Zur Tautomerie des α‐Amino‐pyridins, IV. Mitteilung: Eine Darstellungsmethode des Pyrimidazols und seiner Homologen

Abstract

Nachdem ich die a d e r s t leichte RingschlieQung bei dern a-Aminopyridin unter Bildung von Pyrimidazol-Derivaten festgestellt hattel), war es sehr interessant, die Grundsubstanz dieser Verbindungsreihe, d. h. das Pyrimidazol selbst, zu untersuchen. Schon vor einiger Zeit gelang es mir, die Darstellung dieser Base und ihrer Homologen in ziemlich glatter Weise zu bewerkstelligen. Das Pyrimidazol selbst bildet sich beim Erwarmen des a-Amino-pyridins mit B r o m-ace t alde h y d o d e r des s e n Ace t a1 en in zugeschmolzenen Rohren auf 150-2000. Die sich abspielende Reaktion kann in zwei Richtungen verlaufen, die aber in diesem Fall zu demselben Produkt, dem Pyrimidazol, fuhren: Br.CH, Falls zu der Reaktion anstatt des Brom-acetaldehyds dessen Dime thylacetal genommen wurde, so reagierte der sich gemail3 der Gleichung: C5H6N, + Br . CH,. CH(OCH,), = GH6N,, HBr + z CH,. OH, abspaltende Methylalkohol mit Bromwasserstoff unter Bildung von Methylbromid. Letzteres kann dann, besonders bei hoheren Temperaturen, auf das Aminopyridin methylierend wirken und dementsprechend das Reaktionsprodukt groBere. Mengen methylierter Pyridin? enthalten. Bei der Einwirkung von a-halogenierten Ketonen auf das a-Amino-pyridin bilden sich Homologe des Pyrimidazols, wobei die beiden oben besvrochenen Reaktionsrichtungen zu zwei verschiedenen Isomeren 111 und I? fiihren konnen: -HqH = 111. NH ,CH,.Br CO. CH, HqH = 111. NH ,CH,.Br CO. CH, /a I/ 1. = IV.

Synthesis and tautomerism of aryl- and hetaryl-azo derivatives of bi- and tri-heterocycles

“…The other possible regioisomeric structure 83 was discarded on the basis that reactions of 2-aminopyridine 76 with a-halo-esters were reported to give 84 (Scheme 25) [9,17,18].…”

“…The isomeric structure 9 was discarded on the basis that the isolated product was recovered unchanged after being subjected to oxidation treatment. Structures of type 9 are expected to be oxidized by analogy with 1-methyl-3-phenylhydrazono-1,4-dihydroquinoxalin-2-one 10 which was reported to give 3-phenylazoquinoxaline derivative 11 upon oxidation (Scheme 4) [17][18][19].…”

Preparation of New Nitrogen-Bridged Heterocycles 72. A New Approach to 1-Acyl-3-(substituted methylthio)thieno[3',4':4,5]imidazo[1,5-a]pyridine Derivatives

“…The final R-and Rw-factors after full-matrix least-squares refinements were 0.0612 and 0.0492 respectively for 2462 (IϾ2.00s(I)) observed reflections. 3 . All calculations were performed using the CrystalStructure.…”

“…[2][3][4][5][6][7][8][9][10][11][12] In contrast, imidazo[1,2-a]pyridine derivatives fused with a hetero ring at the 2-and 3-positions have been scarcely reported because of the absence of a suitable preparative method for them. We have recently described the syntheses of 3-alkylthio-1-arylcarbonyl-6,8-dimethylthieno[3Ј,4Ј:4,5]imidazo[1,2-a]pyridines 13) and 3-acyl-4-methylthio-2H-pyrano [2Ј,3Ј:4,5]imidazo[1,2-a]pyridine-2-ones.…”

Synthesis, reactions, and antimicrobial activity of some novel pyrazolo[3,4‐d]pyrimidine, pyrazolo[4,3‐e][1,2,4]triazolo[1,5‐c]pyrimidine, and pyrazolo[4,3‐e][1,2,4]triazolo[3,4‐c]pyrimidine derivatives

“…The results of elemental analyses of the products were found to be consistent with the two isomeric structures 36-38 and 39-41 and its tautomeric structure 42-44 (Scheme 6). Since the imino nitrogen is known to be more basic and in turn more nucleophilic than that of the amino group [29,30] (this reaction seem to follow the same mechanism reported for the reactions of 2-aminopyridine with α-oxohydrazonoyl halides and α-haloketones and esters) (Scheme 7), [31,32] the intermediate 33-35A is not expected to be formed, and accordingly, structure 36-38 was discarded. Furthermore, the electronic absorption spectrum of the isolated products excludes the hydrazone structure 39-41 since they revealed a characteristic absorption maxima at 361 (logε 4.43) and 400 (logε 4.40) nm assignable to an arylazo chromophore.…”