All-Heteroatom-Substituted Carbon Spiro Stereocenters: Synthesis, Resolution, Enantiomeric Stability, and Absolute Configuration.

Chiral tetra-heterosubstituted methanes (i.e., tetraoxa and azatrioxa carbon spiro stereocenters) are synthesized under CpRu catalysis, using cyclic carbonates and carbamates as substrates and α-diazo-β-ketoesters as reagents. Single enantiomers, isolated by chiral stationary phase chromatography, display chiroptical properties, from ∼10 to ∼10, which, together with TD-DFT calculations, provide robust absolute configuration assignments. Crystalline spiro diastereomers were also obtained, confirming further the structural and configurational assignments. Using enantioselective dynamic chromatography, remarkable enantiomerization barriers were determined for the ortho-carbonates and ortho-carbamates, with values of up to 27.6 and 34.6 kcal/mol (half-lives 227 days and >84,000 years at 25 °C, respectively). DFT further elucidates the origin of this large difference pointing toward preferred C-O or C-N bond cleavages in the rate-determining step of the S1-like mechanism.