RACEMIZATION OF CAMPHOR DURING PI SULFONATION

(±--Camphor-8-14C was prepared by a reaction sequence involving 14C-carbonation of 3-methylnorbornanone, reaction of the product with methylmagnesium iodide, followed by rearrangement of the β-hydroxy acid to l,7-dimethylnorbornane-7-carbo-2-lactone. Reduction of the lactone to a glycol followed by selective oxidation of the secondary hydroxyl, replacement of the primary hydroxyl by bromine, and hydrogenolysis of the bromine completed the synthesis. Bartlett's use of 2-acetoxypropenonitrile as a dienophile in the synthesis of norbornanone was extended to 2-acetoxy-2-butenonitrile for the Diels-Alder synthesis of 3-methylnorbornanone, and explicit proof that camphor-π-sulfonic acid is the 9- or anti-sulfonic acid is provided. Racemization of camphor, with and without π sulfonation, is shown to involve two exo-methyl shifts, one before and one after sulfonation, and a 2,6-hydride shift in the actual racemization step. The consequence is an exchange of the 8- and 10-carbons when one enantiomer is converted to the other, the 9-carbon (sulfonated or not) remaining stationary. It appears that sulfonation accelerates the racemization step, and an explanation is offered for this. © 1969, American Chemical Society. All rights reserved.