Abstract
The 7-acetyl-2-aryl-5-bromo-3-(trifluoroacetyl)indoles 1a-d were reacted with hydroxylamine hydrochloride (2.2 equiv.) in the presence of pyridine in ethanol under reflux to afford the corresponding diketo oxime derivatives 2a-d. Beckmann rearrangement of the latter with trifluoroacetic acid under reflux afforded the corresponding 7-acetamido-2-aryl-5-bromo-3-(trifluoroacetyloxime)indoles 3a-d. The structures of the prepared compounds were characterized using a combination of NMR (1H & 13C), IR, and mass spectrometric techniques. The molecular structure of the 3-trifluoroacetyloxime substituted 7-acetamido-2-aryl-5-bromoindoles was unambiguously confirmed by the single crystal X-ray diffraction data of 3d. Structural studies of 3d in the solid state by X-ray crystallography provided evidence of hydrogen bonding networks and π-stacking of the indole moiety. Compound 3d was crystallized in the trigonal space group R-3:H with unit cell dimensions a = 25.1614(13), b = 25.1614(13), c = 17.3032(9) Å, α = β = 90°, γ = 120°, V = 9486.9(11) Å3, Z = 6. The density functional theory (DFT) structural parameters (bond lengths, bond angles, and torsion angles) of the optimized geometry calculated using the B3LYP/6-311G basis set were found to compare favourably with those of the X-ray crystal structure.
Highlights
The chemistry of indole- and isatin-based oximes has been the subject of numerous studies due to their interesting chemical and biological applications [1]
7-acetyl-2-aryl-5-bromo-3-(trifluoroacetyl)indoles 1a-d were subjected to hydroxylamine hydrochloride
Whereas the 2-arylindole-3-acetoximes undergo acid-mediated Beckmann rearrangement into 3-acetamido-2-arylindoles with ease [21], the analogous oximes derived from the trifluoroacetyl derivatives are generally stable towards rearrangement into amides [22]
Summary
The chemistry of indole- and isatin-based oximes has been the subject of numerous studies due to their interesting chemical and biological applications [1]. Oximes generally act as dual hydrogen-bond donors via the –O–H moiety and hydrogen-bond acceptors via the –C=N and the –OH moieties. They are capable of forming dimers as well as oxime-oxime catemers via O–H. Hydrogen bonding plays an important role in drug–receptor interactions, and drug design in chemical and biological processes as well as molecular recognition and the bioactivity of macromolecules [3]. Hydrogen bond formation causes changes in the distances between atoms and the rearrangement of electron densities on the groups involved in the interaction and chemical reactivity of the reaction centres that are directly involved in the hydrogen bonding [4].
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