Urea (0.48 g, 8.0 mmol) and ytterbium(III) triflate (0.26 g, 0.4 mmol) were added to a 1 dram (3.7 mL) glass reaction vial. Then, the 3:1 acetic acid:water solvent mixture (2 mL) was added to the vial; next, the methyl acetoacetate (0.86 mL, 8 mmol) was added, followed by the aldehyde (4 mmol). The stirred solution was heated for 15 minutes at 95 °C and developed an intense yellow colour. The reaction vial was cooled to room temperature; and, its contents were poured into a 25 mL beaker with 2 mL of ice and 5 mL of deionized water to fully precipitate the product. The solid was collected by vacuum filtration using a hirsch filter. The solid was washed with cold water (5 mL), then with toluene (5 mL); removal of the residual solvent by rotary evaporation provided the product in reasonable purity.

Methyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate (1): white solid (0.80 g, 81% yield) M.p. 212-213 °C (lit. 212-214 °C).

¹H NMR (400 MHz, DMSO-d6): δ 2.23 (s, 3H), 3.51 (s, 3H), 5.12 (s, 1H), 7.20-7.32 (m, 5H), 7.76 (s, 1H), 9.23 (s, 1H).
¹³C NMR (100 MHz, DMSO-d6): δ  18.29, 51.27, 54.21, 99.40, 126.61, 127.76, 128.92, 145.10, 149.15, 152.63, 166.28.

F. Damkaci and A. Szymaniak, J. Chem. Ed. 2014, 91 (6), 943. DOI: 10.1021/ed400390k