Of -sigmatropic rearrangement

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 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 ... 

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... 

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. 

The classical Claisen rearrangement is the first and slow step of the isomerization of allyl aryl ethers to orf/to-allylated phenols (Figure 11.41). A cyclohexadienone A is formed in the actual rearrangement step, which is a [3,3]-sigmatropic rearrangement (see Section 11.1 for the nomenclature of sigmatropic rearrangements). Three valence electron pairs are shifted simultaneously in this step. Cyclohexadienone A, a nonaromatic compound, cannot be isolated and tautomerizes immediately to the aromatic and consequently more stable phenol B. 

During the investigation of sigmatropic rearrangements in various systems Radom also covered the cis enol tautomer of DMA using various basis sets, Table 12108. The proton transfer across the barrier was 40 kJ mol-1 [4-31G with configuration interaction (Cl)] which is to be compared with a barrier of 260 kJ mol-1 for proton transfer in 1,3-pentadiene (L) via a similar cyclic intermediate. 

Stereospecificity is a characteristic of sigmatropic rearrangements. For example, the [3,3]-sigmatropic reaction shown in Scheme 8.15 is stereospecific i.e. the stereoisomer of the starting material produces exclusively the stereoisomer of the product shown. 

The initial adduct 9 undergoes a series of sigmatropic rearrangements under the reaction conditions to give tetra- and dihydrothiophene 1-oxides 11 and 1289. The cis relationship between the sulfinyl oxygen and the adjacent methyl group in 11 and 12 is a consequence of the stereoelectron-ic requirements of the addition of the intermediate sulfenic acid 10 to the C-C double bond. 

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