Stereoisomers of oxime ethers of 1,4-naphthoquinone: Synthesis, characterization, X-ray crystal structures, and DFT studies

In the present work, we use the C-3 substituted 2-hydroxy-1,4-naphthoquinones viz. 2-hydroxynaphthalene-1,4-dione (1), 2-hydroxy-3-methylnaphthalene-1,4-dione (2), and 2-chloro-3-hydroxynaphthalene-1,4-dione (3) as starting materials and obtained naphthoquinoneoxime ethers viz. 3-hydroxy-4-(methoxyimino)naphthalen-1(4H)-one (4), 3-hydroxy-4-(methoxyimino)-2-methylnaphthalen-1(4H)-one (5) and 2-chloro-3-hydroxy-4-(methoxyimino)naphthalen-1(4H)-one (6) via a facile and efficient synthesis involving condensation with methoxyamine hydrochloride. X-ray structure showed that 4 belongs to the orthorhombic (P212121) space group, whereas 5 and 6 (P21/c) crystalize in the monoclinic crystal system. The isomer ratios derived from NMR experiments elucidating stereoisomerism in compounds comprised of carbon-nitrogen double bonds have been analyzed; the naphthoquinone oxime ethers 4, 5, and 6 were also characterized employing the ωB97x-D/6-311++G (d, p) level of density functional theory. Theoretical studies reveal different conformational isomers with the 'amphi' and 'syn' isomers as low-lying structures along their energy hierarchy in accordance with the X-ray crystallography data. The spectral characterization of vibrational frequencies and 1H /13C NMR chemical shifts in unison with experiments and theory has been presented.