Epimerization of Lupinine to Epilupinine and viceversa. Reexamination of the Structures of Lupinal and Epilupinal

Although the epimerization of lupinine (I) has been largely investigated, a previously not obsd. compd. of formula C10H17NO was now isolated from the mixt. of alkaloids that remains after the sepn. of epilupinine (II). It is insol. in dry Et2O but sol. in EtOH, from which it is recovered as an Et2O-sol. oil that slowly returns to the Et2O-insol. solid form. For these characteristics and based on GC/MS, 1H-NMR, and IR data, it is considered as the inner salt III of the common enolic form IV of lupinal and epilupinal, with which it is in equil. when standing in soln. The oily form, but not the solid one, is able to improve the conversion of I to II, establishing the role of the aldehydes in the epimerization process. It was obsd. that also II can be converted to I. Finally, the solid lupinal described by Zaboev should be considered as being identical to the now isolated inner salt III, while the oily epilupinal of Wicky and Schumann is, indeed, a mixt. of epilupinal with a minor amt. of lupinal, which, on standing, is converted to the inner salt III of the common enolic form IV.