Substituent dependence of stable conformations in triazolobenzothiazines

From the ¹H and ¹³C NMR spectra of angularly condensed triazolobenzothiazine derivatives it was established that these compounds can be classified into two conformation types, depending on the substituents. The 10-methyl-3-carbethoxy derivatives are cis-anellated, conformationally flexible systems, where one of the aromatic hydrogens (H-9) and one of the methoxy substituents (8-OMe) of the benzothiazine skeleton on the one hand, and the 1-aryl ring on the other, lie close to each other in the preferred conformer. By assuming this conformation, the molecules avoid the steric hindrance of the 10-Me and 1-aryl groups. All compounds unsubstituted on the angular C-10 atom, and the 3-aryl-substituted derivatives of the 10-Me analogues, occur in a different conformation, containing the 1-aryl ring well removed from H-9 and the 8-OMe group.