1,4‐Additions of PhCu · BF3, n‐Bu · BF3 and MeCu · BF3 to the trans‐8‐phenyl‐menthyl enoates 1 proceeded with high chiral induction. Saponification of the resulting esters 2 gave the corresponding enantiomerically pure β‐substituted alkanoic acids 3 and the recovered (−)‐8‐phenylmenthol in good overall yields. Analogous additions to the cis‐crotonate 1 led preferentially to the acids 3 enantiomeric to those obtained from the trans‐crotonate 1, although with lower selectivity. A stereochemical model is proposed consistent with the observed results (Scheme 2, Table).
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