AN NMR SPECTROSCOPIC STUDY OF AZADIRACHTIN AND ITS TRIMETHYL ETHER

The neem tree Azadirachta indica A. Juss and the related Chinaberry tree Melia azedarach L. are attracting considerable interest, particularly because of their insect-repelling properties. The most potent constituent is azadirachtin, a limonoid which exerts strong physiological and phagorepellent activities. Incorporating the earlier chemical and spectroscopic studies carried out by Morgan and coworkers6 and applying the technique of continuous wave proton decoupling in partially relaxed Fourier transform 13C NMR, we proposed structure 1 for azadirachtin in 1975. Since evidence for this complex structure was not convincing, we have been using this molecule as a test sample for the application of modern NMR methods, however, without arriving at a conclusive structure. After proposals of revised structures by Kraus and coworkers, hey and coworkers, and ourselves starting in mid-1985, its structure was finally established as 2 by Kraus, et al. and by Ley, et al. In the following we summarize the results of our NMR studies and some of the unique difficulties encountered during the investigation of this compound and its 7,11,20-trimethyl ether.