Carbocyclizations positional isomerization

Another isomerization reaction of arene oxides is equilibrium with oxe-pins [5], Here, the fused six-membered carbocycle and three-membered oxirane merge to form a seven-membered heterocycle, as shown in Fig. 10.2. An extensive computational and experimental study involving 75 epoxides of monocyclic, bicyclic, and polycyclic aromatic hydrocarbons has revealed much information on the structural factors that influence the reaction rate and position of equilibrium [11], Thus, some compounds were stable as oxepins (e.g., naphthalene 2,3-oxide), while others exhibited a balanced equilibrium... 

As with the carbocyclic systems, the hydrogens on both N and C adopt axial and equatorial positions, and in the change from one conformer to the other the axial H inverts to the equatorial position and vice versa. There are many examples of ring systems in coordination chemistry which can adopt these conformational isomeric positions. Four systems currently being studied are the complexes of 1,2-propylenediamine, 1,2-f/ww-cyclo-hexanediamine, 1,2-fraw.y-cyclopentanediamine, and 2,3-butanediamine, and some reference will be made to each of these. 

The formation of 5-hexenal (reaction 18) is believed to be an intramolecular rearrangement since the addition of oxygen does not cause its suppression. At least in a methyl substituted cyclohexanone the analogous process has been shown to occur by the transfer of a hydrogen atom from the beta position to the carbonyl group before the fission of the six-membered carbocyclic ring (29) as only one of the two possible isomeric heptenals is formed. 

Takahashi and coworkers have employed intramolecular reactions of anions of unsaturated protected cyanohydrins to produce unsaturated carbocyclic systems containing 10-, 14- and 16-membered rings which are convertible into natural products. These reactions occur regiospecifically at the a-position and usually do not involve ( ) — ( isomerization of P,7-double bonds. The preparation of 2-cyclopen-tadecenone (155), which is convertible into the macrocyclic perfumes muscone and exaltone, is illustrative of this methodology (Scheme 76). 

Strained Carbocyclic Systems.— The rearrangement of polycyclic organic compounds catalysed by Ag+ and other metals has been reviewed. The silver(i)-catalysed isomerization of bishomocubanes (1) (R =Me or CHgOAc, R =C02Me) could follow four mechanistic pathways to give the two isomeric snoutane diesters (2) and (3) but the latter is formed preferentially (82% when R =Me 65% when R = CHgOAc) in both cases, The isomerizations of bishomocubanes substituted at the 9,10-positions, the C-4 corner, at C-4 and C-3, and at C-2 and C-3, to the structurally related snoutane follow the second-order rate law... 

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