Synthesis of Novel N-Acylhydrazones and Their C-N/N-N Bond Conformational Characterization by NMR Spectroscopy |
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Authors: | Rubina Munir Noman Javid Muhammad Zia-ur-Rehman Muhammad Zaheer Rahila Huma Ayesha Roohi Muhammad Makshoof Athar |
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Institution: | 1.Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan;2.Department of Chemistry, Kinnaird College for Women, Lahore 54000, Pakistan; (R.H.); (A.R.);3.Department of Chemistry (C-Block), Forman Christian College, Ferozepur Road, Lahore 54600, Pakistan;4.Applied Chemistry Research Centre, PCSIR Laboratories Complex, Lahore 54600, Pakistan; |
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Abstract: | In this article, a synthesis of N’-(benzylidene)-2-(6-methyl-1H-pyrazolo3,4-b]quinolin-1-yl)acetohydrazides and their structural interpretation by NMR experiments is described in an attempt to explain the duplication of some peaks in their 1H- and 13C-NMR spectra. Twenty new 6-methyl-1H-pyrazolo3,4-b]quinoline substituted N-acylhydrazones 6(a–t) were synthesized from 2-chloro-6-methylquinoline-3-carbaldehyde (1) in four steps. 2-Chloro-6-methylquinoline-3-carbaldehyde (1) afforded 6-methyl-1H-pyrazolo3,4-b]quinoline (2), which upon N-alkylation yielded 2-(6-methyl-1H-pyrazolo3,4-b]quinolin-1-yl)acetate (3). The hydrazinolysis of 3 followed by the condensation of resulting 2-(6-methyl-1H-pyrazolo3,4-b]quinolin-1-yl)acetohydrazide (4) with aromatic aldehydes gave N-acylhydrazones 6(a–t). Structures of the synthesized compounds were established by readily available techniques such as FT-IR, NMR and mass spectral studies. The stereochemical behavior of 6(a–t) was studied in dimethyl sulfoxide-d6 solvent by means of 1H NMR and 13C NMR techniques at room temperature. NMR spectra revealed the presence of N’-(benzylidene)-2-(6-methyl-1H-pyrazolo3,4-b]quinolin-1-yl)acetohydrazides as a mixture of two conformers, i.e., E(C=N)(N-N) synperiplanar and E(C=N)(N-N) antiperiplanar at room temperature in DMSO-d6. The ratio of both conformers was also calculated and E(C=N) (N-N) syn-periplanar conformer was established to be in higher percentage in equilibrium with the E(C=N) (N-N) anti-periplanar form. |
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Keywords: | quinoline pyrazolo[3 4-b]quinoline conformer rotamer NMR stereoisomer acylhydrazone |
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