A novel strategy was developed for the synthesis of five different N-fused 1,4-dihydropyridine (1,4 DHP) scaffolds such as imidazopyridine, pyridopyrimidine, benzoimidazopyridine, thiazolopyridine and benzopyridooxazine derivatives by coupling nitroketene S,S-acetal, various nitrogen containing dinucleophiles, malanonitrile and substituted salicylaldehydes/aldehydes in the presence of InCl 3 as catalyst in water-EtOH solvent mixture under reflux condition. Furthermore, mechanism for the formation of 1,4-DHPs was explored through experimental and DFT calculations. DFT studies reveal that the reaction went through lower energy triheterocyclic intermediate than the higher energy chromenoimidazopyridine imtermediate. The attractive features of this protocol include short reaction time, easy separation of the product without chromatographic purification, simple execution with excellent yield and possibility to synthesize structurally diverse 1,4-dihydropyridine derivatives through greener approach.

ObjectivesA regioselective approach has been developed for the synthesis of benzopyranophenazine derivatives using In (OTf)3 as a catalyst in one‐pot synthesis. Further, these synthesized compounds were successfully used as organic corrosion inhibitors on mild steel in 1M H2SO4 solution.MethodsThe synthesized organic inhibitors were confirmed using NMR, and HRMS and their regioselectivity was confirmed by DFT studies using the B3LYP/6‐31G (d, p) basis set. Further, the corrosion, and inhibition efficiencies were confirmed by various methods such as weight loss technique, electrochemical impedance spectroscopy, and potentiodynamic polarization studies.ResultsThe two synthesized compounds MBPPand BBPP show excellent corrosion inhibition properties than the previously reported organic inhibitors, especially, the electron‐donating nature of methoxy‐substituted phenazine derivatives show superior corrosion inhibition properties as 98.96 in 900 ppm than electron withdrawing bromo substituted phenazine derivative in the same concentration which indicates that the electronic structure of the organic inhibitor plays an important role in the corrosion inhibition efficiency of mild steel in an acid medium.ConclusionThe synthesized compound efficiently inhibits the corrosion on the mild steel surface and it forms a barrier on the metal surface. As a result, the phenazine derivatives of both MBPP and BBPP act as very effective corrosion inhibitors and the electron‐donating nature of MBPP shows more inhibition efficiency than BBPP inhibitors.

A rapid and regioselective strategy have developed for synthesis of multi-functional phenazine derivatives such as 3- amino-1- (2-hydroxy phenyl) -1H- benzo[a] pyrano [2,3-c] phenazine -2- carbonitrile benzo-pyrano-phenazine (BPP), 3-amino-1-(2-hydroxy-5-methoxyphenyl)-1H-benzo[a]pyrano[2,3-c]phenazine-2-carbonitrile (MBPP) and 3-amino-1-(5-bromo-2-hydroxyphenyl)-1H-benzo[a]pyrano[2,3-c]phenazine-2-carbonitrile (BBPP) using In(OTf)3 as catalyst in simple one-pot multi-component reaction. Further, the regioselectivity of the product formation was confirmed by DFT studies. In addition, the synthesized compounds were evaluated for their corrosion inhibition strength on mild steel in 1 M hydrochloric acid solution by using weight loss and electrochemical techniques in the first time. Interestingly, the inhibition efficiency was increased up to 98.64 % with an increase in the inhibitor concentration and it was decreased with increases in temperature. These results indicate that our synthesized compounds adopt the Langmuir isotherm model. Further, the polarization measurements clearly indicate that synthesized phenazine derivatives act as mixed-type inhibitors. Also, the electrochemical impedance studies (EIS) strongly revealed that these compounds inhibit corrosion by adsorption mechanism. Moreover, the efficiency of corrosion inhibitors was explained by the surface morphology of the inhibited and uninhibited specimens and was examined using scanning electron microscopy (SEM). Finally, the correlation between the inhibition performance of phenazines and their structural parameters was clearly examined by DFT calculations.