Ezomycin D2

Unique bicyclic octosyl uronic acid nucleosides include ezomycin, malayamycin, and octosyl acid (OA). They are structurally characterized by OA, an unusual 8-carbon furanosyl nucleoside core proposed to be the precursor to polyoxin and nikkomycin. Despite the well-known bioactivity of these nucleoside antibiotics, the biosynthesis of OA has not been elucidated yet. Here we report the two pivotal enzymatic steps in the polyoxin biosynthetic pathway leading to the identification of OA as a key intermediate. Our data suggest that this intermediate is formed via a free radical reaction catalyzed by the radical S-adenosylmethionine (SAM) enzyme, PolH, and using 3'-enolpyruvyl uridine 5'-monophosphate (3'-EUMP) as a substrate. Subsequent dephosphorylation catalyzed by phosphatase PolJ converts the resulting octosyl acid 5'-phosphate (OAP) to OA. These results provide, for the first time, significant in vitro evidence for the biosynthetic origins of the C8 backbone of OA.

[equation--see text] A protected ezomycin octosyl nucleoside was glycosylated at O-6' with a protected ezoaminuroic acid donor to afford, following several functional group modifications, the title compound 1 ( identical with 4-desamino-4-oxoezomycin A(2)).

The ezomycins are Streptomyces-derived antifungal natural products, belonging to the complex peptidyl nucleoside family of antibiotics. Employing D-serine as a chiral platform, we report herein a novel synthetic route to the bicyclic octosyl nucleoside core of the ezomycins. A key step in the sequence involved a stereoselective 6-exo-trig oxymercurationoxidation of a strategic delta-hydroxy alkene derivative, toward construction of the trans-fused furopyran ring system as present in the target products. In contrast to the known carbohydrate-based synthetic routes to the above furopyranyl fragment, the present amino acid chiral template approach is expected to offer a more flexible pathway toward potential SAR-targeted structural/stereochemical modifications of this central bicyclic nucleoside component of the ezomycins.