Sugars reaction with allylsilanes

Glycals serve as activated sugars because of the inherent reactivity of the endocyclic enol ether. Consequently, they have been extremely useful substrates in their complementarity to native activated sugars. Additionally, they have been used to demonstrate versatility not directly available from other sugar derivatives. In a good example of the utility of allylsilanes and silylacetylenes in C-glycoside chemistry, Ichikawa et al. [91] demonstrated a preference for the a anomer in all cases. The results, shown in Scheme 7.17, included a demonstration that a characteristic nOe can be used to confirm the stereochemical outcome of the reaction with hw-trimethylsilylacetylene. 

The same pattern is seen, with only one lone pair, in tetrahydropyrans, when nucleophiles like allylsilanes and allylstannanes attack axially in Lewis acid catalysed reactions in pyranyl sugars.562 The same pattern is also found in imminium ions563 as Stevens demonstrated in several syntheses. He showed that nucleophiles attack from the direction that most easily sets up an anti and axial lone pair, especially if it can create a chair conformation, as in the reduction of the imminium ion 5.143 to give monomorine-I 5.144.564... 

Allylstannanes are much better nucleophiles for dithioacetals than allylsilanes, and dimethyl(methyl-thio)sulfonium fluoroborate is a particularly good catalyst in this reaction. The reaction has been used tellingly in a macrocyclization (Scheme 56). ° Monothioacetals also react with allylstannanes, with cleavage of either the carbon-oxygen or the carbon-sulfur bond, depending upon the choice of Lewis acid (Scheme 57).Sugar thioacetals react in the same way (Scheme 58), but with the opposite anomeric stereoselectivity from that of the corresponding radical chain substitution. ... 

---