By Elimination Reactions

Piperazinediones which contain serine as a subunit sometimes eliminate water spontaneously in the course of their formation by ring closure 37). Picrorocellin (39A), which is a cyclic dipeptide containing 

Elimination reactions have more often been carried out with piperazine-diones containing suitable leaving groups (OH, acetoxy, SCH3) in positions 3 and/or 6. In some cases 102, 168) the hydroxy compound is not isolated, but loses water in situ. Starting compounds for elimination reactions are summarized in Table 2. 

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Preparation of Alkenes by Elimination Reactions of Alcohols and Alkyl Halides... 

Laboratory syntheses of conjugated dienes can be achieved by elimination reactions of unsaturated alcohols and alkyl halides In the two examples that follow the conjugated diene is produced m high yield even though an isolated diene is also possible... 

Bubble columns in series have been used to establish the same effective mix of plug-flow and back-mixing behavior required for Hquid-phase oxidation of cyclohexane, as obtained with staged reactors in series. WeU-mixed behavior has been established with both Hquid and air recycle. The choice of one bubble column reactor was motivated by the need to minimize sticky by-products that accumulated on the walls (93). Here, high air rate also increased conversion by eliminating reaction water from the reactor, thus illustrating that the choice of a reactor system need not always be based on compromise, and solutions to production and maintenance problems are complementary. Unlike the Hquid in most bubble columns, Hquid in this reactor was intentionally weU mixed. 

The stereospecific generation of enamines by -elimination reactions (187) and a vinylogous elimination, which leads to a dienamine (188), have been reported. The loss of an a substituent from a tertiary amine is seen in the generation of enamines by elimination of hydrogen cyanide from benzylic a-aminonitriles (189,190). 

Except for terpene chemistry, the Wagner-Meerwein rearrangement is of limited synthetic importance. It is rather found as an undesired side-reaction with other reactions, for example in the synthesis of alkenes by elimination reactions. 

Just as the chemistry of alkenes is dominated by addition reactions, the preparation of alkenes is dominated by elimination reactions. Additions and eliminations are, in many respects, two sides of the same coin. That is, an addition reaction might involve the addition of HBr or H20 to an alkene to form an alkyl halide or alcohol, whereas an elimination reaction might involve the loss of HBr or H20 from an alkyl halide or alcohol to form an alkene. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

The double bonds in alkenes can be generated by elimination reactions. 

G. Kdbrich and P. Buck, Synthesis of Acetylenes and Polyacetylenes by Elimination Reactions, in H. G. Viehe, Ed., Chemistry of Acetylenes, Marcel Dekker, New York, 1969, pp. 99-168. 

Fig. 4 (a) Chemical structure of tetra-ethyl-ortho-silicate (TEOS). (b) Removal of two oxygen atoms from TEOS. (c) Mechanism depicting how TEOS chemisorbs onto silanol groups (Si-OH) on surface, (d) Formation of Si-O-Si bridges by elimination reactions with neighboring molecules. (Figure adapted from [40])... 

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. 

Ketones can be prepared from special carbanions types by elimination reactions which follow oxygenation. 

Recently organosilicon compounds are being used for the synthesis of olefines by elimination reactions both in acidic and basic conditions. Thus P hydroxysilanes give defines. These reactions have been shown to be highly stereospecific. The acid catalysed elimination taking place by an anti pathway and the base induced elimination taking place by a syn pathway. 

Nucleophilic substitution reactions of halide anions in aprotic solvents are often accompanied by elimination reactions. For instance, reactions of secondary alkyl halides with potassium fluoride solubilized in acetonitrile with the aid of 18-crown-6 [3] give olefins as the main reaction product (Liotta and Harris, 1974). Similarly, the dicyclohexyl-18-crown-6 complex of potassium iodide acted exclusively as a base in its reaction with 2-bromo-octane in DMF (Sam and Simmons, 1974). The strongly basic character of weakly solvated fluoride has been exploited in peptide synthesis (Klausner and Chorev, 1977 Chorev and Klausner, 1976). It was shown that potassium fluoride solubilized... 

Reactions of potassium superoxide solubilized in apolar solvents with crown ethers (see Oxidation reactions, p. 356) are also frequently accompanied by elimination reactions. Thus, in DMSO solution, secondary alkyl bromides only yield olefins when treated with the K02 complex of dicyclohexyl-18-crown-6 (Johnson et al., 1978). Scully and Davis (1978) have studied the elimination of HC1 from N-chloramines with 18-crown-6-solubilized K02, KOH, and KOAc in ether solution (27). High yields of aldimines were obtained with K02,... 

The oxides of thiete and benzothiete have been prepared from halogenated thietane oxides by elimination reactions. (Eqs. 25 and 26)... 

Dioxins, 1,4-oxathiins, and 1,4-dithiins have often been prepared by elimination reactions from saturated analogs as described in CHEC-II(1996) <1996CHEC-II(6)447>. Since then, a synthesis of tetramethyl l,4-dithiin-2,3,5,6-tetracarboxylate 241 has been reported in low yield (12%) by thermal decomposition of the 1,4,2,5-dithiadiazine system 240 in refluxing o-dichlorobenzene in the presence of DMAD <1997J(P1)1157>. Recently, 2,6-divinyl-l,4-dithiin 68 has been isolated from the reaction of l,4-bis(4-bromobut-2-ynyloxy)benzene with an excess of alumina-supported sodium sulfide. The formation of 68 has been presumed to take place via cyclic sulfide 242 <2003S849>. 

Bicyclo[1.1.0]butanes 63 can also be prepared by elimination reactions of cyclobutane derivatives 62. As a result of the nonplanarity of a cyclobutane ring, Cl and C3 are only separated by a distance of approximately 2.1 A. This unique structural feature of cyclobutanes explains the remarkable ease with which they can be transformed into bicyclo[1.1.0]butanes. 

Table 2 summarizes the preparation of bicyclobutanes by elimination reactions.25-29-30... 

The emission spectrum is similar to fluorescence of 9,10-diphenyl-l,4-tiinethoxy anthracene although it is not excited directly. The decomposition of the endoperoxide to (1) and (3) is the excitation producing step in wtcatalyzed cleavage and (2) is activated by energy transfer. The formation of 102 by elimination reaction is a very minor reaction step. 

Pfitzner and Moffatt noted24 that a severe limitation existed for the synthesis of ketopentose nucleosides owing to their instability in certain media, which was attributed to loss of the nitrogenous base by elimination reactions.2... 

Synthesis of a, 3-Didehydro-a-amino Acids by Elimination Reactions... 

Substituted 2H- and AH- thiopyrans are largely confined to alkylated systems, and are usually derived by reduction of corresponding thiopyrylium salts, or by manipulation of dihydro systems either by elimination reactions or Pummerer reactions. The heteroatom substituted compounds which have been reported have usually been made by a [4 + 2] process such as that shown in equations (104) and (111) (71TL2241), and the range of examples is limited. 

Dioxins and 1,4-dithiins have often been prepared by elimination reactions of appropriately substituted saturated analogues. However, in the more recent literature a number of examples of syntheses of 1,4-dithiins are described which utilize reactions involving unsaturated precursors. Examples of 1,4-oxathiins are relatively scarce and no general preparative routes have been developed. 

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