Allylsilanes with acetals

Isoprenylation of carbonyl compounds. In the presence of TiCl, 1 can react as an allylsilane with acetals and acid chlorides (7, 370-371) to give isoprenylated compounds in generally satisfactory yields. Yields with aldehydes tend to be low. Isoprenylation of carbonyl groups is best effected with catalysis with tetrabutylammonium fluoride or io-dotrimethylsilane (10, 216). 

Cyanohydrins and homoallylic acetals. Reaction of MCjSiCN with aldehydes and allylsilanes with acetals are catalyzed by BiBrj at room temperature, whereas SbClj is completely ineffective. 

Catalyst for Reactions of Acetals with Silyl Enol Ethers and Allylsilanes. TMS-I catalyzes the condensation of silyl enol ethers with various acetals (eq 30) and imines, and of allylsilanes with acetals. ... 

The reaction of allylsilanes with acetals to give homoallylic ethers is usually conducted with stoicheiometric amounts of Lewis acids it has now been shown that a catalytic quantity of iodotrimethylsilane is equally effective. ... 

Homoallylk ethers. lodotrimethylsilane catalyzes the allylation of acetals and ketals by allylsilanes, with transposition of the allylic group (equation I). It does not catalyze allylation of aldehydes and ketones. Note that TiCI can catalyze both of these reactions (7, 370-371). In this respect, ISi(CH3)3 resembles (CH3)3SiOTf (this volume). 

Siloxyallyl)silane 24, acting as a synthetic equivalent of acetone a,a -dianion, readily undergoes double alkylation (equation 15)60. An acid-catalyzed transacetalization-ring closure reaction occurs in the reaction of 6-hydroxy substituted allylsilanes 25 with acetals to afford the corresponding trisubstituted tetrahydropyrans in moderate to good yield with high diastereoselectivity (equation 16)61. 

More recently, allylic carbonates 115 have been found to couple with alkenylfluo-rosilanes in the presence of a palladium catalyst to yield the corresponding 1,4-dienes (equation 93)174. The corresponding palladium-catalyzed reaction of allylsilane with allyl acetate 116 has been explored (equation 94)175. 

Photooxygenation of vinylsilane in the presence of acetic anhydride and pyridine furnishes a convenient procedure for the synthesis of a-trimethylsilylenones 185 in moderate to good yields (equation 156)278. Dye-sensitized photooxidations of allylsilane with singlet oxygen produces a mixture of (E)- and (Z)-3-trimethyl-silylallyl hydroperoxides 186 (equation 157)279. 

Sakurai et al. reported the condensation of allylsilane 1 with acetals [8], leading to the preparation of homoallylic ethers 15 (Scheme 13.6). The reaction occurs at —78 °C, in dichloromethane. The yields are usually excellent, even though the condensation is slower than with aldehydes and ketones. 

Another approach towards diastereo- and enantio-enriched homoalyllic ethers was used by Panek et al. during their synthesis of kabiramide C [18]. In this case, optically pure allylsilane 35 was allowed to react with acetal 36, forming the expected syn ether 37 with moderate diastereocontrol (Scheme 13.14). 

In the presence of BF3-OEt2, allyldimethylsilyl enolates react with acetals to give allylated 1,3-diol monoethers.139 A similar tandem aldol-allylation reaction can be performed by strained allylsilyl enolates such as 26 without promoter (Equation (34)).140 Allylsilanes bearing a vinyl ether moiety at the /3-position give rise to another type of tandem aldol-allylation reaction in the presence of a Lewis acid.141... 

The reaction of an allylsilane with an allyl acetate in the presence of TBAF and a palladium catalyst is reported to give a 1,5-diene (Eq. 37) [32]. 

A chiral carbon present in the alcohol portion of acetals might control the stereochemistry of the allylation. A few examples conducted in the presence of a titanium Lewis acid are shown in Eqs (82) [225], (83) [226], and (84) [227]. In contrast, an allylsilane with a chiral auxiliary derived from arabinose on the silicon atom has been used for asymmetric synthesis, although diastereoselectivity was low [228],... 

Allylations. Catalyzed allylation of a-iodo acetals with leaving group dependence on the Lewis acid has been reported. Formation of N-tosyl allylamine by reaction of an allylsilane with (A -tosylimino)phenyliodinane is promoted by Cu(OTf)j. Halide displacement of a polyhalide" occurs in the presence of CuCl. 

Alkylation + allylation. A carbon chain is constructed from an enol silyl ether (including ketene silyl acetal,. ..) with an alkylating agent and an allylsilane with catalysis of InBr3 in CH2CI2 at room temperature. 

The allylsilane 32 was used to prepare dienones via the tin tetrachloride catalyzed condensation with acetals or aldehydes followed by an elimination reaction81 (equation 76). Interestingly, when the condensation is catalyzed with fluoride ion only the a-adduct is observed (equation 77). 

Allylation. The reaction of allylsilanes with carbonyl compounds, acetals, and ortho esters is catalyzed by Cp2Ti(OTf)2. Rapid reaction rates and good conversion... 

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