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The existence of an isomeric form in equilibrium with the major component of FK-520 in polar solutions has been demonstrated. This minor component has been isolated in high yield and purity by a novel crystallization strategy and preparative HPLC. The equilibrium product was characterized by NMR and MS.

The preparation of 4-de-N-methylfortimicin A analogs as well as the preparation of 4-de-N-methyl-4-N-(beta-aminoethyl)-4-N-ethylfortimicin B is reported. It was shown that the 4-N-methyl group in fortimicin analogs is essential for antibacterial activity since neither the 4-de-N-methylfortimicin A nor the 4-de-N-methyl-4-N-(beta-aminoethyl)-4-N-ethylfortimicin B exhibited useful biological activity.

7-Deoxy-4(R)-dihydrospectinomycin (7) has been prepared and its structure firmly established by proton magnetic resonance and high resolution mass spectrometry. This spectinomycin analog is devoid of antibiotic activity.

The structures of fortimicins A and B have been determined by PMR, CMR, mass spectra and CD combined with chemical degradations. Both antibiotics are pseudodisaccha-rides and incorporate a novel aminocyclitol, fortamine. Incontrast to the diaminocyclitol moieties of known aminoglycosides, fortamine is a 1, 4-diamine, contains both N- and O-methyl groups and possesses chiro stereochemistry. Both antibiotics are glycosides of 6-epi-pur-purosamine B, but fort imicin A differs from fortimicin B by being a glycyl amide.