Swiss chemists have performed a complete enantioselective synthesis of one of the tetramantane isomers, constructed from four fused adamantane fragments. To do this, they used a catalytic reaction involving the insertion of a carbene into a carbon-hydrogen bond. Previously, chemists had been unable to obtain such compounds, and they were extracted from fossil fuels, according to the authors of a preprint posted on the Chemrxiv.org portal.
The adamantane molecule is a tricyclic carbon framework, which can be visualized as a small three-dimensional unit of the diamond crystal structure. For this reason, adamantane itself and compounds in which adamantane molecules are fused together are called diamondoids. They are difficult to synthesize in the laboratory because they contain no functional groups and often exist as several similar isomers.
Recently, however, Xiao-Yu Li and Christoph Sparr from the University of Basel have made progress in this direction. They synthesized one of the spatial isomers of tetramantane—skewed tetramantane—in optically pure form. The scientists began by taking 2-bromotriamantane and photochemically reacting it with a benzyl acrylate derivative, resulting in a chain of three carbon atoms being added to the triamantane skeleton. The scientists then modified this chain to contain a group of two nitrogen atoms at its end, which was needed to generate the carbene moiety.
The next stage of the synthesis was key to constructing the tetramantane skeleton. The scientists' idea was to mix the resulting diazo compound with a rhodium catalyst. This, they predicted, would produce a rhodium carbenoid capable of inserting into carbon-hydrogen bonds. The problem was that the carbenoid had many carbon-hydrogen bonds adjacent to it, and the reaction could have resulted in eight different products. Therefore, the chemists had to test many different catalysts and reaction conditions, but ultimately, they obtained the single isomer they needed in optically pure form with a yield of 97%.
The chemists then completed the tetramantane skeleton using ring expansion and other manipulations of functional groups. In the final step, they mixed bromotetramantane with an iridium catalyst, tris(trimethylsilyl)silane, and irradiated the reaction mixture with blue light. This resulted in the detachment of the bromine atom, yielding pure, skewed tetramantane.
The complete enantioselective synthesis of tetramantane required 17 preparative steps, and the scientists confirmed its structure by comparing the obtained NMR spectra with previously published spectra of natural tetramantane. According to the chemists, this is the first targeted synthesis of a diamondoid with four fused adamantane fragments.
Previously, we reported on how chemists completely synthesized the fugu fish poison, tetrodotoxin.
