ortho-Methoxycarbonylethynylphenyl Thioglycosides (MCEPTs): Versatile Glycosyl Donors Enabled by Electron-Withdrawing Substituents and Catalyzed by Gold(I) or Cu(II) Complexes.

With easily accessible and operator-friendly reagents, shelf-stable ortho-methoxycarbonylethynylphenyl thioglycosides were efficiently prepared. Based on these MCEPT glycoside donors, a novel glycosylation protocol featuring mild and catalytic promotion conditions with Au(I) or Cu(II) complexes, expanded substrate scope encompassing challenging donors and acceptors and clinically used pharmaceuticals, and versatility in various strategies for highly efficient synthesis of glycosides has been established. The practicality of the MCEPT glycosylation protocol was fully exhibited by highly efficient and scalable synthesis of surface polysaccharide subunits of Acinetobacter baumannii via latent-active, reagent-controlled divergent orthogonal one-pot and orthogonal one-pot strategies. The underlying reaction mechanism was investigated systematically through control reactions, leading to the isolation and characterization of the vital catalyst species in MCEPT glycosylation, the benzothiophen-3-yl-gold(I) complex. Based on the results obtained both from control reactions and from studies leading to the glycosylation protocol establishment, an operative mechanism was proposed and the effect of the vital catalyst species reactivity on the results of metal-catalyzed alkyne-containing donor-involved glycosylation was disclosed. Moreover, the mechanism for C-glycosylation side product formation from ortho-(substituted)ethynylphenyl thioglycoside donors with electron-donating substituents was also illuminated.

Hydrogen bond activated glycosylation under mild conditions.

Herein, we report a new glycosylation system for the highly efficient and stereoselective formation of glycosidic bonds using glycosyl N-phenyl trifluoroacetimidate (PTFAI) donors and a charged thiourea hydrogen-bond-donor catalyst. The glycosylation protocol features broad substrate scope, controllable stereoselectivity, good to excellent yields and exceptionally mild catalysis conditions. Benefitting from the mild reaction conditions, this new hydrogen bond-mediated glycosylation system in combination with a hydrogen bond-mediated aglycon delivery system provides a reliable method for the synthesis of challenging phenolic glycosides. In addition, a chemoselective glycosylation procedure was developed using different imidate donors (trichloroacetimidates, N-phenyl trifluoroacetimidates, N-4-nitrophenyl trifluoroacetimidates, benzoxazolyl imidates and 6-nitro-benzothiazolyl imidates) and it was applied for a trisaccharide synthesis through a novel one-pot single catalyst strategy.

8-(Methyltosylaminoethynyl)-1-naphthyl (MTAEN) Glycosides: Potent Donors in Glycosides Synthesis.

With 8-(methyltosylaminoethynyl)-1-naphthyl (MTAEN) glycoside as donors, a novel and efficient glycosylation protocol has been established. The MTAEN glycosylation protocol exhibits the merits of shelf-stable donors, mild catalytic promotion conditions, considerably extended substrate scope encompassing both free alcohols, silylated alcohols, nucleobases, primary amides, and C-type nucleophile acceptors, and applicability to various one-pot strategies for highly efficient synthesis of oligosaccharides, such as orthogonal one-pot, single-catalyst one-pot, and acceptor reactivity-controlled one-pot strategies.

Synthetic Investigation toward QS-21 Analogues.

With glycosyl o-alkynylbenzotes as donors, a highly efficient protocol to construct the challenging glycosidic linkages at C3-OH of C23-oxo oleanane triterpenoids is disclosed, on the basis of which different strategies for the highly efficient synthesis of QS-21 analogues with the west-wing trisaccharide of QS-21 have been established.

Reagent-Controlled Divergent Synthesis of C-Glycosides.

By turning on or switching off the directing effect of the C3-OH-located o-diphenylphosphanylbenzoyl (o-DPPB) group in glycals, a reagent-controlled protocol for divergent and regio- and stereoselective syntheses of C-glycosides has been established. In particular, the silence of the directing effect of o-DPPB was achieved by the introduction of a ZnCl2 additive, which is operationally simple and efficient. The flexibility of the novel protocol was exhibited not only by the easy access of both α- and ß-C-glycosides but also by the versatility of the obtained formal Ferrier rearrangement products, which can be easily derivatized to various C-glycoside analogues owing to the embedded multifunctionalities.

Total Synthesis and Structural Revision of Rebaudioside S, a Steviol Glycoside.

The total synthesis of rebaudioside S, a minor steviol glycoside from the leaves of Stevia rebaudiana, was investigated via a modular strategy, culminating not only in the first and highly efficient synthesis of Reb-S and analogues thereof but also in the revision of the originally proposed structure. The modular strategy dictated the application of C2-branched disaccharide Yu donors to forge C-13 steviol glycosidic linkages, posing considerable challenges in stereoselectivity control. Through systematic investigations, the effect of the internal glycosidic linkage configuration on the glycosylation stereoselectivity of 1,2-linked disaccharide donors was disclosed, and the intensified solvent effect by the 4,6-O-benzylidene protecting group was also observed with glucosyl donors. Through the orchestrated application of these favorable effects, the stereoselectivity problems were exquisitely tackled.

Palladium-Catalyzed Cross-Coupling of 2-Iodoglycals with N-Tosylhydrazones: Access to 2-C-Branched Glycoconjugates and Oxadecalins.

A convenient and straightforward synthesis of diverse 2-C-alkenyl-glycosides through a palladium-catalyzed cross-coupling reaction between 2-iodoglycals and N-tosylhydrazones has been developed. Further transformation of 2-C-branched sugars by Diels-Alder reactions provided oxadecalins in good yields.

Total synthesis of scutellarin and apigenin 7-O-ß-d-glucuronide.

A general protocol for direct glucuronic linkages formation featuring Au(I)-catalyzed appropriately protected glucuronyl o-alkynylbenzoate-involved glycosylation reaction, as well as a concise approach for easy access of scutellarein prominent for the mild and efficient hydroxyl group installation via borylation-oxidation sequence from flavanone derivative, has been established, based on which a novel route for scutellarin derivatives preparation has been devised. The developed strategies, among which the stepwise deprotection process was also included, guarantee the high whole synthetic efficiency, and definitely will find broad application in diversity-oriented synthesis of bioactive flavonoid glycosides.

Diversity-Oriented Synthesis of Steviol Glycosides.

With cheap and easily available mixtures of steviol glycosides as starting materials, a practical method for steviol acquisition has been developed, on the basis of which a facile, diversity-oriented, and economic protocol for the synthesis of structurally defined steviol glycosides was established. The novel approach is featured by the highly efficient glycosylation of sterically hindered and acid-sensitive steviol via orchestrated application of Yu glycosylation, Schmidt glycosylation, and PTC glycosylation. Hence, these high-intensity sweeteners and potential lead compounds for drug development are now readily accessible.

Synthetic Access Toward Cycloastragenol Glycosides.

The first efficient synthetic approach toward four types of the cycloartane glycosides, the cycloastragenol 25-O; 3-O; 3,6-O-bis; and 3,25-O-bisglycoside, have been established, which featured the PPY-mediated, concentration-controlled acetylation and Au(I)-catalyzed Yu glycosylation. Through the synthetic investigation, the reactivity sequence of the four OHs in cycloastragenol was fixed for the first time and a detour strategy for the highly efficient removal of bulky pivaloyl protecting groups was discovered.

Synthetic investigation toward apigenin 5-O-glycoside camellianin B as well as the chemical structure revision.

Capitalizing on the Au(i)-catalyzed ortho-alkynylbenzoate glycosylation method, the first total synthesis of the proposed structure of apigenin-5-O-glycoside camellianin B was achieved, wherein three approaches, one linear and two convergent, were established, through which the synthetic structures were firmly corroborated. Meanwhile, through the synthesis of anthentic camellianin B via commercially available camellianin A, the misassigned structures of camellianins A and B were revised.

A Highly Efficient Approach To Construct (epi)-Podophyllotoxin-4-O-glycosidic Linkages as well as Its Application in Concise Syntheses of Etoposide and Teniposide.

By taking full advantage of the mild promotion conditions of an ortho-alkynylbenzoate glycosylation protocol, a highly efficient approach to construct the challenging (epi)-podophyllotoxin 4-O-glycosidic linkages was devised under the activation of a catalytic amount of a Au(I) complex. The novel method enjoys a quite broad substrate scope in terms of both glycosyl donors and podophyllotoxin derivative acceptors, providing the desired glycosides in excellent yields. Based on the new approach, concise syntheses of clinically used anticancer reagents etoposide and teniposide were accomplished, and the overall yields counting from easily available starting materials could reach as high as 18% and 9%, respectively.

Highly efficient synthesis of flavonol 5-O-glycosides with glycosyl ortho-alkynylbenzoates as donors.

With glycosyl ortho-alkynylbenzoates as donors, the highly efficient glycosylation of flavonoid 5-OH which are notorious for their low reactivity due to their involvement in the formation of strong intramolecular H-bonds was achieved under the catalysis of a Au(i) complex. Thus, a series of flavonoid 5-O-glycosides, including a kaempferol 5-O-disaccharide, were synthesized with good to excellent yields.

The chemical synthesis of aryltetralin glycosides.

Led by etoposide and teniposide, the synthesis of aryltetralin glycosides has been experiencing flourishing development in the past five decades. Herein, a review focusing on the total synthesis of aryltetralin glycosides is provided. The main body of this review is composed of two parts, one is the enantioselective synthesis of aryltetralin derivatives and the other one is the construction of key glycosidic linkages. In each part the contents are organised based on the different strategies or protocols applied in the original documents. The total synthesis of aryltetralin glycosides represents the developing direction of this field, and sooner or later will replace the currently applied semi-total synthesis method, using the aglycon residue acquired directly from natural sources. This account provides a comprehensive and deep insight into the field of aryltetralin glycoside synthesis for chemists who have the intention of committing themselves to the development of aryltetralin glycoside medicine.

Synthesis and Evaluation of Novel Erlotinib-NSAID Conjugates as More Comprehensive Anticancer Agents.

A series of novel anticancer agents were designed and synthesized based on coupling of different nonsteroidal anti-inflammatory drugs (NSAIDs) with the epidermal growth-factor receptor (EGFR) tyrosine kinase inhibitor, erlotinib. Both the antiproliferative and pharmacokinetic activity of the target compounds were evaluated using HCC827 and A431 tumor cell lines. Among the derivatives made, compounds 10a, 10c, and 21g showed superb potency, comparable to that of erlotinib. Furthermore, preliminary SAR analysis showed that when the NSAIDs were conjugated via linkage to C-6 OH versus linkage to C-7 OH of the quinazoline nucleus, superior anticancer activity was achieved. Finally, the in vitro pharmacokinetic profile of several conjugates demonstrated the desired dissociation kinetics as the coupled molecules were effectively hydrolyzed, releasing both erlotinib and the specific NSAID in a time-dependent manner. The conjugation strategy represents a unique and simplified approach toward combination therapy, particularly for the treatment of cancers where both EGFR overexpression and inflammation play a direct role in disease progression.

Pd-catalyzed one-pot synthesis of polysubstituted acrylamidines from isocyanides, diazo compounds, and imines.

A novel and efficient Pd-catalyzed one-pot reaction of ethyl diazoacetate, isocyanides, and imines for the synthesis of acrylamidines was developed. The multicomponent reaction may have occurred through an unpredicted ring-opening process of the ketenimine-imine [2 + 2] intermediate to form the acrylamidine products.

An improved procedure for nucleoside synthesis using glycosyl trifluoroacetimidates as donors.

Using glycosyl trifluoroacetimidates as donors and nitromethane (or acetonitrile) as solvent, silylation and subsequent glycosylation were realized in a 'one-pot' procedure to provide the corresponding nucleosides derivatives in high yields. [structure: see text].