Stereoselective total synthesis of skew-tetramantane, diamond's chiral core

Diamond's elementary chiral constituent – skew-tetramantane – features extreme rigidity, stability and a precisely defined geometry, epitomizing the parent structure of a σ-helicene. While skew-tetramantane is naturally occurring in trace fractions in fossil fuels, efforts over several decades for its selective synthesis remained unfruitful. With the recent advances in photo- and transition metal catalysis to tame radical and carbene species, we now devised a targeted total synthesis of skew-tetramantane by means of a stereoselective adamantalogous cage-extension. A first cap attachment was thereby effected by a photocatalytic Giese reaction, while remarkable regio-, diastereo- and enantiocontrol were achieved by an intramolecular C(sp3)−H insertion using Davies' chiral rhodium catalysts. After a Buchner–Curtius–Schlotterbeck ring expansion and a stereoselective Mukaiyama hydration, the fusion to the adamantine skew-tetramantane structure was completed by an intramolecular C(sp3)−H insertion of a nonstabilized carbenoid. Overall, this approach provides access to synthetic skew-tetramantane in isomerically pure form with σ-helicity defined by the catalyst, marking a selective pathway to higher diamondoids.