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It was planned to utilize isoniazid  1  and substituted salicylaldehydes  2a–f  to synthesize some  N '-[( E )-(substituted-2-hydroxyphenyl)methylidene]pyridine-4-carbohydrazides  3a–g.  Compounds  3a–g  on reduction with sodium borohydride in dry methanol with stirring gave  N '-[(substituted-2-hydroxyphenyl) methyl] pyridine-4-carbohydrazides  4a–g  in good yield. The compounds  4a–g  underwent an internal Mannich reaction with formaldehyde (37%) in ethanol for 4–5 h under reflux conditions to give the desired  N -(substituted-2 H -1,3-benzoxazin-3(4 H )-yl)pyridine-4-carboxamides  5a–g . Structures of all the newly synthesized compounds were confirmed by spectral data. All the compounds have been screened for their antibacterial activity against  Bacillus subtilis ,  Staphylococcus aureus ,  Escherichia coli , and  Salmonella typhi , antifungal activity against  Candida albicans ,  Cladosporium oxysporum ,  Aspergillus flavus ,  and Aspergillus niger  and anti-TB activity against  M. tuberculosis . The compound  5a  indicated significant inhibition against  E. coli  and  A. flavus  (MIC-6.25 µg/mL), which was found to be better than the reference drug Gentamycin and Fluconazole respectively. Further, the compound  5d  showed more potent anti-TB activity (MIC-1.56 µg/mL) than the reference drug Ciprofloxacin. The DNA cleavage activity for selected compounds was also carried out by the AGE method. To understand the electronic properties of compounds  4a  and  5a,  DFT calculations at the DFT/B3LYP level using 6-31 G++ (d, p) have been performed to reproduce the geometry and structural features. The non-linear optical property of the compounds  4a  and  5a  was calculated using first-order hyperpolarisability calculation. The calculated HOMO-LUMO analysis explains the charge transfer interaction between the molecules.

Design, Spectroscopic Studies, DFT Calculations and Evaluation of Biological Activity of Novel 1,3-Benzoxazines Encompassing Isoniazid