Glycans are complex compounds consisting of sugars linked glycosidically, existing either as pure polysaccharides or as part of glycoconjugates. They are prevalent in nature and possess important functions in regulating biological pathways. However, their diversity coupled with physiochemical similarities makes it challenging to isolate them in large quantities for biochemical studies, hence hampering progress in glycobiology and glycomedicine. Glycochemistry presents an alternative strategy to obtain pure glycan compounds through artificial synthetic methods. Efforts in glycochemistry have been centered on glycosylation, the key reaction in glycochemistry, especially with regards to anomeric stereoselectivity in polysaccharides and glycoconjugates. In particular, the stereoelectronic and steric properties of glycosyl donors are commonly used to direct the stereoselectivity in glycosylation reactions. Classic glycosylation strategies typically involve saturated glycosyl donors, proceeding either directly using hydrogen bonds and conformational constraints or indirectly by installing moieties covalently through neighboring group participation and intramolecular aglycon delivery. Over the past years, new glycosylation strategies, tapping on the foundations of transition metal catalysis, have emerged. To leverage the power of coordination chemistry, unsaturated glycosyl donors were introduced. Not only are the number of protection/deprotection steps reduced, the resultant unsaturated glycoside provides opportunities for downstream functionalizations, allowing quick access to a variety of sugars, including rare sugars. Alongside the glycosyl donor, an equally important but neglected aspect for targeting stereoselective glycosylation is the glycosyl acceptor. In the case of dual-directing donors, glycosyl acceptors have proved themselves capable of becoming the dominating factor for stereocontrol. Interestingly, rational manipulation or selection of glycosyl acceptors with particular nucleophilicity and p K values can lead to different stereoselectivities, thereby proving the tunability of such acceptors to favor the formation of one anomer over the other stereoselectively. By further venturing beyond substrate controlled stereoselectivity, we are presented with the opportunity to effect stereoselective glycosylation through glycosylating reagents. Of the key reagents, stereoselective catalyst stands out as a greener and efficient alternative to direct stereoselective control with stoichiometric substrates. Recently, investigations into this approach of stereocontrol presented an intriguing range of stereoselectivities, achieved by merely varying the nature of catalysts used. Another crucial effort in glycochemistry is enhancing the efficiencies of glycosylations, by reducing the number of preparative steps before or during glycosylation. Through using transient masking groups or one-pot synthetic strategies, these streamlined approaches provide enormous convenience and practicability for oligosaccharide syntheses. Th...

Stereoselective construction of various O-glycosidic bonds was first achieved by different palladium sources using 3,4-O-carbonate galactal as the donor to reach yields up to 95% under mild conditions. With Pd­(II) catalyst coordination of this glycal donor from the β-face directed by carbonate group, hard nucleophiles (aliphatic alcohols) gave β-glycosides and α-glycosides were obtained from soft nucleophiles (phenols). In contrast, with the Pd(0) catalyst coordinating the donor from the β-face due to steric effect, both hard and soft acceptors could only generate β-glycosides via hydrogen-bond-mediated aglycone delivery.

A palladium-catalyzed one-pot Tsuji-Trost type decarboxylative allylation/Wittig reaction has been developed to synthesize C-vinyl glycosides. Screening of various aldehydes led to formation of β,(E)-selective C-vinyl glycosides with pyridyl group containing aldehydes and β,(Z)-selective C-vinyl glycosides with nonpyridyl aldehydes. A plausible mechanism is proposed based on the coordination effect of the aldehydes.

An efficient and highly stereoselective method for the construction of N-heterocyclic glycosides is reported. This method is based on a palladium-catalyzed allylation which proceeded to provide N-heterocyclic glycosyl compounds in good-to-excellent yields with β- or α-selectivity. Various N-nucleophiles were examined for this reaction and selected N-glycosyl isatin substrates were further elaborated to bis-indole sugars which have potential as antiproliferative drugs.

A gold-catalyzed tandem 1,3-acyloxy migration/Ferrier rearrangement to access 8-oxabicyclo[3.2.1]octanes with high efficiency and complete diastereoselectivity was developed successfully. Resultant compounds could undergo the interrupted Nazarov cyclization to afford diastereomerically pure 11-oxatricyclo[5.3.1.0]undecanes.