Cope eliminations, retro

Addition reactions. At low temperatures a,P-unsaturated acids behave as Michael acceptors toward RLi. The addition of benzenesulfonylmethyllithium to nitrones containing a vinyl group can lead to pyrrolidine Al-oxides after workup, due to a retro-Cope elimination which occurs at room temperature. ... 

The syntheses of (-l-)-a -lycorane (92) and (+)-trianthine (97) using the retro-Cope elimination step 122), and of (-l-)- y-lycorane (100) using the palladium-mediated reaction 123), have been reported. 

If the reaction between cinnamate 1 and hydroxylamines occurred through a concerted mechanism, the process should take place through a cyclic (five-mem-bered ring) transition state, similar to the ones proposed for [3+2] dipolar cycloadditions or retro-Cope eliminations. The cyclic transition state for the concerted addition of deuterated iV-methylhydroxylamine to ethyl cinnamate 1 is represented in Scheme 8.7 (atoms involved in the cyclization colored blue). As the nitrogen atom and the deuterium approach the C=C bond from the same side, the addition is syn, leading to intermediate 12. A rapid proton shift would give compound 3 which can be either isolated or cyclized in the presence of a Lewis acid to the isoxazolidi-none 4 by intramolecular transesterification. The stereochemistry of the products 3 and 4 is determined during the concerted addition step. 

An even milder cycloelimination uses a ring of five atoms 6.28 instead of six, but still involves six electrons. This is no longer a retro-ene reaction, but it is still a retro group transfer and it is allowed in the all-suprafacial mode 6.29. The pyrolysis of N-oxides 6.30 is called the Cope elimination, and typically takes place at 120°, the corresponding elimination of sulfoxides 6.31 (X=S) typically takes place at 80°, and, even easier, the elimination of selenoxides takes place at room temperature or below. All these reactions are affected by functionality making these numbers only rough guides, but they are all reliably syn stereospecific. 

The initial product 2 loses N2 in a retro-DiELS-Alder reaction forming the 3,4-dihydropyridine 3, which aromatizes giving the pyridine derivative 4 by elimination of amine or alcohol. The geometry of the transition state of this [4+2] cycloaddition with inverse electron demand follows from the reaction of 3- or 6-phenyl-1,2,4-triazine 5 or 8 with enamines of cyclopentanone. It is apparently influenced by the secondary orbital interaction between the amino and phenyl groups. 3-Phenyl-1,2,4-triazine 5 favours the transition state 11. It leads first to the 3,4-dihydropyridine 6 which, on oxidation followed by a Cope elimination, affords the 2-phenyldihydrocyclopenta[c]pyridine 7. However, 6-phenyl-1,2,4-triazine 8 favours the transition state 12 leading to 3,4-dihydropyridine 9. Elimination of amine yields 5 -phenyldihydrocyclopenta[c]pyridine 10 ... 

In contrast to the regioselectivity problems with the Cope elimination, the retro reaction, representing an addition to a double bond, may suffer from insufficient face selectivity, as was noticed with hydroxylamine 580. 

The oxy-Cope rearrangement generally is in competition with the retro-ene elimination reaction (6.112). These processes are considered in more detail in section 6.6. On occasions the Claisen rearrangement is also susceptible to side reactions. The first of these, the ortho-ortho Claisen rearrangement, involves... 

The reaction of nitrones with 3-butenylmagnesium bromide was used in the diastereoselective synthesis of cis -2,5-disubstituted pyrrolidines, arising from a Cope retro-elimination (Scheme 2.145) (Table 2.11) (201, 572). 

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