Elimination reactions organic synthesis with

Nitroalkenes with potential leaving groups in (3-position such as a dialkylamino, an alkylthio, or a phenylsulfonyl group undergo addition-elimination reactions with nucleophiles. The chemistry of nitroenamines has been extensively investigated, and their potential utility in organic synthesis has been well established.2613 116 Severin and coworkers have developed the addition of elimination reactions of nitroenamines with carbon nucleophiles in 1960-1970, as exemplified in Eq. 4.94.117... 

Functionalized nitroalkenes are important chenophiles in the Diels-Alder ri example, fE -methyl fi-nitroactylate is an impottant reagent in organic synthesis The nitre group can be readily eliminated the Diels-Alder reaction of fi-nitroactylate is equivalent to that of ethyl propiolate with an inverse regiochemistry fEq. 8.4. ... 

The complete elimination of functional groups is often an undesirable side reaction in organic synthesis, but on the other hand it is a possibility for the recycling of environmentally harmful compounds, for example phenols and haloarenes such as polychlorinated dibenzodioxins (PCDDs or dioxins ). For example, aryl chlorides can be effectively dechlorinated with Pd(0) NPs in tetra-butylammonium salts with almost quantitative conversions also after 19 runs (entry H, Table 1.4) [96]. On the other hand, a C-0 bond cleavage reaction also seems suitable for the fragmentation of sugar-based biomass such as cellulose or cello-biose in that way, sugar monomers and bioalcohol can be derived from renewable resources (entry F, Table 1.4) [164]. 

Nitration of acetylenes with nitryl iodide followed by elimination of HI gives nitroacetylenes, but nitroacetylenes are too thermally unstable to be useful for organic synthesis.77 Recently, nitro-trimethylsilyl-acetylenes are prepared as stable nitroacetylenes by the reaction of bis(trimethylsily)acetylene with nitronium tetrafluoroborate (Eq. 2.39).78... 

Nitroalkenes react with lithium dianions of carboxylic acids or with hthium enolates at -100 °C, and subsequent treatment of the Michael adducts with aqueous acid gives y-keto acids or esters in a one-pot operation, respectively (Eq. 4.52).66 The sequence of Michael addition to nitroalkenes and Nef reaction (Section 6.1) provides a useful tool for organic synthesis. For example, the addition of carbanions derived from sulfones to nitroalkenes followed by the Nef reaction and elimination of the sulfonyl group gives a,P-unsaturated ketones (Eq. 4.53).67... 

Evidently, this approach is not limited to the formation of nitronates, nitroso acetals or enoximes. The rearrangements of these compounds by elimination reactions, the trapping of intermediates and finally their reactions with various reagents are of equal importance. It should be emphasized that silylation of AN as a process in organic chemistry is characterized by an unrivalled completeness and diversity of transformations. Hence, the silylation can be considered as a separate field of application of AN in organic synthesis. 

Simple dienes react readily with good dienophiles in Diels-Alder reactions. Functionalized dienes are also important in organic synthesis. One example which illustrates the versatility of such reagents is l-methoxy-3-trimethylsilyloxy-1,3-butadiene (.Danishefsky s diene) 1 Its Diels-Alder adducts are trimethylsilyl enol ethers which can be readily hydrolyzed to ketones. The /j-mcthoxy group is often eliminated during hydrolysis. 

Chiral alkenyl and cycloalkenyl oxiranes are valuable intermediates in organic synthesis [38]. Their asymmetric synthesis has been accomplished by several methods, including the epoxidation of allyl alcohols in combination with an oxidation and olefination [39a], the epoxidation of dienes [39b,c], the chloroallylation of aldehydes in combination with a 1,2-elimination [39f-h], and the reaction of S-ylides with aldehydes [39i]. Although these methods are efficient for the synthesis of alkenyl oxiranes, they are not well suited for cycloalkenyl oxiranes of the 56 type (Scheme 1.3.21). Therefore we had developed an interest in the asymmetric synthesis of the cycloalkenyl oxiranes 56 from the sulfonimidoyl-substituted homoallyl alcohols 7. It was speculated that the allylic sulfoximine group of 7 could be stereoselectively replaced by a Cl atom with formation of corresponding chlorohydrins 55 which upon base treatment should give the cycloalkenyl oxiranes 56. The feasibility of a Cl substitution of the sulfoximine group had been shown previously in the case of S-alkyl sulfoximines [40]. 

Tertiary aliphatic alcohol linkers have only occasionally been used in solid-phase organic synthesis [73], This might be because of the vigorous conditions required for their acylation. Esterification of resin-bound linker 4 with /V-Fmoc-prolinc [72,74] could not be achieved with the symmetric anhydride in the presence of DMAP (20 h), but required the use of /V-Fmoc-prolyl chloride (10-40% pyridine in DCM, 25 °C, 10-20 h [72]). A further problem with these linkers is that they can undergo elimination, a side reaction that cannot occur with benzyl or trityl linkers. Hence, for most applications in which a nucleophile-resistant linker for carboxylic acids is needed, 2-chlorotri-tyl- or 4-acyltrityl esters will probably be a better choice than ferf-alkyl esters. 

Reactions of alkynyliodonium salts 119 with nucleophiles proceed via an addition-elimination mechanism involving alkylidenecarbenes 120 as key intermediates. Depending on the structure of the alkynyliodonium salt, specific reaction conditions, and the nucleophile employed, this process can lead to a substituted alkyne 121 due to the carbene rearrangement, or to a cyclic product 122 via intramolecular 1,5-carbene insertion (Scheme 50). Both of these reaction pathways have been widely utilized as a synthetic tool for the formation of new C-C bonds. In addition, the transition metal mediated cross-coupling reactions of alkynyliodonium salts are increasingly used in organic synthesis. 

Wittig olefination reaction ( the phosphorus way ) has been a very popular reaction in organic synthesis. However, it is now in competition with Peterson/Chan olefination reaction327 ( the silicon way ). Formally, this latter involves the formation of a (3-silyl heteroatomic anion, which in the absence of an electrophile undergoes a (3-shift of the silyl moiety to the heteroatom (usually oxygen) with final elimination of silylated heteroatomic anion and formation of the olefin. 

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