Acid-base rearrangements

Coldham et al. 1995TL3557 and Somfai 1994CC2785, 1995TL1953 have independently investigated the ring expansion of vinyl aziridines to piperidines. Thus, Coldham et al. have prepared the unsaturated air-piperidines 

268 as single diastereoisomers in fair to good yield from the keto-aziridines 266 using a one-pot, two-step procedure. The stereochemical outcome is rationalized on the basis of the chelated intermediate 267. In a related manner, Somfai has used this protocol for the enantioselective total synthesis of indolizidine 209D 271, starting from aziridine 

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RUZICKA - FUKUSHIMA Rearrangement Base or acid catalyzed rearrangement of I7a-hydroxy-20-keto steroids to 0-tiomo steroids. 

Some of the recent work in contact mechanics is focused on understanding the adhesion of viscoelastic polymers and dynamic contributions to the adhesion energy this work is summarized in Section 5. Sections 6.1 and 6.2 include some of the current applications of contact mechanics in the field of adhesion science. These include possible studies on contact induced interfacial rearrangements and acid-base type of interactions. 

Among azepines and their benzo derivatives, antiaromatic IH-azepine 14a is known as a very unstable (even at -78°C in CDCI3 solution) red oil which in the presence of acid or base rearranges to the marginally... 

Curran s synthesis of ( )-A9(l2)-capnellene [( )-2] is detailed in Schemes 30 and 31. This synthesis commences with the preparation of racemic bicyclic vinyl lactone 147 from ( )-norbomenone [( )-145] by a well-known route.61 Thus, Baeyer-Villiger oxidation of (+)-145 provides unsaturated bicyclic lactone 146, a compound that can be converted to the isomeric fused bicyclic lactone 147 by acid-catalyzed rearrangement. Reaction of 147 with methylmagne-sium bromide/CuBr SMe2 in THF at -20 °C takes the desired course and affords unsaturated carboxylic acid 148 in nearly quantitative yield. Iodolactonization of 148 to 149, followed by base-induced elimination, then provides the methyl-substituted bicyclic vinyl lactone 150 as a single regioisomer in 66% overall yield from 147. 

In order to avoid acid-catalyzed rearrangements of 1,6-oxido-[10]annulene it is recommended that the distillation flask be treated with a base before use. 

Schleyer s Lewis acid-catalyzed rearrangement method, which is based on diamondoid thermodynamic stability during carbocation rearrangements, has had little or no success in synthesizing diamondoids beyond triamantane. In recent years, outstanding successes have been achieved in the synthesis of adamantane and other lower molecular weight diamondoids [42 9]. Some new methods have been developed and the yield has been increased to 60%. 

Autoxidation may in some cases be of preparative use thus reference has already been made to the large-scale production of phenol+ acetone by the acid-catalysed rearrangement of the hydroperoxide from 2-phenylpropane (cumene, p. 128). Another example involves the hydroperoxide (94) obtained by the air oxidation at 70° of tetrahydro-naphthalene (tetralin) the action of base then yields the ketone (a-tetralone, 95), and reductive fission of the 0—0 linkage the alcohol (a-tetralol, 96) ... 

A large number of reaction conditions have been defined for the rearrangement of various allene derivatives into small heterocycles. The rearrangements have been accomplished variously with acids, bases or oxidatively. 

Interesting direct evidence for the reaction pathway of these metal-catalyzed reactions was obtained from stoichiometric reactions of allenylcarbinols such as 61 with the [j7S-CsHsFe(CO)2]+ (Fp+) complex. From the isobutene adduct of Fp+ and 61 the jt-complex 65 was obtained. The complex 65 was suspected to be the product of an acid-catalyzed rearrangement indeed, in the presence of N,N-dimethylaniline as a base, 63 was isolated (Scheme 15.14) [30],... 

In general, the rearrangements of dienes and polyenes can be both thermal and photochemical reactions (the latters are not included in this chapter), and can be catalyzed by acids, bases, metal complexes and enzymes. They can be degenerate processes or occur with the introduction or elimination of functional groups, be accompanied by shifts of multiple bonds or by migrations of atoms or groups and they may lead to cyclizations. 

DR. DALE MARGERUM (Purdue University) You need to be careful because you may not obtain a true Bronsted plot from what you have just described. You don t want to make a plot where each system has a different rearrangement. You want to be certain to have a common type of acid or base with which you are reacting. The literature is full of inverse Bronsted relationships in which there has been a poor choice of reaction acid-base pairs. Much confusion has been generated as a result. 

Aldoses generally undergo benzilic acid-type rearrangements to produce saccharinic acids, as well as reverse aldol (retro-aldol) reactions with j3-elimination, to afford a-dicarbonyl compounds. The products of these reactions are in considerable evidence at elevated temperatures. The conversions of ketoses and alduronic acids, however, are also of definite interest and will be emphasized as well. Furthermore, aldoses undergo anomerization and aldose-ketose isomerization (the Lobry de Bruyn-Alberda van Ekenstein transformation ) in aqueous base. However, both of these isomerizations are more appropriately studied at room temperature, and will be considered only in the context of other mechanisms. 

The stability of polynitroaliphatic compounds to acids, bases and nucleophiles is often linked to the presence of an acidic o -proton(s) which may allow various resonance structures to lead to rearrangement or decomposition. Additionally, the presence of two or more nitro groups on the same carbon atom greatly increases the susceptibility of the carbon-nitrogen bonds to nucleophilic attack. 

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