Sigmatropic Cope rearrangement

MgS04, the tetracycles 2-648 were obtained with excellent diastereoselectivity in reasonable yield. The reaction presumably starts with a condensation of the aldehydes 2-645 with the benzyl-protected amine moiety of 2-644 to give an iminium ion which can subsequently cyclize to afford the spirocyclic intermediates 2-646. A [3,3] sigmatropic Cope rearrangement then forms the nine-membered cyclic enamines 2-647 which, after protonation, act as the starting point for another indole iminium cyclization to provide the tetracycles 2-648 via 2-647. 

In a typical concerted thermal [3,3] sigmatropic (Cope) rearrangement (equation 44), 4,5-dihydrooxepin (70) was formed from cis- 1,2-divinylethylene oxide which was generated in situ from sym-divinylethylene carbonate (63JOC1383). 

Figure 3.5. Reaction lattices of benzene like aromaticity and of the sigmatropic Cope rearrangement.

A reaction in which all first-order changes in bonding relationships take place in concert on a closed curve, i.e., throughout a cyclic array of continually bonded atoms forming a fully conjugated cyclic transition state. An example of a pericyclic reaction is the 3,3-sigmatropic (Cope) rearrangement of hexa-1,5-diene. See Pericyclic Reactions The Diels-Alder Reaction. 

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. 

The aza-Cope/Mannich reaction takes advantage of the facility with which a y,<5-unsaturated itninium ion, such as 6, participates in a [3,3] sigmatropic rearrangement to give an isomeric species which is suitably functionalized for an intramolecular and irreversible Mannich cyclization (see intermediate 7). The aza-Cope rearrangement substrate 6 is simply an unsaturated iminium ion which can be fashioned in a number of ways from a homoallylic... 

King and Harding296 have reported an interesting Sulfo-Cope rearrangement, and have presented evidence for the formation of the unstable sulfene (142) during the gas or liquid phase thermal [3,3]sigmatropic rearrangement of allyl vinyl sulfone (equation 88). 

When 1,5-dienes are heated, they isomerize, in a [3,3] sigmatropic rearrangement known as the Cope rearrangement (not to be confused with the Cope elimination reaction, 17-8)When the diene is symmetrical about the 3,4 bond, we have the unusual situation where a reaction gives a product identical with the starting material ... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

The second largest group of pericyclic domino reactions starts with a sigmatropic rearrangement, which is most often a Claisen or an oxa- and aza-Cope rearrangement however, some processes also exist with a 2,3-sigmatropic rearrangement as the second step. 

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