Complete 1H, 13C and 15N NMR assignment of tirapazamine and related 1,2,4-benzotriazine N-oxides

H-1, C-13 and N-15 NMR measurements (1D and 2D including H-1-N-15 gs-HMBC) have been carried out on 3-amino-1, 2,4-benzotriazine and a series of N-oxides and complete assignments established. N-Oxidation at any position resulted in large upfield shifts of the corresponding N-1 and N-2 resonances and downfield shifts for N-4 with the exception of the 3-amino-1,2,4-benzotriazine 1-oxide in which a small upfield shift of N-4 was observed. Density functional GIAO calculations of the N-15 and (1)3C chemical shifts [B3LYP/6-31G(d)//B3LYP/6-311+G(2d,p)] gave good agreement with experimental values confirming the assignments. The combination of C-13 and N-15 NMR provides an unambiguous method for assigning the H-1 and C-13 resonances of N-oxides of 1,2,4-benzotriazines. Copyright (c) 2006 John Wiley & Sons, Ltd.