Allylic stannanes rearrangement

Upon treatment with a variety of mild Lewis acids, a-oxygenated allylic stannanes rearrange to the y isomers (Eq. 36) [56]. The process is stereospecific and highly regioselective. Thus (S)-a afford (5)-y-allylic stannanes, and vice versa, with essentially no loss of ee. 

Deoxygenation of allyttc alcohols.3 A method for conversion of allylic alcohols to l-alkenes is outlined in equation (I). The first step is an allylic rearrangement of an Oallyl xanthate to 2. The second step is an allyl transfer from sulfur to tin with tri-n-butyltin hydride to give the allylic stannane (3). The last step, destannylation, is a well-known mule to terminal alkcnes.4... 

This chemistry forms the basis of a general method for 1,3-hydroxy transposition in allylic alcohols (equation 7). The starting alcohol is converted by 3,3-sigmatropic rearrangement of the 0-allyl-S-methyldithiocarbonate followed by hydrostannolysis to the allylic stannane, which is oxidized by MCPBA in a completely regiospecific manner. A similar sequence has been reported for allylsilanes. "... 

Tin alkynes rearrange to yield allene products in much the same way as do lithium alkynes, except that the reaction involves a radical mechanism. It is very similar to the reaction of allyl stannanes with alkyl halides, which substitutes the allyl group. Similar reactions are reported for allyl derivatives of cobalt, rhodium and iridium, but this work has not been extended to alkyne derivatives. 

Allyl stannanes have also been prepared via Claisen rearrangement (Scheme 4.85). Ritter showed that the rearrangement of the vinyl stannane glyco-late proceeded through the expected Z-silyl ketene acetal to give the syn pentenoic acid in excellent diastereoselectivity [80]. 

Marshall has developed a collection of chiral a-alkoxy allyl stannanes 152 that provide facile access to various diastereomeric 1,2-diols (Scheme 5.26) [104-106]. For these reagents, Marshall has proposed that stereospecific rearrangement of 152 in the presence of Lewis acids gives the (Z)-y-alkoxy stannanes 153 prior to aldehyde addition. The transition state for the subsequent addition to the aldehyde is formulated as an open acyclic structure, leading to 1,2-syn-configured diols 154 (cf transition state 96) [107, 108]. 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Imidates, rearrangement of, 14, 1 Imines, additions of allyl, allenyl, propargyl stannanes, 64, 1 additions of cyanide, 70, 1 as dienophiles, 65, 2 synthesis, 70, 1 Iminium ions, 39, 2 65, 2 Imino Diels-Alder reactions, 65, 2 Indoles, by Nenitzescu reaction, 20, 3 by reaction with TosMIC, 57, 3 Ionic hydrogenation, 71, 1 Isocyanides, in the Passerini reaction, 65, 1... 

In the presence of Lewis acids allyl silanes and stannanes react with epoxides generally at the sterically less demanding carbon atom. Other electron-rich alkenes, such as ketene acetals, can also be used as nucleophiles. The strong Lewis acids required might, however, also lead to rearrangement of the epoxide before addition of the nucleophile can occur (last reaction, Scheme 4.72). 

Rhodium has also been found to catalyze regioselective acylations of allylic tin derivatives, with chlo-rotris(triphenylphosphine)rhodium(I) being particularly efficient in the terpenoid field. "- Again, the use of nonacidic conditions for stannanes offers advantages over classical conditions both in the elimination of skeletal rearrangement and in the observance of altered regioselectivity by comparison with acylations of silyl substrates (Scheme 28). However, it has not yet been firmly established that rhodium-catalyzed acylations always proceed without rearrangement. 

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