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

Although the epimerization of lupinine ( 1 ) has been largely investigated, a previously not observed compound of formula C₁₀H₁₇NO was now isolated from the mixture of alkaloids that remains after the separation of epilupinine ( 2 ). It is insoluble in dry Et₂O but soluble in EtOH, from which it is recovered as an Et₂O‐soluble oil that slowly returns to the Et₂O‐insoluble solid form. For these characteristics and based on GC/MS, ¹H‐NMR, and IR data, it is considered as the inner salt 6 of the common enolic form 5 of lupinal ( 3 ) and epilupinal ( 4 ), with which it is in equilibrium when standing in solution (see Scheme 1). The oily form, but not the solid one, is able to improve the conversion of 1 to 2 , establishing the role of the aldehydes in the epimerization process. It was observed that also 2 can be converted to 1 . Finally, the solid lupinal described by Zaboev should be considered as being identical to the now isolated inner salt 6 , while the oily epilupinal of Wicky and Schumann is, indeed, a mixture of epilupinal ( 4 ) with a minor amount of lupinal ( 3 ), which, on standing, is converted to the inner salt 6 of the common enolic form 5 .