AbstractThis paper considers the strategy of the StrucEluc expert system application for structure elucidation of new natural products when there is a lack of connectivity information that is characteristic of proton‐deficient molecules. It is shown that in this case, a database search for fragments using a 13C NMR spectrum as input allows an investigator to fill gaps in the recorded data. Algorithms and programs have been developed that allow fragments found in the library and/or proposed by the user to be embedded in the molecular connectivities diagrams built on the basis of 2D NMR data analysis. We demonstrate the structure determination of three alkaloids from the cryptolepine series using the principles of construction and application of a user fragment library. The approach described appears to be the most efficient means of structure elucidation for natural products with 2D NMR spectra characterized by sparse responses. Copyright © 2003 John Wiley & Sons, Ltd.