Group-elimination

A variety of ring syntheses have been devized which depend on carbanion addition to an activated double bond. The examples depicted in Scheme 74 illustrate the use of inter alia cyano and nitro groups which are subsequently eliminated. In appropriate instances the inclusion of additional eliminable groups ensures the formation of fully aromatized products. 

Very simple compounds such as alkyl-substituted tetrahydrofurans can be dehydrogenated by chlorosulfonic acid or by oleum, these reagents having some capacity for oxidation, and furylium ions are produced (Section III,B), but the technique has not been exploited synthetically.19 The presence of an easily eliminated group helps indeed, the terpenoid furan solidagenone might well be an artefact formed during isolation from an unstable hydrofuran... 

Enzymes of this group catalyze elimination of y substituents from amino acids as illustrated in Fig. 14-7. Eliminated groups may be replaced by other substituents, either in the a or the P positions. The ketimine formed initially by such an enzyme (step a) undergoes elimination of the y substituent (P with respect to the C = N group) along with a proton from the P position of the original amino acid to form an... 

Various aspects of this reaction have been summarized by Kocienski.2 It is particularly useful for synthesis of disubstituted alkenes and conjugated dienes and trienes. It fails with some trisubstituted alkenes and most tetrasubstituted. alkenes because the precursors are unstable. One advantage of this route is that rra/is-alkenes are formed preferentially or exclusively. Yields are highest when the eliminated groups can adopt a /rons-coplanar arrangement. 

A planar, anti arrangement of the eliminated groups (H and L) is preferred. The reaction rate depends on the concentration of both the base (B) and the substrate. This mechanism can be important regardless of whether L is attached to a primary, secondary, or tertiary carbon. 

The main method of producing 1,2-benzisoxazoles with the nitro group in the arylene fragment of the molecule is intermolecular condensation in an alkaline medium of the corresponding oxymes containing an easily eliminated group in the ortho-position (Scheme 2.60) [119, 298, 415-424],... 

Regiochemistry of enamide photocyclization in this synthesis can also be controlled by the use of an ortho substituent that acts as an eliminating group on cyclization to the root of the substituent. Thus, the omethoxyl and bromo groups are used for this regioselective cyclization as exemplified by a number of total syntheses of alkaloids (19,20) (Scheme 37). 

Protection of aldoses at the non-anomeric positions makes it possible to use many of the common procedures in organic chemistry for oxidizing lactols as shown with mannofura-nose 1 and glucopyranose 3 (O Table 1). The reactions can be divided into three main categories oxidations mediated by activated dimethyl sulfoxide (DMSO), oxidations with chromi-um(VI) oxides, and oxidations catalyzed by ruthenium oxides. The DMSO-mediated oxidations of alcohols can be promoted by several activators [27]. With the partially protected aldoses the activation has mainly been achieved with acetic anhydride and oxalyl chloride. Competing /3-elimination does usually not occur unless the eliminating group is an ester, e. g., an acetate or a benzoate [27]. 

Alkenes are formed by the thermal decomposition of esters, xanthates, amine oxides, sulfoxides, and selenoxides that contain at least one (3-hydrogen atom. These elimination reactions require a cw-configuration of the eliminated group and hydrogen and proceed by a concerted process. If more than one (3-hydrogen is present, mixtures of alkenes are generally formed. Since these reactions proceed via cyclic transition states, conformational effects play an important role in determining the composition of the alkene product. 

In the E2 mechanism, the eliminated groups are anti-periplanar to each other, and this gives rise to //// -elimination, In general, a //-elimination will result in... 

The activated monomer mechanism was based on the assumption that the major part of propagation reaction was carried out by NCA species rather than NHCOOH or -NH Cwith CO elimination) group at the growing chain end. 

From these results, the most likely key intermediate that interacts with fluorophores is dioxetanone (II) still bearing the eliminating group rather than dioxetanedione (I) in this system. Recently, the six-membered ring was suggested by Lee et aW and B. Rechard et al reported that the key intermediate was a dioxetanedione in the certain peroxyoxalate chemiluminescence reactions. However, we believe that the results obtained in the present study can be applied to the general peroxyoxalate chemiluminescence. 

Effectively two singly bonded atoms or groups are removed from adjacent carbon atoms, and a double bond is formed between the carbon atoms to which the eliminated groups had been attached. Some examples of P-eliminations are listed in Table 1.1. 

In all three reactions one of the eliminated groups is a hydrogen atom. This is the most common type of b-elimination... 

When the eliminated groups are on adjacent carbon atoms, the degree of unsaturation is increased for example... 

E2 elimination occurs preferentially from a conformation in which the eliminated groups are opposed to each other in an antiperiplanar conformation. This process is known as anfi-elimination. 

Early synthetic observations illustrated the great preference for an anti-orientation of the elimination groups . In this conformation the electron pair released from the beta carbon hydrogen bond enters the C -octet on the side remote from the leaving group. Repulsion energy between the electron pairs is thus minimised in the transition state and by analogy to the known stereochemistry of Sn2 reactions, the Cg-H electrons are most favourably disposed... 

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