From Rhodium's page Synthesis of Barbiturates (https://www.thevespiary.org/rhodium/Rhodium/chemistry/barbiturates.html (https://www.thevespiary.org/rhodium/Rhodium/chemistry/barbiturates.html)
):
"Pentobarbitone. (Called Nembutal or Pentobarbital.) 26.7 g of clean metallic sodium are dissolved in 400 g of anhydrous (dry) ethanol. To this, a solution of 100 g of 1-methyl butyl-ethyl malonic ester and 37.2 g of dry urea is added. The mixture is heated for 4 to 6 hours in an autoclave, or refluxed for 20 to 40 hours. The alcohol is then removed by distillation. The residue is dissolved in water and this aqueous solution is acidified with hydrochloric acid. The precipitated product is filtered, washed with cold water, and recrystallized from boiling water. Yield: depends on your ability to exclude H2O from the beginning of the reaction, mp: 127-130°C."
I've not yet been able to figure out how one would prepare this ester. Most discussions of malonic esters in organic chemistry involve the preparation of carboxylic acids.
Ethyl Diethylmalonate -> Ethyl 1-Methylbutyl Diethylmalonate
Patent US1842293 (http://l2.espacenet.com/dips/viewer?PN=US1842293&CY=gb&LG=en&DB=EPD)