A-Alkoxystannanes

Tributyl[( )-l-(methoxymethoxy)-2-butenyl]stannane (4) is available by the addition of tri-butyltin lithium to 2-butenal followed by treatment with chloromethoxymenthane 03. The resulting diastereomers were separated by chromatography to give stereochemically homogeneous a-alkoxystannanes 5 and 6104. 

To examine whether the breakdown in equilibration arose from a slow rate of equilibration or if the equilibrium does not favor the equatorial alkyllithium,the time course of equilibration was investigated (Scheme 34). The diastereomeri-cally pure a-alkoxystannanes 198 were prepared in analogy to Linderman s procedures 170], and their equilibrations were examined. Transmetallation and alkylation under kinetic control (entries 1 and 2) provided the expected equatorial and axial adducts, respectively, with no detectable minor isomer. Attempted equilibrations of the axial stannanes (entries 4 and 6) led to mixtures of axial and equatorial products. The equatorial stannane gave only the equatorial adduct under both equilibration conditions (entries 3 and 5). From these data, the authors concluded that the equilibration does take place under these conditions, and that the equilibrium lies essentially completely toward the equatorial alkyllithium. The observed product ratios in entries 4 and 6 can be attributed to a slow rate of equilibration. 

The exchange reactions of a-alkoxystannanes occur with retention of configuration at the carbon-metal bond.76... 

TABLE 19. HOMO level, energy values of several a-alkoxystannanes and silanes, with comparison to alkylmetals and to dimethyl... 

Electrooxidation of a-alkoxystannanes on a preparative scale can therefore be carried out at lower potentials, with cleavage of the C—Sn bond. Nucleophilic attack, e.g. by methanol, butanol or an amine on carbon at that a position, gives the product in high yields, 90-95% diether or 55% aminoether. A number of examples have been given ... 

The regioselectivity can be better controlled if a-alkoxystannanes are used as substrates. This modification is named the Wittig-Still Rearrangement . Here, the intermediate organolithium compound is produced through transmetallation ... 

Reactions of Mi4-anions with aldehydes resulted in facile 1,2-additions leading to a-metallated alcohols (equations 15716 24 and 1589). When the aldehyde was highly conjugated (equation 158), the nucleophilic addition did not occur and a SET reaction took place (see Section V.E). In the case of tin, the reaction was used for the synthesis of a-stannylalcohol (equation 159)4b and then applied to the preparation of a-alkoxystannanes (equations 160 and 161)104,171 and a -iodoslann anes1 °2.172. 

In 1980, Still discovered that transmetallation of chiral a-alkoxystannanes is stereospecific,25 and this result set the scene for the exploitatoin of chiral a-alkoxyorganolithiums. Not only are the organolithiums formed stereospecifically, but they are also configurationally stable over periods of at least minutes at -30 °C. Transmetallation of syn- and anti-38, quenching with acetone, gave clearly different products 40 each with no trace of the other. 

Table 2. Electrochemical Oxidation Potentials for 2-Substituted 1,3-Dithianes and a-Alkoxystannanes...

A number of structurally diverse ketones have been reduced using BINAL-H. Some of the results are summarized in Table 1. Aryl alkyl ketones, alkynic ketones, and a, -unsaturated ketones are reduced to alcohols with good to excellent % ee, while aliphatic ketones give products with lower optical purities. The asymmetric reduction of a number of acylstannanes with (7) gives synthetically valuable a-alkoxystannanes with high optical purities after protection of the initially formed unstable alcohols as their MOM or BOM ethers. ... 

The InCl3-promoted reaction of enantiomerically enriched a-alkoxystannanes 305 with achiral aldehydes produces a mixture of homoallylic alcohols with a high degree of relative diastereoselectivity and excellent enantioselectivity (Scheme 10-106). A plausible explanation first invokes the formation of indium reagent 306 produced via anti Se2 attack of lnCl3 on the a-alkoxystannane. Addition to the aldehyde can then occur via the chair-like transition structure xxxvii to afford the anti homoallylic alcohol preferentially. 

The scope of this transformation has been extended by employing enantiomerically pure aldehydes for the synthesis of the eight stereoisomeric hexoses [206]. The stereochemical outcome of these reactions is controlled by the configuration of the chiral a-alkoxystannane. 

Yamamoto has reported impressive results for the intra-molecular cyclizations of y-alkoxyallenylstannanes leading to five, six, and seven-membered ethers. " Oxidation of 347 provided an intermediate aldehyde 348 for in situ cyclization to the tranx-tetrahydrofuran 350 (Scheme 5.2.74, top). However, transmetalation of the a-alkoxystannane 351 with SnCU prodnced the Se trichlorostannane 352 which afforded the dx-tetrahydrofuran 354 via the closed transition state 353 (Scheme 5.2.74, bottom). 

Two related reports have appeared of new alcohol preparations involving synthetic equivalents of the umpolung species (3). One approach utilizes a-alkoxystannanes (4), readily available from carbonyl compounds (Scheme 4), to generate a-alkoxy-organolithiums which will react with electrophiles. In the... 

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