Synthesis and solvolysis of tricyclo [ 3.2.1.0 2,4] oct-8-7l derivatives
Creators: name: Haywood-Farmer, John Stewart
Publisher: name: University of British Columbia
Year: 2011

description: Some of the evidence supporting the olefin-like character of cyclopropyl groups, the double-bond assisted solvolysis of 7-norbornenyl derivatives and the ability of the cyclopropyl group to anchimerically assist ionization is reviewed. With regard to the high rate of solvolysis of 7-norbornenyl esters, the similarity of olefins and cyclopropyl groups and the steric features of the tricyclo [3.2.1.0 ²'⁴] octane ring system, a solvolytic study of tricyclo [3.2.1.0²'⁴] oct-8-yl derivatives was undertaken to determine whether a suitably oriented cyclopropyl group can also provide a large anchimeric assistance to ionization. The synthesis of the four isomeric tricyclo [3.2.1.0²'⁴] octan- 8-ols (II-OH, VIII-OH, XVI-OH and XVII-OH) is described. The cuprous chloride-catalyzed addition of diazomethane to anti-7-norbornenol gave pure exo-anti-tricyclo [3.2.1.0²'⁴] octan-8-ol (II-OH). Addition of diazomethane to 7,7-dimethoxynorbornene gave exo-8,8-dimethoxytricyclo [3.2.1.0²'⁴] octane (Vl) which was hydrolyzed to the corresponding ketone (VII) with acid. Reduction of the exo ketone afforded pure exo-syn-tricyclo [3.2.1.0²'⁴] octan-8-ol (VIII-OH). Attempts to prepare endo methylene adducts from norbornenes failed, although they were obtained from norbornadienes. The endo-tricyclo [3.2.1.0²'⁴] octan-8-ols were therefore prepared by a different route. Reaction' of 5,5-dimethoxy-l,2,3,'f-tetrachlorocyclopentadiene with cyclo- propene gave the endo Diels-Alder adduct (Xii). Dechlorination of this compound with metallic sodium, followed by catalytic hydrogenation, gave endo-8,8-dimethoxytricyclo [3.2.1.0²'⁴] octane (XIV). This ketal, upon acid hydrolysis, afforded the endo ketone (XV) which was reduced by a variety of reagents to give mixtures of the two endo alcohols. Preparative gas-liquid phase chromatography (glpc) gave pure samples of endo-syn and endo-anti-tricyclo [3.2.1.0²'⁴] octan-8-ols (XVI-OH and XVII-OH). Acetolysis of the p-bromobenzenesulfonates (brosylates) of the endo-syn and exo-syn alcohols in acetic acid and hydrolysis of the p-nitro- benzoates of the endo-anti alcohol and of anti-T-norbornenol in 70% aqueous dioxane gave relative rates of ionization of 37, 10⁴, 10¹² and 10⁹, respectively, compared to the solvolytic rate of a 7-norbornyl derivative at 100°. The exo-anti derivative (II-OBs) had previously been shown to have a relative solvolytic rate of 1.65 under these conditions. Acetolysis of the exo-syn and endo-syn brosylates gave rise to mixtures of entirely rearranged products which were not identified. Hydrolysis of the endo-anti p-nitrobenzoate in 70% aqueous dioxane gave rise only to endo-tricyclo [5-1.0.0⁴'⁸] oct-3-yi (XVIIl) products. The kinetic and product study data are consistent with the proposal that the endo-anti derivatives ionize to the 2,4-ethanotrishomocyclopropenyl cation. The stereochemistry of the reactions by which the tricycle [3.2.1.0²'⁴] octane ring system is formed, the effect of the stereochemistry of this ring system on the spectral characteristics of its derivatives and the mechanistic implications of the kinetic and product study data are discussed. [Symbols and illustrations omitted].