Epimerization of lupinine to epilupinine and vice versa. Reexamination of the structures of lupinal and epilupinal

Though the epimerization of lupinine has been largely investigated, a not previously observed compound of formula C10H17NO has been now isolated from the mixture of alkaloids that remains after the separation of epi-lupinine. It is insoluble in dry ether, but it is soluble in ethanol, from which is recovered as an ether soluble oil that slowly returns to the ether insoluble solid form. For these characteristics and on the base of GC-MS, 1H-NMR and IR spectra, it is considered as the internal salt of the common enolic form of lupinal and epi-lupinal, with which it is in equilibrium when standing in solution. The oily form, but not the solid one, is able to improve the conversion of lupinine to epi-lupinine, proving the role of the aldehydes in the epimerization process. It was observed that also epi-lupinine can be converted to lupinine. Finally, the solid lupinal described by Zaboev should be considered coincident with the now isolated enol internal salt, while the oily epi-lupinal of Wicky and Schumann is indeed a mixture of epi-lupinal with a minor amount of lupinal, that on standing is converted to the internal salt of the common enolic form.