Rearrangements Involving Functional Groups

Both conjugated and isolated dienes are usually accessible by extension of the methods suitable for mono-olefins. Allylic functions for ehmination may be produced by double bond introduction a to a functional group or by allylic substitution of an olefin. Both reduction of allylic systems to mono-olefins and elimination to give dienes, may involve rearrangement. 

An advantage of Tiffeneau ring enlargement and rearrangement of 17a-hydroxy-20-keto steroids for preparing of D-homosteroids is the ready availability of the starting 17-ketones. Both procedures involve mild conditions which allow a wide variety of suitably protected functional groups to be present. 

Selected additional reactions, transformations, or rearrangements of three-membered ring sulfones and sulfoxides will follow. The criteria for selection is the direct or indirect involvement of the functional groups in the reaction. 

Saccharinic acid formation has been studied for several years. The four-step reaction proceeds rapidly in alkaline solution because of basic catalysis, particularly in the last two steps. Initially formed is an enediol that can undergo j8-elimination of a functional group, usually a hydroxyl group. The final two steps involve tautomerization to an a,j8-dicarbonyl intermediate followed by a benzilic acid rearrangement. This sequence is shown in Scheme 6 for the formation of the a- and j8 -xylometasac-charinic acids (30) by way of 3-deoxy-D-g/ycero-pentos-2-ulose (29). 

Reduction of p-ketoesters in aqueous ethanolic sulphuric acid leads to removal of both functional groups and the formation of a hydrocarbon. This reaction which was discovered in 1907 [117] and recognised in 1912 [118] as involving a skeletal rearrangement is now termed the Tafel rearrangement. Conversions of the type 26 into 27 occur in 30 - 60 % yields [118,119] and the hydrocarbon is easily separated... 

Because of the strongly acidic and rather vigorous conditions required to effect hydration of most alkenes, these conditions are only applicable to molecules that have no acid-sensitive functional groups. Also, because of the involvement of cationic intermediates, rearrangements can occur in systems where a more stable cation would result by aryl, alkyl, or hydrogen migration. A much milder and more general procedure for alkene hydration is discussed in the next section. 

Functional group transformations that rely on substitution by the SN1 mechanism are not as generally applicable as those of the SN2 type. Hindered substrates are prone to elimination, and rearrangement is possible when carbocation intermediates are involved. Only in cases in which elimination is impossible are SN1 reactions used for functional group transformations. 

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