Alkylation of carbohydrates

The first part of this section will present the different possible reactions enabling the formation of aUcyl glucosides. This will be followed by discussion of the different types of catalyst used for the synthesis, with special emphasis on solid catalysts. 

---

These reactions are particularly useful in the regioselective acylation and alkylation of carbohydrates, when the selectivity depends on the nature of the solvent, and of the dimers or higher oligomers which are present. 

Organotin ether derivatives continue to be utilized in the regioselective acylation and alkylation of carbohydrates. The 3-O-acylation of benzyl a-L-lyxopyranoside used the dibutylstannylene method while a polymer supported aryl(butyl)tin dichloride reagent has been used catalytically in the selective 6-0-... 

Similarly, carbohydrates often exist in an equilibrium between a cyclic h niacetal form and a carbonyl form with the latter in an extremely low quantity (14). Therefore, for alkylation of carbohydrate derivatives in organic solvent, extensive functional group transformation and derivatization is required to generate the desired carbonyl. However, in the aqueous reaction, the hemiacetal-form and the carbonyl form is in a constant equilibrium. And the carbonyl form will react, which drives the equilibrium until all the starting material has reacted (eq. 7). 

Other reactions of carbohydrates include those of alcohols, carboxylic acids, and their derivatives. Alkylation of carbohydrate hydroxyl groups leads to ethers. Acylation of their hydroxyl groups produces esters. Alkylation and acylation reactions are sometimes used to protect carbohydrate hydroxyl groups from reaction while a transformation occurs elsewhere. Hydrolysis reactions are involved in converting ester and lactone derivatives of carbohydrates back to their polyhydroxy form. Enolization of aldehydes and ketones leads to epimerization and interconversion of aldoses and ketoses. Addition reactions of aldehydes and ketones are useful, too, such as the addition of ammonia derivatives in osazone formation, and of cyanide in the Kiliani-Fischer synthesis. Hydrolysis of nitriles from the Kiliani-Fischer synthesis leads to carboxylic acids. 

Chan L, Taylor MS (2011) Regioselective alkylation of carbohydrate derivatives catalyzed by a diarylborinic acid derivative. Org Lett 13 3090-3093... 

Scheme 2 Regioselective alkylation of carbohydrate derivatives cata zed by a diaryl-...

Selective Acylations and Alkylations of Carbohydrate Derivatives Catalyzed by Diphenylborinate Ester 1... 

Organotin ethers continue to be employed in selective alkylations. In a study of the Bu2SnO-mediated alkylation of lactosides 1 and 2 aimed at obtaining the 3 -0-ethers, the penta-benzyl ether 2 afforded better yields than the corresponding benzoate 1, and siniilar conditions have been employed for the regioselective benzylation of octyl P-D-galactopyianoside at 0-3 Dibutyltin dimethoxide has been used in place of dibutyltin oxide for selective alkylation of carbohydrate diols. ... 

---