Oximes of 3,7-dimethylocta-2,6-dienal: green synthesis, preparative separation of all diastereomers and complete assignment of 1H and 13C NMR spectra

Abstract A mixture of four diastereomeric oximes was prepared starting from citral, an inexpensive mixture of neral and geranial ((Z)- and (E)-3,7-dimethylocta-2,6-dienal, respectively), in a solvent-free mechano-chemical reaction with NH2OH•HCl/NaOH, at room temperature. All stereoisomeric oximes ((1E,2E), (1Z,2E), (1E,2Z), and (1Z,2Z)) were isolated in pure state by isocratic silica-gel column chromatography. For the first time, the oximes were separated and individually characterized by 1D- and 2D-NMR spectroscopy in two deuterated solvents. Their spectra were fully assigned and mutually compared. Coupling constant values of higher-order multiplets belonging to the -CH2CH2- moiety were determined from (iterative) spin simulations. A careful analysis of the NOESY spectra of all diastereomers revealed that the oximes formed hydrogen-bonded adducts in CDCl3. Under the oximation conditions, the configuration of the C2-C3 double bond in the conjugated enals did not change. Prolonged standing of pure oximes of citral in either CDCl3 or DMSO-d6 led to isomerization of the C=N bond while keeping the conjugated C=C bond configuration unaltered. The equilibrium ratio of the anti- to syn-oximes in solution corresponded to their ratio in the original reaction mixture. The conditions of GC were sufficient to cause dehydration of the oximes to geranyl or neryl nitriles, and/or isomerization to occur in the heated areas of the instrument, giving more complex chromatograms than expected and being dependent on the identity of the analyzed oxime.