Decomposition of Usnic Acid. VII. Pyrolysis of Dihydrousnic Acid. (3). Isodihydrousnic Acid.

Abstract

The structure of (+) isodihydrousnic acid (IDHU), C₁₈H₁₈O₇, m.p. 128°, [α]_D+57.5°, which was obtained from (-) dihydrousnic acid (DHU), C₁₈H₁₈O₇, m.p. 149°, [α]_D-86.8°, by heating above 200°was studied. IDHU was separated from DHU by recrystallization from MeOH or by paper electrophoresis. DHU and IDHU were distinguished by their Cu-complex salts which gave markedly different optical rotation. DHU and IDHU are interconvertible to form an equilibrium mixture in pyridine or in aq. alkaline solution, which showed an equilibrium value of optical rotation. The former suggestion that DHU and IDHU would be cis-trans isomers at B/C ring fusion has now been revised. The N.M.R. spectra showed a cis-ring fusion in both DHU and IDHU. The chemical and N.M.R. spectral studies of DHU, IDHU and their model compounds resulted that a rearrangement of A-ring occurs by the cleavage of B-ring in the course of interconversion of DHU and IDHU. The N.M.R. spectra of IDHU showed that it exists in an equilibrium state of different situation of hydrogen bonding, which is represented as in the formulas (A) and (B).