Allylsilane-Aldehyde Cycloadditions

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. 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

The class of 3-silyl-substituted reagents provides, upon addition with aldehydes, allylic silanes that offer many options for further derivatization. Oxidative processes are described in previous sections (see the sections on Preparation of 1,2-Diols and 1,4-Diols). If the appropriate silicon substituents are chosen, formal [3+2] cycloadditions with aldehydes can be promoted under Lewis acid catalysis. For example, the mismatched addition of the Z-3-propyl-3-benzhydryldimethyl allylsilane 183 to an a-benzyloxy aldehyde proceeds with low diastereofacial selectivity in favor of product 184 however, after protection of the secondary alcohol, an efficient [3+2] annulation provides the polysubsubstituted furan 185 in good yield and acceptable stereoselectivity (Scheme 24). ° The latter is brought forward to a tricyclic unit found in the antitumor natural product angelmicin B. 

Another approach leading to pyrrolidine 210 via [3 + 2] cycloaddition is the reaction of A-protected a-amino aldehydes with allylsilanes in the presence of a catalytic amount of BF3 OEt2 (equation 172). No 1,2-silyl group migration occurs in these annulation processes313. 

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... 

The [2 + 2]-cycloaddition of allylsilanes is applicable to the synthesis of substituted oxetanes from aldehydes and ketoesters (Equation (49)).176,193 The BF3-OEt2-promoted reaction of iV-tosylaldimines with allylsilanes gives [3 + 2]-cycloadducts, 3-silylpyrrolidines (vide supra).178 In contrast, iV-acylaldimines are converted into [2 + 21-cycloadducts under similar conditions.194... 

The BF3-catalyzed reaction of a-aminoaldehydes with 10 is valuable for highly stereoselective synthesis of 2,3,5-trisubstituted pyrrolidines with all -cis configurations (Equation (50)).197 The stereochemical outcome like chelation-controlled stereochemistry might result from the inherent conformational arrangement of the aldehyde-BF3 complex. />-Quinoneimines, o-quinones, and a-alkoxyhydroperoxides undergo similar types of [3 + 2]-cycloadditions with allylsilanes to afford dihydro indoles, dihydrobenzofurans, and 1,2-dioxolanes, respectively.164,175,198... 

Three modes of reaction of aldehydes with allylsilane bearing sterically demanding silyl substituents are mediated by the proper choice of Lewis acid (Eq. 39) [66a]. Thus, influenced by SnC, allyl-t-butyldimethylsilane reacts with aldehyde in 2 1 stoichiometry to afford a ketone derivative. In contrast, use of BF3 OEt2 leads to the formation of a 1,3-dioxane derivative, which is a 1 2 adduct. Furthermore, ZrCU-pro-moted [2 -r 2] cycloaddition of allylsilane and aldehyde fnmishes oxetanes in good yields [66b]. 

The [3+2] cycloaddition of allylsilanes is applicable to imines and iminium salts [419, 484, 485], Highly substituted pyrrolidines can be synthesized with high diastereo and enantio control by reaction of homochiral crotylsilanes with carbon-nitrogen double bond generated in situ from acetals and methyl carbamate (Scheme 10.174) [419]. The cycloaddition to N-tosylaldimines of aromatic aldehydes proceeds with excellent 2,4-ds selectivity whereas the stereoselectivity with aliphatic aldimines is rather low [484]. With N-tosylaldimines, fhe formation of [2+2] adducts is not observed (vide infra). 

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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