V

(S)-2,3-diamino-propionie acid (64)

penicillin G (65)

co2h cephalosporin C (66)

Scheme 1.5.11. L-Dap 64 and related j-lactam antibiotics.

b-amino acids, for example Map 68, to ''complicated'' aromatic compounds, such as Adda 74 [114], has been isolated from freshwater and marine cyanobacteria (Schemes 1.5.12 and 1.5.13) [115]. a-Methylation and a, g or ¿-hydroxylations are typical features of these cyanobacterial b-amino acids.

Often marine peptides or depsipeptides with b-amino acid substructures were originally derived from invertebrates, for example sponges, tunicates, or mollusks [116]. Thus, initially it was not clear whether the isolated organism, symbionts thereof, or prey species were the real producers of the isolated secondary metabolites [117]. Because of the complex networks within marine ecosystems the biosynthesis of natural products does not have to take place in a single organism. These interspecies networks might lead to multistep interspecies metabolism of bioactive natural products, as they move through the ''marine food web'' [133]. It must generally be stated that numerous unique secondary metabolites (or their biogenetic precursors) isolated from marine invertebrates may well be produced by microbial (fungal, bacterial) symbionts. For many b-amino acid-containing compounds, however, cyanobacterial biosynthesis has already been proven by microbiological investigations. A second indicator of such non-host biosynthesis is structural similarity of the natural product in question to further metabolites originating without doubt from cyanobacteria.

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