Conjugate addition of Grignard reagents to N-(α,β-unsaturated)acylpyrazoles.: Diastereoselective β-alkylation using 3-phenyl-l-menthopyrazole

The conjugate additions of N-(alpha,beta-unsaturated)acylpyrazoles were carried out by the treatment with Grignard reagents in the presence of cuprous halides. The reaction of 2-(alpha,beta-unsaturated)acyl-3-phenyl-l-menthopyrazoles 3a-h occurred in higher chemical yields and with asymmetric inductions on beta-position, where the addition of magnesium bromide as a Lewis acid influenced to the yields and the diastereoselectivities. In the case of alpha-methylated 2-(alpha,beta-unsaturated)acyl-3-phenyl-l-menthopyrazoles 3i-n, the excellent asymmetric induction on the alpha-position was also observed through the diastereofacial protonation.