Solid State Auto-Inversion of C‑Centrochirality: Enantioselective Total Synthesis of Furocarbazolones (−)-epi-Claulansine D and (−)-Claulansine D and Pyranocarbazolone (+)-epi-Claulansine C

Starting from dimethyl (E)-2-{[(1-tert-butoxycarbonyl)-1H-indol-3-yl]­methylene}­succinate and (R)-2,2,5,5-tetramethyl-1,3-dioxolane-4-carbaldehyde, facile synthesis of (−)-epi-claulansine D was accomplished via condensation and two intramolecular cyclizations. The (−)-epi-claulansine D in the solid state exists in a metastable form, and after an induction period of 30–90 days, it underwent complete epimerization to exclusively deliver the desired natural product (−)-claulansine D in quantitative yield. The witnessed inversion of C-centrochirality in the solid state is conceptually novel and takes place for relatively higher crystal stability reasons. Base-catalyzed ring expansion of both (±)/(−)-epi-claulansine D and (±)/(−)-claulansine D resulted in (±)/(+)-epi-claulansine C in very good yields.