Silyl sulfines

The geometry of the silyl sulfines 30 was determined by LIS measurements. During chromatographic purification on silica gel, a partial protiodesilylation to the corresponding thioaldehydes 5-oxides was observed (vide infra). 

Protiodesilylation of thioacylsilane 5-oxides 30 (silyl sulfines) offers a unique mild synthetic path to thioaldehyde 5-oxides 40 (monosubstituted sulfines), which... 

SO2 Activation. At low temperatures, l-alkoxy-l,3-dienes add to sulfur dioxide activated by TBDMS triflate to generate a zwitterionic intermediate that can be quenched with silyl enol ethers to give, y-unsaturated silyl sulfinates. DesUylation and reaction with methyl iodide provide a stereoselective route to polyfunctional sulfones in an overall four-component coupling sequence (eq 50). ... 

TBDMS triflate also catalyzes the ene reaction of sulfur dioxide with silyl enol ethers. The silyl sulfinate intermediates can be halogenated to the corresponding sulfonyl halide, and subsequently trapped with an amine or alcohol to give sulfonamides or sulfonate esters, respectively (eq 51). ... 

The mechanism of the Pd-catalyzed desulfinylation of silyl sulfinate 162 into 163 has not been established yet. Probably the Pd(0) intermediate (reduction of Pd(OAc)2 by PhaP) undergoes C—S bond oxidative addition with formation of an (allyl)Pd-S02SiMe3 species that loses SO2, followed by protolysis with I -PrOH. This generates an intermediate of type (allyl)Pd-H that undergoes regio- and stereoselective (3-insertion of hydride to provide the final product. 

The lithio derivatives of dibenzofuran undergo the expected reactions. Carbonation yields carboxylic acids,reaction with sulfur dioxide yields sulfinic acids, and reaction with methyl sulfate yields methyl compounds. Alkylation can also be achieved by treatment of 4-lithio-dibenzofuran with alkyl halides. Silylation can be achieved with chlorotrimethyl- or chlorotriphenylsilane. " " Reaction of lithiodibenzo-furans with acetaldehyde and with benzonitrile take the expected course. 

In the sulfur literature only a few examples of thioaldehyde S-oxides 99 (mono-substituted sulfines) have been reported due to the low stability of these compounds, and they were not prepared by oxidative methods since the starting thioaldehydes are also very unstable. These problems were circumvented by using silyl thioketones249,250 as precursors which, by controlled oxidation with mCPBA, afforded the corresponding thioacylsilane S-oxides 98. These sulfines were subsequently desilylated with BU4NF (equation 104) and the stereochemistry of this process has been studied in detail408. This indirect route has been applied to the preparation of aromatic and aliphatic, not enethi-olizable, thioaldehyde S-oxides. 

Fig. 1.47. Myers synthesis of atkanes from aldehydes via the sulfonylated hydrazones B and their silyl derivatives C. This procedure provides a skeletongenerating three-step synthesis of alkanes from aldehydes. In terms of Figure 1.2, the chain reaction F —> I involved is a "substitution reaction by fragmentation." (The sulfonylated hydrazide anion, which here results from the addition of R3-Li to the sulfonyl hydrazone C, does—at this reaction s temperature of-78 °C—not release toluene sulfinate. Compare the different behavior of the sulfonylated hydrazide anion C in the reaction of Figure 17.69, where a mesitylene sulfinate residue is eliminated at a temperature that is higher by 160 °C).

A method that makes available aromatic and aliphatic aldehyde derived sulfin-imines 47, for the first time, was recently introduced by Davis and co-workers.23,36 This one-pot procedure entails treatment of the Andersen reagent 40 with LiHMDS to generate 44 which subsequently reacts with the lithium methoxide by-product to produce silyl sulfinamide anion 46. Reaction of 46 with the aldehyde in a Peterson-type olefination reaction affords the sulfinimine 47 in >96% ee. This method was highly effective for the preparation of arylidene sulfmamides 47 (R = aryl) which were usually obtained in 60-76% yield although the alkyl counterparts... 

Vinyl anion equivalents. After the derived sulfonyl anions react with electrophiles, elimination of the sulfonyl/silyl groups may be induced. With an epoxide as electrophile the treatment with NaH in refluxing hexane actually triggers a C/0-transsilylation to prelude expulsion of the sulfinate anion. 

An alternative method for incorporating a sulfone moiety into a carbon framework for the RBR involved an elegant four-component coupling between sulfur dioxide, silyl enol ethers 19 and 20, and iodide 21 (Scheme 8.71. The mechanism presumably comprises addition of 1-siloxy-1,3-diene 19 to sulfur dioxide, reaction of the resulting aldehyde with silyl enol ether 20, and then attack of the resulting sulfinate on the a ky iodide 21. The product 22, obtained as four diastereomers, was converted by the RBR to an intermediate that comprised a formal total synthesis of apoptolidin (see Section R.fiT... 

Peterson-type reaction of a-silyl carbanions with sulfur dioxide gives sulfines 268. In this case, a C=S double bond is formed with elimination of lithium silanolate (Scheme 2.160) [443, 444]. This method can be applied to the synthesis of phos-phorylated sulfines and a,/i-unsaturated sulfines [445, 446]. 

Scheme 2.161. Peterson-type reaction of a-silyl carbanions with sulfines giving the corresponding alkenes.

Robin and Huet have reported a general method for the synthesis of 8-lactones as weU as lactones with several ring sizes via oxidation of the corresponding lactol [87] (Scheme 43). Alkylation of sulfone 211 with bromoacetaldehyde dimethyl acetal, cleavage of the silyl ether, and treatment with acetic acid afforded lactol 212. Oxidation of lactol 212 with PCC furnished the corresponding saturated 8-lactone 213, which underwent elimination of sulfinic acid in the presence of DBU to provide lactone 214. 

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