Structure of Manasse's dimer from endo-2-hydroxyepicamphor by NOE difference spectroscopy

Reduction of (1R)‐(+)‐camphorquinone by zinc in acetic acid yields a mixture of endo‐2‐hydroxyepicamphor and 3‐hydroxycamphor. The former undergoes reaction in anhydrous methanol with HCl gas to furnish a stable crystalline solid, first reported in 1902. High‐resolution electron impact mass spectrometry showed the solid to be a dimer of formula C22H36O4. The 13C{1H} spectrum showed 11 discrete environments, in keeping with a symmetrical dimer. High‐field 1H NOE difference experiments were used to determine not only the connectivity but also the stereochemistry of the system. All of the NOE experiments are consistent with only one structure, that of the symmetrical dimer. In addition, 2D carbon‐proton correlation experiments (HETCOR) were used to assign unambiguously its 13C{1H} spectrum.