Cycloaddition, tetrahydrofuran allylsilane-aldehyde

Allylsilane-aldehyde [3+2]-cycloadditions are a very powerful means of generating tetrahydrofurans as they can be used to give either 2,5-cis- or 2 -trans-isomers depending upon the Lewis acid used to promote the reaction [30]. As an example of the power of these reactions, Roush and Micalizio described the reaction of allylsilane 108 (available from the condensatimi of allylborane 107 with aldehyde 106) with aldehyde 109 in the presence of BF3-Et20 to give c -tetrahydrofuran 110 in 78 % yield and in >20 1 dr (Scheme 30) [31], In a complementary fashion, the use of SnCLt as the Lewis acid gave an 85 % yield (dr = 20 1) of trara-tetrahydrofuran 111. 

The synthesis of polysubstituted tetrahydrofurans was also achieved in a stereoselective manner by a formal [3+2] cycloaddition of an allylsilane with a-triethylsiloxy aldehydes. An example showing the mechanism is illustrated <02JA3608>. In another approach, allylsilane was also allowed to react with a-keto esters in a [3+2] annulation reaction, providing highly substituted tetrahydrofurans in good yields as single diastereomers <02OL2945>. 

The Lewis acid-promoted reaction of aldehydes with a-substituted allylsilanes forms 3-silyltetrahydrofurans in good to high yields.169-172 The use of homochiral allylsilanes is very valuable for highly diastereo- and enantio-selective syntheses of tri- and tetrasubstituted tetrahydrofurans (Equation (43)). Catalytic asymmetric [3-1-21-cycloaddition of a-substituted allenylsilanes to aldehydes can be achieved by a chiral scandium complex.173... 

Allylation adducts of ( )-Y-(dimethylphenylsilyl)allylboronate 47 selectively undergo addition reactions with aldehydes. Lewis acid catalyzed [3 + 2] cycloaddition reaction of these allylsilanes with various aldehydes, e.g., 55, provides tetrahydrofuran adducts with high diastereoselectivity. ... 

Cycloadditions have become a valuable means of generating tetrahydrofurans. They typically involve the condensation of an aldehyde with a 1,3-dipole in an overall [3+2]-cycloaddition process. With one exception, the focus of this section is on tetrahydrofuran formation from dipolar cycloadditions of either push-pull cyclopropanes or allylsilanes with aldehydes. 

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