In the resolution enhanced 600 MHz 1H NMR spectrum of 1 in C6D6, the downfield proton resonance at δ 9.73 with a coupling constant of 7.43 Hz (3J11,12), 1.12 Hz (4J11,13) and 0.65 Hz (5J11,14) was assigned to H-11 of the D ring. This could be explained based on the anisotropic effect of 7-CO group to H-11 due to space proximity in the nearly planar structure established by the X-ray crystallography. Similarly, the downfield proton resonance signal at δ 9.29 (doublet of triplet, 3J4,3 = 7.87 Hz; 4J4,2 = 5J4,1 = 0.98 Hz) to H-4 of the a ring A. The assigned H-4 and H-11 resonances constitute are the entry point for identification of spin network of the A and D ring based on the COSY correlations. Identification the B ring spin system was made on the basis of the key HMBC correlation between quaternary carbon resonance at δ 57.6 (C-27) and three bond away proton resonance at δ 7.33 (H-29, B ring) which highly overlaps with H-12 and H-13 resonances of the D ring. The remaining three protons of the B ring (H-30, H-31 and H-32) easily identified by series of 1D TOCSY spectra.
The gHMBC correlation with the quaternary sp3 carbon resonance at δ 57.6 allowed the assignment of the proton signal at δ 7.33 to the hydrogen three bonds away, which in turn allowed the assignment of the remaining three hydrogens on the same benzene ring at δ 6.84 (ortho), 6.75 (meta), and 7.46 (para) based on 2D TOCSY and COSY correlations.
The gHMBC spectrum revealed a three bond correlation between H-32 at δ 7.46 and the carbonyl resonance (34-CO) at δ 187.74 and four bond correlations between H-31 at δ 6.74 and 34-CO at δ 187.74 are exclusive. Similarly, the contour plot of the gHMBC spectrum revealed three bond gHMBC correlations between H-1 (δ 7.83) and the carbonyl resonance at δ 190.31 (7-CO). Long-range cross peaks via four bond were observed also in a contour plot of gHMBC spectrum between H-2 at δ 7.46 and the carbonyl resonance (7-CO) at δ 190.31.
Using the gHMBC technique, the proton signal at δ 7.46 was used to locate the 13C chemical shift of the neighboring carbonyl carbon at δ 187.7. Similarly, the chemical shift of the hydrogen three bonds away from the other carbonyl carbon at δ 190.3 was identified at δ 7.84, which in turn allowed the assignment of the remaining three hydrogens on the same benzene ring at δ 6.97 (ortho), 7.19 (meta), and 9.29 (para). The most downfield proton signal at δ 9.73 was assigned to the hydrogen on the remaining benzene ring closest to the carbonyl carbon with a chemical shift at δ 190.3. The last three proton signals at δ 7.32 (ortho), 7.34 (meta), and 8.82 (para) were then assigned by COSY.
The C ring proton resonances (H-20, H-21, H-22, H-23) identified based on the COSY cross peaks and 1D TOCSY spectra. The upfiled proton resonance at δ 6.33 assigned to H-23 (C ring) that lies in the shielding zone of the Phenyl ring according to the X-ray structure of 1. For the same reason, the H-22 proton appear in the 1H NMR spectrum in upfield region (δ 6.60, H-22).
