Sugar acetates, rearrangements

Concerning the mechanism of the rearrangement of sugar acetates by aluminum chloride, there is very little to be added at the present time. The reaction proceeds equally as well with the a-octaacetate of cellobiose as it does with the /3-octaacetate of lactose.Under the most favorable conditions so far discovered, it appears that octaacetyl-lactose is converted to about equal amounts of acetochlorolactose and acetochloroneolactose similar results were obtained with octaacetyl-cellobiose. Although Kunz and Hudson believed acetochlorolactose to be the primary reaction product, which was transformed subsequently to the isomeric neolactose derivative, later experiments by Richtmyer and Hudson did not substantiate this view. In an uncompleted study of the action of a mixture of aluminum and phosphorus chlorides upon pentaacetyl-D-glucose, Richtmyer and Hudson have demon.strated that both D-altrose and n-mannose derivatives are formed by rearrangement of the D-glucose molecule. 

During a study devoted to fragmentation of sugar acetals in the presence of butyllithium [142], the formation of unsaturated pyranoses (36-38) from L-rhamnose derivative 35 was observed and explained in terms of anionic species rearrangement (O Scheme 12). 

Subsequently this route to 3-deoxy sugars was developed by further work in our laboratory by Ferrier and Sankey (23). They demonstrated that although tetra-O-acetyl-2-hydroxy-D-galactal undergoes rearrangement in boiling acetic acid, and that reaction is speeded up and goes to completion if catalytic amounts of methanesulfonic acid are added, the... 

Thiamine pyrophosphate is a coenzyme for several enzymes involved in carbohydrate metabolism. These enzymes either catalyze the decarboxylation of oi-keto acids or the rearrangement of the carbon skeletons of certain sugars. A particularly important example is provided by the conversion of pyruvic acid, an oi-keto acid, to acetic acid. The pyruvate dehydrogenase complex catalyzes this reaction. This is the key reaction that links the degradation of sugars to the citric acid cycle and fatty acid synthesis (chapters 16 and 18) ... 

The first example of a methoxy-group participation in sugars was reported by Lemieux and Fraser-Reid 7) who observed 1,3,4,6-tetra-O-acetyl-2-0-methyl-D-glucop3U-anose (4) as one of the products of bro-minolysis of methyl 3,4,6-tii-0-acetyl-2-deoxy-2-iodo- -D-mannopyra-noside ffj in the presence of silver acetate. The cyclic methoxonium ion (2) was indicated to be an intermediate in this rearrangement, giving rise to the product either by direct nucleophilic attack of acetate ion or... 

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