Cope rearrangement alkyl substitution

The oxy-Cope and anionic-oxy-Cope rearrangements involve highly ordered cyclic transition states, so the asymmetry is almost completely transferred from the substrate to the product. Most commonly in acyclic systems as in other [3,3]-sigmatroplc rearrangements, the transition states are chairlike and the substituents adopt a quasiequatorial position to minimize unfavorable steric interactions. In unsubstituted substrates the diastereoselection is low, tout the introduction of an alkyl substituent at C4 improves the diastereoselectivity. In (Z)-1-substituted alkenes there is preference for the oxyanionic bond to be pseudo-equatorial, whereas in ( )-1-substituted alkenes it tends to... 

Cyclopropanation reactions with alkyl 2-diazobut-3-enoates (entries 3-6) occur with a remarkably high diastereoselectivity. When 1,3-dienes (acyclic 1,3-dienes,cyclopen-tadiene, furan, " substituted pyrroles ) are cyclopropanated with these reagents, the CK-divinylcyclopropane derivatives are formed with high or complete diastereoselectivity, but they usually undergo a spontaneous Cope rearrangement under the reaction conditions. 

Based on the study of the thermal 3-aza-Cope rearrangement of iV-allylketene Af,0-acetals, Kurth and coworkers have developed a similar methodology to the asymmetric synthesis of C(a)- and C( )-substituted-4-pentenoic acids by using a chiral auxiliary. Prepared from the alkylation of oxazolines 179 with tosylate esters 180, followed by neutralization with n-butyllithium in THF, iV-allylketene. /V,0-acetals of 182 rearrange without isolation at 180 °C to 2-butenylisoxazolines (183) with 79-92% diastereoselectivity (d.e,). Enantiomeric excess (e.e.%) reaches as high as 98%. In... 

A detailed reexamination of the thermal rearrangement of 1-methyl-imidazole originally reported by Wallach has been extended to encompass a wide range of 1-substituted imidazoles. The reaction appears to be of potential synthetic utility since it leads easily to 2-alkyl- and 2-arylimidazoles. It is an irreversible reaction, uncatalyzed, intramolecular, and does not involve radicals [l,5]-sigmatropic shifts, as shown in Scheme 40, are probably implicated. The major product is the 2-substituted imidazole (134), but small amounts of the 4- (or 5-) isomer (135) are also formed. A 1-allyl substituent migrates with equal facility to both 2- and 4(or 5)-positions, suggesting that a Cope-type rearrangement may also be involved for this substituent. ... 

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