A-bond rearrangements

Cephalotaxine Alkaloids.— The structures of this group have been elucidated mainly by n.m.r. and mass spectroscopyand confirmed by an X-ray crystallographic analysis.This class of compounds, exemplified by cephalotaxine (23), bears an intriguing resemblance to the Erythrina alkaloids and may be derived by a dienol-phenol rearrangement, a bond migration, and reduction from the partially reduced form of a recognised precursor (22). The first step has ample in vivo and in vitro precedent in related alkaloid series. 

Fig. 3. Dissociation mechanism for the replacement of Cl from m-[Co(en)20HCl]". Without rearrangement, a- bonding of the p orbital with the vacated

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. 

The breaking of a strategic bond and the generation of synthesis precursors defines a synthesis reaction. In the simplest case, the reaction is already known from literature. In most cases, however, the rcaaion step obtained has to be generalised in order to find any similar and successfully performed reactions with a similar substituent pattern or with a similar rearrangement of bonds. One way of generalizing a reaction is to identify the reaction center and the reaction substructure of the reaction. This defines a reaction type. 

Sigmatropic rearrangement (Section 24 13) Migration of a a bond from one end of a conjugated tt electron system to the other The Claisen rearrangement is an example... 

The description of the nonclassical norbomyl cation developed by Wnstein implies that the nonclassical ion is stabilized, relative to a secondary ion, by C—C a bond delocalization. H. C. Brown of Purdue University put forward an alternative interpreta-tioiL He argued that all the available data were consistent with describing the intermediate as a rapidly equilibrating classical ion. The 1,2-shift that interconverts the two ions was presumed to be rapid relative to capture of the nucleophile. Such a rapid rearrangement would account for the isolation of racemic product, and Brown proposed that die rapid migration would lead to preferential approach of the nucleophile fiom the exo direction. 

Rearrangement (Section 5.13) Intramolecular migration of an atom, a group, or a bond from one atom to another. 

Mechanistically this reaction is described as a concerted pericyclic [3,3] sigma-tropic rearrangement. A carbon-oxygen bond is cleaved and a carbon-carbon bond is formed. In a subsequent step the initial product 4 tautomerizes to the stable aromatic allylphenol 3 ... 

Isomerization reactions occur frequently in catalytic cracking, and infrequently in thermal cracking. In both, breaking of a bond is via beta-scission. However, in catalytic cracking, carbocations tend to rearrange to form tertiary ions. Tertiary ions are more stable than secondary and primary ions they shift around and crack to produce branched molecules (Equation 4-10). (In thermal cracking, free radicals yield normal or straight chain compounds.)... 

Both Cope and Claisen rearrangements involve reorganization of an odd number of electron pairs (two tt bonds and one a bond), and both react by suprafacial pathways (Figure 30.13). 

Two other theories as to the mechanism of the benzidine rearrangement have been advocated at various times. The first is the rc-complex mechanism first put forward and subsequently argued by Dewar (see ref. 1 pp 333-343). The theory is based on the heterolysis of the mono-protonated hydrazo compound to form a n-complex, i.e. the formation of a delocalised covalent it bond between the two rings which are held parallel to each other. The rings are free to rotate and product formation is thought of as occurring by formation of a localised a-bond between appropriate centres. Originally the mechanism was proposed for the one-proton catalysis but was later modified as in (18) to include two-protons, viz. 

Both cis- and rrans-l-arylsulfonyl-2-arylsulfenyl propenes (56) underwent a Smiles rearrangement under electron impact at 20 and 70 eV and formed a diarylsulfide ion [M — 104]+ (equation 27a)39 through a process where a bond between the R C H group and the sulfide sulfur is formed and a rearomatization occurs by a loss of the neutral thiirene dioxide or a simultaneous expulsion of SOz and propyne. The ion m/z 148 was also obtained from all of the sulfonyl-sulfides, 56 (equation 27b) and here the loss of R2 seemed to be related to the bond strength39. In addition to the above compounds 56 exhibited some simple cleavages before and after sulfone-sulfinate rearrangements. 

The first step in the nonreversible degradation reactions is the formation of a reactive a-dicarbonyl species through the p-elimination of a hydroxide ion. The subsequent reaction pathways to all degradation products can be described by just five reaction types, namely, p-elimination, benzilic acid rearrangement, a-dicarbonyl cleavage, aldol condensation, and retro-aldol condensation (see Fig. 7).31 Retro-aldol condensation and a-dicarbonyl cleavage involve C-C bond... 

Bimolecular processes are reactions in which two reactant molecules collide to form two or more product molecules. In most cases the reaction involves a rather simple rearrangement of bonds in the two molecules ... 

A sigmatropic rearrangement is defined as migration, in an uncatalyzed intramolecular process, of a a bond, adjacent to one or more n systems, to a new position in a molecule, with the n systems becoming reorganized in the process. Examples... 

A dyotropic rearrangement is an uncatalyzed process in which two a bonds simultaneously migrate intramolecularly. There are two types. The above is an example of type 1, which consists of reactions in which the two a bonds interchange positions. In type 2, the two a bonds do not interchange positions. An example is... 

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