O-Glycosyl Trichloroacetimidates as Glycosyl Donors and Platinum(IV) Chloride as a Dual Catalyst Permitting Stereo- and Regioselective Glycosidations

Abstract

Stereo- and regioselective glycosidation is still a great challenge in carbohydrate chemistry. Among the tremendous endeavors in this area, acid–base catalysis, available to O-glycosyl trichloroacetimidate activation, is of particular interest. It features an alternative glycosidation pathway initiated by the sequential binding between the catalyst and the glycosyl acceptor and then with the glycosyl donor. Through the formation of a catalyst–acceptor adduct, increased proton acidity and oxygen nucleophilicity are reached, thus enforcing donor activation and concomitant acceptor transfer. As the reaction takes place via an SN2-type transition state, essentially stereoselectivity is granted. Platinum­(IV) chloride is an ideal catalyst in this regard; it is a weak Lewis acid, yet it possesses high affinity to the glycosyl acceptor hydroxy group due to its ligand-binding capacity, thus affording from the α-configurated glycosyl donor the glycosidation product predominantly in β-configuration. In addition, the capacity of platinum­(IV) chloride to bidentate ligation is another advantage, as it enhances nucleophilicity differences between hydroxy groups, thus permitting regioselective glycosidation, as observed for various 1,2-cis-, 1,3-, and 1,2-trans-diol and 1,2,3-triol moieties present in carbohydrate acceptors. This observation found application in an efficient saponin natural product synthesis.

Preparation of Tea Aroma Precursor Glycosides: An Efficient and Sustainable Approach via Chemical Glycosidation

“…Recently, we found for some catalysts an alternative reaction course (the “acid–base catalysis”) . Namely, a catalyst with high oxophilicity first binds the acceptor ROH to form a RO–Cat–H adduct, and this adduct then enforces an acid–base-catalyzed intramolecular glycosidation (path ii). − It can be envisioned that orthoester formation will be circumvented if the glycosidation takes place via the acid–base catalysis pathway.…”

A Stereoselective Glycosylation Approach to the Construction of 1,2‐trans‐β‐d‐Glycosidic Linkages and Convergent Synthesis of Saponins

“…Lewis acids capable of effecting the 1,2-trans-β-selective glycosylation with glycosyl α-trichloroacetimidate donors have been identified. Those include AuCl 3 , [24] PtCl 4 , [25] Pd-(PhCN) 2 (OTf) 2 /Pd(CH 3 CN) 4 (BF 4 ) 2 , [26] and FeCl 3 , [27] as reported by the research groups of Schmidt, Peng, Nguyan, and Ghosh, respectively.…”

Gold(III) Chloride─tert-Butyl Cyanide Catalyzed Glycosidations with O-Glycosyl Trihaloacetimidates

“…Along with our ongoing interest in the development of innovative and efficient glycosidation strategies, we discovered that transition metal salts, for instance, Gold(III), 18 Gold(I) 18 and Platinum(IV) 19 salts, displayed superior reactivity in acid─base catalysis glycosidations even at low temperatures, which accommodated a broad scope of glycosyl donors and acceptors. However, the current methods are not fully satisfactory in some aspects: (1) O-glycosyl trichloroacetimidate donors with single configuration were required for high stereoselectivity via this SN2-type glycosidations.…”

“…However, the current methods are not fully satisfactory in some aspects: (1) O-glycosyl trichloroacetimidate donors with single configuration were required for high stereoselectivity via this SN2-type glycosidations. 10,[14][15][16][17][18][19] Although single β-or -O-glycosyl trichloroacetimidate donors can be readily obtained via kinetic or thermodynamic reaction control, respectively, there remain sometimes difficulties in their generation and purification in order to access the desired pure anomers. (2) The stereoselectivity of the glycosidation of glycosyl acceptors with steric hindrance is not yet satisfactory.…”

“…(3) As the acid─base catalysts were mild acids, other O-glycosyl imidate donors with lower reactivity, for instance, O-glycosyl N-phenyl trifloroacetimidate donors 20,21 did not work as well under these glycosidation conditions. 19 (4) Metal catalysts are commonly poisoned in the presence of thioglycosides, 22 thus limiting the scope of the application of these methods.…”