2-Unsubstituted Imidazole -Oxides as Novel Precursors of Chiral 3-Alkoxyimidazol-2-ylidenes Derived from -1,2-Diaminocyclohexane and Other Chiral Amino Compounds.

'Desymmetrization' of 1,2-diaminocyclohexane by treatment with α,ω-dihalogenated alkylation reagents leads to mono-NH derivatives ('primary-tertiary diamines'). Upon reaction with formaldehyde, these products formed monomeric formaldimines. Subsequently, reactions of the formaldimines with α-hydroxyiminoketones led to the corresponding 2-unsubstituted imidazole -oxide derivatives, which were used here as new substrates for the in situ generation of chiral imidazol-2-ylidenes. Upon O-selective benzylation, new chiral imidazolium salts were obtained, which were deprotonated by treatment with triethylamine in the presence of elemental sulfur. Under these conditions, the intermediate imidazol-2-ylidenes were trapped by elemental sulfur, yielding the corresponding chiral non-enolizable imidazole-2-thiones in good yields. Analogous reaction sequences, starting with imidazole -oxides derived from enantiopure primary amines, amino alcohols, and amino acids, leading to the corresponding 3-alkoxyimidazole-2-thiones were also studied.