In the present study, we report the synthesis of different condensed pyrimidine derivatives by the exploitation of addition reactions to π-deficient systems or active methylene building blocks. For instance, the Diels–Alder reaction of pyrimidinethione 1 and benzalacetophenone 2 yielded pyridopyrimidine 5 (Scheme 1). On the other hand, the Michael addition of active methylene containing compounds to 1 produced derivatives 8, 10, 11, 12 and 14 (Scheme 2). The alkylation of 1 by ethyl chloroacetate yielded 16. Furthermore, a triazinoquinazoline 19 was obtained through the cyclic condensation reaction of 1 with benzilmonohydrazone (Scheme 3). All reactions proceeded in good to moderate yields and the resulting compounds were characterized by different spectroscopic techniques.