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at room temperature in the presence of imidazole as ax-
ial ligand. As shown in Table 1, oxidation of 4-isopropyl
derivative (alkyl moiety may be responsible for generat-
ing stable carbocation) was accompanied by expulsion
of this substituent and gave dealkylated pyridine deriva-
tive (entry 13). All reactions were completed during the
appropriate time and gave only the corresponding pyri-
dine derivative. The results are summarized in Table 1.
plate or silica gel column (eluent: CCl4–Et2O). The identi-
ties of products were confirmed by mp, IR and 1H NMR
spectral data.
4. Conclusions
Mn(III) porphyrin/IOÀ4 catalytic system has the
following advantages in the oxidation of Hantzsch
1,4-dihydropyridines to their corresponding pyridine
derivatives: (i) understanding the action of cytochrome
P-450 in the oxidation of Hantzsch 1,4-dihydropyri-
dines to their corresponding pyridine derivatives, (ii)
short reaction time, (iii) high efficiency for oxidation
of Hantzsch 1,4-dihydropyridines to their correspond-
ing pyridine derivatives and (iv) mild reaction condi-
tions. Therefore, the present method could be a
useful addition to the available methods in organic
synthesis.
In the absence of Mn(TPP)Cl catalyst, (Bu4N)IO4 has
poor ability to oxidize 1,4-dihydropyridines at room
temperature (6–10% yields).
2.2. Effect of solvent on the oxidation of 4-phenyl
derivative of 1,4-dihydropyridine
Among methanol, acetone, acetonitrile, chloroform and
dichloromethane, CH2Cl2 was chosen as the reaction
medium, because manganese(III) porphyrin complex,
1,4-dihydropyridine derivatives and oxidant are highly
soluble in this solvent and higher pyridine derivative
yields were observed.
Acknowledgments
2.3. Effect of axial ligand on the oxidation of 4-phenyl
derivative of 1,4-dihydropyridine
The partial support of this work by Yasouj University
Council of Research is acknowledged.
One important aspect of this catalytic system is the modi-
fication of the oxidation rate by addition of a small
amount of imidazole to the reaction mixture. The form-
ation of their corresponding pyridine derivatives in the
absence of axial ligand is slow and the yields are always
below 10%, whereas the amount of product reaches 97%
in the catalyzed reaction with the imidazole as the axial
base.
References and notes
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1984, 1967.
The effect of different axial ligands upon the oxidation
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3. Experimental
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Tetraphenylporphyrin was prepared and metallated
according to reported procedure.14 All Hantzsch
1,4-dihydropyridines were synthesized by the reported
procedures.15 1H NMR spectra were obtained with a
Brucker AW80 (80 MHz) spectrometer.
9. Traylor, T. G.; Fann, W. P.; Bandyopadhyay, D. J. Am.
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3.1. General procedure for oxidation of Hantzsch 1,4-
dihydropyridines to their corresponding pyridine
derivatives
12. Triggle, D. J. In Comprehensive Medicinal Chemistry;
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All of the reactions were carried out at room tempera-
ture in a 25 mL flask equipped with a magnetic stirring
bar. To a solution of Hantzsch 1,4-dihydropyridine
(1 mmol), Mn(TPP)Cl (0.02 mmol) and imidazole
(0.067 mmol) in CH2Cl2 (10 mL) was added tetra-n-
butylammonium periodate (2 mmol). Progress of the
reaction was monitored by TLC. After the reaction
was completed, the product was purified by silica gel