Base-catalysed cyclocondensation of (E)-3-benzylideneflavanones with thiourea has been found to be highly stereoselective, producing (4S*,5S*)-diphenyl-1,3,4,5-tetrahydro-2H-[1]benzopyrano[4,3-d] pyrimidine-2-thiones in high yield, the structure of one of which has been determined by X-ray crystallography. Just like 4,6-diaryl-3, 4-dihydropyrimidine-2(1H)-thiones, the said products also undergo monoacetylation on treatment with AC2O. The site of acetylation in both of these classes of compound are established by detailed NMR spectral studies.