Trans- s. Isomers, Rearrangement

Rearrangement of sulfoxides 38a, b exhibited the interplay of several conformational factors. Both diastereomers afford predominant axial (trans) alcohol, but with opposite absolute configuration. The (R, R)-diastereomer strongly prefers the exo-transition state, whereas the (R, S)-isomer prefers the endo conformation. Hoffmann interprets these results in terms of an approximately 3-fold preference for the exo-transition state but a 6-fold preference for formation of an axial bond, these effects reinforcing each other in one isomer but opposing each other in the second. 

While use of the William s sludge catalyst, discussed above, and the exo-trans-exo isomer of 14 dramatically improves the isolable yield of diamantane (from 1 to 10 %), 20>31) the preparative utility of this reaction is limited. This problem is overcome using a different precursor. Hydrogenation of the Binor-S dimer of norbornadiene followed by rearrangement (Eq. (7)) gives diamantane in an average overall yield of 65 %3l ... 

Silver s role in the valence isomerization of bicyclobutane to butadiene, however, is most striking and deserves special attention. Masamune et cd., studying the silver perchlorate-catalyzed valence isomerization of exo.exo-and e (7o,ew-2,4-dimethylbicyclobutane, reported largely stereospedfic conversion to trans,trans- and ds,7ra s-2,4-hexadiene, respectively 49). At 5 °C, the isomerization of the ew,e o-bicyclobutane was 77% stereospecific, while the endo,exo isomer was 99% specific. Significantly, both isomers rearranged along the symmetry-forbidden Ya a + a2a] path in preference to alternative, symmetry-allowed [[Pg.85]

Photochemical Fe(CO)s-induced rearrangement of silylated allyl amine 9 gave N-silylated enamine 10, which on subsequent Cu-catalyzed cyclopropanation by methyl diazoacetate afforded cyclopropane derivative 11. The use of an optically active catalyst gave an asymmetric induction of 56% ee for the cis isomer and 20% ee for the trans isomer. Further acid-induced ring cleavage afforded the j8-formyl ester 12, whereas reduction and desilylation produced aminocyclopropane carboxylic acid 13 (equation 2). 

In these syntheses, Peppard had to rely on the very weak trans effect in the cobalt complex [Co(NH3) i (SO3) 2 ]-. The cis-dichloro-cis-diammineethylenediaminecobalt(III) ion is asymmetric, and Peppard partially resolved it into its enantiomeric forms by adsorption on quartz ground to 100 mesh. This method of resolution is frequently, but not always, effective. Before it can be fully utilized, we will need to learn a great deal more about the principles of adsorption. The preparation of Peppard s isomer s is particularly interesting because cobalt(III) complexes rearrange easily, whereas the platinum compounds do not. 

The situation is more complicated in the case of the pinacolic rearrangement of the isomeric cyclopentane-1,2-diols (Bunton and Carr, 1963b). In aqueous perchloric acid, cis-1, 2-dimethylcyclopentane-l,2-diol (19) is converted into a mixture of the polymeric cyclopentadiene). No O18 is found in the unrearranged cis-diol. The trans-1,2-dimethylcyclopentane-1,2-diol (20), on the other... 

Possible synthetic routes to frinvestigated using the /J-hydroxy-t/3-phosphane oxide alkene inversion procedure. A stepwise approach via the epoxy-trans-cyclononene derivative failed. However, consecutive double elimination involving the bis-/ -hydroxy-) 5-phosphane oxides 11 and 12 gives cis- and trails- 1,2-di-vinylcyclopentanes which are believed to be formed by a rapid Cope rearrangement of parallel and crossed r[Pg.362]

Isomers, Rearrangement Trans... s. a. Interdiange Transannular s. Cycloaddition, transannular. Ring closure, -Transesterification... 

Cyclopropane derivatives can be prepared by several methods. Michael addition of the enolate of ethyl chloroacetate to ethyl acrylate generated the cyclopropane ring in 7.223 via addition to form a carbanion and internal expulsion of the chlorine moietyl Manipulation of functional groups allowed selective reduction to 7.224 and conversion to 7.225 (as a mixture of cis- and trans-isomers). Rearrangement and hydrolysis led to c/s-2-(2-amino-l-cyclopropyl)ethanoic acid, 7,226. The analogous cyclobutane derivatives were also prepared by a similar route. 

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