Determination of isokinetic ratios necessary for equimolar incorporation of carboxylic acids in the solid-phase synthesis of mixture-based combinatorial libraries

The methods used to study the relative reaction rates of 45 different aliphatic and aromatic carboxylic acids when coupled to resin-bound amino acid amides is described. Competition experiments involving the coupling of incoming carboxylic acids to resin-bound amino acid amides were performed. The relative composition of each N-acylated amino acid amide in the resulting mixtures was compared to controls prepared by physically mixing equal aliquots of individual compounds in order to study the relative reaction rates of the incoming carboxylic acids. The ratios of the incoming carboxylic acids were then iteratively adjusted to yield as close to equimolar products as possible. As expected, the steric and electronic nature of the incoming carboxylic acids was found to influence their relative reaction rates. The steric hindrance of the resin-bound amino acid appears to have a proportional effect on the reaction rates of the incoming carboxylic acids. N-acylated amino acid amides in the final mixtures, prepared using the final isokinetic ratios, were found to be approximately equimolar.