Grignard additions of methylmagnesium iodide to fenchone and dehydrofenchone : Configurations of 1,2,3,3-tetramethylnorbornan-2-ols and 1,2,3,3,-tetramethyl-5-norbornen-2-ols

Abstract

In the performed work the structures of the tertiary alcohols formed in the Grignard additions of fenchone and methylmagnesium iodide as well as of dehydrofenchone and methylmagnesium iodide have been investigated. It has been shown by structural proof that the alcohol obtained as main product (95 % of the alcohol part) of the Grignard addition of fenchone is 2-exo-methyl-2-endo-fenchol. The by-product of the reaction (5 % of the alcohol part) is 2-endo-methyl-2-exo-fenchol. The obtained result is in agreement with the results of analogous investigations performed on other ketones of the bicyclo[2.2.1]heptane series and thus strength tens the conception of steric factors in these compounds controlling the course of the addition reaction in question. In the Grignard addition of optically active dehydrofenchone synthesized in connection with this work tertiary alcohol isomers are formed in the ratio 57:43. The main product is 2-exo-methyl-2-endo-dehydrofenchol and the byproduct 2-endo-methyl-2-exo-dehydrofenchol. The structures of the compounds have been confirmed by spectrometric and chemical methods. Although the result deviates noticeably from the general stereochcmical course of the Grignard additions of other ketones of the bicyclo[2.2.1]heptene series, it anyhow gives strong support to the significance of steric factors in this reaction.