Tautomeric equilibria in the reaction products of asymmetric 1,3-diamines with β-dicarbonyl compounds

The reaction products of 1,3-butanediamine and 2-methyl-2,4-pentanediamine with beta-keto aldehydes were shown by H-1 and C-13 NMR spectroscopy to exist as tautomeric mixtures in solutions, comprising one cyclic and two open-chain forms due to the non-equivalence of the amino groups. The chain products exist as Z- and E-isomers. After equilibration, the products from 1,3-butanediamine contain relatively less of the cyclic form than those from 2-methyl-2,4-pentanediamine. The products of 2-methyl-2,4-pentanediamine with p-substituted aroylacetaldehydes, exhibit a linear correlation between log K of the ring-chain equilibria and Hammett's sigma values of the aromatic ring substituents. alpha-Substitution of beta-keto aldehydes notably increased the relative amounts of the chain E-isomers in their condensation products and also resulted in the formation of two diastereomers for each of the cyclic products. No ring-chain equilibria were observed in the products of 1,3-butanediamine and 2-methyl-2,4-pentanediamine with beta-diketones, beta-keto esters, or beta-keto amides. (c) 2006 Elsevier Ltd. All rights reserved.