Unified Total Synthesis of Polyoxins J, L, and Fluorinated Analogues on the Basis of Decarbonylative Radical Coupling Reactions

Visible-Light-Mediated Synthesis of C-Alkyl Glycosides via Glycosyl Radical Addition and Aryl Migration

A visible-light-induced glycoarylation of activated olefins has been accomplished. Glycosyl radicals are generated via radical transfer strategies between (TMS)3SiOH and glycosyl bromides. Subsequent radical translocation and rapid 1,4-aryl migration form β-sugar amide derivatives, and eight types of sugars are compatible with this reaction. Further, the cascade reaction produced a quaternary carbon center with good functional group adaptability and high regioselectivity in mild conditions.

Visible-light-mediated α-amino alkylation of ketimines and aldimines for the synthesis of 1,2-diamines

A visible-light-mediated protocol to prepare 1,2-diamines has been successfully explored based on the photoredox/Brønsted acid co-catalyzed α-amino alkylations of imines with tertiary amines. Both ketimines and aldimines are applicable to this transformation. Various 1,2-diamines with different functional groups were produced in moderate to excellent yields. Moreover, this approach could be performed on a gram scale, showing its practicality.

Tellurium in carbohydrate synthesis

In this article, we discuss about the influence of tellurium in carbohydrate synthesis. Mainly the chapter focuses on the importance of the tellurium during the synthesis of glycosides and during the oxidation of glucose.

Total Syntheses of Hikosamine and Hikizimycin

Hikizimycin (1) is a potent anthelmintic and antibacterial natural product. The core 4-amino-4-deoxyundecose sugar (hikosamine) of 1 consists of an 11-carbon linear chain substituted with one amino group and 10 hydroxy groups. The C1 and C6O positions of the 10 contiguous stereocenters are further appended by a cytosine base and a 3-amino-3-deoxyglucose sugar (kanosamine), respectively. Since the structural determination in the early 1970s, synthetic chemists have been attracted by this exceedingly complex structure and have investigated the full chemical construction of 1. These synthetic efforts culminated in four syntheses of the protected hikosamines and two total syntheses of 1. In this Perspective, we summarize the strategies and tactics utilized in these syntheses to showcase the evolution of modern natural product synthesis.

Glycosyl-Radical-Based Synthesis of C-Alkyl Glycosides via Photomediated Defluorinative gem-Difluoroallylation

We have developed a stereoselective, glycosyl radical-based method for the synthesis of C-alkyl glycosides via a photomediated defluorinative gem-difluoroallylation reaction. We demonstrate for the first time that glycosyl radicals, generated from glycosyl bromides, can readily participate in a photomediated radical polar crossover process, affording a diverse array of gem-difluoroalkene containing C-glycosides. Notable features of this method include scalability, mild conditions, broad substrate scope, and suitability for the late-stage modification of complex molecules.

Enzymatic synthesis of fluorinated compounds

Fluorinated compounds are widely used in the fields of molecular imaging, pharmaceuticals, and materials. Fluorinated natural products in nature are rare, and the introduction of fluorine atoms into organic compound molecules can give these compounds new functions and make them have better performance. Therefore, the synthesis of fluorides has attracted more and more attention from biologists and chemists. Even so, achieving selective fluorination is still a huge challenge under mild conditions. In this review, the research progress of enzymatic synthesis of fluorinated compounds is summarized since 2015, including cytochrome P450 enzymes, aldolases, fluoroacetyl coenzyme A thioesterases, lipases, transaminases, reductive aminases, purine nucleoside phosphorylases, polyketide synthases, fluoroacetate dehalogenases, tyrosine phenol-lyases, glycosidases, fluorinases, and multienzyme system. Of all enzyme-catalyzed synthesis methods, the direct formation of the C-F bond by fluorinase is the most effective and promising method. The structure and catalytic mechanism of fluorinase are introduced to understand fluorobiochemistry. Furthermore, the distribution, applications, and future development trends of fluorinated compounds are also outlined. Hopefully, this review will help researchers to understand the significance of enzymatic methods for the synthesis of fluorinated compounds and find or create excellent fluoride synthase in future research.Key points• Fluorinated compounds are distributed in plants and microorganisms, and are used in imaging, medicine, materials science.• Enzyme catalysis is essential for the synthesis of fluorinated compounds.• The loop structure of fluorinase is the key to forming the C-F bond.

1,2-trans-Stereoselective Synthesis of C-Glycosides of 2-Deoxy-2-amino-sugars Involving Glycosyl Radicals

We report for the first time that the imidate radical can be efficiently added to glycals to generate glycosyl radicals, based on which a general, toxic-reagent-free synthesis of C-glycosides of 2-deoxy-2-amino sugars has been developed. Complementary to previous strategies, the reaction is 1,2-trans-stereoselective and could use aryl alkenes as substrates. The late-stage functionalization and density functional theory calculations are reported.

Convergent Assembly of Highly Oxygenated Natural Products Enabled by Intermolecular Radical Reactions

Natural products with a high ratio of sp³-hybridized atoms and oxygen-substituted stereogenic centers represent privileged structures for the development of pharmaceuticals and chemical probes. The multiple oxygen functionalities of these natural products endow their potent and selective biological activities, although they significantly heighten the challenge of their chemical assemblies. We focused on developing efficient strategies for the total syntheses of this biologically and chemically important class of molecules. A convergent strategy is more advantageous than a linear strategy for designing a shorter synthetic route because a convergent strategy enables direct coupling of functionalized fragments whereas a linear strategy involves stepwise construction of a molecule from its terminus. Radical reactions are preferred over polar reactions for the coupling of heavily functionalized and sp³-rich fragments, as they allow for C(sp³)-C(sp³) coupling without damaging diverse polar functional groups. With these considerations in mind, we designed radical-based convergent strategies for assembling highly oxygenated natural products. Here we summarize the concise total syntheses of asimicin (1, antibiotic activity), 1-hydroxytaxinine (2, cytotoxicity), polyoxins (3, antifungal activity), and hikizimycin (4, anthelmintic activity) as representative examples. Retrosynthetic disconnection at the central part of these molecules produces highly substituted α-alkoxy radicals as synthons. In the synthetic direction, the α-alkoxy radicals were generated from the corresponding α-alkoxyacyl tellurides in a unified fashion, and then utilized for four distinct coupling reactions. Formation of the Et radical from Et₃B and O₂ homolytically cleaves the C-Te bond of α-alkoxyacyl telluride, and the facile expulsion of carbon monoxide from the acyl radical leads to the α-alkoxy radical. Dimerization of the stabilized α-alkoxy radical resulted in the core structure of 1 with 10 contiguous stereocenters. The coupling adduct was derivatized to 1 through the attachment of two different carbon chains (17 steps as the longest linear sequence). Alternatively, intermolecular addition reactions of the α-alkoxy radicals to electron-deficient C═C, C═N, and C═O bonds, followed by Et₃B-mediated radical termination, led to the core structures of 2, 3, and 4, respectively. Intermolecular coupling between the α-alkoxy radical and the cyclohexenone derivative and intramolecular pinacol coupling gave rise to the 6/8/6-fused ring system of 2, which was transformed to 2 (26 steps). The two amino acid moieties of 3 were prepared by combining the α-alkoxy radical and the oxime and were then condensed to complete the synthesis of 3 (11 steps). Furthermore, a combination of α-alkoxyacyl telluride and Et₃B/O₂ realized a novel addition reaction of α-alkoxy radicals to aldehydes. This method was incorporated in the construction of the core 4-amino-5-deoxyundecose with 10 contiguous stereocenters, which was fabricated with two appendage structures to deliver 4. The four total syntheses described here demonstrate the versatility and robustness of intermolecular radical reactions. These syntheses will also provide new insights for retrosynthetic analyses in the field of organic chemistry and streamline synthetic routes to various bioactive natural products with multiple oxygen functionalities.

Fragment Coupling Reactions in Total Synthesis That Form Carbon-Carbon Bonds via Carbanionic or Free Radical Intermediates

Fragment coupling reactions that form carbon-carbon bonds are valuable transformations in synthetic design. Advances in metal-catalyzed cross-coupling reactions in the early 2000s brought a high level of predictability and reliability to carbon-carbon bond constructions involving the union of unsaturated fragments. By comparison, recent years have witnessed an increase in fragment couplings proceeding via carbanionic and open-shell (free radical) intermediates. The latter has been driven by advances in methods to generate and utilize carbon-centered radicals under mild conditions. In this Review, we survey a selection of recent syntheses that have implemented carbanion- or radical-based fragment couplings to form carbon-carbon bonds. We aim to highlight the strategic value of these disconnections in their respective settings and to identify extensible lessons from each example that might be instructive to students.

Et3 B/Et2 AlCl/O2 -Mediated Radical Coupling Reaction between α-Alkoxyacyl Tellurides and 2-Hydroxybenzaldehyde Derivatives

A newly devised radical-based strategy enabled coupling between multiply oxygenated α-alkoxyacyl tellurides and 2-hydroxybenzaldehyde derivatives. A reagent combination of Et₃ B, Et₂ AlCl, and O₂ promoted the formation of the α-alkoxy carbon radical from the α-alkoxyacyl telluride and the addition of the radical to the carbonyl group of 2-hydroxybenzaldehyde. The reaction chemo- and stereoselectively forged the hindered C-C bond between two oxygen-functionalized carbons at ambient temperature. The method was applied to the preparation of 12 coupling adducts with three to six contiguous stereocenters and to the concise synthesis of an antitumor compound, LLY-283.

Transition Metal-Mediated C-C Single Bond Cleavage: Making the Cut in Total Synthesis

Transition-metal-mediated cleavage of C-C single bonds can enable entirely new retrosynthetic disconnections in the total synthesis of natural products. Given that C-C bond cleavage inherently alters the carbon framework of a compound, and that, under transition-metal catalysis, the generated organometallic or radical intermediate is primed for further complexity-building reactivity, C-C bond-cleavage events have the potential to drastically and rapidly remodel skeletal frameworks. The recent acceleration of the use of transition-metal-mediated cleavage of C-C single bonds in total synthesis can be ascribed to a communal recognition of this fact. In this Review, we highlight ten selected total syntheses from 2014 to 2019 that illustrate how transition-metal-mediated cleavage of C-C single bonds at either the core or the periphery of synthetic intermediates can streamline synthetic efforts.

Total Synthesis of Diospyrodin and Its Three Diastereomers

Antibacterial diospyrodin (1) was synthesized in 13 steps. Et₃B and O₂ promoted the formation of an α-alkoxy carbon radical from l-ribose-derived α-alkoxyacyl telluride 5, which reacted with d-glucose-derived aldehyde 4. The radical addition realized the convergent assembly of the contiguously hydroxylated carbon-chain of 3-α and greatly contributed to streamlining the synthetic route. Compound 3-α was transformed not only to 1 but also to its three diastereomers by functional group manipulations.

Convergent Total Synthesis of Hikizimycin Enabled by Intermolecular Radical Addition to Aldehyde

Hikizimycin (1), which exhibits powerful anthelmintic activity, has the most densely functionalized structure among nucleoside antibiotics. A central 4-amino-4-deoxyundecose of 1 possesses 10 contiguous stereocenters on a C1-C11 linear chain and is decorated with a cytosine base at C1 and a 3-amino-3-deoxyglucose at C6-OH. These distinctive structural features of 1 make it an extremely challenging target for de novo construction. Herein, we report a convergent total synthesis of 1 from four known components: 3-azide-3-deoxyglucose derivative 4, bis-TMS-cytosine 5, d-mannose 9, and d-galactose derivative 10. We first designed and devised a novel radical coupling reaction between multiply hydroxylated aldehydes and α-alkoxyacyl tellurides. The generality and efficiency of this process was demonstrated by the coupling of 7c and 8, which were readily accessible from two hexoses, 9 and 10, respectively. Et₃B and O₂ rapidly induced decarbonylative radical formation from α-alkoxyacyl telluride 8, and intermolecular addition of the generated α-alkoxy radical to aldehyde 7c yielded 4-amino-4-deoxyundecose 6-α with installation of the desired C5,6-stereocenters. Subsequent attachments of the cytosine with 5 and of the 3-azide-3-deoxyglucose with 4 were realized through selective activation of the C1-acetal and selective deprotection of the C6-hydroxy group. Finally, the 3 amino and 10 hydroxy groups were liberated in a single step to deliver the target 1. Thus, the combination of the newly developed radical-coupling and protective-group strategies minimized the functional group manipulations and thereby enabled the synthesis of 1 from 10 in only 17 steps. The present total synthesis demonstrates the versatility of intermolecular radical addition to aldehyde for the first time and offers a new strategic design for multistep target-oriented syntheses of various nucleoside antibiotics and other bioactive natural products.

Recent development in the synthesis of C-glycosides involving glycosyl radicals

C-Glycosylation involving glycosyl radical intermediates is a particularly effective approach to access C-glycosides, which are core units of a great number of natural products, bioactive compounds and marketed drugs.

Synthesis of MeBmt and related derivatives via syn-selective ATH-DKR

The unusual α-amino, β-hydroxy acid MeBmt is a key structural feature of cyclosporin A, an important naturally occurring immunosuppressant and antiviral agent. We present a convergent synthesis of MeBmt which relies on new aspects of dynamic kinetic resolution (DKR) to establish simultaneously the chirality at C(2) and C(3). We also show that this route is applicable to the synthesis of other derivatives.

[Development of Synthetic Strategies for Densely Oxygenated Natural Products: Total Synthesis of Lactacystin and Zaragozic Acid C Using Photochemical C(sp3)-H Functionalization]

This review describes two novel synthetic routes from (S)-pyroglutaminol to (+)-lactacystin, a potent inhibitor of the 20S proteasome and from d-gluconolactone derivative to zaragozic acid C, a potent squalene synthase inhibitor. In lactacystin synthesis, the photoinduced intermolecular C(sp³)-H alkynylation and intramolecular C(sp³)-H acylation chemoselectively and stereoselectively constructed the tetrasubstituted and trisubstituted carbon centers, respectively. In the synthesis of zaragozic acid C, the stereoselective installation of the two contiguous tetrasubstituted carbons was achieved by the photochemical intramolecular C(sp³)-H acylation of a densely oxygenated intermediate.

Forging C(sp3)-C(sp3) Bonds with Carbon-Centered Radicals in the Synthesis of Complex Molecules

Radical fragment coupling reactions that unite intricate subunits have become an important class of transformations within the arena of complex molecule synthesis. This Perspective highlights some of the early contributions in this area, as well as more modern applications of radical fragment couplings in the preparation of natural products. Additionally, emphasis is placed on contemporary advances that allow for radical generation under mild conditions as a driving force for the implementation of radical fragment couplings in total synthesis.

Transition metal-free, visible-light-mediated construction of α,β-diamino esters via decarboxylative radical addition at room temperature

A metal-free photoredox catalyzed decarboxylative radical coupling of free-carboxylic acids and glyoxylic oximes was developed to synthesize α,β-diamino acids.

An ATP-Dependent Ligase with Substrate Flexibility Involved in Assembly of the Peptidyl Nucleoside Antibiotic Polyoxin

Polyoxin (POL) is an unusual peptidyl nucleoside antibiotic, in which the peptidyl moiety and nucleoside skeleton are linked by an amide bond. However, their biosynthesis remains poorly understood. Here, we report the deciphering of PolG as an ATP-dependent ligase responsible for the assembly of POL. A polG mutant is capable of accumulating multiple intermediates, including the peptidyl moiety (carbamoylpolyoxamic acid [CPOAA]) and the nucleoside skeletons (POL-C and the previously overlooked thymine POL-C). We further demonstrate that PolG employs an ATP-dependent mechanism for amide bond formation and that the generation of the hybrid nucleoside antibiotic POL-N is also governed by PolG. Finally, we determined that the deduced ATP-binding sites are functionally essential for PolG and that they are highly conserved in a number of related ATP-dependent ligases. These insights have allowed us to propose a catalytic mechanism for the assembly of peptidyl nucleoside antibiotic via an acyl-phosphate intermediate and have opened the way for the combinatorial biosynthesis/pathway engineering of this group of nucleoside antibiotics.IMPORTANCE POL is well known for its remarkable antifungal bioactivities and unusual structural features. Actually, elucidation of the POL assembly logic not only provides the enzymatic basis for further biosynthetic understanding of related peptidyl nucleoside antibiotics but also contributes to the rational generation of more hybrid nucleoside antibiotics via synthetic biology strategy.

Total Synthesis of (-)-Chromodorolide B By a Computationally-Guided Radical Addition/Cyclization/Fragmentation Cascade

The first total synthesis of a chromodorolide marine diterpenoid is described. The core of the diterpenoid is constructed by a bimolecular radical addition/cyclization/fragmentation cascade that unites two complex fragments and forms two C-C bonds and four contiguous stereogenic centers of (-)-chromodorolide B in a single step. This coupling step is initiated by visible-light photocatalytic fragmentation of a redox-active ester, which can be accomplished in the presence of an iridium or a less-precious electron-rich dicyanobenzene photocatalyst, and employs equimolar amounts of the two addends. Computational studies guided the development of this central step of the synthesis and provide insight into the origin of the observed stereoselectivity.

Synthesis of the Tetracyclic Structure of Batrachotoxin Enabled by Bridgehead Radical Coupling and Pd/Ni-Promoted Ullmann Reaction

The steroidal ABCD-ring system of the potent neurotoxin batrachotoxin was efficiently assembled in a convergent fashion. Bridgehead radical coupling between the simple AB-ring and D-ring fragments (3 and 4) formed the sterically congested linkage at the C9-oxygen-attached tetrasubstituted carbon. The C-ring was then cyclized by the Pd/Ni-promoted Ullmann reaction of the vinyl triflate and vinyl bromide of 19, giving rise to tetracyclic structure 1.