The Biosynthesis of the Benzoxazole in Nataxazole Proceeds via an Unstable Ester and has Synthetic Utility

Abstract

Heterocycles, a class of molecules that includes oxazoles, constitute one of the most common building blocks in current pharmaceuticals and are common in medicinally important natural products. The antitumor natural product nataxazole is a model for a large class of benzoxazole‐containing molecules that are made by a pathway that is not characterized. We report structural, biochemical, and chemical evidence that benzoxazole biosynthesis proceeds through an ester generated by an ATP‐dependent adenylating enzyme. The ester rearranges via a tetrahedral hemiorthoamide to yield an amide, which is a shunt product and not, as previously thought, an intermediate in the pathway. A second zinc‐dependent enzyme catalyzes the formation of hemiorthoamide from the ester but, by shuttling protons, the enzyme eliminates water, a reverse hydrolysis reaction, to yield the benzoxazole and avoids the amide. These insights have allowed us to harness the pathway to synthesize a series of novel halogenated benzoxazoles.

Myxococcus xanthus as Host for the Production of Benzoxazoles

“…In presence of MxcE, the areas of all detected amide and product peaks were increased. As expected from a previous study, [6] only NatL2 and NatAM were needed for the in vitro synthesis of the 3‐HBA‐derived benzoxazole ( 2 c ). Still, the presence of MxcE improved the production level of 2 c by 46 % (Table S3).…”

“…Previous studies indicated that only two enzymes are required for benzoxazole biosynthesis from 3‐HAA, namely an ATP‐dependent ligase and an amidohydrolase [6] . The two genes natL2 and natAM from the nataxazole biosynthetic gene cluster of Streptomyces sp.…”

“…Benzoxazole‐containing compounds are used in different therapeutic areas, as exemplified by the approved drugs chlorzoxazone (muscle relaxant), tafamidis (transthyretin stabilizer) or suvorexant (treatment of insomnia) [9] . Their potent biological activities combined with the understanding of benzoxazole biosynthesis have stimulated the biotechnological production of benzoxazole analogs [6–8,10–12] . Recently, the genes involved in nataxazole biosynthesis were expressed in the bacterium Escherichia coli and, upon feeding of 3‐HAA together with other aryl carboxylic acids, various benzoxazoles were generated, albeit in low titers [12] …”

“…Upon enzymatic linkage of 3‐hydroxyanthranilic acid (3‐HAA) with another aryl carboxylic acid via an ester bond, an amidohydrolase catalyzes the formation of a hemiorthoamide intermediate and a subsequent dehydration to give a benzoxazole (Figure 1A). [6] Natural products such as the antibiotics caboxamycin and A‐33853 or the anticancer agent nataxazole are synthesized in this way (Figure 1B). [6,7] A distinct assembly strategy is pursued in the biosynthesis of the closoxazoles, which were recently identified from the anaerobic bacterium Clostridium cavendishii DSM21758 [8] .…”

Synthesis and Biological Evaluation of 2‐(Halophenyl)benzoxazole‐5‐Carboxylic Acids as Potential Anti‐Inflammatory and Cytotoxic Agent with Molecular Docking Studies

“…For this purpose, benzoxazole derivatives have attracted a lot of attention in recent years based on their usage as intermediates in the creation of novel biological materials. The compounds having the presence of benzoxazole skeleton in synthetically important and naturally occurring molecules that possess anti-inflammatory [4][5] and anticancer activity [6][7][8][9][10][11][12] are displayed in Figure 1.…”