Unlimited non-linear selectivity effects in systems of independent parallel reactions as a basis for new chemical separation techniques

Abstract There exist two basic types of kinetically controlled selective reactions. They differ fundamentally from each other with respect to the time dependence of the relative concentrations of the participating reactants. We call the first type, systems of dependent parallel reactions. They have in common the same ensemble of starting materials. The second type, systems of independent parallel reactions, have starting material ensembles that are distinguishable. The selectivity of dependent parallel reactions is limited by the ratio of their rate constants, whereas the selectivity of independent parallel reactions. An example is the recently developed synthesis of isomerically pure oligopeptides by similar manner. This follows from an analysis of the solutions of the corresponding differential equation systems. The non-linear and unlimited selectivity effects of independent parallel reactions may serve as a basis for new techniques to prepare pure chemical compounds from mixtures of chemical analogs that would be hard to separate by other means. Under suitable conditions this is possible, even if the reagents are also mixtures of chemical analogs. One of the most effective ways to synthesize isomerically pure chemical compounds is by beginning with a selective system of dependent parallel reactions, and then letting its products participate in a subsequent selective system of independent parallel reactions. An example, is the recently developed synthesis of isomerically pure oligopeptides by a stereoselective 4-component-condensation (4CC), followed by purification of the desirable diastereomer by preferential acidolysis of the other diastereomer (confer Scheme 6). Note that in such asymmetric syntheses the chiral reference system is exploited twice, first in the dependent parallel 4CC, and then in the independent parallel acidolysis. The overall stereoselectivity of such syntheses corresponds to the product of the selectivities of the consecutive steps.