Rearrangement migration

Sesquiterpenes are formed by the head-to-tad arrangement of three isoprene units (15 carbon atoms) there are, however, many exceptions to the rule. Because of the complexity and diversity of the substances produced in nature, it is not surprising that there are many examples of skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids. 

Problem 30.9 When a 2,6-disubslituted allyl phenyl ether is heated in an attempted Claisen rearrangement, migration occurs to give the />-allyl product as the result of two sequential pericyclic reactions. Explain. 

STEVENS REARRANGEMENT. Migration of an alkyl group from a quaternary ammonium salt to an adjacent carbanionic center on treatment with strong base. The product is a rearranged tertiary amine, sulfonium, or sulfide. 

Skeletal rearrangement of amides [equations (77)-(80)] appears to be the reverse of the known thermal processes, but in generalizing this type of rearrangement, migration from oxygen to sulphur was included and this is in the same direction expected for a thermal process. Thus, in terms of pyrolysis, there is no self-consistent analogy with the generalization of equation (76). 

Hofmann rearrangement. Migration to electron-deficient nitrogen... 

Phenols —>quinones. Useful catalysts include polymer-supported vanadium salts and (PhjPljRuClj. The Rh-catalyzed reaction can oxidize p-substituted phenols, giving 4-t-butylperoxy-2,5-cyclohexadienones, which undergo rearrangement (migration of the substituent) to afford the quinones on treatment with TiCl. ... 

Sigmatropic rearrangement Migration of a a bond from one end of a conjugated tt eleetron system to the other. The Claisen rearrangement is an example. 

Geoisomers s. cw- rnn -Isomers Geometry, retention 27, 981 Geospecificity s. a. Addition, geospecific, Homoallyl rearrangement, -, Migration,... 

Wiechell rearrangement homoallyl rearrangement migration position shift retroallyl rearrangement snatoin —... 

Sesquiterpenoids contain three isoprene units and the precursor for them all, in nature, is famesyl pyrophosphate as shown in Fig. 8.3. Because there are now three double bonds in the molecule, as opposed to the two of monoterpenoids, the variety of possible cyclic structures is much greater, as shown in Fig. 8.6. Skeletal rearrangements, migrations of methyl groups, and even loss of carbon atoms to produce norsesquiterpenoids all contribute further to the variety. There are probably >3000 sesquiterpenes that have been isolated and identified in nature. A large number of sesquiterpenoids possess interesting biological activities, but most are of academic interest only and have no commercial application outside folk medicine. 

The first of the emunerated pathways proceeds via transamination to p-hydroxy-phenylpyruvate then there is oxidation to the quinol and rearrangement (migration of the side chain) to the p-dihydroxy derivative. This seemingly unusual step... 

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