Sugar acetates, rearrangements with

The exolendo selectivity of the 1,3-dipolar addition of nitrones was regulated by the titanium salt as shown in Eq. (156) [398,399]. Aza-Cope rearrangement of a sugar-derived substrate initiated by the acetal cleavage with TiCl4 proceeded diastereoselec-tively to give the open-chain product (Eq. 157) [400]. 

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. 

In comparison to some of the other activation methods however, the dimethyl sulfoxide-acetic anhydride procedure has certain disadvantages. The method often requires the use of long reaction times (1 24 h), which can result in many side reactions, especially with sensitive substrates. Notable in this respect is that it is not uncommon for this procedure to result in the formation of substantial yields of the thiomethyl ethers obtained from the Pummerer rearrangement product as described above. In fact upon attempted oxidation of cholesterol with this system, the major product obtained was the corresponding (methylthio)methyl ether. Acetates may also be formed if the alcohol is unhindered. For example the sugar derivative (9) reacts under these conditions to form an enol acetate (derived from the requir carbonyl compound) in 40% yield contaminated with 30% of the acetate (10 equation S). ... 

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