Prokaryotic Glycoproteins
A large number of proteins and lipids in biological systems are glycosylated. Comparisons of well characterized protein sequence data base entries indicate that more than half of all proteins in nature will eventually be identified as glycoproteins. Only
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Contents I. Introduction 2. Isolation and Structure Elucidation .............................. 2.1. Spirobisnaphthalenes with Two Oxygen Bridges . . . . . . . . . . . . . . . . . . 2.2. Spirobisnaphthalenes with Three Oxygen Bridges (Preussomerins) . . . . . 2.3. Spirobisnaphthalenes with Two Oxygen Bridges and One C-C Bridge (Spiroxins) ...................................... 2.4. Determination of Relative and Absolute Stereochemistry . . . . . . . . . . ..
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3. Biological Activity .........................................
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4. Biosynthesis ..............................................
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5. Synthesis ................................................ 5.1. Biomimetic Type Approach .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Spiroketalization Approach ................................
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Acknowledgements ...........................................
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References .................................................
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1. Introduction In the last decade, a structurally diverse class of new bioactive natural products was isolated from fungi in which two naphthalene units are fused together via oxidative coupling. These compounds attracted attention because of their antifungal, antibacterial, and antitumoral
D. P. Chakraborty et al., Progress in the Chemistry of Organic Natural Products © Springer-Verlag Wien 2003
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K. Krohn
activity. In addition, several enzymes, namely: phospholipase D, DNA gyrase, and Ras famesyl-protein transferase, were also found to be inhibited with high selectivity. It has been known for a long time that nature can dimerize simple naphthalene building blocks through one or more C-C bonds to construct a variety of secondary metabolites. Examples for these compounds include the neopodine glycosides (1), the sphaerolones (2), the stemphytriols (1), and the stemphyltoxins (3). More highly substituted compounds of this class are represented in the perylenequinones (4, 5) as exemplified by the elsinochromenes (6) (for a review on some C-Cdimeric naphthalenes see Gill and Steglich in (7». The biosynthetic origin of this type of naphthalene "dimers" is well established (see for example (8». In this overview, we will focus on the more recently elucidated structures where the naphthalene units are linked by two or three oxygen atoms. In most of these "dimers" the naphthalene cores are partially modified (saturated or oxidized) and the oxygen atoms are also incorporated into the acetal bridges. As one naphthalene part is generally linked to a decalin system via a spiroketal bridge, this class of compounds is referred to as "spirobisnaphthalenes" (9), or, more specifically, "bisnaphthospiroketals" in case of the spiroxins, (10) or "spiroacetal-linked naphthodiepoxydecalinones" in case of the diepoxins (11). Reflecting on the relatively small number of carbon atoms, the spirobisnaphthalene type of natural products belong one of the most diverse classes of secondary metabolites. They are an excellent example of how flexible nature is in producing numerous d
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