Diels-Alder reactions and Claisen rearrangement

Another new development in enzymatic reactions is the use of monoclonal antibodies as catalysts. These antibodies show regio- and stereoselectivity, substrate specificity, and rate acceleration. Hilvert has discussed three different reactions that are catalyzed by these antibodies decarboxylation, Diels-Alder reactions, and Claisen rearrangements. This catalysis is a new field with tremendous possibilities. 

Tandem Diels-Alder reaction and Claisen rearrangement... 

Classical organic reactions that have been carried out in water include, among others, the Diels-Alder reaction, the Claisen rearrangement, aldol condensations, Michael additions, and nucleophilic substitutions. In the Diels-Alder reaction, for example, water has been found to increase the reaction rate and to enhance the endoselectivity 120). Two reviews summarize the results for organic reactions in water 121). 

The influence of the hydrophobic effect on the aqueous pericychc reactions can be compared with the effect of catalytic antibodies. Antibodies have been found to catalyze Diels-Alder reactions, hetero-Diels-Alder reactions, and Claisen and oxy-Cope rearrangements. It is suggested that antibodies catalyze these reactions by acting as an entropy trap, primarily through binding and orienting the substrates in the cyclic conformations. 

These bulky aluminium aryloxides will also promote a variety of carbon-carbon bond forming reactions with a high degree of regio and stereoselectivity. Examples include Michael addition to a,/S-unsaturated ketones, Diels-Alder additions, and Claisen rearrangements. In the case of Diels-Alder reaction, ATPH promoted the exo-selective condensation of a,/6-unsaturated ketones with dienophiles. The Claisen rearrangement can be catalysed by ATPH and its more Lewis acidic 4-bromo derivative. A series of chiral aluminium aryloxides were also synthesized and have been applied to asymmetric Claisen rearrangements, aldol reactions, and aldehyde alkylations. ... 

Soldermann, N. Velker, J. Vallat, O. Stoekli-Evans, H. Neier, R. "Application of the Novel Tandem Process Diels-Alder Reaction/lreland-Claisen Rearrangement to the Synthesis of rac-Juvabione and /ac-EpiJuvabione" Helv. Chim. Acta 2000, 63, 2266-2276. 

Catalysis by lithium perchlorate in dichloromethane Diels-Alder reactions and 1,3-Claisen rearrangements [100]... 

The ene reaction is an electrocyclic reaction similar to the Diels-Alder reaction and the Claisen rearrangement. In this reaction, a hydrogen atom is participating in the electrocyclic process. The mechanism, illustrated below using arrow pushing,... 

Other Uses. Reagent 1 has been used for enantioselec-tive enolborination, albeit with poor (1.1 1) selectivity. Similar bis-sulfonamide-derived boron Lewis acids have been used for aldol additions, "" ester-Mannich reactions, Diels-Alder reactions, Ireland-Claisen reactions, and [2,3]-Wittig rearrangements. Similar bis-sulfonamide-derived aluminum Lewis acids have been used for aldol additions, Ehels-Alder... 

Diels-Alder reactions and 1,3-Claisen rearrangements. Lithium perchlorate in CH2CI2 apparently mimics the LiC104-Et20 system in the catalysis of many reactions, including the Diels-Alder reactions and 1,3-Claisen rearrangements. 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

Oxazoles readily participate in cycloaddition reactions as dienophiles and as dienes in Diels-Alder reactions, and suitably substituted oxazoles participate in sigmatropic rearrangements (e.g., aza-Claisen rearrangements). In particular, the Diels-Alder reaction of oxazoles is one of the most widely explored and synthetically useful reactions, and as such, it has been used extensively both in natural product syntheses and to convert oxazoles to other heterocyclic ring systems. For example, a partial list of heterocyclic systems readily accessible from oxazoles via Diels-Alder reactions or other cycloadditions include pyridines hydroxy-pyridines isoindoles pyridazines tetrahydronaphthyridines benzo[h]-l,6-naphthyridines benzopyrano[3,4-b]pyridines 2-substituted, 2,4-disubstituted,... 

Organic compounds mostly consist of just ten to a dozen non-metallic elements including C, H, N, P, O, S, and halogens. This may be one of the main reasons why chemists, until relatively recently, tended to rely heavily on those reactions involving only non-metallic elements. Many of them including the Diels-Alder reaction, the Claisen and Cope rearrangements continue to be important. Even so, their combined synthetic scope has been rather limited. 

Overall stability should increase. For example, conjugation may increase, especially aromatization, single bonds may form at the expense of the double bonds (as in the Diels-Alder reaction), and double bonds to oxygen may form at the expense of double bonds to carbon (as in the Claisen rearrangement). 

A number of important concerted organic reactions proceed through isopolar transition states examples are the Diels-Alder reaction, and the Cope and Claisen rearrangements. These processes are in fact specific cases of a whole family of reactions which have been termed pericyclic processes (Woodard and Hoffmann, 1969). Concerted pericyclic reactions proceeding via polar transition states are also well-known. 

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