Allylic stannyl ethers

Transmetallation can be employed in order to avoid the use of strongly basic conditions. One such variant is the [2,3]-Wittig-Still rearrangement wherein stannyl ethers can be converted to homoallylic alcohols. Several examples of this tranformation in the synthesis of amino acid components of bioactive polyoxins have been reported by Ghosh. In their synthesis of 5-0-carbomylpolyoxamic acid, a bioactive amino acid nucleoside, E and Z-allylic stannyl ethers, such as 45, derived from an isopropylidene L-threitol derivative, were subjected to the [2,3]-Wittig-Still rearrangement. 

The -allylic stannyl ether derivative gave better syn-diastereoselectivity (5.4 1) than the Z-isomer (2 1) this is most probably due to the competing electronically favoured and sterically favoured transition... 

On the other hand, the corresponding tin precursor (63) undergoes smooth cycloaddition with a wide variety of aldehydes to produce the desired methylene-tetrahydrofnran in good yields [32, 33]. Thus prenylaldehyde reacts with (63) to give cleanly the cycloadduct (64), whereas the reaction with the silyl precursor (1) yields only decomposition products (Scheme 2.20) [31]. This smooth cycloaddition is attributed to the improved reactivity of the stannyl ether (65) towards the 7t-allyl ligand. Although the reactions of (63) with aldehydes are quite robust, the use of a tin reagent as precursor for TMM presents drawbacks such as cost, stability, toxicity, and difficult purification of products. 

In 1980 Trost and Keinan reported on allylic alkylations of tin enolates such as 33 catalyzed by tetrakis(triphenylphosphine)palladium (equation 12). The stannyl ethers led to a rapid and clean monoaUcylation with high regioselectivity. Thereby, alkylation generally occurred at the less substituted end of the allyl moiety with formation... 

A stepwise Stille reaction enables the synthesis of unsymmetrical 3,3-diaryl-2-propen-l-ol derivatives. The first reaction occurs by replacement of the (Z)-substituted stannyl group of 3,3-bis(tributylstannyl)allyl methoxymethyl ether. ... 

SET photochemistry is involved in the reaction between the enones (371) and the a-stannyl ethers (372) in methanol. The products are the 3-sub-stituted cycloalkanes (373) which arise from addition of aryloxymethyl radicals to the enones. Irradiation (X > 400 nm) of the stannanes (374) in the presence of the ketones and aldehydes (375) affords two products identified as (376) and (377). The former of these is dominant and the reaction arises by an electron transfer from the stannane to the ketone. The resultant stannane radical-cation undergoes fission to yield an alkoxy allyl radical and the tin cation. The alkoxyalkyl radical adds to the carbonyl radical-anion with a preference for... 

As shown in the case of cinnamaldehyde, addition to the carbonyl functionality occurs in preference to attack at the electron-poor double bond. The extent to which stereogenic aldehydes can control the stereochemical course of these cycloadditions is discussed in Section 1.6.1.2.3.2. The success of the tin-based cocatalyst to induce reaction is explained in terms of the formation of an intermediate stannyl ether which exhibits greater reactivity towards 7t-allyl palladium cations than free alkoxide ions. The addition to ketones is less general and one of the more successful examples is given. 

Sigmatropic rearrangements. The Claisen rearrangement of allyl vinyl ethers is promoted by i-BUjAl. Furthermore, the carbonyl group of the products also undergoes reduction to furnish 4-alkenoIs. A stannyl group is lost in the products of distannyl- and silylstannyl-substituted allyl vinyl ethers when the rearrangement is conducted in the presence of i-BUjAL... 

The use of stannylene acetals and stannyl ethers is widely used for achieving selective benzylation, the selectivity paralleling that observed for allylation (see section 2.3.2, Allyl ethers). 

The importance of the base used in the [2,3]-Wittig reaction is made clear by the work of Mitchell and coworkers who have reported the rearrangements of silyl- and stannyl-substituted diallyl ethers. They found that the use of LDA on such substrates (38, 41, 43) leads to [2,3]-Wittig reaction while employing lithium diethylamide can result in an allyl-vinyl ether rearrangement instead. 

Cycloadditions of TMM to aldehydes allow the preparation of methylene tetrahydrofurans. The challenge was to overcome the poor nucleophilicity of the intermediate alkoxide, resulting from the Pd-TMM nucleophilic addition to the aldehyde. The latter must attack intramolecularly the r-allyl moiety to generate the cycloadduct and liberate the palladium catalyst for the next cycle. The solution is to add trialkyltin acetates (Me3SnOAc or BusSnOAc) to the reaction mixture. This leads to the formation of stannyl ethers that react readily with 7r-allylpalladium cation intermediates. MesSnOAc gives better results than other tin derivatives and only 5-10 mol % is necessary (eq 27). 

Stannylation of lithiated allyl ethers gives (Z)-3-alkoxyallylstannanes (1)115,116, whereas mixtures of (Z)- and ( )-tributyl(3-methoxy-2-propenyl)stannanes (2) were obtained from free-radical addition of tributyltin hydride to l-methoxy-l,2-propadienel16. 

Alkoxyallylstannanes can be generated in situ by stannylation of allyl ethers or by 1,3-isomerization of isomers, and trapped by boron trifluoride-diethyl ether complex induced addition to aldehydes to give syn-diol derivatives 13,120. 3-Alkylthioallylstannanes can similarly be generated and trapped84. 

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

The benzylidene derivative (94) has been converted into the alcohol (95) [97, 98] using the diborane — trimethylamine — aluminium chloride reagent [99] and into the diol (96) [95, 100, 101]. Veyrieres has converted methyl P-lactoside (and the corresponding allyl lactoside) into the 3-O-allyl ether (97) in good yield [102] by alkylation of the dibutylstannylene derivative in the presence of tetrabutylammonium iodide [103, 104], and this was converted into the alcohol (98) and the triol (99) [105]. Veyrieres [106] has also converted (97) into the per-p-bromobenzyl derivative and deallyl-ated the product to give a derivative with a free 3 -hydroxyl group. The diol (100) [107] has been converted by the stannylation procedure [108, 109] into the alcohol (101) [110, 111]. The partially acetylated benzyl P-lactoside (103) [101,112, 113] has been converted into the alcohol (104) via the orthoacetate [113]. 

Free radical additions of phenylthio or stannyl radicals to 2-alkenyl 2-siloxycyclo-propanes afford similar products although a completely different mechanism is operative 84). This direct generation of protected y-oxoesters 144 and 145 is of interest since the silyl enol ether function might be usable for regioselective C-C-bond formation and the allyl stannane moiety in 145 could be activated for subsequent transformations. Yet further examples have to demonstrate utility and scope of this mode of ring opening. 

Anodic oxidation of homo allyltrimethylsilylmethyl ethers 238 or homo allyl trimethyl-stannyl methyl ethers in the presence of tetrabutylammonium tetrafluoroborate results in the formation of fluorine- containing tetrahydropyrans 239249(equation 131). The process involves formation of a resonance stabilized carbocation and its intramolecular cycliza-tion by the participation of a neighboring vinyl group, followed by attack of fluoride ion. This process is a convenient way to form the C—F bond involving electrochemical steps. 

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