Acetolysis selective

In this Section, hydrolysis, acetolysis, and isomerization of acetals are considered. Selective deprotection of acetals may also be achieved through halogenation, hydrogenolysis, ozonolysis, and photolysis, but these topics, are covered in an accompanying article in this Series, on the reactivity of cyclic acetals of aldoses and aldosides,3 and will not be discussed here. 

In general, only methylene acetals have been selectively depro-tected in this way on acetolysis, benzylidene and isopropylidene acetals are readily converted into the corresponding peracetates of the... 

Selective acetolysis has been achieved in useful yields with some... 

It is intended that the present Chapter shall be complementary to Haines s article in this Volume8 that deals with the selective deprotection of protected sugars. The reader interested in all aspects of the hydrolysis, the alcoholysis, the acetolysis, and the isomerization of cyclic acetals pf sugars should refer to Haines s article. A review has also been published concerning the formation and migration of cyclic acetals of carbohydrates.80... 

The differences in the rates of hydrolysis of various linkage types by a particular glycosidase can be used to provide information about this aspect of structure. Jack-bean a-D-mannosidase cleaves a-(l- 2) and a-(l- 6) linkages much faster than a-(l -> 3). Oligosaccharides, obtained by endo-N-acetyl-/J-D-glucosaminidase hydrolysis of ovalbumin, were subjected to acetolysis, which selectively cleaved the a-(l - 6) bonds. A tetrasaccharide isolated after this treatment was then incubated with jack-bean o-d-... 

For the synthesis of the D-C portion, two different concepts were followed either by modification of laminaribiose (166) [89] or by a stereospecific P, 1 ->3-glycosylation [20]. Laminaran is isolated from seaweeds or from Poria cocos Wolf, degraded by selective acetolysis, and the lower oligomers separated by preparative HPLC [90]. Following acetylation, the heptaacetyl laminaribiosyl bromide is prepared and transformed into the disaccharide glycal 167 by the classical approach in 93 % yield. The 2-deoxy-2-iodo-a-glycoside is formed by application of the NIS procedure after deprotection and subsequent 4,6-0-benzylidenation, the precursor 168 for the radical formation of the 6,6 -dibromo-6,6 -dideoxy derivative is at hand. This compound may be further reduced to methyl-3-0-(P-D-chinovosyl)-a-D-olivoside (169). 

Antibodies can be used for detection of carbohydrate sequences in biological materials. For example, a mannotetraose (Manal-3Manal-2Manal-2Man) obtained by selective acetolysis of yeast mannan was derivatized with phenethylamine and diazotized to edestin (3). Specificity of the anti-mannotetraose antibodies obtained was studied by the radioimmunoassay methods just described. Results of these studies are summarized in Table II. 

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... 

The influence of bulky substituents in the 3 positions of vinyl substrates also appears to increase the selectivity of the vinyl cation (Rappoport and Gal, 1973a, 1970). This effect was attributed to shielding of the cationic intermediate from the incoming nucleophile. Thus, in the acetolysis of vinyl bromides, common ion return for two )3 substituents decreases in the order anisyl, anisyl > anisyl, H > methyl, methyl > H, H (Rappoport and Gal, 1970). 

The large amounts of five- and six-membered cyclic tosylates, products of internal return (49% yield in one case 12a of Table 3) points also to a highly selective intermediate, although the reason for such a remarkable selectivity towards external nucleophiles is not clear. On the other hand, acetolysis of 6-phenyl-5-hexynyl brosylate, which is apparently anchimerically assisted by the triple bond (4>bs./ caic. = 1 6) yields only the five-membered ring product in addition to open-chain solvolysis products (13 of Table 3). The effect of the phenyl group in orienting the oyclization reaction would indicate that intermediate species like 43 may become important when R =Ph. 

The selectivity toward bromide versus acetate ions, the amounts of E/Z isomerized starting material and product, the amount of anisyl 1,2-shift, and the nature of the capturing nucleophile in the acetolysis are all in quantitative agreement with the corresponding thermolysis reactions. The experimental results for nucleophilic vinylic photosubstitution support the involvement of an intermediate linear free vinyl cation which is thermally relaxed. 

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