De Novo Synthesis of Furanose Sugars: Catalytic Asymmetric Synthesis of Apiose and Apiose-Containing Oligosaccharides

Asymmetric Pd/Organoboron-Catalyzed Site-Selective Carbohydrate Functionalization with Alkoxyallenes Involving Noncovalent Stereocontrol

Herein, we demonstrate the robustness of a synergistic chiral Pd/organoboron system in tackling a challenging suite of site-, regio-, enantio- and diastereoselectivity issues across a considerable palette of biologically relevant carbohydrate polyols, when prochiral alkoxyallenes were employed as electrophiles. In view of the burgeoning role of noncovalent interactions (NCIs) in stereoselective carbohydrate synthesis, our mechanistic experiments and DFT modeling of the reaction path unexpectedly revealed that NCIs such as hydrogen bonding and CH-π interactions between the resting states of the Pd-π-allyl complex and the borinate saccharide are critically involved in the stereoselectivity control. Our strategy thus illuminates the untapped potential of harnessing NCIs in the context of transition metal catalysis to tackle stereoselectivity challenges in carbohydrate functionalization.

Total Synthesis of the Purported Structure of Branched Resin Glycosides Merremoside G and H2

The first total synthesis of the purported structure of branched resin glycosides merremoside G and H2 is accomplished. A signature step is represented by the sequential transition-metal-catalyzed coupling of stable trisaccharide homoallylic alcohol and monosaccharide alkoxyallene to afford the pentasaccharide skeleton. This de novo strategy is conducted under mild conditions with no need of preactivation. In addition, it allows for efficient preparation of the target compounds in combination with late-stage functionalization.

Access to Chiral O,O-Acetals Enabled by Palladium-Catalyzed Asymmetric Addition of Oximes to Alkoxyallenes

Enantiomerically pure acyclic O,O-acetal compounds (up to 97 % ee) have been accessed through chemo-, regio- and enantioselective palladium-catalyzed addition of oximes to alkoxyallenes. DFT calculations support that a protonative hydropalladation pathway is favourable, in which the hydrogen bonding interaction between the amide group of the diphosphine ligand and the alkoxyallene is critical for the highly stereoselective formation of the dioxygenated stereogenic center.

Insights into the missing apiosylation step in flavonoid apiosides biosynthesis of Leguminosae plants

Apiose is a natural pentose containing an unusual branched-chain structure. Apiosides are bioactive natural products widely present in the plant kingdom. However, little is known on the key apiosylation reaction in the biosynthetic pathways of apiosides. In this work, we discover an apiosyltransferase GuApiGT from Glycyrrhiza uralensis. GuApiGT could efficiently catalyze 2″-O-apiosylation of flavonoid glycosides, and exhibits strict selectivity towards UDP-apiose. We further solve the crystal structure of GuApiGT, determine a key sugar-binding motif (RLGSDH) through structural analysis and theoretical calculations, and obtain mutants with altered sugar selectivity through protein engineering. Moreover, we discover 121 candidate apiosyltransferase genes from Leguminosae plants, and identify the functions of 4 enzymes. Finally, we introduce GuApiGT and its upstream genes into Nicotiana benthamiana, and complete de novo biosynthesis of a series of flavonoid apiosides. This work reports an efficient phenolic apiosyltransferase, and reveals mechanisms for its sugar donor selectivity.

Total Synthesis of Starfish Cyclic Steroid Glycosides

Starfishes have evolved with a special type of secondary metabolites, namely starfish saponins, to ward off various predators and parasites; among them, the starfish cyclic steroid glycosides stand out structurally, featuring a unique 16-membered ring formed by bridging the steroidal C3 and C6 with a trisaccharide. The rigid cyclic scaffold and the congested and vulnerable steroid-sugar etherate linkage present an unprecedented synthetic challenge. Here we report a collective total synthesis of the major starfish cyclic steroid glycosides, namely luzonicosides A (1) and D (2) and sepositoside A (3), with an innovative approach, which entails a de novo construction of the ether-linked hexopyranosyl units, use of olefinic pyranoses as sugar precursors, and a decisive ring-closing glycosylation under the mild gold(I)-catalyzed conditions.

Anomeric Stereoauxiliary Cleavage of the C−N Bond of d ‐Glucosamine for the Preparation of Imidazo[1,5‐a]pyridines

The targeted cleavage of the C−N bonds of alkyl primary amines in sustainable compounds of biomass according to a metal‐free pathway and the conjunction of nitrogen in the synthesis of imidazo[1,5‐a]pyridines are still highly challenging. Despite tremendous progress in the synthesis of imidazo[1,5‐a]pyridines over the past decade, many of them can still not be efficiently prepared. Herein, we report an anomeric stereoauxiliary approach for the synthesis of a wide range of imidazo[1,5‐a]pyridines after cleaving the C−N bond of d‐glucosamine (α‐2° amine) from biobased resources. This new approach expands the scope of readily accessible imidazo[1,5‐a]pyridines relative to existing state‐of‐the‐art methods. A key strategic advantage of this approach is that the α‐anomer of d‐glucosamine enables C−N bond cleavage via a seven‐membered ring transition state. By using this novel method, a series of imidazo[1,5‐a]pyridine derivatives (>80 examples) was synthesized from pyridine ketones (including para‐dipyridine ketone) and aldehydes (including para‐dialdehyde). Imidazo[1,5‐a]pyridine derivatives containing diverse important deuterated C(sp²)−H and C(sp³)−H bonds were also efficiently achieved.

Mechanism of Sugar Ring Contraction and Closure Catalyzed by UDP-d-apiose/UDP-d-xylose Synthase (UAXS)

Uridine diphosphate (UDP)-apiose/UDP-xylose synthase (UAXS) is a member of the short-chain dehydrogenase/reductase superfamily (SDR), which catalyzes the ring contraction and closure of UDP-d-glucuronic acid (UDP-GlcA), affording UDP-apiose and UDP-xylose. UAXS is a special enzyme that integrates ring-opening, decarboxylation, rearrangement, and ring closure/contraction in a single active site. Recently, the ternary complex structure of UAXS was crystallized from Arabidopsis thaliana. In this work, to gain insights into the detailed formation mechanism of UDP-apiose and UDP-xylose, an enzyme-substrate reactant model has been constructed and quantum mechanical/molecular mechanical (QM/MM) calculations have been performed. Our calculation results reveal that the reaction starts from the C4-OH oxidation, which is accompanied by the conformational transformation of the sugar ring from chair type to boat type. The sugar ring-opening is prior to decarboxylation, and the deprotonation of the C2-OH group is the prerequisite for sugar ring-opening. Moreover, the keto-enol tautomerization of the decarboxylated intermediate is a necessary step for ring closure/contraction. Based on our calculation results, more UDP-apiose product was expected, which is in line with the experimental observation. Three titratable residues, Tyr185, Cys100, and Cys140, steer the reaction by proton transfer from or to UDP-GlcA, and Arg182, Glu141, and D337 constitute a proton conduit for sugar C2-OH deprotonation. Although Thr139 and Tyr105 are not directly involved in the enzymatic reaction, they are responsible for promoting the catalysis by forming hydrogen-bonding interactions with GlcA. Our calculations may provide useful information for understanding the catalysis of the SDR family.

LC-QTOF-MS/MS Based Molecular Networking Approach for the Isolation of α-Glucosidase Inhibitors and Virucidal Agents from Coccinia grandis (L.) Voigt

Coccinia grandis or ivy gourd is an edible plant. Its leaves and fruits are used as vegetable in many countries. Many works on antidiabetic activity of a crude extract of C. grandis, i.e., in vitro, in vivo, and clinical trials studies, have been reported. Profiles of the antidiabetic compounds were previously proposed by using LC-MS or GC-MS. However, the compounds responsible for antidiabetic activity have rarely been isolated and characterized by analysis of 1D and 2D NMR data. In the present work, UHPLC-ESI-QTOF-MS/MS analysis and GNPS molecular networking were used to guide the isolation of α-glucosidase inhibitors from an extract of C. grandis leaves. Seven flavonoid glycosides including rutin (1), kaempferol 3-O-rutinoside (2) or nicotiflorin, kaempferol 3-O-robinobioside (3), quercetin 3-O-robinobioside (4), quercetin 3-O-β-D-apiofuranosyl-(1→2)-[α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside (5) or CTN-986, kaempferol 3-O-β-D-api-furanosyl-(1→2)-[α-L-rhamnopyranosyl-(1→6)]-β-D-glucopyranoside (6), and kaempferol 3-O-β-D-apiofuranosyl-(1→2)-[α-L-rhamnopyranosyl-(1→6)]-β-D-galactopyranoside (7) were isolated from C. grandis leaves. This is the first report of glycosides containing apiose sugar in the genus Coccinia. These glycosides exhibited remarkable α-glucosidase inhibitory activity, being 4.4–10.3 times more potent than acarbose. Moreover, they also displayed virucidal activity against influenza A virus H1N1, as revealed by the ASTM E1053-20 method.

Convergent Synthesis of Tetrasaccharide Fragment of Cervimycin K

A new synthetic approach toward oligosaccharides consisting only of 2,3,6-trideoxypyranoglycosides is reported. The key feature is highlighted by the convergent approach that allows the introduction of the aglycon moiety in the late stage of the synthesis. As an illustrative example, the tetrasaccharide portion of cervimycin K was prepared as cyclohexyl glycoside.

Catalytic Asymmetric Synthesis of Hexahydro-furofuran-3-ol and Its Pyran Derivatives

The catalytic asymmetric synthesis of hexahydro-furofuran-3-ol, a key fragment of HIV protease inhibitors, is reported. A signature event is represented by the sequential metal catalysis that combines the Pd-catalyzed asymmetric hydroalkoxylation of ene-alkoxyallene and ring-closing metathesis (RCM). Notably, this unprecedented and highly chemoselective approach allows for a unified access to pyranofuranol and furopyranol derivatives.

Deciphering the enzymatic mechanism of sugar ring contraction in UDP-apiose biosynthesis

D-Apiose is a C-branched pentose sugar important for plant cell wall development. Its biosynthesis as UDP-D-apiose involves decarboxylation of the UDP-D-glucuronic acid precursor coupled to pyranosyl-to-furanosyl sugar ring contraction. This unusual multistep reaction is catalyzed within a single active site by UDP-D-apiose/UDP-D-xylose synthase (UAXS). Here, we decipher the UAXS catalytic mechanism based on crystal structures of the enzyme from Arabidopsis thaliana, molecular dynamics simulations expanded by QM/MM calculations, and mutational-mechanistic analyses. Our studies show how UAXS uniquely integrates a classical catalytic cycle of oxidation and reduction by a tightly bound nicotinamide coenzyme with retro-aldol/aldol chemistry for the sugar ring contraction. They further demonstrate that decarboxylation occurs only after the sugar ring opening and identify the thiol group of Cys100 in steering the sugar skeleton rearrangement by proton transfer to and from the C3’. The mechanistic features of UAXS highlight the evolutionary expansion of the basic catalytic apparatus of short-chain dehydrogenases/reductases for functional versatility in sugar biosynthesis.

Synthesis of Deoxyaminosugar Cyclohexyl-l-callipeltose and Its Diastereomer Using Pd-Catalyzed Asymmetric Hydroalkoxylation

Cyclohexyl-l-callipeltose, an aminodeoxysugar subunit of Callipeltoside A, was synthesized in six steps and 40% overall yield from readily available (S)-4-methylpent-4-en-2-ol and cyclohexyloxyallene. The signature step is represented by Pd-catalyzed asymmetric intermolecular hydroalkoxylation that generates the key dihydropyran intermediate upon combination with the ring-closing metathesis reaction. Notably, an unnatural diastereomer of the target compound could also be obtained with comparable efficiency simply by using the enantiomeric ligand.

Homogeneous Catalysis: A Powerful Technology for the Modification of Important Biomolecules

Homogeneous catalysis plays an important and ubiquitous role in the synthesis of simple and complex molecules, including drug compounds, natural products, and agrochemicals. In recent years, the wide-reaching importance of homogeneous catalysis has made it an indispensable tool for the modification of biomolecules, such as carbohydrates (sugars), amino acids, peptides, nucleosides, nucleotides, and steroids. Such a synthetic strategy offers several advantages, which have led to the development of new molecules of biological relevance at a rapid rate relative to the number of available synthetic methods. Given the powerful nature of homogeneous catalysis in effecting these synthetic transformations, this Focus Review has been compiled to highlight these important developments.

Catalytic asymmetric synthesis of geminal-dicarboxylates

Stereogenic acetals, spiroacetals and ketals are well-studied stereochemical features that bear two heteroatoms at a common carbon atom. These stereocenters are normally found in cyclic structures while linear (or acyclic) analogues bearing two heteroatoms are rare. Chiral geminal-dicarboxylates are illustrative, there is no current way to access this class of compounds while controlling the stereochemistry at the carbon center bound to two oxygen atoms. Here we report a rhodium-catalysed asymmetric carboxylation of ester-containing allylic bromides to form stereogenic carbon centers bearing two different carboxylates with high yields and enantioselectivities. The products, which are surprisingly stable to a variety of acidic and basic conditions, can be manipulated with no loss of enantiomeric purity as demonstrated by ring closing metathesis reactions to form chiral lactones, which have been extensively used as building blocks in asymmetric synthesis.

Generation and Rearrangement of N,O-Dialkenylhydroxylamines for the Synthesis of 2-Aminotetrahydrofurans

A new diastereoselective route to 2-aminotetrahydrofurans has been developed from N,O-dialkenylhydroxylamines. These intermediates undergo a spontaneous C-C bond-forming [3,3]-sigmatropic rearrangement followed by a C-O bond-forming cyclization. A copper-catalyzed N-alkenylation of an N-Boc-hydroxylamine with alkenyl iodides, and a base-promoted addition of the resulting N-hydroxyenamines to an electron-deficient allene, provide modular access to these novel rearrangement precursors. The scope of this de novo synthesis of simple nucleoside analogues has been explored to reveal trends in diastereoselectivity and reactivity. In addition, a base-promoted ring-opening and Mannich reaction has been discovered to covert 2-aminotetrahydrofurans to cyclopentyl β-aminoacid derivatives or cyclopentenones.

Natural products and ring-closing metathesis: synthesis of sterically congested olefins

This review highlights RCM reactions towards the synthesis of sterically congested natural products throughout the recent evolution of catalysts.

Enantioselective indium(i)-catalyzed [4 + 2] annulation of alkoxyallenes and β,γ-unsaturated α-keto esters

An indium(i)–chiral phosphoric acid complex was found to catalyze the enantioselective [4 + 2] annulation reaction of β,γ-unsaturated α-keto esters with alkoxyallenes, affording cyclic O,O-acetals in good yields and with high regio- and stereo-selectivities.