Tautomerism troubles: Proton transfer modifies the stereochemical assignments in diastereoisomeric structures of spirocyclic 5-methyl-2H-imidazol-4- amine dimers

Blade, Helen, Horton, Peter N., Morrison, James A., Orton, James B., Sullivan, Rachel A. and Coles, Simon J. (2021) Tautomerism troubles: Proton transfer modifies the stereochemical assignments in diastereoisomeric structures of spirocyclic 5-methyl-2H-imidazol-4- amine dimers. Acta Crystallographica Section E: Crystallographic Communications, 77, 1311-1315. (doi:10.1107/S205698902100668X).

Abstract

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, C₃₆H₄₁Br₂N₅O₂, 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.

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Contributors

Author: Peter N. Horton

Author: Simon J. Coles

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