Highly Efficient and Mild Method for Regioselective De-O-benzylation of Saccharides by Co₂(CO)₈-Et₃SiH−CO Reagent System

Abstract

A highly efficient and mild method for the de-O-benzylation of protected saccharides was developed by transforming terminal benzyl ethers into silyl ethers using Co(2)(CO)(8)-Et(3)SiH under 1 atm of CO. The method was successfully used for the de-O-benzylation of perbenzylated monosaccharides with various anomeric protecting groups, as well as natural disaccharides and trisaccharides such as sucrose, raffinose, and melezitose in good yields (>80%).

An Easy and Versatile Approach for the Regioselective De‐O‐benzylation of Protected Sugars Based on the I2/Et3SiH Combined System

“…[16] The primary positions can be de-Obenzylated (and concomitantly triethylsilylated) with excellent regioselectivity on prolonged exposure of benzylated mono-, di-and trisaccharides to the Et 3 SiH/A C H T U N G T R E N N U N G [Co 2 (CO) 8 ] combined system, at high temperature under a CO atmo-A C H T U N G T R E N N U N G sphere. [17] Acetolysis is another approach that often leads to selective de-O-benzylation (and concomitant acetylation) of the primary positions, but in this case undesired cleavage of glycosidic bonds might occur. [18] Trimethylsilyl iodide (TMSI) has also been reported to produce selective de-Obenzylation of primary carbinols (via the corresponding trimethylsilyl ethers).…”

A mild and efficient method for the selective cleavage of primary p-methoxybenzyl protecting group of saccharides by Co2(CO)8–Me2PhSiH–CO system

“…14, 15 In order to probe the potential similarity of mechanism between the de-OPMB and de-OBn process which we reported previously, 9,10 triethylsilyl ether intermediate 1c (Table 1, entry 1) which is stable to silica gel chromatography (see SI). Solvents, temperature, and equivalents of Co 2 (CO) 8 were then screened.…”

“…Previous work in our laboratory has identified Co 2 (CO) 8 -hydrosilane-CO system as an effective method for the selective cleavage of benzyl ether. 9, 10 Various substrates have been tested to show its broad applications and high functional groups tolerance. 10 Since PMB ether is similar to benzyl ether, we presume this mild method is also applicable for the selective cleavage of PMB ether.…”

“…[9][10][11][12] The reactivity of hydrosilane determines the reaction rate of the cleavage process. Therefore, five hydrosilanes were firstly screened in the presence of 2.0 equiv of Co 2 (CO) 8 in benzene under balloon pressure of CO at 50°C (Table 1, entries 1-6).…”

“…Therefore, based on previous study and formation of this byproduct 16 and also silyl ether intermediate 1c (Table 1, entry 1), we proposed a mechanism for this system as shown in Scheme 2: catalytic species Co(CO) 4 SiR 3 is generated in situ to substitute PMB group as the silicon atom is strongly electrophilic, and the resultant PMBCo(CO) 4 goes through insertion of CO, oxidative addition with hydrosilane and reductive elimination to regenerate catalytic species. 9,11 MeO OH…”