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X-Ray crystallography and 500 MHz proton NMR spectroscopy have been used to investigate the conformations of four diterpenoid lactones isolated from extracts of the shrub Andrographis paniculata. The crystal structure of neoandrographolide (2) was refined using a blocked-diagonal least squares technique with 340 parameters and 2018 reflections to yield an R value of 0.049. Full-matrix least-squares refinement for 14-deoxyandrographolide (3) using 102 parameters and 893 reflections yielded an R value of 0.083. The proton NMR spectra of andrographolide (1), neoandrographolide, 14-deoxyandrographolide, and 14-deoxy-11,12-didehydroandrographolide (4) have been assigned and the conformations of the lactone and glucose side-chains deduced from proton coupling constants.
Medforth et al. (Mon,) studied this question.