Acetolysis benzyl ethers

Regioselective debenzylation can be achieved by treatment with Lewis acids such as ferric chloride and S11CI4 or under acetolysis conditions with acetic anhydride and sulfuric acid, and several examples are depicted in Scheme 2.3.9 Acetolysis results in cleavage of the most acid-sensitive benzyl group. In general, primary benzyl ethers can be selectively acetolysed in the presence of secondary benzyl ethers. The regioselectivity of the reaction can be explained as follows sulfuric acid protonates acetic anhydride followed by the formation of an acetyl ion and acetic acid. The acetyl ion reacts with the sterically most accessible oxygen which is at... 

Acetylation. Rapid peracetylation of carbohydrates can be effected in the presence of iodine. With more iodine and longer reaction times the selective acetolysis of primary benzyl ethers is achieved. Perbenzyl ethers of mono- and disaccharides undergo group exchange (OBn —> OAc) at the primary carbon atoms on treatment with ACjO-HOAc-ZnClj. 

Following the observation that benzyl ethers are cleaved by acetic anhydride in the presence of sulfuric acid or perchloric acid, Allerton and Fletcher S extended the work to carbohydrate systems. 2,5-Di-O-benzyl-l,3 4,6-di-0-methylene-D-mannitol (12) gave 1,3,4,6-tetra-0-acetyl-2, 5-di-O-benzyl-D-mannitol in the presence of 0.09% of sulfuric acid, but, with 2.4% of the acid, the hexaacetate was formed. With 1,6-di-O-benzyl-2,4 3,5-di-0-methylene-L-iditol (13), brief acetolysis gave the di-0-acetyl-di-O-methylene derivative longer treatment gave the hexaacetate. 

Anhydro rings of the 1,3,6, and 3,4 types have all been opened by acetolysis. The first class of linkage can be broken specifically in the presence of a disaccharide (1— )-linkage. - As mentioned previously (see p. 16), it is also possible to cleave a 1,6-anhydro ring in the presence of benzyl ether groups by using very mild conditions. ... 

In order to minimize such possible side-reactions as anomerization of glycosides and acetolysis of acetal structures, alternative techniques of acetylation have been studied. Whereas basic catalysis leads to severe complications in certain nitro sugar derivatives (see Section IV,2a and c pp. 124, 129), boron trifluoride etherate has proved to be an excellent agent for satisfactory acetylation of glycosides, O-benzyl-idenated glycosides, and disaccharides that contain a deoxynitro function.148,149... 

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