Alcohols, reaction with glycals

The glycals are easily converted into the 1,2-dihalo-derivatives, which in principle can act as glycosyl donors. However, these derivatives have not found wide application in glycoside synthesis, mainly because of the low facial selectivity in the initial addition of the electrophilic species [143-145]. In an example of a successful application, 2-deoxy-2-bromo-a-D-glucopyranosyl bromide [146] has been shown to give predominantly the 2-deoxy-P-D-glucopyranosides in silver-triflate-promoted reactions with alcohols. 

Glycal assembly method (Section 25.9) a method of polysaccharide synthesis in which a glycal is converted into its epoxide, which is then opened by reaction with an alcohol. 

As already mentioned, unsaturated sugars, particularly glycals, are readily available compounds. Perrier rearrangement of glycals in the presence of Lewis acids and alcohols easily takes place to provide 2,3-unsaturated carbohydrates. The allylic esters obtained this way can be engaged in different reactions with organometallic reagents. 

In recent years the discovery of novel methods of asymmetric synthesis has greatly increased the ability of organic chemists to synthesize optically active sugars. For example, the asymmetric epoxidation reaction discovered by Katsuki and Sharpless [142] was recently used as the key step in a synthesis of D-oleandrose 118 from divinyl carbinol 119 by Hatakeyama et al. [143]. An alternative approach to asymmetric synthesis of oleandrose was taken by Danishefsky et al. [144,32] in their synthesis of avermectin which is the first, and currently the only, reported total synthesis of an avermectin. The key step of this synthesis was a cyclocondensation reaction of optically active diene 121 with acetaldehyde catalyzed by the optically active Lewis acid (-h)-Eu(hfc)3 [145]. The resulting chiral pyrone was then elaborated to methyl-L-oleandroside 113. This was further converted to the disaccharide glycal 122 by a 4 step sequence in which glycoside formation was accomplished by iV-iodosuccinimide mediated addition of the alcohol to a glycal followed by tributyltin hydride... 

Rearrangement reactions of glycal derivatives continue to be of interest, trl- -acetyl-D-glucal giving the a-glycoside (7) under standard conditions with boron trifluorlde as catalyst and the appropriate alcohol. Periodate oxidation gave the o-vlnyl... 

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