Sulfur ylides. rearrangement

The allylic position of the sulfide group in the cycloadducts may be employed for 2,3-sigmatropic reactions, as demonstrated by the alkenation/sulfur ylide rearrangement sequence (144) (145). A... 

Cyclization of the diazo compound (108) with a copper catalyst affords the clavulanic acid derivatives (110) and (111), possibly via rearrangement of the sulfur ylide (109) (80H(14)1999). Similar reactions have been reported in the recent literature (80H(14)1967, 81H(16)1305, 80TL31). 

The major limitation of asymmetric sulfur ylide epoxidations is that only aromatic vinylepoxides can be formed efficiently and with high selectivity. When an aliphatic aldehyde is allowed to react with a semistabilized or nonstabilized sulfur ylide, poor diastereoselectivities and yields are observed, due to problems in controlling the ylide conformation and competing ylide rearrangement reactions [71]. However, some racemic, aliphatic vinylepoxides have been successfully formed by sulfur ylide epoxidations, although varying diastereoselectivities were observed [78-80],... 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Sulfur ylides can also transfer substituted methylene units, such as isopropylidene (Entries 10 and 11) or cyclopropylidene (Entries 12 and 13). The oxaspiropentanes formed by reaction of aldehydes and ketones with diphenylsulfonium cyclopropylide are useful intermediates in a number of transformations such as acid-catalyzed rearrangement to cyclobutanones.285... 

Olefins analogous to 158 and 159 were also isolated from the CuS04-catalyzed decomposition of ethyl diazoacetate in the presence of 2-isopropenyl-2-methyl-1,3-dithiane (total yield 56%, E Z — 4 1) a butadiene was absent from the reaction mixture 161). With dimethyl diazomalonate instead of ethyl diazoacetate, only the Z-olefin resulting from a [2,3]-sigmatropic rearrangement of the corresponding sulfur ylide was obtained in 36 % yield 161). When the same procedure was applied to... 

Intramolecular sulfur ylide formation and subsequent [2,3]-sigmatropic rearrangement has been utilized in construction of ring systems, as demonstrated by the total synthesis of (+)-/3-elemenone 118 and (+)-eleman-8/ , 12-olide 117 (Scheme a-diazo (3-kcto esters 113 as the starting material, the core structure is formed... 

Sulfur ylides containing a benzylic group (analogous to 19) undergo an analogous rearrangement.254... 

Boekelheide and his collaborators [407] have described a two-step sequence for transforming sulfide linkages to carbon-carbon double bonds — Stevens rearrangement of sulfur ylides and Hofmann elimination — which they found particularly useful for the synthesis of cyclophane derivatives, such as the [2.2]metaparacyclophane-l,9-diene shown. The Ramberg-Backlund rearrangement (see Section 4.3.2) was unsatisfactory for such highly strained molecules. 

We have mentioned previously as a stable ylide dimethyl sulfonium fluorenide (see Section 2.9). It yielded the first example (1938) of a [2,3] sigmatropic rearrangement of a sulfur ylide [505, 506] (Sommelet-Hauser rearrangement). 

Ketocarbenes (1) are usually generated from the corresponding diazo compounds (3).s Other sources which are occasionally used are a,a-dibromo compounds (4),9 sulfur ylides (5)10 and iodonium ylides (6 Scheme 2).11 The thermal or photochemical decomposition of diazo compounds in the presence of ir-systems is often complicated by indiscriminate side reactions, such as Wolff rearrangements,12 C—H insertions and hydride migrations. To avoid such problems, the use of metal-catalyzed decomposition of diazo compounds is generally preferred.1 2... 

The intermediacy of such oxaspiropentanes has been proposed in the addition of diazomethane to ketonesi0) and in the reaction of dimethyloxosulfonium methylide with a-haloketones55). In contrast to phosphorous ylides, sulfur ylides usually condense with carbonyl compounds to yield epoxides, thus reaction of the N,N-dimethylaminophenyloxosulfonium cyclopropylide 99 with cyclohexanone produced the dispiroepoxide 100 which rearranged to the spiro [3.5] nonan-l-one 101 upon isolation by gas chromatography, Eq. (29) S6). 

Sulfur ylides can undergo a [2.3] sigmatropic rearrangement which has been very useful in organic synthesis. Scheme V/16 shows some fundamental applications of this reaction. 

A third mechanism is also proposed in which ion pairs are formed instead of radical pairs. A variant of the Stevens rearrangement is the rearrangement of sulfur ylides. 

Sulfur ylides undergo a number of rearrangements such as [2,3]-sigmatropic rearrangement , as shown in Scheme 3.30. 

In sharp contrast to the well-studied [2,3]sigmatropic rearrangement of allylic sulfur ylides [42], the study of allylic selenium ylides is quite limited [43]. Furthermore, its asymmetric version using optically active allylic selenium ylides has not been developed until quite recently. In this section, some examples of the asymmetric [2,3]sigmatropic rearrangement via optically active allylic selenium ylides are described. 

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