The natural analogues of thiazolo [4,5-d] pyrimidines are purine bases and their derivatives which are present in nucleic acids, some vitamins and coenzymes. Thiazolo [4,5-d] pyrimidine derivatives became increasingly important as their spectrum of activity in medicine expands. Some basic structures that demonstrated therapeutic activity have been described. In this paper, the synthesis of 11 new thiazolo [4,5-d] pyrimidine derivatives was carried out in order to develop new biologically active compounds. The reaction of cyanoacetamide synthesis was carried out first followed by the reaction of active methylene nitriles with sulfur and phenyl isothiocyanate and then, using cyclocondensation reactions in which the second ring was closed, a recognizable bicyclic structure was achieved. The cyclocondensation reactions were carried out with two different aromatic aldehydes (benzaldehyde and 4- fluorobenzaldehyde) to produce two different products which were used for further synthesis and allowed the single synthetic pathway to be divided into two parallel synthetic pathways. Chlorination reactions followed in which the oxygen atom was replaced by a chlorine atom at position 7, a hydrazine reaction at which a chlorine atom is replaced by a hydrazine moiety and finally, by the acylation reaction in which nucleophilic substitution occurs on the primary amine of the hydrazine group with reactants such as different acyl chloride / bromide or anhydride. Thin layer chromatography (TLC) was used to monitor the synthesis. Different techniques were used for the analysis of synthesized compounds for the purpose of confirming the structure obtained: elemental analysis technique, mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy ( 1 H-NMR, 13 C-NMR).