Of isoborneol

Isoborneol yields camphor on oxidation, but it yields camphene on dehydration much more readily than borneol does. If a solution of isoborneol in benzene be heated with chloride of zinc for an hour, an almost quantitative yield of camphene is obtained. Pure borneol under the same conditions is practically unchanged. 

Propose a mechanistic pathway for the biosynthesis of isoborneol. A carbo-cation rearrangement is needed at one point in the scheme. 

Bornene has been prepared from the reaction of 2-bromo-bornane-3-carboxyIic acid with aqueous sodium bicarbonate,6 by pyrolysis of isoborneol methyl xanthate,7 and by the j8-elimination of hydrogen chloride from bornyl chloride with sodium alkoxides in various solvents.2... 

Camphor is produced by fractional distillation and crystallization of camphor oil or, synthetically, by dehydrogenation of isoborneol (from isobornyl acetate, see p. 73) over a copper catalyst. 

Method A To an ice-cold solution of 8.0 g (0.06 mol) of anhyd AICI, in 60 mL of F.t,0 are added 15 mL (0.015 mol) of a 1 M ethereal soln of LiAIH4. After a few minutes, 9.3 g (0.06 mol) of (—)-isoborneol (or borneol) in 1 5 mL of Ei,0 is added dropwise until the evolution of gas ceases. A slight excess of the alcohol is generally used to avoid any unreacted hydride. The reaction vessel is removed from the ice-bath and 0.05 mol of the ketone in 20 mL of F.t,0 is added within 15 min. The mixture is stirred at r.t. for 30 min and then cautiously decomposed with icc-cold 4 N H2SG4. The ethereal layer is separated, washed free of mineral acid and the solvent is evaporated through a Vigreux column. The crude products are analyzed by GC. For results and yields see Table 8, entries 1-68. 

In a thermodynamically controlled reaction, 3-oxo-e.vo-tricyclo[4.2.1.02 s]nonane is converted to e. vo-3-hydroxy-e.vo-tricyclo[4.2.1.02,5]nonane (85-95%). With sodium borohydride the ewfo-alcohol is obtained185. For example, the Meerwein-Ponndorf-Verley reduction of camphor yields a 70 30167 (75 2525, 69 31 186) mixture of the exo- and endol,7,7-trimethylbi-cyclo[2.2.1]heptan-2-ols (isoborncol and borneol). The equilibrium has been established to give a 29 71 mixture of isoborneol and borneol183, giving another example where the Meerwein-Ponndorf-Verley reduction leads predominantly to the less stable product. 

Brit. pat. 803,178 (1958), Prepn of the /-form by reduction of d-camphor with lithium aluminum hydride Trevoy, Brown, J. Am. Chem. Soc. 71, 1675 (1949). Separation of isoborneol from its ewfo-isomer, borneol, via the p-nitro-benxoare deriv Truett, Moulton, foe. cit. Resolution of the d/-form Pickard, Litflebury, foe, cit. Kenyon, Priston, ibid. 127, 1472 (1925). Configuration (isoborneol = c o-form borneol = eflrfo-form) Toivonen et al, Acta Chem. Scand. 3, 991 (1949), Review J. L. Simonsen, The Terpenes vol. 11 (University Press, Cambridge, 2nd ed., 1949) pp 365-367 A. R, Pinder, The Chemistry of the Terpenes (Chapman Hall, London, I960) pp 22-24, Id, 103, 105-107, 111-... 

Stock Standard Solutions (SS) Accurately weigh about 0.5g camphore, 0.075 g of isoborneol and borneol into a 5 mL volumetric flask, diluted to volume with ethyl acetate. 

Carbon-13 spectrum of isoborneol, CDCIg. (Small peaks at 9,19, 30, and 43 are due to impurities.)... 

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