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We synthesized novel pyrido[2,3- b ]pyrazin based heterocyclic compounds (4-7) and their chemical structures were ascertained by spectral techniques (NMR, FT-IR). Besides experimental investigation, density functional theory (DFT) computations with B3LYP/6-31G(d,p) level of theory were executed to obtain spectroscopic and electronic properties. Nonlinear optical (NLO) properties, frontier molecular orbitals (FMOs), UV-visible, vibrational analysis, natural bond orbitals (NBOs), transition density matrix (TDM) and density of states (DOS) analyses of molecules (4-7) were accomplished at B3LYP/6-31G (d,p) level. Global reactivity parameters (GRPs) were correlated with the band gap ( E gap ) values; compound 7 with lower E gap (3.444 eV), exhibited smaller value of hardness (1.722 eV) with greater softness value (0.290 eV -1 ). The dipole moment ( μ ), average polarizability 〈 α 〉, first ( β tot ) and second 〈 γ 〉 hyper-polarizabilities were calculated for compounds (4-7). Compound 7 showed less E gap , highest absorption wavelength and remarkable NLO response. The highest 〈 α 〉, β tot and 〈 γ 〉 values for compound 7 were observed as 3.90 × 10 -23 , 15.6 × 10 -30 and 6.63 × 10 -35 esu, respectively. High NLO response revealed that pyrido[2,3- b ]pyrazin based heterocyclic compounds had very remarkable contributions towards NLO technological applications. Further compounds (4-7) are utilized for the first time in electrochemical sensing of DNA, i n vitro antioxidant and antiurease activity., Competing Interests: There are no conflicts of interest to declare., (This journal is © The Royal Society of Chemistry.)

In the present study, we reported the efficient synthesis of novel, heterocyclic, coumarin-based pyrano-chromene derivatives, 2-amino-8-methyl-5-oxo-4-[2-(2-oxo-2 H -chromen-3-ylmethoxy)-phenyl]-4 H ,5 H -pyrano[3,2- c ]chromene-3-carbonitrile (4a) and 2-amino-8-methyl-5-oxo-4-[2-(2-oxo-2 H -chromen-3-ylmethoxy)-phenyl]-4 H ,5 H -pyrano[3,2- c ]chromene-3-carboxylic acid methyl ester (4b). The chemical structures of synthesized compounds were resolved by employing various spectroscopic techniques like UV-Vis, FT-IR, 1 H & 13 C NMR, and single crystal X-ray diffraction (SC-XRD) analysis. The compounds; 4a and 4b, with appealing π-bonded skeleton were further analyzed in terms of their electronic and structural aspects using an integral approach of density functional theory (DFT) and time-dependent DFT (TD/DFT). The methodology: M06-2X/6-31G(d,p) level of theory was applied to compare their experimental data with theoretical outcomes using quantum chemical analysis. The frontier molecular orbitals (FMOs) study revealed that, 4a possesses a low band gap (5.168 eV) as compared to 4b (6.308 eV). Global reactivity parameters were associated with E gap values as 4a, with the lowest band gap showed the smaller value of hardness (0.094 eV) and a larger value of softness (5.266 eV). The non-linear optical (NLO) insight exhibited that, the average polarizability 〈 α 〉 and second hyperpolarizability ( γ tot ) were observed in 4a as 6.77005 × 10 -23 and 0.145 × 10 4 esu, respectively. Overall, the computational studies suggest that the investigated compounds have distinct NLO properties., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

In this work, we report the efficient synthesis of novel (hydroxybenzoyl)pyrido[2,3- d ]pyrimidine heterocycle derivatives: 6-(2-hydroxy-5-methylbenzoyl)-1-methylpyrido[2,3- d ]pyrimidine-2,4(1 H ,3 H )-dione (6a), 6-(5-fluoro-2-hydroxybenzoyl)-1-methylpyrido[2,3- d ]pyrimidine-2,4(1 H ,3 H )-dione (6b), 6-(5-ethyl-2-hydroxybenzoyl)-1-methylpyrido[2,3- d ]pyrimidine-2,4(1 H ,3 H )-dione (6c) and 6-(2-hydroxy-5-isopropylbenzoyl)-1-methylpyrido[2,3- d ]pyrimidine-2,4(1 H ,3 H )-dione (6d). The chemical structures of the title compounds were ascertained by spectral techniques including 1 H, 13 C NMR, UV-visible and FT-IR spectroscopy as well as single-crystal X-ray diffraction analysis. Additionally, density functional theory (DFT) and time-dependent (TD-DFT) computation were adopted to analyze the electronic structures of 6a-d. Compounds 6a-d were computed in the ground state for FT-IR spectroscopic and natural bond orbital (NBO) analysis by DFT/B3LYP with the 6-311+G(d,p) basis set. UV-vis spectroscopic and HOMO and LUMO energy values for 6a-d were determined via TD-DFT/B3LYP with the 6-311+G(d,p) basis set. The optimized geometric parameters, UV-vis findings, and vibrational frequencies indicate good consistency with the experimental data. NBO analysis was conducted to explore the interactions and charge transfer among different orbitals in the title compounds. The HOMO and LUMO band gap (Δ E ) values for 6a-d were found to be 3.93, 3.91, 4.10 and 3.91 eV, respectively. Molecular electrostatic potential (MEP) analysis explored the reactivity of the title compounds by predicting their nucleophilic as well as electrophilic sites., Competing Interests: The authors declare that they have no conflicts of interest., (This journal is © The Royal Society of Chemistry.)