Different substituted diesters of thiazolopyrimidine were prepared by the treatment of 3,4 dihydropyrimidine2-thione with 𝛼-haloesters using ethanol under reflux condition affording 71-85% yield. IR, ¹HNMR, ¹³CNMR and elemental analyses were used for the characterization of these compounds. The crystal and molecular structure of one of the product, 5-phenyl-3,7-dimethyl-5H-thiazolo[3,2-a]pyrimidine-2,6-dicarboxylic acid diethyl ester (3e) was verified by single crystal X-ray diffraction method. The antimicrobial activity was evaluated against four bacterial strains and one fungal species. Few of the derivatives exhibited antibacterial and antifungal activities.

The compounds: 11-trimethylsilyloxy-1,2,3,4,4a,9a-hexahydro-1,4-etheno-anthraquinone and 4-benzyl-8-trimethylsilyloxy-4-aza-tricyclo[5.2.2.0]undec-8-ene-3,5-dione were synthesized by the Diels-Alder [${}_4\pi_s +_2 \pi_s$] cycloaddition reaction of 2-(trimethylsilyloxy)-1,3-cyclohexadiene with naphthaquinoneand 𝑁-benzylmaleimide under ultrasonic conditions. The crystal structure analysis was done using single crystal X-ray diffraction method. In both the compounds, the trimethylsilyloxy- and naphthaquinone/𝑁-benzylmaleimide moieties are endo- to the bicyclic ring.

The compounds, 7-methyl-3,5-diphenyl-5𝐻-thiazolo[3,2-a]pyrimidine-6-carboxylic acid ethyl ester (1), 3-amino-2-cyano-7-methyl-5-phenyl-5𝐻-thiazolo[3,2-𝑎]pyrimidine-6-carboxylic acid methyl ester (2), 2-dimethylaminomethylene-7-methyl-3-oxo-5-phenyl-2,3-dihydro-5𝐻-thiazolo[3,2-𝑎]pyrimidine-6-carboxylic acid ethyl ester (3), 2-(3-cyano-benzylidene)-5-(4-hydroxy-phenyl)-7-methyl-3-oxo-2,3-dihydro-5𝐻-thiazolo[3,2-𝑎]pyrimidine-6-carboxylic acid methyl ester; with 𝑁,𝑁-dimethyl-formamide (4) and 3-ethoxycarbonylmethyl-5-(4-hydroxy-3-methoxy-phenyl)-7-methyl-5𝐻-thiazolo[3,2-𝑎]pyrimidine-6-carboxylic acid methyl ester (5) have been synthesized and their structures evaluated crystallographically. Compound 1 crystallizes in the space group $P^¯_ı$ with Z=8, with four molecules in the asymmetric unit. Compound 2 also crystallizes in the space group $P^¯_ı$ with Z=4 wherein asymmetric unit accommodates two molecules. Compound 3 belongs to $P2_1$/c with Z=4, compound 4 crystallizes in 𝑃bc2₁ with Z= 4 and compound 5 belongs to $P^¯_ı$ with Z=2. In all the above compounds, the aryl ring positioned at C5 of thiazolopyrimidine ring is almost perpendicular. In the case of compounds with substituted phenyl ring, aryl group-up conformation predominates. However, for compounds with unsubstituted phenyl ring, aryl group-down conformation is adopted. By varying the substituents at positions C2, C3, C6 and on the aryl at C5 in the main molecular scaffold of (1-5), we have observed significant differences in the intermolecular interaction patterns. The packing features of the compounds are controlled by C-H…O, C-H…N, N-H…N O-H…N, C-H$\ldots \pi$ and $\pi\ldots \pi$ weak interactions.

A new ammonium acetate-assisted, convenient and efficient procedure for the synthesis of arylidene derivatives of thiazolopyrimidine is described. The main advantages of this protocol is that it is economical, short reaction time, commonly available chemicals, and ease of isolation of products. In addition, a new series of thiazole-fused pyrimidines were synthesized and hydrolysis of one of its arylidene derivative studied. All the compounds were characterized by analytical and spectroscopic methods. Further, the structure of hydrolyzed product and two other compounds were confirmed by X-ray crystal structure analysis. The crystal structures are stabilized by intermolecular C-H...O, C-H...N, C-H... 𝜋 and 𝜋 ...𝜋 weak interactions. The anti-microbial screening was done on the compounds in order to test their anti-bacterial and anti-fungal activities.