Of sigmatropic

Fig. 11.6. Classification of sigmatropic hydrogen shifts with respect to basis set orbitals.

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

Thomson - v Click Organic Interactive to predict products from a variety of sigmatropic rearrangement reactions. 

Analogs of sigmatropic rearrangements in which a cyclopropane ring replaces... 

Woodward and Hoffmann have given the name of sigmatropic rearrangements to such reactions because they involve the movement of a sigma bond which is adjacent to one or more it system. 

This second class of sigmatropic rearrangements can again be subdivided here we find several classical named reactions that, again with propargylic substrates, lead to allenic products. 

As a result of these heavy-atom KIE experiments the principal features of the benzidine rearrangements have now been firmly established. The two main products arise from two parallel reactions one of which is concerted and the other is not. Other concerted processes have been identified and all of the concerted processes can be readily classified in the terminology of sigmatropic rearrangements within the general class of percyclic reactions. 

The synthetic applications of sigmatropic rearrangements to the synthesis of dissonant molecules (mainly 1,6-D) were extensively studied by Evans [22]. Although such a methodology meets the first two requirements of "logical disconnections", it does not meet the third one since it lacks simplicity. 

The 1,3-hydrogen shift in propene is one of the model reactions which, despite the fact that it does not occur, is used to illustrate the principles involved in the W.-H. treatment of sigmatropic reactions. The situation for the neutral system... 

The mechanistic basis of sigmatropic rearrangements was introduced in Chapter 11 of Part A. The sigmatropic process that is most widely applied in synthesis is the [3,3] sigmatropic rearrangement. The principles of orbital symmetry establish that concerted... 

Analogs of sigmatropic rearrangements in which a cyclopropane ring replaces one of the double bonds are also known, e.g.,418... 

Scheme 24 Concept of Sigmatropic Rearrangement for the Stereospecific Synthesis of ip[CH=CH] Isosteres...

The approach of the two orbitals on the same side of the surface is called supra , that on opposite sides is called antara . The above rules, 1 and 2, also apply in the case of sigmatropic rearrangements, since the changes of orbital symmetries are the same on going from a HOMO to a LUMO or from a chain of 4n to a chain of 4n + 2 tt electrons. 

Exercise 21-21 The Cope rearrangement is a type of sigmatropic rearrangement that occurs with 1,5-dienes. An example is the rearrangement of 3-methyl-1,5-hexa-diene to 1,5-heptatriene ... 

The pericyclic theory has proved enormously fruitful for the field of sigmatropic reactions. In addition to providing a deeper understanding of previously known phenomena, it has led to the discovery of a variety of new rearrangements. Our treatment here, as in the previous sections, is not intended to be comprehensive. 

A variety of sigmatropic rearrangements through six-electron transition states are known. In contrast to the rare [l,3]-migrations, [l,5]-shifts of hydrogen in dienes, suprafacially allowed, occur readily. The experiment outlined in Equation 12.88 confirms the predicted stereochemistry.139 The authors estimated the... 

The symmetry and the corresponding selection rules of sigmatropic reactions have been studied.113... 

This reaction does not create a ring but there is an important group of sigmatropic rearrangements that make five-membered rings—the vinyl cyclopropane to cyclopentene rearrangement. 

This facile type of sigmatropic of rearrangement appears general. Julia and his group [96-98] have prepared and characterised a number of unsaturated sulfines. Instead of an oxidation route to the starting sulfoxides, they performed the reaction of vinyl Grignard reagents with allyl sulfenates at -78 °C. In most cases the subsequent [3.3] shift is so rapid that the intermediate unsaturated sulfoxides were not even evidenced sulfines were isolated instead. 

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