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In the course of our screening program for new inhibitors of dipeptidyl peptidase III (EC 3.4.14.4), we discovered fluostatins A and B (1 and 2, Fig. 1) from the culture broth of Streptomyces sp. TA-3391. The taxonomy of the producing strain, fermentation, isolation, physico-chemical properties and biological properties have been described in the preceding paper1). Here we report on the structure determination of 1 and 2 on the basis of spectroscopic studies, chemical studies and X-ray crystallographic analysis. The physico-chemical properties of 1 and 2 were described in the previous paper1). The molecular formulae of 1 (C18H10O5) and 2 (C18H14O6) were determined by interpretation of the HRFAB-MS, NMR spectral analyses and elemental analyses. The 1H and 13C NMR spectra of 2 showed the presence of twelve protons and eighteen carbons. The DEPT experiment revealed that these carbons consisted of two carbonyls, twelve other sp2 and four sp3 carbons. The carbons bearing protons were assigned by a Heteronuclear Single Quantum Coherence (HSQC) experiment. These data are summarized in Table 1. Two partial structures of 2 were established on the basis of 1H-1H coupling constants in the 1H NMR spectrum and Heteronuclear Multiple Bond Correlation (HMBC) experiment (Fig. 2-b). A 4-bond coupling

Fluostatins A and B, New Inhibitors of Dipeptidyl Peptidase III, Produced by Streptomyces sp. TA-3391. II. Structure Determination.