Photoinduced Aromatization of Unsymmetrically Substituted 1,4‐Dihydropyridines

Abstract

Irradiation of unsymmetrically substituted 1,4-dihydropyridines (1b-1j) while bubbling oxygen or argon through the solution resulted in aromatization to the corresponding pyridine derivatives (3b-3j). Compound 1a with 2-nitrophenyl substituent in 4-position undergoes elimination of water upon irradiation under both oxygen and argon atmospheres and formation of 3a with 2-nitrosophenyl substituent. On the other hand, irradiation of the compounds 1e, 1k and 1l with 4-hydroxy-3-methoxyphenyl, 5-methyl-2-furyl and 2-furyl substituent in this position, respectively, resulted in the expulsion of these substituents and formation of a pyridine derivative unsubstituted in position 4, namely compound 2. Chloroform as a solvent causes the photo-oxidation of these compounds by an electron transfer mechanism which is supported by the formation of dichloromethane according to GC analysis and presence of acid (HCl) in the solution after irradiation.

Light-induced Free Radical Oxidation of 2-Oxo-1,2,3,4-tetrahydropyrimidines

“…This observation is possibly the reason why the concurrent removal of a hydrogen atom from the allylic position (6-CH 3 ) is not observed in the present study. It should also be noted that the removal of a benzylic or secondary alkyl substituent on the 4-position of the heterocyclic ring has not been observed as it was found in the case of 1,4-dihydropyridines [12,13].…”

“…There are a wide variety of methods to synthesize pyrimidinones [8], but the efficient synthesis of 1,2-dihydropyrimidinones by oxidation of 1,2,3,4-tetrahydropyrimidinones has rarely been explored [9 -11]. In contrast to Hantzsch-type 1,4-dihydropyridines, whose aromatization to the corresponding pyridines, either thermally [12] or photochemically [13], is typically facile, the dehydrogenation of 1,2,3,4-tetrahydropyrimidinones is known to be non-trivial [9,11]. Various powerful and mild oxidants such as chloranil, DDQ, KMnO 4 /clay, MnO 2 , PCC, NaNO 2 /AcOH [9] or RuCl 3 /O 2 in AcOH [14], Co(NO 3 ) 2 ·6H 2 O/K 2 S 2 O 8 [9], and also dehydrogenating agents such as Pd/C [8], Br 2 [15] c 2012 Verlag der Zeitschrift für Naturforschung, Tübingen · http://znaturforsch.com and sulfur [16] have been used for this purpose, but none of these reagents is ideal, particularly due to their safety profile, lower yield and difficulty in product isolation.…”

Substitution effects on the NMR and DFT studies of 4,6‐diaryl‐2‐oxo‐1,2,3,4‐tetrahydropyrimidines

“…[4][5][6] In our previous studies, we were interested in elucidating the parameters affecting the rate of thermal and photochemical oxidation of some nitrogen containing heterocyclic compounds, e.g. 1,4-dihydropyridines (DHPs), [7] 2-oxo-1,2,3,4-tetrahydropyrimidines (THPMs) [8] and 2,3-dihydroquinazolinones (DHQZs). [9] The results of experimental work concerning the steric and electronic effects of the substituent on the light-induced oxidation, [8] thermal oxidation [10] and also computational studies [11,12] in the THPM system indicate that the electronic nature and steric effect of the additional substituent, on the aryl ring attached to the C 4 -atom, influence the electron density of the heterocyclic ring through the σ bond, while the same effects are observed via conjugation of the C 5 -substituent with the C 5 ¼C 6 double bond (Scheme 1).…”

Study of the Synthesis of Some Biginelli-type Products Catalyzed by Nano-ZrO2

“…Pyrimidine cores with extended π systems have interesting fluorescence properties, and similar compounds are useful in the development of advanced electronic and photonic materials [2]. Although various methods for the dehydrogenation of specific 1,4-dihydropyridines have been reported in the literature [3], 3,4-dihydropyrimidin-2(1H)-ones are highly stable toward mild and powerful oxidizing reagents. Recently, Memarian and coworkers have developed the best methods for the oxidation of these compounds [4 -10].…”