A Three-Step Synthesis of 4-Hydroxy-2H-benzo[g]chromen-2-one from Beta-Naphthol

Introduction: Coumarins are naturally occurring lactones with documented biological activities, including antimicrobial properties. The fusion of a coumarin ring with aromatic systems represents an approach for obtaining compounds with potential biological activity. This paper reports the synthesis of 4-hydroxy-2H-benzo[g]chromen-2-one (B-4), a naphtho-fused coumarin derivative, from commercially available beta-naphthol.

Methods: The synthesis involved three steps. First, beta-naphthol was esterified with acetic anhydride using p-toluenesulfonic acid to yield naphthalen-2-yl acetate (B-2). Second, tartaric acid was esterified with methanol to obtain dimethyl tartrate (B-3). The key step was condensation of B-2 and B-3 using ferric chloride (FeCl₃) as a Lewis acid catalyst in 1,4-dioxane to afford the target coumarin. All compounds were characterised by TLC and IR spectroscopy.

Results: Esters B-2 and B-3 were obtained in 88% and 80% yields, respectively. The condensation reaction produced B-4 as a pale yellow solid in 70% yield. IR spectroscopy was consistent with the proposed structure, showing a lactone carbonyl absorption at 1617 cm⁻¹ and an OH stretch at 3400 cm⁻¹. Preliminary screening indicated that alternative Lewis acid catalysts (AlCl₃, ZnCl₂) and solvents did not yield the desired product under the conditions tested.

Conclusions: A coumarin derivative was synthesised via a three-step route with an overall yield of approximately 49%. The use of FeCl₃ in 1,4-dioxane proved effective for the condensation step. This work presents a straightforward synthetic pathway for accessing naphtho-fused coumarin systems, paving the way for future pharmacological evaluation. Full characterisation by ¹H NMR, ¹³C NMR, and MS, as well as biological activity assessment, recommended for future studies.