Sulfon-ylide

Various other routes have been reported to give the 1,4-benzothiazine ring, some of which are summarized below. Synthesis of the 1,4-benzothiazine 1,1-dioxides (105) from benzaldimine and methylene sulfone ylide has been reported.147... 

The sulfonated ylide nickel complex 4 (see structures 1—4, reactions 64A) activated with Et2AlOEt co-oligomerizes ethylene with higher olefins, for example, 1-hexene, 1-heptene, 1-octene, and 1-decene. Olefin insertion into the Ni—H bond is nonselective. The chain propagation step, a-olefin insertion into the nickel-alkyl bond, is also nonselective. The Ni—C bond reactivity in the olefin insertion reaction depends on the structure of an alkyl group bonded to the nickel ion. 

The Julia-Lythgoc olefination operates by addition of alkyl sulfone anions to carbonyl compounds and subsequent reductive deoxysulfonation (P. Kocienski, 1985). In comparison with the Wittig reaction, it has several advantages sulfones are often more readily available than phosphorus ylides, and it was often successful when the Wittig olefination failed. The elimination step yields exclusively or predominantly the more stable trans olefin stereoisomer. 

There have been two general approaches to the direct asymmetric epoxidation of carbonyl-containing compounds (Scheme 1.2) ylide-mediated epoxidation for the construction of aryl and vinyl epoxides, and a-halo enolate epoxidation (Darzens reaction) for the construction of epoxy esters, acids, amides, and sulfones. 

Oxidative cleavage of oxosulfonium ylides as well as of sulfoximines leads to sulfone formation. In the course of oxidations of dialkoxy sulfuranes(IV) by hydrogen peroxide" or t-butyl hydroperoxide , sulfone formation takes place (equation 99). 

Aldol addition and related reactions of enolates and enolate equivalents are the subject of the first part of Chapter 2. These reactions provide powerful methods for controlling the stereochemistry in reactions that form hydroxyl- and methyl-substituted structures, such as those found in many antibiotics. We will see how the choice of the nucleophile, the other reagents (such as Lewis acids), and adjustment of reaction conditions can be used to control stereochemistry. We discuss the role of open, cyclic, and chelated transition structures in determining stereochemistry, and will also see how chiral auxiliaries and chiral catalysts can control the enantiose-lectivity of these reactions. Intramolecular aldol reactions, including the Robinson annulation are discussed. Other reactions included in Chapter 2 include Mannich, carbon acylation, and olefination reactions. The reactivity of other carbon nucleophiles including phosphonium ylides, phosphonate carbanions, sulfone anions, sulfonium ylides, and sulfoxonium ylides are also considered. 

Among the olefination reactions, those of phosphonium ylides, phosphonate anions, silylmethyl anions, and sulfone anions are discussed. This chapter also includes a section on conjugate addition of carbon nucleophiles to a, (J-unsaturated carbonyl compounds. The reactions in this chapter are among the most important and general of the carbon-carbon bond-forming reactions. 

Oxidation of sulfur atom of pyrazolo[l,5-c]thiazole 64 into sulfoxide 65 followed by Pummerer-type dehydration furnished the transient nonclassical pyrazolo[ 1,5-c]thiazole, the thiocarbonyl ylide 67, which could react with various dipolarophiles such as Ar-pheny 1 ma 1 eiinide (Equations 27 and 28) <2000T10011>. In an excess of oxidizing agent, pyrazolo[l,5-c]thiazole 64 was readily converted to sulfone 66 (Equation 27) <2001J(P1)1795>. 

A series of analogous py rrolo[ 2,1 -c [ 1,4]oxazine-8-carboxy latcs 188 and 189 (Scheme 28) were obtained by cycloaddition of azomethine ylide 187 with dipolarophiles. The ylide was formed by /(-toluene sulfonic acid-mediated reaction of the benzotriazolyl chiral morpholinone 186, which can be considered as a stable crystalline azomethine ylide precursor <2001SL1841>. This procedure was applied also to morpholinone 190 that generated ylide 191 by reaction with... 

Sulfonium ylides R2S=CR 2 [672,673] and metallated sulfones [674-676] can cyclopropanate simple alkenes upon catalysis with copper and nickel complexes (Table 3.6). Because of the increased nucleophilicity and basicity of these ylides, compared with diazoalkanes, these reagents are prone to numerous side-reactions,... 

The established mechanism of sulfide photo-oxidation in solution invokes the novel formation of two intermediates a persulfoxide. A, and a hydroperoxy sulfonium ylide, B, (Fig. 13A) [25], In this mechanism the sulfide substrate intercepts the second intermediate, k o, and does not competitively inhibit the predominant sulfone forming pathway, kso2. As a consequence, the sulfone/sulfoxide ratio is independent of sulfide concentration. In contrast, the results in the zeolite are inconsistent with this mechanism but are consistent with the trapping of a single intermediate with both sulfide and adventitious sulfoxide (Fig. 13B). 

The second class of benzo-fused heterocycles accessible from benzofuroxans are benzimidazole oxides. In this case only one carbon from the co-reactant is incorporated in the product. With primary nitroalkanes 2-substituted l-hydroxybenzimidazole-3-oxides (46) are formed via displacement of nitrite, and / -sulfones behave similarly. The nitrile group of a-cyanoacetamides is likewise eliminated to alford 2-amide derivatives (46 R = CONRjX and the corresponding esters are formed in addition to the expected quinoxaline dioxides from acetoacetate esters. Under similar conditions secondary nitroalkyl compounds afford 2,2-disubstituted 2//-benzimidazole-1,3-dioxides (47). Benzimidazoles can also result from reaction of benzofuroxans with phosphorus ylides <86T3631>, nitrones (85H(23)1625>, and diazo compounds <75TL3577>. 

These compounds come from methylenation reactions of the corresponding carbonyl derivative by means of an ylide. Several experimental conditions have been described. In most cases, CF2Br2 and HMPT (hexamethyl phosphorotriamide) are employed. The reaction occurs with aldehydes as well as with ketones in the furanose and pyranose series. The reaction can also be performed with lactones the fluoromethyl group is then introduced in the anomeric position. With these substrates, the Julia olefmation, which uses difluoromethyl sulfone, has also been reported to be an efficient method. Some examples of these reactions are shown in Figure 6.24. 

Chiral exocyclic alkenes such as 112, also having the chiral center two bonds away from the reacting alkene moiety, have been used in highly diastereoselective reactions with azomethine ylides, and have been used as the key reaction for the asymmetric synthesis of (5)-(—)-cucurbitine (Scheme 12.37) (169). The aryl sulfone 113 was used in a 1,3-dipolar cycloaddition reaction with acyclic nitrones. In 113, the chiral center is located four bonds apart from alkene, and as a result, only moderate diastereoselectivities of 36-56% de were obtained in these reactions (170). 

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