Studies on Orientation of Cyclization in Thiazolo‐Quinazoline Heterocyclic System Through NMR, DFT, and X‐ray Diffraction

Abstract

1‐(4‐Methoxyphenyl)‐1,2,5,6‐tetrahydrobenzo[f]quinazoline‐3(4H)‐thione 3, obtained by the condensation of 2‐tetralone 1, 4‐methoxy benzaldehyde 2 with thiourea, on reaction with chloroacetic acid and 1,2‐dibromoethane furnishes compounds 12‐(4‐Methoxyphenyl)‐9,12‐dihydro‐5H‐benzo[f]thiazolo[2,3‐b]quinazolin‐10(6H)‐one 4 and 12‐(4‐Methoxyphenyl)‐‐6,9,10,12‐tetrahydro‐5H‐benzo[f]thiazolo[2,3‐b]quinazoline 5 and not their possible isomers 7 and 8, respectively. The regiochemistry of the cyclized products and their structure is established by elemental analysis, 1H NMR, 13C NMR, IR, and mass spectral data. Density functional theory (DFT) calculations have been carried out for compounds 4 and 5 and their isomers 7 and 8 with Jaguar version 6.5112 using Becke three parameter Lee‐Yang‐Parr density functional method and 6‐31G** basis set. X‐ray diffraction studies indicate that compound 4 crystallizes in the triclinic space group P‐1, with Z = 4 and cell parameters a = 9.5558(8) Å, b = 9.6363(4) Å, c = 20.9728(11) Å, α = 87.652(4)˚, β = 77.283(5)˚, and γ = 70.437(5)˚. 1H and 13C NMR of compounds 4 and 5 and their isomers 7 and 8, respectively, have been calculated and correlated with experimental results. 2‐Arylidene derivatives of 4 were obtained by two routes, and their structure was established by spectral data. The lowest energy optimized geometry of the compound 4 in gas phase is consistent with that obtained by X‐ray crystallographic studies.