One‐Pot Synthesis of 5,7,8,9,9a,10‐Hexahydro‐8‐thioxotetrahydropyrido[2,3‐d : 6,5‐d′]dipyrimidine‐2,4,6(1H,3H,5aH)‐triones via a Four‐Component Coupling Reaction of Aldehydes, Amines, Barbituric Acids, and Thiouracil