Computational NMR Study of Cryptolepis Alkaloids: Could the Structural Misassignment of Cryptospirolepine Have Been Avoided?

It is demonstrated that by using modern computational methods, it is possible to identify potential structural misassignments for even molecules as complex as cryptospirolepine. While methods that rely on anisotropic nuclear magnetic resonance (NMR) methods can also be used to offer a powerful orthogonal means of structure elucidation and verification, the present computational study should be feasible in most chemistry departments with access to sophisticated computational facilities. From the overall correlations calculated against experimental 1H and 13C NMR chemical shifts in the series of thirteen Cryptolepis alkaloids, it follows that, generally, they provide a very high level of correlation. The corrected mean absolute error (CMAE) is found to be at the level of 0.13 and 1.5 ppm for, proton and carbon chemical shifts, respectively, which is only about 2 and 1% of their ranges. Based on the performed calculations, some spectral reassignments are suggested, and a number of missing (unknown) experimental chemical shifts are predicted.