Stannyl ethers

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. 

Remarkably, the addition of only 5-10 mol% of Me3SnOAc to the reactions of the sOyl precursor (1) with aldehydes also cleanly produce the cycloadducts in excellent yields [31]. It then appears that the capping of the alkoxide (61) to form the stannyl ether in situ is efficient enough that only a catalytic amount of Me3SnOAc is sufficient to facilitate reaction. This tin-ejfecC greatly enhances the... 

An oxidative Prins cyclisation of the allylsilane-tethered a-stannyl ethers offers a route to tetrahydropyrans avoiding the need for catalysis by a Lewis acid (Scheme 17) . 

The electrolysis of unsaturated a-tributylstannyl ethers 12 yields cyclic compounds by a similar reaction, involving in this case carbon-carbon bond formation. Here, too, the evidence points to a facile ET with formation of an a-stannyl ether radical cation. This in turn cleaves to the tributylstannyl radical and an unsaturated ene-oxonium residue. 

A bromide was introduced in the reaction instead of a fluoride in performing the anodic oxidation of a-stannyl ethers in dibromomethane solvent with tetrabutyl-ammonium perchlorate as the electrolyte (Scheme 19) [28]. The bromide ion was generated by the reduction of the solvent at the cathode of an undivided cell. 

While most of the examples studied employed a stannyl group as the group 14 metal, the use of an a-silyl ether was also examined. In this case, the same cyclic product was formed as in the corresponding a-stannyl ether substrate. However, the yield of product obtained was higher when the tin based starting material was employed (83% vs 66%). 

Finally, the reactions were examined in order to determine their compatibility with the initiation of Friedel-Crafts cyclizations (Scheme 19, Eq. 2). Both the use of an a-stannyl ether and an a-stannyl amide substrates led to cyclized product. 

Oxygen-Substituted Organometallic Compounds From a-Stannyl Ethers... 

The enantiomerically pure a-stannyl ether, (R)-l-benzyloxymethoxy-l-tributylstannyl-propane, can be obtained by resolution of the precursor compound. The tin - lithium exchange reaction, as well as the electrophilic substitution, occurred with retention of configuration to give (I )-2-(benzyloxymethoxy) butane only28. A later study examined more examples and also confirmed this result27. 

The most general way to obtain chiral a-stannylated ethers today consists of the asymmetric reduction of acylstannanes34,35,36 using the 2,2 -dihydroxy-l,T-binaphthyl-modified lithium aluminum hydride (BINAL-H) reagent37 and etherification of the crude alcohol with chloro-methoxymethane. 

Transmetalation of the a-stannylated ethers to the lithium compound and methylation using dimethyl sulfate occurs with high stereoselectivity (80-90% ee)35,36. 

These reactions are discussed elsewhere in this volume. See Section D.l.1.1.1.1.1.1. From a-Stannyl Ethers. 

Selectivity can be an overriding commodity in cases where reactivity is dictated by logic and accepted concepts. Such is the case with stannylene acetals of diols and trialkyl-stannyl ethers of alcohols. Enhanced nucleophilicity of oxygen attached to tin and well-documented stereoelectrorric effects associated with methine carbon atoms of trialkyhin ethers lead to remarkably selective reactions of (7-substitution and oxidation in polyhydroxy compounds. 

Photochemical aryloxymethylation of enones was expected to proceed by employing a-stannyl ethers in a similar fashion to stannyl sulfides. During our study, Steckhan and co-workers reported the alkoxymethylation of electron-deficient olefins with a-silyl ethers in the presence of sensitizers. On irradiation of a mixture of a-silyl ether 70 and strongly electron deficient alkene 71 in the presence of DCA and BP as sensitizers, alkoxymethyl product 72 was obtained (Scheme 29). This reaction mechanism is explained in a similar manner as mentioned in the reaction of a-silyl carbamates 57 (Scheme 24). 

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... 

Enol siiyl ethers undergo Pd-catalyzed coupling with aromatic bromides in the presence of tributyltin fluoride, which converts the enol silyl ethers into the stannyl ethers or a-stannyl ketones regarded as real active species chemoselective a-arylation of terminal ketones is possible (equation 110). ... 

Aldol reaction. The jyn-selectivity for the condensation of cyclohexenyl tributyl-stannyl ether with benzaldehyde is the best among various triflates M(OTf) (M= Ag, Cu, Zn, Sn, Y, Sc, and TMS). Stannyl enol ethers seem to be more reactive than the corresponding silyl enol ethers as donors for aldol reactions, at least in a system catalyzed by a complexed CuCOTOj. 

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