Development of highly sensitive NMR spectroscopic techniques and their application to structural analysis of natural products

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In order to develop highly sensitive NMR techniques, which enable to obtain satisfactory structural information of complicated natural products only available in a small quantity, we attempted to create new pulse sequences by modification of 2D-NMR techniques to 1D-NMR version. During these process, we found that an irradiation field with very high selectivity, which could be created by the use time sharing gated decoupling (TSD), gave very good results for these purposes. By employing this irradiation field and 90゜ composite pulse, we succeeded in developing a new noe technique named time sharing gated decoupling noe (TSD-NOE) which has several advantages over conventional noe experiments.As a next step, we applied TSD for obtaining decoupling difference spectra and could develop new pulse techniques named TSD-COSY and TSD-relayed-COSY. In these techniques, a highly selective irradiation field created by TSD was employed to saturate a specific proton to cause the decrease of signal intensities of the protons coupled to the saturated protons. After further detailed experiments, we succeeded in set up best experimental conditions which relied upon several experimental factors such as delay time and field strength.Next we developed a new technique named selective HOHAHA-CH which facilitated to reveal the relationship of a given proton and the carbons attached to the protons involved in the same proton spin system. Thus, by application of this technique, it has become possible to carry out very efficiently the structural analytical works of complex molecules. The structure of a new nucleoside antibiotic portmicin was determined by application of this new pulse sequence. In addition, we reported that HMBC wasparticularly useful for structural studies of natural products such as polyketides and terpenes with many methyl groups.