Sustainable aqueous-mediated one-pot synthesis of 2-hydroxy-5-oxo-5,6,7,8-tetrahydro-4H-chromene and coumarin derivatives using taurine as the green bioorganic catalyst

In this study, two taurine-catalyzed, aqueous-mediated methodologies were reported for the synthesis of 4-aryl-2-hydroxy-7,7-dimethyl-5,6,7,8-tetrahydro-4H-chromen-5-one (3a-j) and 3-substituted coumarin derivatives (3′a-f) under reflux conditions. These methodologies entail the use of substituted aromatic aldehydes, diethyl malonate, and dimedone as reagents for the sustainable synthesis of 2-hydroxy-tetrahydro-4H-chromene derivatives via the Knoevenagel-Michael cascade, as well as the use of substituted salicylaldehyde with diethyl malonate or ethyl 3-oxobutanoate for the efficient synthesis of coumarin derivatives via the Knoevenagel-transesterification reaction. These strategies emphasize environmental sustainability and are designed as one-pot processes, ensuring simplicity and convenience. The methods deliver high yields and are cost-effective, significantly reducing reaction times, enhancing step efficiency, and are successful at the multigram level. Notably, the taurine catalyst demonstrates exceptional reusability, maintaining its activity for up to five cycles without notable degradation. The catalyst is easily recoverable through filtration, and the overall procedure is streamlined, requiring no additional purification steps. These attributes collectively contribute to the green chemistry paradigm by minimizing waste and energy consumption, thus offering a practical and efficient approach to the synthesis of these important organic compounds.