The role of the putative P450 monooxygenase OxyD and the chlorination time point in the biosynthesis of the glycopeptide antibiotic balhimycin produced by Amycolatopsis balhimycina were analyzed. The oxyD gene is located directly downstream of the bhp (perhydrolase) and bpsD (nonribosomal peptide synthetase D) genes, which are involved in the synthesis of the balhimycin building block ␤-hydroxytyrosine (␤-HT). Reverse transcriptase experiments revealed that bhp, bpsD, and oxyD form an operon. oxyD was inactivated by an in-frame deletion, and the resulting mutant was unable to produce an active compound. Balhimycin production could be restored (i) by complementation with an oxyD gene, (ii) in cross-feeding studies using A. balhimycina JR1 (a null mutant with a block in the biosynthesis pathway of the building blocks hydroxy-and dihydroxyphenylglycine) as an excretor of the missing precursor, and (iii) by supplementation of ␤-HT in the growth medium. These data demonstrated an essential role of OxyD in the formation pathway of this amino acid. Liquid chromatography-electrospray ionization-mass spectrometry analysis indicated the biosynthesis of completely chlorinated balhimycin by the oxyD mutant when culture filtrates were supplemented with nonchlorinated ␤-HT. In contrast, supplementation with 3-chloro-␤-HT did not restore balhimycin production. These results indicated that the chlorination time point was later than the stage of free ␤-HT, most likely during heptapeptide synthesis.

In the mutasynthetic approach, the DeltadpgA mutant of the vancomycin-type glycopeptide antibiotic producer Amycolatopsis balhimycina, which is deficient in the synthesis of 3,5-dihydroxyphenylglycine (DPg), was supplemented with synthetic DPg analogues to obtain the corresponding modified glycopeptides. Sterically more demanding 3,5-disubstituted methoxy derivatives as well as monosubstituted DPg analogues were accepted as substrates. These facts indicate that steric and electronic requirements suffice in several cases for the oxidative closure of the AB ring, thus leading to the generation of novel antibiotically active glycopeptide derivatives. The results represent a further step in evaluating the potential of mutasynthesis for peptidic secondary metabolites.

Natural products represent an important source of drugs in a number of therapeutic fields, e.g. antiinfectives and cancer therapy. Natural products are considered as biologically validated lead structures, and evolution of compounds with novel or enhanced biological properties is expected from the generation of structural diversity in natural product libraries. However, natural products are often structurally complex, thus precluding reasonable synthetic access for further structure-activity relationship studies. As a consequence, natural product research involves semisynthetic or biotechnological approaches. Among the latter are mutasynthesis (also known as mutational biosynthesis) and precursor-directed biosynthesis, which are based on the cellular uptake and incorporation into complex antibiotics of relatively simple biosynthetic building blocks. This appealing idea, which has been applied almost exclusively to bacteria and fungi as producing organisms, elegantly circumvents labourious total chemical synthesis approaches and exploits the biosynthetic machinery of the microorganism. The recent revitalization of mutasynthesis is based on advancements in both chemical syntheses and molecular biology, which have provided a broader available substrate range combined with the generation of directed biosynthesis mutants. As an important tool in supporting combinatorial biosynthesis, mutasynthesis will further impact the future development of novel secondary metabolite structures.

Mutasynthesis—a very potent tool? The two chlorine atoms attached to the aglycon of vancomycin‐type glycopeptide antibiotics (see structure) have considerable influence on the antibiotic activity of each compound. By combining molecular genetic methods and chemical synthesis these chlorine atoms can be replaced by fluorine atoms. The described approach may also be applicable to modifying other parts of the glycopeptide molecule.

A new depsipeptide, cyanopeptolin 963 A (1), was isolated from an axenic strain of the toxic freshwater cyanobacterium Microcystis PCC 7806. The structure of this compound was elucidated by chemical and spectroscopic analyses, including high-resolution ESI-FTICR-MS, 2-D NMR, and GC-MS of the hydrolysate. The major structural difference compared to previously characterized cyanopeptolins of this strain is the replacement of the basic amino acid in position 3 by L-tyrosine. Compound 1 displayed inhibitory activity against chymotrypsin with an IC50 value of 0.9 microM.

For Abstract see ChemInform Abstract in Full Text.

Palladium-catalyzed alpha-arylation and vinylation of dioxolane (S,S)-I, easily obtained from (S)-mandelic acid, proceeds with high yields and excellent diastereoselectivity at room temperature employing commercially available P(t-Bu)(3).HBF(4) and Pd(OAc)(2) as a catalytic precursor system. This method displays general utility for a large variety of aryl, heteroaryl, and vinyl bromides.

Professor Anton Rieker zum 70. Geburtstag gewidmetDas Glycopeptid-Antibiotikum Vancomycin (Schema 1) wird seit mehr als 30 Jahren bei Enterokokken-und Staphylokokken-Infektionen eingesetzt und hat den Status eines ZUSCHRIFTEN Das Boratom an der Spitze von 3 u ist von seinen vier Nachbarn planar umgeben. In 3 a, 3 b, 3 c und 3 d bildet die Ebene des B 3 -Dreiecks mit der Ebene C,B1,B4 an der Spitze des Dreiecks Winkel von 22, 26, 14.6 bzw. 2.18, die Boratome B1 sind also z. T. nahezu planar-tetrakoordiniert. Vollst‰ndige Planarit‰t wird in 3 a und 3 c durch den Elektronenmangel des Boratoms im Sechsring [12] verhindert: Im Modell 3 e mit tetraedrisch-koordiniertem B4 betr‰gt der genannte Interplanarwinkel nur 3.28, in 3 d mit doppelt gebundenem, trikoordiniertem B4 nur 2.18. Die um 248 st‰rkere Abweichung von der Planarit‰t in 3 b gegen¸ber der in 3 d ist auf sterische Hinderung durch die Trimethylsilylgruppe an B4 zur¸ckzuf¸hren. Im Modell 3 f ohne Methylenbr¸cken ist B1 exakt planar-tetrakoordiniert. Die ersten Triboracyclopropanate zeigen also im Wesentlichen die vorhergesagten Strukturen.