Abstract 6‐Amino‐2,3‐dihydro‐2‐thioxo‐4(1 H )‐pyrimidinone ( 1 ) reacts in boiling DMF with α,/β‐unsaturated ketones 2 yielding the pyrido[2,3‐ d ]pyrimidine systems 5 and 6, respectively. The orientation in the addition process can be determined by nmr measurements, especially by NOE difference spectroscopy. The products do not correspond to a normal Skraup or Doebner‐v. Miller synthesis.