Sigmatropic arrangement

The principles of orbital symmetry conservation establish that concerted suprafacial [3, 3]-sigmatropic arrangements are allowed processes. Based on these principles and the results of numerous experiments their stereochemistry is highly predictable. 

The use of a [2,3] Wittig-Still sigmatropic arrangement for the synthesis of 48 was also investigated. The advantage of this reaction is... 

In accord with the experimental findings a mechanism via a [5,5]-sigmatropic rearrangement has been formulated. In a first step the hydrazobenzene is proto-nated to the dicationic species 3, in which the phenyl groups can arrange in such a way to allow for rearrangement ... 

Thermal sigmatropic 1,5 hydrogen shifts are quite common in certain allene and diene systems, cis- 1,3-Dienes have the proper geometric arrangement to undergo a thermally allowed suprafacial hydrogen shift, cis-1,3-Hexadiene, for instance, gives... 

We cannot draw a complete chair as we would need a six-membered ring for that (see discussion of [3,3]-sigmatropic rearrangements above), but the part that is to become the new k bond can be in a chair-like part of the five-mem bered ring. The substituent R prefers an equatorial position and the resulting tram arrangement of the groups is outlined in black. 

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