Some Examples of Sigmatropic Rearrangements

The field of allenes bearing phosphorus is dominated by sigmatropic rearrangements (see Section 1.3.1). Nevertheless, some examples of prototropic rearrangements are known. 

In sharp contrast to the well-studied [2,3]sigmatropic rearrangement of allylic sulfur ylides [42], the study of allylic selenium ylides is quite limited [43]. Furthermore, its asymmetric version using optically active allylic selenium ylides has not been developed until quite recently. In this section, some examples of the asymmetric [2,3]sigmatropic rearrangement via optically active allylic selenium ylides are described. 

Hydrogen is not the only group which can migrate, and there are therefore many other kinds of sigmatropic rearrangement. As usual, those involving a total of (4n + 2) electrons are allowed in the all-suprafacial mode, and these are the common reactions. Here are some examples.136-142... 

Scheme 6.11 gives some examples of [3,3]-sigmatropic rearrangements for both aliphatic and aromatic systems. 

Besides the Ireland-Claisen [3,3]-sigmatropic rearrangement/RCM sequence, some examples of Johnson-Claisen/RCM were reported by Ghosh and Maity [5]. 

The Fischer indole synthesis is commonly recognized as one of the most powerful and versatile tools for construction of indoles, as reflected by the amount of recent publications in which it has been used successfully. The usual approach involves initial preparation of a phenylhydrazone 323 from a suitable phenylhydrazine and an enolizable carbonyl compound, followed by an acid-induced cyclization to the target indole 324, featuring a [3,3]-sigmatropic rearrangement as the key step (Scheme 38). Some selected new examples of indoles prepared using standard Fischer conditions are collected in Table 2. 

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