The'H NMR spectra (Table I, supplementary material) of these salts indicate that they exist in the anti conformation since the iminium proton is coupled to H-4 (J= 0.6-0.7 Hz) but not to H- 5. This is unlike most pyrrole-2-carboxaldehydes for which the syn conformer predominates at room temperature (JCH0, HS= 1 Hz) but analogous to that observed for certain 1- acylpyrrole-2-carboxaldehydes (see ref 3, pp 286-289, 474-475). It is also noteworthy that ...
