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Tetrahedron Letters 2005, 46, 6875-6878. To a solution of cyclopent-2-enone (99.82 g, 1216 mmol) and dimethyl malonate (560 mL, 4887 mmol) in dry toluene (1000 mL) was added 2,3,4,6,7,8-hexahydro-1H-pyrimido[1,2-a]pyrimidine (5.10 g, 36.6 mmol). The reaction was stirred at room temperature under nitrogen for 16 hrs, then concentrated under reduced pressure to ˜½ volume. The crude product was filtered through a short path of silica gel (3.5×4 cm), eluting with ethyl acetate. The filtrate was concentrated under vacuum pump rotary evaporation (70-75° C. water bath) for 2 hrs, affording the target compound (247.7 g, 1098 mmol, 90% yield). 1H NMR (500 MHz, CDCl3) δ ppm 3.76 (3 H, s), 3.74 (3 H, s), 3.37 (1 H, d, J=9.46 Hz), 2.80-2.91 (1 H, m), 2.49 (1 H, dd, J=18.31, 7.63 Hz), 2.29-2.37 (1 H, m), 2.15-2.28 (2 H, m), 2.00 (1 H, ddd, J=18.46, 10.99, 1.37 Hz), 1.58-1.71 (1 H, m).

1. concentrationconcentrated under reduced pressure
2. filtrationThe crude product was filtered through a short path of silica gel (3.5×4 cm)
3. washeluting with ethyl acetate
4. concentrationThe filtrate was concentrated under vacuum pump rotary evaporation (70-75° C. water bath) for 2 hrs