0022-4766/14/5508-1521 © 2014 by Pleiades Publishing, Ltd. 1521 Journal of Structural Chemistry. Vol. 55, No. 8, pp. 1521-1526, 2014. Original Russian Text © 2014 H. Nagarajaiah, M.I.A. Khazi, A.Y. Khan, N. Fathima, I.A.M. Khazi, N.S. Begum. SYNTHESIS AND CRYSTAL STRUCTURE ANALYSIS OF TWO THIAZOLO[3,2-a]PYRIMIDINE DERIVATIVES H. Nagarajaiah 1 , M.I.A. Khazi 2 , A.Y. Khan 2 , N. Fathima 1 , I.A.M. Khazi 2 , N.S. Begum 1 UDC 548.737 Synthesis of 5R*-(3-methoxy-phenyl)-3,7-dimethyl-5H-thiazolo[3,2-a]pyrimidine-2,6-dicarboxylic acid diethyl ester (2a) and 3,7-dimethyl-5S*-thiophen-2-yl-5H-thiazolo[3,2-a]pyrimidine-2,6-dicarboxylic acid diethyl ester (2b) are achieved by the cyclocondensation of 3,4-dihydropyrimidine-2-thione derivative with α-haloester. Preliminary analysis was done spectroscopically by means of 1 H NMR spectra, mass spectra and elemental analyses. Further the structures were confirmed by X-ray crystal structure analysis. The two molecules are not identical in configuration. In both the compounds the central pyrimidine ring adopts a conformation which is best described as an intermediate between a boat and screw boat form. The crystal structure is stabilized by intermolecular C–H…O and C–H…π weak interactions. DOI: 10.1134/S0022476614080204 Keywords: thiazolo[3,2-a]pyrimidine derivative, crystal structure, C–H…O and C–H…π weak interactions. INTRODUCTION Dihydropyrimidines represent a heterocyclic system with remarkable pharmacological efficiency and are described as potent mimics of dihydropyridine calcium channel blockers [1-5]. In recent years, several biological activities including antiviral, antitumor, antibacterial, and antiinflammatory activities has been ascribed to these partly reduced pyrimidine derivatives. More recently, appropriately functionalized DHPMs have emerged as orally active antihypertensive agents [6-8]. By performing pharmacological studies with uniquely designed single-enantiomer DHPMs, it was established that calcium channel modulation (antagonist versus agonist activity ) is dependent on the absolute configuration at chiral carbon atom, whereby the orientation of the aryl group substituted to chiral carbon atom acts as a ″molecular switch″ between antagonist (aryl-group up) and agonist (aryl-group down) activity [9]. Furthermore, in the receptor-bound conformation the substituted aryl ring should be positioned axially, perpendicular to, and bisecting the boat-like dihydropyridine/pyrimidine ring, with the 4-aryl substituent preferring the synperiplanar orientation [10]. A cis-carbonyl ester orientation was also found mandatory for optimum calcium channel modulators activity. Thus, pyrimidines have been subjected to a large variety of structural modifications in order to synthesize derivatives with different biological properties. In the course of our structural studies of the family of thiazolopyrimidine derivatives, we report here the structures of two such compounds, namely 5R*-(3-methoxy-phenyl)-3,7-dimethyl-5H-thiazolo[3,2-a]pyrimidine-2,6-dicarboxylic acid diethyl ester (2a) and 3,7-dimethyl-5S*-thiophen-2-yl-5H-thiazolo[3,2-a]pyrimidine-2,6-dicarboxylic acid diethyl ester (2b). 1 Department of Chemistry, Bangalore University, Bangalore-560001, India; noorsb05@gmail.com, noorsb@rediffmail.com (N.S. Begum). 2 Department of Chemistry, Karnatak University, Dharwad-580003, India. The text was submitted by the authors in English. Zhurnal Strukturnoi Khimii, Vol. 55, Supplement 2, pp. S343-S348, 2014. Original article submitted May, 10, 2013.