Cope rearrangement double bond configuration

The stereochemistry of acyclic anionic oxy-Cope rearrangements is consistent with a chair TS having a conformation that favors equatorial placement of both alkyl and oxy substituents and minimizes the number of 1,3-diaxial interactions.214 For the reactions shown below, the double-bond configuration is correctly predicted on the basis of the most stable TS available in the first three reactions. In the fourth reaction, the TSs are of comparable energy and a 2 1 mixture of E- and Z-isomers is formed. 

Scheme V/9. A repeatable version of the Cope rearrangement [2]. The configuration of the double bonds in V/55 is unknown.

The stereochemical features of the Claisen rearrangement are very similar to those described for the Cope rearrangement, and reliable stereochemical predictions can be made on the basis of the preference for a chairlike transition state. The major product has the -configuration at the newly formed double bond because of the preference for placing the larger substituent in the pseudoequatorial position in the transition state. ... 

The Cope rearrangement is the conversion of a 1,5-hexadiene derivative to an isomeric 1,5-hexadiene by the [3,3] sigmatropic mechanism. The reaction is both stereospecific and stereoselective. It is stereospecific in that a Z or E configurational relationship at either double bond is maintained in the transition state and governs the stereochemical relationship at the newly formed single bond in the product.137 However, the relationship depends upon the conformation of the transition state. When a chair transition state is favored, the EyE- and Z,Z-dienes lead to anli-3,4-diastereomcrs whereas the E,Z and Z,/i-isomcrs give the 3,4-syn product. Transition-state conformation also... 

A tandem cyclopropanation/Cope rearrangement sequence precluded the isolation of the bi-cyclic c -l,2-divinylcyclopropane9, when ( , )-hexa-2,4-dienyl 2-diazo-4-phenylbut-3-enoate and related dienylic esters were catalytically decomposed. When the double bond next to the tether had the Z configuration, however, the diastereomeric tra 5-l,2-divinylcyclopropane was formed and isolated it underwent the Cope rearrangement leading to the same product 10 only at 140 C. ... 

The next sequence relies on four mechanistic steps without any catalyst (Scheme 5.51) [134], First, a disubstituted vinyl-aziridine 145 adds to dimethyl-acetylenedicarboxylate to form a divinyl-aziridine 146, an appropriate substrate for an aza-Cope rearrangement. Delocalization of the double bonds of the resulting azepine is supposed to promote a disrotatory electrocyclization to the azabicyclo-[3.2.0]-heptane 147, which tautomerizes to the final product 148. The initial chirality of the aziridine and of the (Z)-configurated double bond control the full overall diastereoselectivity of the process thanks to the conservation of orbital symmetry. 

As dienes 220 and 222 demonstrate, the configurations of the two double bonds again determine the configuration at the sp3-centers. However, it has to be emphasized here that for the Cope rearrangement the phenomenon of charge acceleration turns out to be of particular importance [84]. 

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