The Role of Cytochrome†P450 AbyV in the Final Stages of Abyssomicin†C Biosynthesis.

Abyssomicin C and its atropisomer are potent inhibitors of bacterial folate metabolism. They possess complex polycyclic structures, and their biosynthesis has been shown to involve several unusual enzymatic transformations. Using a combination of synthesis and in vitro assays we reveal that AbyV, a cytochrome P450 enzyme from the aby gene cluster, catalyses a key late-stage epoxidation required for the installation of the characteristic ether-bridged core of abyssomicin C. The X-ray crystal structure of AbyV has been determined, which in combination with molecular dynamics simulations provides a structural framework for our functional data. This work demonstrates the power of combining selective carbon-13 labelling with NMR spectroscopy as a sensitive tool to interrogate enzyme-catalysed reactions in vitro with no need for purification.

The Role of Cytochrome†P450 AbyV in the Final Stages of Abyssomicin†C Biosynthesis.

Abyssomicin C and its atropisomer are potent inhibitors of bacterial folate metabolism. They possess complex polycyclic structures, and their biosynthesis has been shown to involve several unusual enzymatic transformations. Using a combination of synthesis and in vitro assays we reveal that AbyV, a cytochrome P450 enzyme from the aby gene cluster, catalyses a key late-stage epoxidation required for the installation of the characteristic ether-bridged core of abyssomicin C. The X-ray crystal structure of AbyV has been determined, which in combination with molecular dynamics simulations provides a structural framework for our functional data. This work demonstrates the power of combining selective carbon-13 labelling with NMR spectroscopy as a sensitive tool to interrogate enzyme-catalysed reactions in vitro with no need for purification.

Enantioselective Organocatalytic Synthesis of Bicyclic Resorcinols via an Intramolecular Friedel-Crafts-Type 1,4-Addition: Access to Cannabidiol Analogues.

The organocatalytic transformation of resorcinols is extremely rare. In this article, we report a highly enantioselective, organocatalytic intramolecular cyclization of these systems by a Friedel-Crafts-type 1,4-addition using a Jørgensen-Hayashi-like organocatalyst with a large silyl protecting group, and show that heat improves reaction yield with virtually no detriment to enantioselectivity. A variety of bicyclic resorcinols were obtained with excellent enantioselectivities (up to 94%). To show the utility of these constructs, and as part of a wider project involving the synthesis of cannabinoid-like compounds, the resorcinol formed was used to generate both 'normal' and 'abnormal' cannabidiol (CBD) derivatives which were shown to have anticonvulsant activity.

General Method for the Synthesis of α- or ß-Deoxyaminoglycosides Bearing Basic Nitrogen.

The introduction of glycosides bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, because many aminoglycosides lack a C2 substituent, diastereomeric mixtures of O-glycosides are often produced. Herein, we present a method for the synthesis of α- or ß- 2,3,6-trideoxy-3-amino- and 2,4,6-trideoxy-4-amino O-glycosides from a common precursor. Our strategy proceeds by the reductive lithiation of thiophenyl glycoside donors and trapping of the resulting anomeric anions with 2-methyltetrahydropyranyl peroxides. We apply this strategy to the synthesis of α- and ß-forosamine, pyrrolosamine, acosamine, and ristosamine derivatives using primary and secondary peroxides as electrophiles. α-Linked products are obtained in 60-96% yield and with >50:1 selectivity. ß-Linked products are obtained in 45-94% yield and with 1.7->50:1 stereoselectivity. Contrary to donors bearing an equatorial amine substituent, donors bearing an axial amine substituent favored ß-products at low temperatures. This work establishes a general strategy to synthesize O-glycosides bearing a basic nitrogen.

Addendum: Synthesis and reactivity of precolibactin 886.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Synthesis and reactivity of precolibactin 886.

The clb gene cluster encodes the biosynthesis of metabolites known as precolibactins and colibactins. The clb pathway is found in gut commensal Escherichia coli, and clb metabolites are thought to initiate colorectal cancer via DNA crosslinking. Here we report confirmation of the structural assignment of the complex clb product precolibactin 886 via a biomimetic synthetic pathway. We show that an α-ketoimine linear precursor undergoes spontaneous cyclization to precolibactin 886 on HPLC purification. Studies of this α-ketoimine and the related α-dicarbonyl revealed that these compounds are unexpectedly susceptible to nucleophilic cleavage under mildly basic conditions. This cleavage pathway forms other known clb metabolites or biosynthetic intermediates and explains the difficulties in isolating fully mature biosynthetic products. This cleavage also accounts for a recently identified colibactin-adenine adduct. The colibactin peptidase ClbP deacylates synthetic precolibactin 886 to form a non-genotoxic pyridone, which suggests precolibactin 886 lies off the path of the major biosynthetic route.

Programmable Synthesis of 2-Deoxyglycosides.

Control of glycoside bond stereochemistry is the central challenge in the synthesis of oligosaccharides. 2-Deoxyglycosides, which lack a C2 substituent to guide stereoselectivity, are among the most difficult classes of glycoside bond constructions. Here we present a method to synthesize 2-deoxysaccharides with specified glycoside bond stereochemistry using a nucleophilic carbohydrate residue and the synthetic equivalent of an alcohol electrophile. Because the configuration of the nucleophile can be precisely controlled, both α- and ß-glycosides can be synthesized from the same starting material in nearly all cases examined. Stereoselectivities in these reactions are often greater than 50:1 and yields typically exceed 70%. This strategy is amenable to the stereocontrolled syntheses of trisaccharide diastereomers, and a tetrasaccharide. This method may be extensible to other classes of carbohydrates.

Cloning, expression, and purification of intact polyketide synthase modules.

Polyketides are a structurally and functionally diverse family of bioactive natural products that have proven to be a rich source of pharmaceutical and agrochemical lead compounds. Many polyketides are biosynthesized by large multifunctional megaenzymes termed type I modular polyketide synthases (PKSs). These systems possess a distinctive assembly line-like architecture, comprising a series of linearly arranged, multidomain extension modules, housed in sequence within giant polypeptide chains. Due to their inherently modular structures, PKSs represent attractive targets for reengineering, enabling access to functionally optimized "nonnatural" natural products. In this chapter we describe methods for the molecular cloning, recombinant over-expression, and purification of intact PKS modules and multimodular PKS polypeptides. The usefulness of these methods is demonstrated by applying them to the study of the abyssomicin C PKS, a >1MDa multimodular synthase responsible for the biosynthesis of a polyketide antimicrobial lead compound.

An Esterase-like Lyase Catalyzes Acetate Elimination in Spirotetronate/Spirotetramate Biosynthesis.

Spirotetronate and spirotetramate natural products include a multitude of compounds with potent antimicrobial and antitumor activities. Their biosynthesis incorporates many unusual biocatalytic steps, including regio- and stereo-specific modifications, cyclizations promoted by Diels-Alderases, and acetylation-elimination reactions. Here we focus on the acetate elimination catalyzed by AbyA5, implicated in the formation of the key Diels-Alder substrate to give the spirocyclic system of the antibiotic abyssomicin C. Using synthetic substrate analogues, it is shown that AbyA5 catalyzes stereospecific acetate elimination, establishing the (R)-tetronate acetate as a biosynthetic intermediate. The X-ray crystal structure of AbyA5, the first of an acetate-eliminating enzyme, reveals a deviant acetyl esterase fold. Molecular dynamics simulations and enzyme assays show the use of a His-Ser dyad to catalyze either elimination or hydrolysis, via disparate mechanisms, under substrate control.

The Catalytic Mechanism of a Natural Diels-Alderase Revealed in Molecular Detail.

The Diels-Alder reaction, a [4 + 2] cycloaddition of a conjugated diene to a dienophile, is one of the most powerful reactions in synthetic chemistry. Biocatalysts capable of unlocking new and efficient Diels-Alder reactions would have major impact. Here we present a molecular-level description of the reaction mechanism of the spirotetronate cyclase AbyU, an enzyme shown here to be a bona fide natural Diels-Alderase. Using enzyme assays, X-ray crystal structures, and simulations of the reaction in the enzyme, we reveal how linear substrate chains are contorted within the AbyU active site to facilitate a transannular pericyclic reaction. This study provides compelling evidence for the existence of a natural enzyme evolved to catalyze a Diels-Alder reaction and shows how catalysis is achieved.

Tilting and Tumbling in Transmembrane Anion Carriers: Activity Tuning through n-Alkyl Substitution.

Anion transport by synthetic carriers (anionophores) holds promise for medical applications, especially the treatment of cystic fibrosis. Among the factors which determine carrier activity, the size and disposition of alkyl groups is proving remarkably important. Herein we describe a series of dithioureidodecalin anionophores, in which alkyl substituents on one face are varied from C0 to C10 in two-carbon steps. Activities increase then decrease as the chain length grows, peaking quite sharply at C6 . Molecular dynamics simulations showed the transporter chloride complexes releasing chloride as they approach the membrane-aqueous interface. The free transporter then stays at the interface, adopting an orientation that depends on the alkyl substituent. If chloride release is prevented, the complex is positioned similarly. Longer chains tilt the binding site away from the interface, potentially freeing the transporter or complex to move through the membrane. However, chains which are too long can also slow transport by inhibiting movement, and especially reorientation, within the phospholipid bilayer.