Preparation and characterization of new chiral nitronyl nitroxides bearing a stereogenic center in the imidazolyl framework.

A synthetic procedure for optically active and racemic alpha-nitronyl nitroxides (alpha-NNs) having a stereogenic center at the 4-position of the imidazolyl ring is described. This procedure consists of (1) the synthesis of a dissymmetric vic-dinitro compound by Kornblum reaction, (2) the enantiomeric resolution of the racemate by a diastereomer method for obtaining the optically active sample, (3) the quick reduction of the optically active or racemic vic-dinitro compound to the bis(hydroxyamino) derivative with Al/Hg, (4) the solvent-free condensation of the bis(hydroxyamino) compound with an aldehyde to give the 1,3-dihydroxyimidazolidine, and (5) the final oxidation of the alpha-NN precursor with aqueous NaIO(4). The absolute configuration of the optically active alpha-NNs was assigned by correlating with the X-ray crystal structure of the (-)-(1S,4R)-camphanic acid ester derivative of the optically active vic-dinitro compound. The molecular conformation of the optically active alpha-NNs was found to be folded both in solution and in the solid state by CD spectroscopy and energy minimization with the Monte Carlo method. The magnetic properties of both optically active and racemic alpha-NNs in solution and in the solid state were characterized by EPR spectroscopy and magnetic susceptibility measurement, respectively.