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 CN bond while keeping the conjugated CC 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.