Synthesis of potential early-stage intermediates in the biosynthesis of FR900482 and mitomycin C

Tracing of Acyl Carrier Protein-channeled Mitomycin Intermediates in Streptomyces caespitosus Facilitates Characterization of the Biosynthetic Steps for AHBA-GlcN Formation and Processing

Mitomycins are a family of naturally occurring, potent alkylating agents in which the C member has been clinically used for cancer chemotherapy for over 5 decades. In Streptomyces caespitosus, mitomycins are derived from an N-glycoside composed of a 3-amino-5-hydroxybenzoic acid (AHBA) unit and a d-glucosamine (GlcN) unit; however, how this N-glycoside is formed and rearranged to a mitosane, for example, the compact polycyclic ring system of mitomycin C, remains elusive. Benefiting from the development of a method used to trace the mitomycin intermediates that accumulate on an acyl carrier protein (ACP), we here dissect the enzymatic steps for AHBA-GlcN formation and processing to underlie the mitosane structure. Following the N-glycosylation of AHBA with activated N-acetyl-GlcN, deacetylation occurs on ACP to provide AHBA-GlcN. Then, the sugar portion of this N-glycoside is transformed into a linear aminodiol that terminates with an epoxyethane, yielding an ACP-channeled intermediate that is ready for mitosane formation through crosslinking between the AHBA and linearized sugar units. This transformation is unusual and relies on the functional association of a dihydronicotinamide adenine dinucleotide (phosphate)-dependent protein with a radical S-adenosyl-l-methionine protein. Characterization of these ACP-based enzymatic steps for AHBA-GlcN formation and processing sheds light on the poorly understood biosynthetic pathway of mitomycins.

Characterization of Acyl Carrier Protein-Dependent Glycosyltransferase in Mitomycin C Biosynthesis

Mitomycins make up a group of antitumor natural products that are biosynthesized from aminohydroxybenzoic acid (AHBA) and N-acetylglucosamine (GlcNAc). While the biosynthetic gene cluster was reported two decades ago, the mechanism by which the two building blocks, AHBA and GlcNAc, are coupled during biosynthesis remained uncharacterized. Here we report evidence that AHBA is first loaded onto an MmcB acyl carrier protein (ACP) by a MitE acyl ACP synthetase, followed by a transfer of GlcNAc from UDP-GlcNAc by MitB. The results suggest that the early steps of mitomycin biosynthesis proceed via intermediates linked to MmcB.

Biomimetic Thioesters as Probes for Enzymatic Assembly Lines: Synthesis, Applications, and Challenges

Thioesters play essential roles in many biosynthetic pathways to fatty acids, esters, polyketides, and non-ribosomal peptides. Coenzyme A (CoA) and related phosphopantetheine thioesters are typically employed as activated acyl units for diverse C-C, C-O, and C-N coupling reactions. To study and control these enzymatic assembly lines in vitro and in vivo structurally simplified analogs such as N-acetylcysteamine (NAC) thioesters have been developed. This review gives an overview on experimental strategies enabled by synthetic NAC thioesters, such as the elucidation of complex biosynthetic pathways and enzyme mechanisms as well as precursor-directed biosynthesis and mutasynthesis. The review also summarizes synthetic protocols and protection group strategies to access these versatile synthetic tools, which are reactive and often unstable compounds. In addition, alternative phosphopantetheine thioester mimics are presented that can be used as protein tags or suicide inhibitors for protein crosslinking and off-loading probes to elucidate polyketide intermediates.

Natural products synthesis: enabling tools to penetrate Nature's secrets of biogenesis and biomechanism

Selected examples from our laboratory of how synthetic technology platforms developed for the total synthesis of several disparate families of natural products was harnessed to penetrate biomechanistic and/or biosynthetic queries is discussed. Unexpected discoveries of biomechanistic reactivity and/or penetrating the biogenesis of naturally occurring substances were made possible through access to substances available only through chemical synthesis. Hypothesis-driven total synthesis programs are emerging as very useful conceptual templates for penetrating and exploiting the inherent reactivity of biologically active natural substances. In many instances, new enabling synthetic technologies were required to be developed. The examples demonstrate the often untapped richness of complex molecule synthesis to provide powerful tools to understand, manipulate and exploit Nature's vast and creative palette of secondary metabolites.

Monosaccharide-NAIM derivatives for D-, L-configurational analysis

The D-, L-enantiomeric pairs of common monosaccharides (xylose, ribose, rhamnose, arabinose, fucose, glucose, mannose, galactose, N-acetylgalactosamine, glucuronic acid and galacturonic acid) were derivatized with 2,3-naphthalenediamine to form the corresponding D-, L-aldo-NAIM derivatives. A simple and facile capillary electrophoretic method was established for sugar composition analysis by simultaneously determining the migration times of these aldo-NAIMs using borate buffer at high pH (100 mM, pH 9.0). The methodology is also applicable to sialic acid (ketose monosaccharides). The quantitation level of the proposed method was in the 10~500 ppm range and the LOD was 1 ppm. The enantioseparation of D, L pairs of aldo-NAIMs were also achieved by using modified sulfated−α−cyclodextrin as the chiral selector in phosphate buffer (300 mM, pH 3.0). In addition, the combination by reductive amination of amino-aldo-NAIM agent and D-, L-enantiomeric pairs of monosaccharides formed a diastereomeric pair for saccharide configuration analysis. Aldo-NAIM derivatives are thus shown to be rapid and efficient agents for analyzing saccharide compositions and configurations with good linearity and short analysis times via capillary electrophoresis.

Aziridinomitosanes via lactam cyclization

Aziridinomitosane ketones 4 and 24 are accessed by internal acyl anion equivalent-lactam cyclization of 29 in a convergent route. The key aziridinolactam 6 is prepared by tin-lithium exchange via the lithiated aziridine 11.

Mitomycins syntheses: a recent update

Mitomycins are a class of very potent antibacterial and anti-cancer compounds having a broad activity against a range of tumours. They have been used in clinics since the 1960's, and the challenges represented by their total synthesis have challenged generations of chemists. Despite these chemical and medicinal features, these compounds, in racemic form, have succumbed to total synthesis only four times over the last 30 years.