Preparation of β-Substituted γ-Keto Esters by the Grignard Reaction on N-Acylpyrazoles
摘要:
Various gamma -keto esters were prepared by either the alcoholysis of N-(4-oxoalkcanoyl)pyrazoles or the Grignard replacement of pyrazole moiety of 4-(N-pyrazolyl)-4-oxoalkanoic esters. By using 3-phenyl-l-menthopyrazole as a chiral auxiliary, beta -substituted gamma -keto esters were enantioselectively obtained.
Diastereoselective .alpha.-Alkylation of 2-Acyl-3-phenyl-l-menthopyrazoles
摘要:
N-Acylpyrazoles were alpha-alkylated in good yields by the treatment with alkyl halides after metalation with LDA or LiHMDS, In the case of chiral N-acylpyrazoles, e.g., 2-acyl-3-phenyl-l-menthopyrazoles (4), the alpha-alkylation was highly diastereoselective. The subsequent alpha-alkylation products could be converted into esters in good yield in the presence of BF3.OEt(2) without the loss of the optical purity.
Conjugate addition of grignard reagents to<i>N</i>-(α,β-unsaturated)acylpyrazoles. Diastereoselective β-alkylation using 3-phenyl-<i>l</i>-menthopyrazole
The conjugate additions of N-(α,β-unsaturated)acylpyrazoles were carried out by the treatment with Grignardreagents in the presence of cuprous halides. The reaction of 2-(α,β-unsaturated)acyl-3-phenyl-l-menthopyrazoles 3a-h occurred in higher chemical yields and with asymmetric inductions on β-position, where the addition of magnesium bromide as a Lewis acid influenced to the yields and the diastereoselectivities
Enantiomerically enriched preparation of enolizable β-keto amides. Diastereoselective α-acylation and subsequent aminolysis of 2-acyl-3-phenyl-l-menthopyrazoles
After deprotonation with LDA, 2-acyl-3-phenyl-l-menthopyrazoles (13) were diastereomerically α-acylated to give N-(3-phenyl-l-menthopyrazolyl) β-ketoamides (14–19). The subsequentamides were converted into the corresponding N-alkyl amides (21–24) retaining their enantiomericenrichment on the α-position. These are the first examples of enolizableβ-keto acid derivatives having only one chiral center
The aldol reaction of 1-acyl-3,5-dimethylpyrazoles (1) was kinetically controlled with syn stereoselectivity through lithium enolate intermediate using LDA. On the contrary, the anti stereoselective aldol reaction of 1 was caused by the action of DIEA in the presence of MgBr2 under the thermodynamic control. in the formation of syn-aldol products using 3-phenyl-1-menthopyrazole as a chiral auxiliary, the diastereoselectivity was observed up to 81% de with the predominant configuration of 2'S form.
Diastereomeric Preparation of a- and b-Phenylthio Substituted N-Acylpyrazoles
作者:Choji Kashima、Katsumi Takahashi、Akira Hosomi
DOI:10.3987/com-95-s14
日期:——
When 3-phenyl-l-menthopyrazole was used as a chiral auxiliary, the diastereomeric preparation of alpha- and beta-phenylthio-substituted N-acylpyrazoles (4 and 9) was accomplished in high yield by the alpha-sulfenylation using diphenyl disulfide and the conjugate addition of thiophenol, respectively.
Mechanistic Insight into Catalytic Aerobic Chemoselective α-Oxidation of Acylpyrazoles