Tautomerism troubles: proton transfer modifies the stereochemical assignments in diastereoisomeric structures of spiro-cyclic 5-methyl-2 H -imidazol-4-amine dimers.

During the racemization of a novel pharmaceutical spiro-cyclic imidazole-amine compound, namely, 6'-bromo- N -(6'-bromo-4-meth-oxy-4''-methyl-3' H -di-spiro[cyclo-hexane-1,2'-indene-1',2''-imidazol]-5''-yl)-4-meth-oxy-4''-methyl-3' H -di-spiro-[cyclo-hexane-1,2'-indene-1',2''-imidazol]-5''-imine, C 36 H 41 Br 2 N 5 O 2 , two impurities were isolated. These impurities were clearly dimers from mass spectroscopic analysis, however single-crystal diffraction characterization was required for the assignment of stereochemistry. The single-crystal diffraction results revealed subtly different structures to those proposed, due to an unexpected proton transfer. The dimers contain four stereocentres, but two of primary inter-est, and are centrosymmetric, so after careful structure refinement and close inspection it was possible to unambiguously assign the stereochemistry of both the homochiral [( S ),( S )- and ( R ),( R )-] and the heterochiral [( S ),( R )- and ( R ),( S )-] compounds.