Development of the traceless phenylhydrazide linker for solid-phase synthesis

The hydrazide group is a new oxidatively cleavable traceless linker for solid-phase chemistry. It can be readily introduced by hydrazide formation between a carboxy-functionalized resin and different substituted hydrazines. In order to achieve high yields in this step, new carboxylic acid resins were developed that are not prone to undesired imide formation upon activation of the carboxylic acid. The polymer-bound acyl hydrazides were successfully employed in various transformations, namely Heck, Suzuki, Sonogashira, and Stille couplings, as well as Wittig and Grignard reactions. Traceless release of the coupling products from the solid support is achieved selectively under mild conditions and in high purity by oxidation of the aryl hydrazides to acyl diazenes with Cu-II salts or N-bromosuccinimide (NBS) and subsequent nucleophilic attack of the acyl diazene intermediates. Traceless cleavage by oxidation with NBS can be carried out as a two-step process in which stable acyl diazenes are first generated by treatment with NBS in the absence of a nucleophile. After removal of the reagents by simple resin washing, the traceless release is effected by the addition of methanol, which leads to products of high purity without any additional separation steps.