Sporidesmins. XV, The ¹³C nuclear magnetic resonance spectra of sporidesmin and sporidesmin-D. The evidence in the spectra for strain imposed by an epidithio bridge

Abstract

The proton-noise-decoupled ¹³C N.M.R. spectra of sporidesmin and sporidesmin-D are presented. The detailed assignments were worked out from the undecoupled spectrum of sporidesmin-D. The 3-methyl resonance moved downfield (by 7.3 ppm) upon cleavage of the -S-S- bridge in sporidesmin while the peaks of the two quaternary carbons to which the sulphurs are bound moved upfield (by 4.7 ppm). Lactam N-methyl and indoline N-methyl proton resonance decoupling showed that their p.m.r. assignments should be reversed. The electro-negativity of nitrogen, sulphur and carbonyl groups increased the coupling constants of the 3-CH₃ by 7Hz and of the 11-CH by 10 Hz. The carbon of the lactam N-methyl was shielded by 14 ppm. There were six three-bond couplings, four HCN¹³C and two HCO¹³C, evident but no non-aromatic HCC¹³C couplings.