Regiochemical tagging: A new tool for structural characterization of isomeric components in combinatorial mixtures

In this contribution we present a new combined synthetic and analytical strategy (regiochemical ragging) that allows facile determination of complete structure, including substituent position and regiochemistry, of mass-redundant components in complex combinatorial mixtures. The libraries of components (oxime ethers) are formed by the reaction of a mixture of substituents (aldehydes) with a scaffold containing several chemically similar attachment points (aminooxy groups), The structure of the resulting library components can then be determined from a combination of single MS and the tandem (MS/MS) spectra. Determination of the unique isomeric motif for each component is made possible via the following features of library design: (1) pan of the scafforld moiety, "transferable group" (the nitrogen atom from the oxime group) is transferred to the: substituent during fragmentation in the tandem experiment, (2) transferable groups on the scaffold differ from each other by either isotopic labels or fragmentation energies, and (3) mass-redundant substituents are isotopically labeled to create at least a 2 mass unit difference between them. The components of thr resulting library thus become labeled with different mass- and energy tags, which allows for precise regiochemical assignment of the functional group positions on the scaffold and substituents by mass spectrometry. The approach has been used to create and analyze a mixture of 27 isomeric compounds, each containing three boronic acid groups. The combination of the MS and MS/MS spectra of the tagged mixture has yielded a unique and structurally definitive signature of each component. Applications of the regiochemical tagging techniques to rapid synthesis and screening of combinatorial mixtures are discussed.