Naturally occurring aminoglycosides are amino sugars consisting of an aglycon unit, which can be cyclic or open chain. The aglycon is either streptamine, its related aminocyclitols, cyclitol, monoaminocyclitol, or a noncyclitol moiety. Therefore, these natural products can be divided into three major classes: (1) aminoglycosides containing streptamine and its related aminocyclitols, (2) aminoglycosides containing cyclitol and/or monoaminocyclitol, and (3) aminoglycosides having a noncyclitol moiety in their structures.
A. Aminocyclitol Aminoglycosides Containing Streptamine and Its Derivatives
The aglycon unit in aminocyclitol aminoglycosides that contain streptamine and its derivatives is 2-deoxystreptamine (compounds 1-3 ) 1 (2-DOS), streptamine 2, or streptidine 3.
Most of the aminocyclitol aminoglycosides with high antibacterial activity have the 2-deoxystreptamine as the aglycon unit. The carbon atom bearing the amino group with the R configuration is assigned number 1 in the structure. The most active aminoglycosides are those with their sugar units substituted at positions 4 and 5, or 4 and 6, of the aglycon unit. By convention, the sugar unit attached to the 4-position
of 2-DOS is designated by the primed (') numbers and those attached to the 5- or 6-positions are indicated by the double-primed (") numbers.
(a) 4-Substituted 2-Deoxystreptamine. Among 4-substituted-2-deoxystreptamine aminoglycosides, three pseudotrisaccharides, apramycin, oxyapramycin (14) , and saccharocin (15) (compounds 13-15 ) [27,28], show more potency than neamine (4) and are active against many resistant bacterial strains. Members of this group of pseudodisaccharides (compounds 4-12 ) are minor metabolites of Actinomycetes and show weak antibacterial activity [29-35].
(b) 5-Substituted 2-Deoxystreptamine. These aminoglycoside antibiotics have an unusual spiro ring junction in their structures. They show weak antibacterial activity rr<
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