Cope rearrangement 3,3 -sigmatropic rearrangement

FIGURE 4 Ectocarpene as the product of a [3.3]-sigmatropic rearrangement. The fatty acid accommodates to the active center of the enzyme in a U-shaped fashion. Decarboxylation in conjunction with loss of the C(8) HR hydrogen atom yields, after cyclization between C(4) and C(6) of the precursor, the thermolabile (lS,2R)-cyclopropane. A subsequent spontaneous [3.3]-sigmatropic rearrangement (Cope rearrangement) proceeds via the cis-endo transition state and yields (6S )-ectocarpene. 

Ring Expansion/Con traction via Sigmatropic Rearrangements - Cope Rearrangement... 

Another example of a synthesis of a 1,6-D system is found in the key step of the synthesis of juvabione (38), also by Evans [26], which proceeds through a base catalysed [3,3]-sigmatropic oxy-Cope rearrangement (Scheme 5.23). 

The non-classical version of the intermediate carbenium ion (also referred to as a carbonium ion) resulting upon dissociation of the pyrophosphate anion from famesylpyrophosphate explains the cyclization to several cyclic carbenium ions as demonstrated for some sesquiterpenes (Fig. 3). Additional diversity arises from 1,2-hydride and 1,2-alkyl shifts (Wagner-Meerweest rearrangements) sbA sigmatropic reactions (COPE rearrangements) on the one hand, and on the other hand from the formation of diastereomers and enantiomers provided that the cyclizations generate new asymmetric carbon atoms (Fig. 3)... 

The propargyhc esters 33 of (fS-indolyl)acetic acid undergo Au(I)/Ag(I)-catalyzed isomerization to the tetracyclic lactone 34, which is interpreted as a tandem (3,3)-sigmatropic oxa-CopE rearrangement followed by an intramolecular (2 + 2)-cydoaddition [155] ... 

Azabicyclo[3.3.1]nonenes are common structure types present in a variety of different alkaloids [67]. An elegant and fast approach to these structure types starts from oxazolidines 203 harboring a vinylsilane in the side chain. From this vinylsilane the branched aUylsilane is stereospecifically generated in a [3,3]-sigmatropic aza-Cope rearrangement Acid-catalyzed iminium cychzation leads to the formation of the desired 9-azabicyclo[3.3.1]nonene 204. 

SECTION 7.3. [3,3]-SIGMATROPIC REARRANGEMENTS COPE AND CLAISEN REARRANGEMENTS... 

In this compound one a-bond is flanked by a 7i-system on either side. It is possible that 3, 5-sigmatropic (supra-antara) might have taken place to give a threo-product. On the other hand 3, 3-sigmatropic shift (Cope-rearrangement) may give an eiythro-product. Transition states for both the possibilities are given below (Fig. 8.1.). 

The most important sigmatropic rearrangements from the synthetic point of view are the [3,3] processes involving carbon-carbon bonds. The thermal rearrangement of 1,5-dienes by [3,3] sigmatropy is called the Cope rearrangement. The reaction establishes equilibrium between the two 1,5-dienes and proceeds in the thermodynamically favored direction. The conversion of 24 to 25 provides an example ... 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

Cope rearrangement (Section 30.8) The sigmatropic rearrangement of a 1,5-hexadiene. 

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