The megalomicins. Part 7. A structural revision by carbon-13 nuclear magnetic resonance and X-ray crystallography. Synthesis and conformational analysis of 3-dimethylamino- and 3-azido-D- and -L-hexopyranosides, and the crystal structure of 4″-O-(4-lodobenzoyl)megalomicin A

An X-ray crystallographic study on 4″-O-(4-iodobenzoyl)megalomicin A has led to the revision of the structures of the megalomicins and the XK-41 antibiotics. Crystals are orthorhombic, space group P212121 with a= 12.669(2), b= 19.501 (6), c= 25.741 (9)A, and Z= 4. The structure was solved by the heavy-atom technique, and 1 812 observed reflections led to a final R of 0.095. The novel amino-sugar previously thought to be D-rhodosamine has been shown to have the L-configuration and is therefore renamed L-megosamine. It has also been shown to be glycosidically attached to the tertiary 6-hydroxy group. The 13C n.m.r. and circular dichroism (c.d.) parameters of these macrolides are described. The syntheses of methyl α- and β-D-rhodosaminide, methyl α- and β-D-megosaminide, methyl α- and β-L-megosaminide, methyl α- and β-D-angolosaminide, and methyl 2,3,6-trideoxy-3(dimethylamino)-α-D-xylo-hexopyranoside are described and their conformations and 13C n.m.r. parameters are discussed. Methyl α-D- and -L-amicetoside, methyl α-D- and -L-cineruloside and other model 4-oxopyranosides and pyrans have been synthesized. Their c.d. properties have been determined and they have been shown to exhibit Anti-Octant behaviour.