I hope following may work for you.

link.springer.com/content/pdf/10.1631%2Fjzus.B0820079.pdf

www.mdpi.com/1420-3049/17/9/10846/pdf

www.sciencedirect.com/science/article/pii/S1878535214000732

Dal,

See

--- Starcevic, Stefan; Brozic, Petra; Turk, Samo; Cesar, Jozko; Lanisnik Rizner, Tea; Gobec, Stanislav J. Med. Chem. (2011), 54(1), 248-261. and references cited therein.

Good luck

You could try one of the methods I've attached...

All the best.

The knoevenagel condensation you are using is one of the best ways of obtaining your product. Just because you haven't observed a solid yet, it doesn't mean the reaction hasn't worked. The transesterification may not have gone to completion; try heating it up and cooling down again.

Repeat the reaction, use toluene or benzene as a solvent at reflux and a dean stark apparatus to drive off water and ethanol formed in the reaction. Monitor by TLC.

I totally agree with Dr. Eric Harris comments

and

See also some old but valid literature

The Fluorescence of Some Coumarins1

Charles E. Wheelock

J. Am. Chem. Soc., 1959, 81 (6), pp 1348–1352

DOI: 10.1021/ja01515a021

Improved synthesis of some hydroxycoumarins

M. Bulut, Ç. Erk

Dyes and Pigments, Volume 30, Issue 2, 1996, Pages 99–104

Thank you for the suggestions.

Follow the methods explained in the attachment but start by

2,4-dihydroxybenzaldehyde, after the first step of condensation,

undergo acetylation with acetic anhydride or acetyl chloride.

Undergo the cyclization reaction and hydrolyze the acetyl protecting ester.

Good luck

synthesize 3-acetyl-7-hydroxycoumarin

 React 2,4-dihydroxy benzaldehyde  with acetoacetic ester & the cyclize  with out isolation of the intermediate product with PPA or Conc. sulphuric acid   , neutralize the reaction mix. with Na2CO3 & keep for 24 hrs.,  white solid separates out .

Dear Dal N. l

You can follow the following paper.

www.mdpi.com/1420-3049/17/9/10846/pdf

Good Luck

Prof. Dr. Talaat El-Emary

Do the reaction in Methanol as a solvent and 2 drops of piperidine as a catalyst. U will get it. I followed the same procedure and got the product in good yields.

2,4-dihydroxybenzaldehyde and ethyl acetoacetate in the presence of conc. H2SO4 in 00C.

Follow the procedure from this paper:

Org. Biomol. Chem, 2011, 9, 6943-6948

check this paper

Journal of Medicinal Chemistry, 2011, Vol. 54, No. 1

Dr @Devulapally Srikrishna do you mind sharing the reference for the reaction that you suggest? 

@ Dal N. L. Please provide ur e-mail ID i will forward it to you.