Acetals cyclic, acetolysis

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. 

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... 

One of the few efficient reactions to have been reported of 5-bromohexose derivatives is the acetolysis [mercury(II) acetate in acetic acid] of methyl tetra-0-acetyl-5-bromo-/ -D-glucopyranuronate (10), which affords the crystalline methyl 5-acetoxy-tetra-O-acetyl-a-L-idopyranuronate (126) this was assigned the inverted configuration at C-5 on the grounds of a major optical rotational change during its formation. Likewise, methyl tetra-0-acetyL/f-L-xy/o-hexulopyranosonate (127) is obtained in excellent yield from the bromide 8, and it, too, is dextrorotatory and is produced from a levorotatory bromide, conceivably by way of a cyclic 4,5-acetoxonium ion. Compound 127 also differs in expected ways from its known C-5 epimer, and, on deprotection, affords L-ascorbic acid.21 28... 

The observation that added sodium acetate gives rise to no kinetic salt effect at all in the acetolysis of dineophylmercury has already been put forward32 as evidence that the acetolysis proceeds by mechanism SE2(cyclic). 

The method most likely to provide information concerning the nature of the reaction intermediates is that of detailed, kinetic analysis under a variety of carefully controlled conditions. It is evident that, in the application of this method, there is an extensive field of investigation to be surveyed, not only of the action of acyl trifiuoroacetates on hydroxylic compounds under the influence of different media and catalysts, but also of the peculiar differences between the ring-opening reactions of cyclic acetals with this type of reagent and with that which is employed in the Hudson acetolysis procedure. 

Other examples of neighboring group cyclization that give no cyclic products are often results of kinetic versus thermodynamic control. The isolation of the bicyclic product (7) instead of a tricyclic acetate similar to (6) in the acetolysis of (3) is such an example. In summary, then, Eq. (28) cannot be used to predict products (i.e., there may be a rate/product discrepancy) when the routes of kinetic control and thermodynamic control are of similar energies, and the noncyclic product is thermodynamically favored. 

The reaction of D-glucose phenylhydrazone with glacial acetic acid has been examined iV-fructosyl-A -glucosylphenylhydrazine was formed at 30 °C, whereas 1,2-dideoxy-l -phenylimino-2-phenylhydrazono-D-ara6//io-hexose was obtained after brief treatment at 100 2-(D-arfli //jo-Tetrahydroxybutyl)quinoxaline was also formed. The acetolysis of the 2,3,4,6-tetra-0-acetyl-j8-D-glucopyranosyl 2,4-dinitrophenylhydrazine (154) yielded a mixture of acyclic and cyclic hydrazones... 

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