The title compound, 2,7-dimethyl-3-chloro-4H-pyrido[1,2-a] pyrimidin-4-one crystallized in the Triclinic crystal system with space group P1 having unit cell parameters: a = 7.2701(3) b = 7.5248(4) c= 9.6031(4)Å α = 105.975(3)° β = 105.735(3)° γ = 96.943(3)°, Z = 2. A crystal of dimensions 0.3 × 0.2 × 0.2 mm was used for data collection on Bruker CCD area-detector diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å) in Ф and ω scan mode. Data were collected at 293 K. A total of 8011 reflections were scanned for θ ranging from 2.33 < θ < 25.0° and out of these reflections, 3212 were found unique (-8 ≤ h ≤ 8, -8 ≤ k ≤ 8, -11 ≤ l ≤ 11). Intensities of 2352 reflections had I > 2σ(I) and were taken as observed. Data were corrected for Lorentz-polarization and multi-scan absorption corrections (Tmin = 0.989, Tmax = 0.994). The π-π interactions were observed between the asymmetric molecules. The pyrido-pyrimidine moiety is involved only in π-π stacking interactions in the structure and not in any C-H…π contacts. [ABSTRACT FROM AUTHOR]