Microwave assisted one-pot green synthesis of cinnoline derivatives inside natural sporopollenin microcapsules

We present a green and efficient approach for the synthesis of novel cinnoline derivatives inside natural Lycopodium clavatum sporopollenin (LCS) microcapsules via a one-pot microwave (MW) assisted reaction for the first time. We also propose the concept that the robust micrometre-sized sporopollenin microcapsules can act as MW microreactors. We demonstrate the feasibility of this concept by in situ synthesising 8-hydroxy-7-nitro-6-(3-nitrophenyl)-3-oxo-2-(p-tolyl)-2,3,5,6-tetrahydrocinnoline-4-carbonitrile inside the LCS microcapsules via a microwave (MW) assisted reaction of ethyl 5-cyano-4-methyl-6-oxo-1-(p-tolyl)-1,6-dihydropyridazine-3-carboxylate with 1-nitro-2-phenylethylene in the presence of piperidine as a base at 100 °C for 20 minutes. The LCS microparticles are extensively characterised before and after the MW induced reaction using several techniques. The formation of the cinnoline compound inside the LCS microcapsules is confirmed by laser scanning confocal microscopy (LSCM), X-ray diffraction (XRD) and fourier-transform infrared spectroscopy (FTIR) analyses. Using liquid chromatography-mass spectrometry (LCMS) analyses, we show that the structural integrity of the cinnoline compound, recovered from the cinnoline loaded (cinn-loaded) LCS, is preserved. The pure cinnoline is found to show promising optical properties with two λ(max) absorption peaks at 310 and 610 nm. Both the pure cinnoline and cinn-loaded LCS show promising antibacterial activity against Pseudomonas aeruginosa (Gram-negative) and Bacillus cereus (Gram-negative) human pathogenic bacterial strains. The successful MW induced reaction of the prominent cinnoline derivative inside the biocompatible LCS microreactors can open up intriguing applications in materials and pharmaceutical sciences.