A simple, facile and convenient practical method for the room temperature one-pot synthesis of a series of potentially biologically active coumarin-3-carboxylic acids has been developed in water from the Knoevenagel condensation and intramolecular cyclization of diverse 2hydroxybenzaldehydes with Meldrum's acid using either potassium carbonate or sodium azide as commercially available, cheap and eco-friendly catalyst. The salient features of the present protocol are mild reaction conditions, good to excellent yields, high atom-economy, environmentally benignity, easy isolation of products, without column chromatography, clean reaction profiles and applicability towards large-scale synthesis.General. Infrared spectra were recorded using a Shimadzu (FT-IR 8400S) FT-IR spectrophotometer using KBr disc. 1 H and 13 C NMR spectra were obtained at 400 MHz and 100 MHz respectively, using Bruker DRX-400 spectrometer and DMSO-d 6 as the solvent.Mass spectra (TOF-MS) were measured on a QTOF Micro mass spectrometer. Elemental analyses were performed with an Elementar Vario EL III Carlo Erba 1108 micro-analyzer instrument. X-ray data were collected on a Bruker single crystal X-ray difractometer (model:APEX 2). Melting point was recorded on a Chemiline CL-726 melting point apparatus and is uncorrected. Thin Layer Chromatography (TLC) was performed using silica gel 60 F 254 (Merck) plates.
General procedure for the synthesis of coumarin-3-carboxylic acids (3): An oven-driedscrew cap test tube was first charged with a magnetic stir bar, salicylaldehydes (1, 1.0 mmol), Meldrum's acid (2, 1.0 mmol; 144 mg), distilled water (5 mL) and K 2 CO 3 (20 mol%; 28 mg).The whole mixture was then stirred vigorously at room temperature (24-26 ο C) for 20 h to complete the conversion. Upon completion, the resulting solution was acidified with chilled acid-water, when solid mass precipitated out, filtered off and thoroughly washed with water to obtain almost pure product of coumarin-3-carboxylic acids 3. For further purification, the products were crystallized from ethyl acetate. All the products 3 were also synthesized repeating the same reaction using sodium azide (50 mol%; 33 mg) as the catalyst in place of potassium carbonate. The structure of each purified product was confirmed by its analytical as well as spectral studies including FT-IR, 1 H NMR, 13 C NMR and TOF-MS. For few entries, 2D-NMR ( 1 H-1 H COSY, HMQC and HMBC) spectral properties were also studied.In addition, single X-ray crystallographic studies for one entry, 6-methoxycoumarin-3carboxylic acid (3f), was performed.