Shuting Cai

Regio- and Stereoselective Synthesis of 2-Deoxy-C-aryl Glycosides via Palladium Catalyzed Decarboxylative Reactions

An efficient and versatile approach for the synthesis of 2-deoxy-C-aryl glycosides is reported. This strategy is based on a palladium-catalyzed decarboxylative Heck coupling reaction of benzoic acids and glycals. A wide variety of glycals and benzoic acids have been screened, and all these reactions could afford the desired C-aryl glycoside products in moderate to good yields with exclusive regio- and stereoselectivities.

Access to quinolines through gold-catalyzed intermolecular cycloaddition of 2-aminoaryl carbonyls and internal alkynes

A facile and general method leading to polyfunctionalized quinolines was developed. In the presence of a highly efficient combination encompassing (PPh)(3)AuCl and AgOTf, the reactions between 2-aminocarbonyls and an array of internal alkynes proceeded smoothly to afford quinoline derivatives in good to excellent yields (up to 93%).

Palladium-catalyzed direct cross-coupling reaction of glycals with activated alkenes.

An efficient method for a Pd(OAc)(2)-catalyzed cross-coupling reaction of glycals with activated alkenes under mild conditions has been developed. This transformation provides an expedient synthetic method to C(2)-functionalized glycals, which are common structural building blocks in natural products and other biologically active compounds. The reaction scope includes different kinds of carbohydrates, protecting groups and substituents on alkene. Moderate to excellent yields and pure E configuration selectivity were obtained.

Stereoselective β‐C‐Glycosylation by a Palladium‐Catalyzed Decarboxylative Allylation: Formal Synthesis of Aspergillide A

The efficient stereoselective construction of glycosidic linkages is indubitably a principal focus in carbohydrate chemistry because it is necessary for the construction of natural glycoconjugates. Among the wide variety of glycosylation methods, the Ferrier reaction has received considerable attention as it provides convenient and direct access to 2,3unsaturated glycosides from glycals. However, the stringent requirement of glycosyl acceptors generally confines the reaction to specific nucleophiles with strong reactivity and the utilization of stoichiometric amounts of a Lewis acid is inevitable in some cases. In addition, the dominant anomeric effect leads to the stereoselective generation of a-glycosides for Ferrier-type O-glycosylation and therefore accents the rigidity of the reaction. On the other hand, most results from Ferrier C-glycosylation reactions remain mediocre as only moderate a-selectivity has been achieved. The pursuit of high b-selectivity is also extremely challenging and judging from the lack of substantial reports, the barriers surrounding this problem has not been solved. [Scheme 1, Eq. (1)]. Consequently, the combination of limitations surrounding the Ferrier reaction prompted researchers to develop other methods to synthesize 2,3-unsaturated glycosides in exclusive selectivity, especially b-selectivity. Recent demonstrations on the efficiency of palladiumcatalyzed coupling reactions have stimulated considerable interest in applying this strategy to carbohydrates, particularly for the synthesis of 2,3-unsaturated glycosides. One of the successful and prominent examples is the Heck-type glycosylation of glycals with arylboronic acids or aryl halides by transition-metal insertion and reductive elimination [Scheme 1, Eq. (2)]. The allylic feature of glycals also encouraged chemists to pursue the applicability of palladium-catalyzed allylic alkylation in glycosylation reactions [Scheme 1, Eq. (3)]. However, the formation of Pd p-allyl species in glycal systems has long been recognized as tedious and difficult. To overcome this challenge, the more activated pyranone system was generated and additional activators were employed. Following the removal of this hurdle, great strides were made in decarboxylative allylation (DcA). 10] In particular, intramolecular decarboxylative allylation has developed into an area of great potential among the transition-metal-catalyzed decarboxylative coupling reactions which have drawn considerable attention in the area of C C bond formation. The Tunge, Trost, and Stoltz groups, for instance, have reported a series of catalytic decarboxylative allylation and benzylation reactions. Inspired by these reports, we envisioned that the palladiumcatalyzed decarboxylation of the C-3 ester of glycal would be helpful in the formation of a Pd p-allyl intermediate which might accomplish the desired C-glycosylation with high stereoselectivity [Scheme 1, Eq. (4)]. In a continuation of our work on developing efficient glycosylation methods, we report herein on a palladium-catalyzed stereoand regioselective C-glycosylation by means of intramolecular decarboxylative coupling. In initial studies, the decarboxylative coupling reaction of compound 1a was carried out in the presence of a catalytic amount of [Pd(PPh3)4] in DMF at 80 8C for 12 h. To our delight, the regiospecific coupling product 2a was obtained in 50% yield with a b/a ratio of 6:1 (Table 1, entry 1). To improve the yield and selectivity, various Pd catalysts were screened with the 1,2-bis(diphenylphosphino)ethane (DPPE) ligand in DMF (Table 1, entries 2–4). We found that the reaction catalyzed by Pd(OAc)2 gave better yield and Scheme 1. Various types of glycosylation.

N-Heterocyclic Carbene Catalyzed C-Glycosylation: A Concise Approach from Stetter Reaction

Described herein is the first example of an organocatalytic approach for acylanion addition to the anomeric carbon of 2-nitroglucal using an N-heterocyclic carbene catalyst. Control over the reaction conditions gives β-selective and nitro-eliminated C-glycosides, providing opportunities to produce new classes of C-glycoside.

(S)-Camphorsulfonic acid catalyzed highly stereoselective synthesis of pseudoglycosides.

A mild and efficient synthesis of pseudoglycosides has been developed using metal free (S)-camphorsulfonic acid. (S)-CSA acts as an excellent catalyst for conversion of 2,4,6-tri-O-acetyl-D-glucal to 2,3-unsaturated O-glycosides. A wide range of biologically active natural products, alcohols and thiols could be coupled with glucal to give the desired pseudoglycosides in good to excellent yields with exclusive alpha-stereoselectivity.

Total Synthesis of Sialic Acid by a Sequential Rhodium‐Catalyzed Aziridination and Barbier Allylation of D‐Glycal

The synthesisof 2-amino-2-deoxypyranoside residues is chal-lenging in two ways: the selective functionaliza-tion of the nitrogen moiety at the C2-position,and the formation of the glycosidic bond withappropriate glycosyl acceptors. Nowadays, gly-cals are often employed as versatile startingmaterials for such syntheses. Numerous methodshave been developed for the direct introductionof a nitrogen substituent at the C2-position ofglycals. These approaches involve inter- or intra-molecular addition of a nitrogen atom to a glycalscaffold, which generates an aziridine intermedi-ate, followed by ring opening with a nucleophile.These methods have been developed as generalpathways to generate 2-amino sugars.

Green glycosylation promoted by reusable biomass carbonaceous solid acid: an easy access to β-stereoselective terpene galactosides

An efficient green protocol has been developed for the atom economic glycosylation of unprotected, unactivated glycosyl donors and glycosylation of glycosyl trichloroacetimidates with the aid of reusable eco-friendly biomass carbonaceous solid acid as catalyst.

Facile synthesis of carbohydrate-integrated isoxazolines through tandem [4+1] cycloaddition and rearrangement of 2-nitroglycals

Carbohydrate-integrated isoxazolines were synthesized from 2-nitroglycals and sulfur ylides, with the aid of 1-phenylthiourea catalyst. The reactions proceeded via [4+1] annulations and upon subsequent rearrangement, afforded the corresponding isoxazolines in high yields with excellent diastereoselectivities (up to 95% de).

[3 + 2] Cycloaddition on Carbohydrate Templates: Stereoselective Synthesis of Pyrrolidines

Pyrrolidine derivatives were prepared in high diastereoselectivities and good yields via a [3 + 2] cycloaddition of a tert-butyldimethylsilyl protected carbohydrate-based allene with a diverse range of imines. The subsequent removal of the carbohydrate auxiliary afforded a variety of pyrrolidines with excellent enantioselectivities (up to 99% ee). Selective reduction of the pyrrolidines further demonstrated the potential of this strategy.