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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 8, Number 3
March 20, 1987 

Structural Studies on Cyclic Compounds. Substituent Effects on the Reducing Ability of Dihydropyridines
Kwanghee Koh Park*, Gyeoung Un Moon, Ki-Sung Kwon
The reaction between 1-benzyl-3-carbamoyl-1,4-dihydropyridine (BNAH) 1 and various 1-arylpyridinium salts 2, and the reaction between 1-(4-methylphenyl)-1,4-dihydropyridine 4b and 1-aryl-3-carbamoylpyridinium (1-arylnicotinamide) salts 5 were carried out. The extents of reaction in equilibrium were estimated by nmr integration data. The equilibrium constants for the reactions, K, and the standard Gibbs free energy changes for the reduction of the pyridinium salts to the corresponding 1,4-dihydropyridines ΔG。' were evaluated. The Hammett plot of log K for the reaction between 1 and 2, and ΔG。' against σp of the substituents in 1-aryl moiety shows linear correlation with the reaction constant ρ of 9.4 (for log K vs σp) and -54.5 KJ/mole (for ΔG。' vs σp). It was found that 1-aryl-1,4-dihydropyridines have much higher reducing power than the corresponding 1-aryl-1,4-dihydronicotinamides, and the power is affected greatly by the electron-withdrawing ability of the substituents in aryl group. The reactions were utilized for preparation of 1,4-dihydropyridines bearing highly electron-withdrawing groups such as 4-nitrophenyl and 2,4-dinitrophenyl, which could not be obtained by conventional dithionite reduction of the corresponding pyridinium salts due to the base-labile nature of the salts.
168 - 170
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