Trityl radicals with a combination of the orthogonal functional groups ethyne and carboxyl: Synthesis without a statistical step and EPR characterization.

Finland trityl radical (FTR) shows very attractive EPR spectroscopical properties for a manifold of applications. For most of its applications only one chemically reactive functional group is needed. The presence of three equally reactive carboxyl groups leads to FTR modifications through reactions which give statistical mixtures of onefold, twofold and threefold modified, and unmodified FTR. To avoid the side effects of such a statistical reaction - limited yields and sepa-ration challenges - we took a route to FTR-type trityl radicals with scaffold assembly by addition of an aryllithium with one type of substituent to a diarylketone with another type of substituent. This gave the two FTR-type trityl radicals 1 and 2 which carry a combination of the chemically orthogonal groups, carboxyl and triisopropylsilylethynyl. Standard column chromatography was sufficient for product isolation on all stages, whereby polar tagging helped. The EPR spectroscopical properties of the trityl radicals 1 and 2 in ethanol were determined in X and W band. Their g anisotropy and T1 and T2 relaxation times make them as good a spin label as the benchmark FTR. The paper discloses also details on the synthesis of building blocks used for FTR preparation and an improved access to the bare FTR scaffold.