Green Condensation of Aromatic Aldehydes and Barbituric Acid Enabled by Spiral Gas–Solid Flow

This paper presents a novel spiral gas–solid two-phase flow (S-GSF) synthetic methodology for establishing carbon–carbon double bonds during mechanochemical synthesis, thus enabling the solvent-free continuous production of barbituric-acid-derived Knoevenagel adducts. The applicability of this method in the Knoevenagel condensation reaction was verified by the condensation of aromatic aldehydes with malononitrile. Employing the condensation product between veratraldehyde and barbituric acid (compound 1) as a model system, we confirmed that this approach is a viable alternative to conventional mechanochemical synthesis protocols. Structural elucidation of the synthesized barbiturate derivatives was accomplished through Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and ultraviolet–visible (UV–vis) absorption spectrometry. Analytical results confirmed the successful fabrication of compound 1 at a production rate of 1 g/min within a 3 min reaction interval, demonstrating a substantial temporal reduction compared to the 90 min duration required by twin-screw extrusion methodologies. Furthermore, we performed a kinetic evaluation of the reaction mechanism through time-dependent UV–vis spectral monitoring and found that the formation of compound 1 follows the Prout-Tompkins (B1) nucleation–growth kinetic paradigm. The S-GSF technique exhibits a remarkable capability to construct conjugated olefinic systems under solvent-free conditions, showing promising potential for advancement in organic synthesis applications and offering innovative solutions for continuous process intensification.