Synthesis and Mass Spectrometry Studies of Branched Oxime Ether Libraries. Mapping the Substitution Motif via Linker Stability and Fragmentation Pattern

The oxime ether chemistry has recently been used as a convenient approach to preparing potentially highly diverse combinatorial libraries. The synthetically easiest way to form the libraries is convergent, i.e., via reaction of a branched scaffold containing two or more aminooxy linker groups, with a variety of carbonyl substituents. We show here that such reactions between aldehydes and ketones of different structure with the scaffolds containing different types of aminooxy groups can lead to the formation of virtually all expected components in the model mixtures 1−3 formed from three scaffolds (7−9) and eight substituents (R1−R8). One important problem with the branched libraries is that the libraries formed from the more complex scaffolds, such as 11, contain multiple regioisomers. The results of extensive analysis of a variety of library components by mass spectrometry presented here show that the differences in the MS−MS fragmentation energies for different linkers yield regiochemical information essential for identification of individual library components.