Purpurosamines

The other component parts of streptomycin, namely L-streptose and the aminosugar 2-deoxy-2-methylamino-L-glucose (A -methyl-L-glucosamine), are also ultimately derived from o-glucose. Gentamicin Ci contains two aminosugars, L-garosamine and D-purpurosamine. 

Purpurosamine B (252), a component of gentamicin C2, belongs to a new class of amino sugars, the 2,6-diamino-2,3,4,6-tetradeoxyaldo-... 

Carbohydrate antibiotics Antibiotics that contain one or more amino sugars within the molecule are called carbohydrate antibiotics. For example, gentamicin is composed of three different units purpurosamine, 2-deoxystreptamine and garosamine. Other examples include streptomycin and neomycin. 

The hetero-Diels-Alder reaction of activated butadienes with carbonyl compounds is a convenient method for the preparation of precursors of sugars. Up to three chiral centers are created simultaneously. The high-pressure [4 + 2]cycloaddition of l-methoxybuta-1,3-diene 32 to N-mono- and N,N-diprotected alaninals was investigated [42-45]. The Eu(fod)3-mediated reaction of 32 with alaninal 25 gave a mixture of four diastereoisomers, which was then subjected to acidic isomerization, leading to the thermodynamically more stable pair of adducts syn-33 and anti-34, with predominance of the latter isomer (Scheme 12). The N-monoprotected alaninals reacted with a moderate ryn-diastereoselectivity. This method was used in the synthesis of purpurosamines (see Sec. DI.C). 

A. Golfbiowski, U. Jacobsson, and J. Jurczak, High-pressure approach to the total synthesis of 6-epi-D-purpurosamine B, Tetrahedron 43 3063 (1987). 

T. Suami, Y. Honda, T. Kato, M. Masu, and K. Matsuzawa, Synthesis of methyl 2,6-di-A-acetyl-2,3,4,6,7-pentadeoxy-L-iyxo-heptopyranoside derivative of 6-epi-D-purpurosamine B, Bull. Chem. Soc. Jpn. 53 1372 (1980). 

Figure 7.12. The structure of gentamicins. Ring I is purpurosamine, [I is 2-deoxystreptamine and III is garosamine. The arrows indicate where this group of antibiotics can he N-acetylated by (a) kanamycin acetyltransferase (b) gentamicin acetyltransferase I and (c) gentamicin acetyltransferase II...

Several years later Danishefsky introduced l-methoxy-3-trimethylsilyloxy-l,3-butadiene, a highly reactive diene, which upon reaction with (not activated) aldehydes catalyzed with mild Lewis acids [(Eu(fod)3] afforded cyclic a, -unsaturated ketones [19]. Another method involved reaction of l,4-di-alkoxy(acyloxy)-butadiene with an activated heterodienophile, which led to more functionalized derivatives (Schmidt) [20]. This methodology may be illustrated by the classical synthesis of the precursor of purpurosamine B (3) and higher sugar... 

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