Sugars reaction with allylstannanes

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. ... 

Sugar allylstannanes have been prepared by the homolytic reduction of allylic dithiocarbonates,255 and by the reaction of Bu3SnCu with an allylic mesylate or bromide 256 257 the relative yields of isomers depend on the steric demands of the sugars (Equations (90) and (91)). 

The addition of allyltributylstannane to a chirally modified aldehyde 111 proceeds in high yield and moderate diastereoselectivity to give the horaoallylic alcohols 112 and 113 (Scheme 10-46) [86]. When the methyl group on the sugar is not present, the reaction proceeds in higher diastereoselectivity (95%) and excellent yield. The formation and reaction of a 2/1 complex (aldehyde/Lewis acid) with the allylstannane can account for the observed selectivity. 

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... 

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