The other fragment, glutacondialdehyde or its rnonoanil, gives azulenes, as shown by Ziegler and Hafner, as well as Konig; these compounds can also be prepared from alkylpyridinium salts with cyclopentadienylsodium (Hafner). Syntheses of labile aldehydes which are otherwise dijjjcult to obtain and of a-ketocarboxylic acids, starting from pqiridinium salts and proceeding via nitrones or a-cyanoanils, are illustrated by many wcw examples. Benzimidazoles, benzothiazoles, and amidines are accessible from pyridinium salts via nitrones or a-cyanoanils; cyanoanils can be transformed into guinoxalines (including xanthopterine), perimidines, etc. Furthermore, nitrosonaphthols and nitrosonaphthylamines react with pyridinium salts containing an active iizethylene group on the nitrogq leading to new ring syslems. Further syn fhetic possihilities arise from nucleophilic &isplacements of the pyridine in pyriditiium salts by SN2 reactions. Combination of the route via nitrones with the methods of Arndt-Eistert, Ortoleva-King, or with chloromethylation constitutes a significant enlargement of its range of'application. The forniation of a-cyanoanils from diazoketones permits synthesis of a-keto-0-umino acids from a-amino acids, or of phenylglyoxylic acids from benzoic acids by chain extension.
Die aktivierende Wirkung des Pyridinium‐Restes auf eine benachbarte N‐Methylen‐ oder N‐Methin‐Gruppe befähigt viele Pyridiniumbetaine, deren verschiedene Typen näher besprochen werden, in einer den β‐Diketonen vergleichbaren Weise zu Kondensationen mit Aldehyden, Nitroso‐Verbindungen, Diazoniumsalzen usw. Dabei kann vielfach der Pyridinium‐Rest spontan oder in einer zweiten Umsetzung abgespalten werden, so daß in oft vorteilhafter Weise Carbonsäuren, Nitrone, Aldehyde, Ketone, α‐Ketoaldehyde, α‐Ketosäuren, Isatogene usw. zugänglich werden. Auch nach anderen Reaktionsmechanismen sind Pyridiniumsalze nicht selten wichtige Zwischenprodukte für Synthesen.
Es werden mit Hilfe der neuen Synthese di-, tri-und tetrasubstituierte Pyridine bereitet. Auch lassen sich die Ringsysteme etwa des 7-Aza-fluoranthens, des 4-Aza-fluorens und des 4-Aza-phenanthrens aufbauen. Ferner werden substituierte 2-Hydroxy-, 2-Mercapto-und 2-Amino-pyridine nach dem neuen Prinzip dargestellt.Die Pyridinsynthese mit Hilfe von Cyclimmoniumsalzen1) ist nicht auf die Darstellung von Triaryl-pyridinen beschrankt. Die vorliegende Arbeit bringt einige Beispiele, wie man sie auch fur die Synthese anders substituierter Pyridine nutzbar machen und wie man durch sie zu kondensierten Heterocyclen gelangen kann. Di-substituierte Pyridine entstehen nach unserer Synthese auf verschiedenen Wegen. Aus Zimtaldehyd und dem Salz I wurde in maDiger Ausbeute (25% d. Th., bezogen auf I) das 2.4-Diphenyl-pyridin als Pikrat gewonnen; bei Verwendung von Zimtaldehyd-hydrazon entsteht es gar nur zu 18% d. Th. Grundsatzlich kann man auch statt der a.P-ungesattigten Carbonylverbindungen die zugehorigen P-Keto-MannichBasen heranziehen, die ja im Reaktionsgeschehen in jene iibergehen3). Zum Beispiel erhalt man aus w-Dimerhylamino-propiophenon (11) und I in Eisessig/Ammonium-N(CH3)Z 1 ) 1.
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