Rearrangement eliminations

R = H) undergoes a variety of enzyme-catalyzed free-radical intramolecular cyclization reactions, followed by late-stage oxidations, eliminations, rearrangements, and O- and N-alkylations. Working from this generalization as an organizing principle, the majority of known AmaryUidaceae alkaloids can be divided into eight stmctural classes (47). 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

CgH (n = 6, 7, 8). A novel collision-induced isomerization of CgH7 (10a), which has a sttained allenic bond, to (lOyS) has been reported to occur upon SIFT injection of (10a) at elevated kinetic energies (KE) and collision with helium. In contrast, radical anions (9) and (11) undergo electron detachment upon collisional excitation with helium. Bimolecular reactions of the ions with NO, NO2, SO2, COS, CS2, and O2 have been examined. The remarkable formation of CN on reaction of (11) with NO has been attributed to cycloaddition of NO to the triple bond followed by eliminative rearrangement. 

Problem 3.6 Classify the following as substitution, addition, elimination, rearrangement, or redox reactions. (A reaction may have more than one designation.)... 

Competition, reaction Elimination, rearrangement Elimination, especially with 3° RX in strong Bronsted base... 

A TBAF-induced elimination/rearrangement of certain 4- /t-butyldimethylsilyloxy-2-amino-l-aza-bicyclo[4.1.0]-hept-3-enes led to substituted l//-azepin-4(7//)-one derivatives in low to fair yields rearrangement to N-substituted pyrid-(177)-ones was a substituent-dependent competing process <2007T11167>. 

Diphenylvinyl ethyl ether was not always the sole product 100, 91 and 40% of it are formed when X = F, Cl and Br, respectively. Diphenylacetylene formed in an elimination-rearrangement process... 

The kRrlka ratios for the elimination-rearrangement process at 120° are 20,46 and 41 for R = H, Me and MeO, respectively. Bond-breaking is therefore important, and slow elimination of halide ion from vinyl carbanions, which are formed in pre-equilibrium, seems plausible (Jones and Damico, 1963). 

Nucleophilic substitutions at alkene carbons also proceed with retention in cases which are not obscured or complicated by competing addition, elimination, rearrangement, etc. (Patai and Rappoport, 1964). Apart from two studies (Jones et al., 1960 Miller and Yonan, 1957), the major contributions were first made in Italy (Landini and Montanari, 1967), e.g. 

The entries in rows 3-5 in Table 14.1 refer to one-step eliminations/rearrangements of this type in which oxenium ions are avoided. 

Reaction of iV-(2-cyanophenyl)benzimidoyl chloride 66 with sterically hindered thioureas gives intermediate l-[(2-cyanophenylimino)phenylmethyl]thioureas 67 (see also Section 14.08.7.4.2), which are prone to intramolecular S-attack and give 2-phenylquinazoline-4(377)-thione 68 as a final elimination/rearrangement product (Scheme 13 <2002MOL96 . 

AU four of the elementary reactions in a cationic polymerization involve electrophilic or cationic intermediates. Thus, initiation, propagation, transfer, and termination may be classified as either nucleophilic substitution, electrophilic addition, elimination, rearrangement, or possibly as a pericyclic reaction. Initiation occurs in alkene polymerizations by either addition of acid to the alkene, or by ionization of a covalent initiator followed by addition of the resulting carbocationic intermediate to an olefin s double bond. Although initiation is an electrophilic addition (AdE) reaction in... 

Investigations into the stereochemistry of the carbon to which the nucleophile becomes attached are hampered by the fact that a carbonium ion or incipient carbonium ion is formed in the reaction, and that elimination, rearrangement and/or racemization processes frequently occur so that stereochemical information is lost. The best studies are those of LaLonde who has shown that various bicydo[n. 1.0]alkanes undergo internal carbon-carbon bond cleavage with the nucleophile entering predominately with twem oM (Eq. (13)). 

The two-electron process in the electrodecarboxylation of carboxylates has been intensively investigated from the mechanistic and synthetic viewpoints. Significant progress of such carbenium reactions has been recognized in alkoxylation, acetoxylation, acetamida-tion, olefin formation ( 6-elimination), rearrangement, C-C bond cleavage, and so on. Typical reaction patterns of the electrogenerated cation intermediates and their synthetic potentialities are discussed next. 

Reductive deoxygenation-rearrangement of 2-yne-l,4-diols to 1,3-dienes is a useful synthetic procedure since a large variety of ynediols are available in a few steps by sequential reaction at both ends of acetylene with aldehydes. Acetylenic 1,4-diols can be deoxygenated reductively by lithium aluminium hydride to form conjugated dienes of high stereoisomeric purity (equation 9). A modification to this procedure is the use of acetylenic 1,4-diol mono-THP derivative. Allenic tertiary alcohols which are intermediates in the reaction can be separated and subjected to reductive elimination rearrangement... 

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