suggesting the presence of an iminium double bond
between N-1 and C-2.7 Furthermore, according to the MS analysis
and the chemical shift of C-2 (δC 187.4), an oxygen anion rather
than a hydroxy group should be connected to C-2. The presence of
an oxygen anion and a quaternary nitrogen atom (N-4) was essential
for stability and led to the inference that compound 1 was an inner
salt.