Complete assignment of the 1H and 13C NMR spectra and conformational analysis of bonellin dimethyl ester

The use of a high-field NMR instrument (v(H-1) = 500 MHz) and 2-dimensional NMR techniques (HMQC, HMBC, ROESY) enabled us to fully assign the H-1 and C-13 chemical shifts of bonellin dimethyl ester. The P-pyrrolic proton of C-3 appeared as a broad singlet at 8 = 8.93, whereas that of C-8 gave a quartet with 8 = 8.69 and (4)J(H-H) = 11.281 Hz. The C-2(1) methyl protons appeared as a doublet with 6 = 3.55 and (4)J(H-H) = 11.071 Hz, while the C-7(1) methyl protons afforded a doublet with 6 = 3.51 and (4)J(H-H) = 11.281 Hz. These results suggest that the P-pyrrolic carbons of ring A belong to the aromatic 18 it-electron [18]diazaannulene delocalization pathway, whereas those of ring B remain outside the aromatic pathway. The broadening of the C-3 beta-pyrrolic proton signal can be attributed to the allylic 3-CH - 2(1)-CH3 coupling and the 3-CH - 21-NH coupling. At 330 K, the tautomeric exchange 21-NHa - 23-NHb is fast and only one broad signal at 8 = -2.49 is seen for these protons. The ROESY spectrum showed clear correlation signals between the 18(2)-CH3, and 17(1)-CH2 protons, the 18(2)-CH3 and 17(4)-CH3 protons, as well as between the 18(1)-CH3 and 17-CH protons. These results are compatible with the previous assignment that the absolute configuration at C-17 is S. Application of spin simulation enabled us to determine the chemical shifts and the (3)J(H-H) coupling constants of the 17-propionate side-chain. The (3)J(H-H)-values were. used to calculate the populations for the 17(1)-17 and 17(2) 17(1) rotamers. A relatively high population value of 0.41 was found for the 17(1)-17 g-rotamer, whose methoxycarbonylmethyl group points to the C-15 methine-bridge. This was interpreted as explaining the high tendency of bonellin to form anhydrobonellin. The rotational freedoms in the 13-propionate side-chain were studied by measuring the 1H NMR spectra of the side-chain at temperatures between 300 and 195 K. At 300 K, the 13(1) - and 13(2) -CH2 proton signals appeared as deceptively simple triplets, which at 195 K were split into complex multiplets. At 195 K, the signal arising from the 13(1)-CH2 protons exhibited more splitting, which indicates that these protons have less rotational freedom than the 13(2) -CH2 protons. Copyright (c) 2004 Society of Porphyrins T Phthalocyanines.