Carbocyclizations olefin isomerization

The asymmetric synthesis of (+)-Codeine 432 devised by White and colleagues included a Beckmann rearrangement to introduce the nitrogen atom in the carbocyclic structure (equation 182). Even though two isomeric lactams 430 and 431 were obtained as a result of the rearrangement, the preferential migration of the bridgehead carbon atom produced 430 as the predominant isomer. The synthesis of the non-natural enantiomer of Codeine was completed after oxidation, olefin formation and reduction. 

During our further studies of ketone catalysts, ketone 16 was found to be highly enantioselective for a number of acyclic and cyclic d.s-olefins (Table 10.6).73-74 It is important to note that the epoxidation is stereospecific with no isomerization observed in the epoxidation of acyclic systems. Ketone 16 also provides encouragingly high ee s for certain terminal olefins, particularly styrenes.74-75 In general, ketones 1 and 16 have complementary substrate scopes. In our subsequent study of the conformational and electronic effects of ketone catalysts on epoxidation, ketone 17, a carbocyclic analog of 16, was found to be highly enantioselective for various styrenes (Table 10.7).76... 

Radical cyclization reactions have become a powerful tool for the constniction of carbocyclic and heterocyclic frameworks. Irradiation of 368 in the presence of benzophenone (BP) for 20 min provided the spiro 1,3-dithiolane 370 in 63% yield along with the substtate (14%) via the radical intermediate 369 (Scheme 47). The reaction carried out in the presence of acetone or acetophenone, instead of BP, resulted only in isomerization of the olefin geometry <1996T9713>. 

Unstable metallacycloalkane intermediates play an important role in a variety of transition-metal-catalyzed isomerizations or rearrangements of strained carbocyclic systems and olefin metathesis reactions ... 

The 1 1 complex derived from phenyltungsten trichloride and aluminium trichloride is an effective catalyst for diene-cyclobutane metathetical interconversions. Thus, the tetracyclic compounds (291) and (292) were respectively isomerized to the dienes (293) and (294). Rather more surprising was the virtually quantitative formation of the cyclobutanoid compound (296) from (295). Reaction of norbomadiene with 2,2 -bipyridyl(cyclo-octa-l,5-diene)nickel at 25°C yielded the exo-trans,endo-metal o-carbocyclic (297) which, on treatment with an activated olefin (e.g. maleic anhydride), afforded the cyclo-dimer (298 predominantly exo-trans,endo) in good yield by displacement of the hydrocarbon moiety. Catalytic conversions can also be achieved. 

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