Transition state for the Cope rearrangment

There is a second possible transition state for the Cope rearrangement in which the transition state adopts a boatlike geometry ... 

The vinylcarbenoid [3-1-4] cycloaddition was applicable to the short stereoselective synthesis of ( )-tremulenolide A 73 and ( )-tremulenediol A 74 (Scheme 14.7) [81]. The key step is the cyclopropanation between the cyclic vinyldiazoacetate 69 and the functionalized diene 70, which occurs selectively at the ds-double bond in 70. Because of the crowded transition state for the Cope rearrangement of the divinylcyclopropane 71, forcing conditions are required to form the fused cycloheptadiene 72. The stereo-... 

The success of the Claisen-Cope rearrangement need not be limited to the production of aldehydes via enol ethers. Allylic alcohol (58) is successively transposed into a mixture of allylic isomers (59 Scheme 4), and is subjected to an orthoester Claisen rearrangement at 150 "C to provide ester (61). The moderate temperature of the Claisen step permits the isolation of an intermediate (c/. Scheme 3) prior to the final Cope rearrangement (195 C) to. y-unsaturated esters (60). The esters (60) are a 55 45 mixture of ( )- and (Z)-double bond isomers owing to the near equal steric bulk of the methyl and acetic acid residues in the transition state for the Cope rearrangement. ... 

This problem was intensively studied both experimentally and theoretically. The quantum chemical calculations were carried out using various methods at different levels. The earlier calculations for the Cope reanangement based on a CASSCF wave function for six electrons in the bonds rearranged were found to overestimate the diradical character of the wave funclion- --. More recently, MP2 methods for the multireference wave function have been developed whose application to an estimate of the energy of the chair transition state has been described. AMI calculations of altemative transition states for the Cope rearrangement of 1,5-hexadiene derivatives have been discussed by Dewar and colleagues i -217... 

It is well established that steric effects hinder the Cope rearrangement of divinylcyclopropanes. An interesting example of this steric effect is seen in the reaction of 33 with cis- and trans-l-acetoxy-butadiene (Scheme 13). ° The reaction of 33 with trans-1-acetoxy-l, 3-butadiene leads cleanly to the [3+4] annulation product 34 in 67% yield. In contrast, the product from the reaction of 33 with c/j-l-ace-toxy- 1,3-butadiene is the cw-divinylcyclopropane 35 (80% yield), and high temperatures (220 °C) are required to convert 35 to the [3+4] annulation product 36. The effect of alkene geometry on the stereochemistry and the rate of reaction is readily explained by considering the boat transition state for the Cope rearrangement of divinylcyclo-propanes (structure 37). A trans diene substituent (Y) would generate a trans product (34), whereas a cis substituent (X) would lead to a cis... 

Fig. 18. Geometry of the transition state for the Cope rearrangement optimized at the MC-SCF/3-2IG level. (Reprinted with permission from Osamura et al. ...

By analyzing the possible chair- and boatlike transition states for the Cope rearrangement and by taking into account the lower energy of a chair conformation, one can predict the preferential stereochemistry of the products. 

FIGURE 20.51 The transition state for the Cope rearrangement involves the migration of one allyl radical across another. 

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