Effective Organotin-Mediated Regioselective Functionalization of Unprotected Carbohydrates

Rh(II) and Chiral Phosphoric Acid Co-catalyzed Selective O-H Insertions for Stereodivergent O-Alkylation of Glycosides

Carbohydrates are synthetically challenging molecules with essential biological functions in all living systems. The selective synthesis and modification of carbohydrates are crucial for investigating their biological functions. Controlling chemo-, regio-, and stereoselectivity is a central theme in carbohydrate synthesis. Achieving the full set of stereoisomers of carbohydrate derivatives would significantly enhance the efficiency of building compound libraries for biological studies and drug discovery. However, the selective functionalization of seemingly identical hydroxyl groups in carbohydrates remains a long-standing challenge in organic chemistry. In carbohydrate synthesis, achieving precise control of both relative configurations in catalyst-controlled reactions that create a new stereocenter presents a significant synthetic challenge. Herein, we developed an efficient method for the stereodivergent O-alkylation of carbohydrate hydroxyl groups via Rh(II)/chiral phosphoric acid-cocatalyzed insertion of metal carbenoids. This system is mild and robust, offering excellent selectivity across a broad range of substrates with high regio- and stereoselectivity. Furthermore, this strategy opens up vast opportunities for stereodivergent synthesis.

Catalyst-Controlled, Site-Selective Functionalization of Levoglucosan Analogues Mediated by Chiral Zinc Diamine Complexes

In this work, we developed a site-selective functionalization of levoglucosan analogues. This strategy used simple zinc diamine complexes for regioselective functionalization at O2 or O4. Successful transformations of monotosyl analogues allowed the preparation of useful intermediates in carbohydrate chemistry, such as four protected Černý epoxides and three protected amino sugars. This unique method represents a rare use of zinc for the desymmetrization of 1,3-diols.

Enantioconvergent and Site-Selective Etherification of Carbohydrate Polyols through Chiral Copper Radical Catalysis

Going beyond currently reported two electron transformations that formed the core backdrop of asymmetric catalytic site-selective carbohydrate polyol functionalizations, we herein report a seminal demonstration of an enantioconvergent copper catalyzed site-selective etherification of minimally protected saccharides through a single-electron radical pathway. Further, this strategy paves a rare strategy, through which a carboxamide scaffold that is present in some glycomimetics of pharmacological relevance, can be selectively introduced. In light of the burgeoning interest in chiral radical catalysis, and the virtual absence of such stereocontrol broadly in carbohydrate synthesis, our strategy showcased the unknown capability of chiral radical copper catalysis as a contemporary tool to address the formidable site-selectivity challenge on a remarkable palette of naturally occurring saccharides. When reducing sugars were employed, a further dynamic kinetic resolution type glycosylation can be activated by the catalytic system to selectively generate the challenging β-O-glycosides.

Silicomolybdic Acid Cluster as Biocompatible Catalyst for One-Pot Tandem Synthesis of Orthogonally Protected Glycosides

The present paper describes a new and practical approach for the one-pot preparation of O-isopropylidene derivatives and also orthogonally protected S- and O-glycosides from the corresponding unprotected saccharides by employing 2 mol % of a silicomolybdic acid (SMA) cluster as a versatile and biocompatible catalyst. The present protocol is applicable to two-step one-pot tandem transformations, which include the O-isopropylidation, spiroketal functionalization, 4,6-O-arylidene acetalations, and arylidene acetylation processes under relatively mild reaction conditions. One-pot sequential transformations, low catalyst loading, rapid transformation, high to excellent reaction yields, mild reaction conditions, and a nontoxic biocompatible workup procedure are the notable advantages of devised protocol.

Trivalent dialkylaminopyridine-catalyzed site-selective mono-O-acylation of partially-protected pyranosides

This work demonstrates trivalent tris-(3-N-methyl-N-pyridyl propyl)amine (1) catalyzing the site-selective mono-O-acylation of glycopyranosides. Different acid anhydrides were used for the acylation of monosaccharides, mediated by catalyst 1, at a loading of 1.5 mol%; the extent of site-selectivity and the yields of mono-O-acylation products were assessed. The reactions were performed between 2 and 10 h, depending on the nature of the acid anhydride, where the bulkier pivalic anhydride required a longer duration for acylation. The glycopyranosides are maintained as diols and triols, and from a set of experiments, the site-selectivity of acylations was observed to follow the intrinsic reactivities and stereochemistry of hydroxy functionalities. The trivalent catalyst 1 mediates the reactions with excellent site-selectivities for mono-O-acylation product formation in the studied glycopyranosides, in comparison to the monovalent N,N-dimethylamino pyridine (DMAP) catalyst. This study illustrates the benefits of the multivalency of catalytic moieties in catalysis.

Regioselective Deacetylation of Peracetylated Deoxy-C-glycopyranosides by Boron Trichloride (BCl3)

A general approach for regioselective deacetylation at sugar 3-OH of peracetylated 6-deoxy-C-glucopyranosides mediated by BCl3 was developed. The approach could be extended to other sugar-derived 6-deoxy-C-glycopyranosides, such as those derived from mannose, galactose, and rhamnose, with deacetylation occurring at varied sugar hydroxyl groups, and further extended to 4-deoxy-C-glucopyranosides with deacetylation occurring at sugar 3-OH. The approach would enable access to synthetically challenging carbohydrate derivatives. A possible mechanism of the regioselectivity was proposed.

Advances in glycoside and oligosaccharide synthesis

The structural complexity of glycans poses a serious challenge in the chemical synthesis of glycosides, oligosaccharides and glycoconjugates. Glycan complexity, determined by composition, connectivity, and configuration far exceeds what nature achieves with nucleic acids and proteins. Consequently, glycoside synthesis ranks among the most complex tasks in organic synthesis, despite involving only a simple type of bond-forming reaction. Here, we introduce the fundamental principles of glycoside bond formation and summarize recent advances in glycoside bond formation and oligosaccharide synthesis.