Among the available reports on base-catalysed cyclocondensation of chalcones (1) with thiourea, two 1,2 state that the products are 4,6-diaryl-5,6-dihydropyrimidine-2(1H)thiones (2), while a third 3 claims that they are 4,6-diaryl-3,4dihydropyrimidine-2(1H)-thiones (3). The spectral data of the products, however, gave convincing evidence that the last structure is actually correct. 3 (E)-3-Benzylideneflavanones (4), the synthesis and reactions of which we have recently studied [4][5][6][7][8] , possess a part structure which is similar to the chalcones (1). So we endeavoured to synthesise a new type of fused heterocycle possessing a flavonoid and a pyrimidine moiety, both endowed with interesting physiological activities [9][10][11][12] , by cyclocondensation of 4 with thiourea. Since the target system contains two stereogenic centres, our interest was also to determine the configuration of the major / only product of the reaction (which may be 5 or its diastereomer). Furthermore, on acetylation 3 gives a monoacetyl derivative. Without giving any concrete chemical or spectral evidence, Al-Hajjar et al. 3 assigned the structure 6 to this derivative. So, in connection with synthesis of the said new heterocycles, we intended to observe the outcome of their acetylation reaction and to ascertain the acetylation site(s). The results of these studies are presented in this paper.
Results and discussionWhen each of the (E)-3-benzylideneflavanones 4a-d was treated with thiourea in methanol in presence of sodium methoxide, a single product was obtained in very good yield. Analytical and spectral data of the products from 4a-d (see Experimental) definitely showed that one diastereomeric dl pair of the desired cyclocondensation product was formed in all cases. The reaction was thus highly stereoselective. All the products formed good quality crystals from methanol. An X-ray crystallographic analysis of the product from 4a established its stereostructure as 5a. The other compounds were therefore Synthesis of (4S*,5S*)-diphenyl-1,3,4,5-tetrahydro-2H-[1]benzopyrano [4,3-d]pyrimidine-2-thiones by base-catalysed cyclocondensation of (E)-3-benzylideneflavanones with thiourea, and determination of their monoacetylation site