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.

The clb gene cluster encodes the biosynthesis of metabolites known as precolibactins and colibactins. The clb pathway is found in gut commensal E. coli, and clb metabolites are thought to initiate colorectal cancer via DNA cross-linking. Here we report confirmation of the structural assignment of the complex clb product precolibactin 886 via a biomimetic synthetic pathway. We show that a α-ketoimine linear precursor undergoes spontaneous cyclization to precolibactin 886 upon 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, suggesting precolibactin 886 lies off-path of the major biosynthetic route.

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 αand βglycosides 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. Our method offers a fundamentally new approach to O-glycoside synthesis to enable downstream biochemical and natural product applications.

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 β- 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 β-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. β-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 β-products at low temperatures. This work establishes a general strategy to synthesize O-glycosides bearing a basic nitrogen.

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.