Stereoselective reactions. XXI. Asymmetric alkylation of α-alkyl β-keto esters to α,α-dialkyl β-keto esters having either (R)- or (S)-chiral quaternary center depending on the solvent system
摘要:
Asymmetric alkylation reaction of chiral enamines prepared from alpha-alkyl beta-keto esters and (S)-valine tert-butyl ester leading to either enantiomer is described. Lithiated chiral enamines can be alkylated with alkyl halides in a toluene solvent in the presence of HMPA to give, after hydrolysis, alpha,alpha-dialkyl beta-keto esters in 70-99%ee. The reactions in the presence of THF, dioxolane, or trimethylamine. instead of HMPA, afford the corresponding antipodes with enantiomeric purities of 44-92%ee. The present method provides a procedure for the synthesis of both enantiomers of alpha,alpha-dialkyl beta-keto esters in high enantiomeric purities starting from the same chiral enamines.
Asymmetric oxidation of β-ketoesters with benzoyl peroxide; enantioselective formation of protected tertiary alcohols
作者:Junning Lee、Shunichi Oya、John K. Snyder
DOI:10.1016/s0040-4039(00)79421-4
日期:1991.10
Asymmetric oxidation of β-ketoesters by the benzoylperoxide quench of their lithioenamines formed with (S)-valine-t-butyl esters has been accomplished with good enantioselectivity for the formation of tertiary benzoate esters. This procedure thus enables generation of tertiary alcohols in protected form.
Construction of contiguous quaternary and tertiary carbon centres via the asymmetric Michael reaction
作者:Kiyoshi Tomioka、K?suke Yasuda、Kenji Koga
DOI:10.1039/c39870001345
日期:——
Michaelreactions of chiral lithioenamines of α-alkyl β-oxo esters with methyl ethylidenemalonate afforded, after hydrolysis, adducts having contiguousquaternary and tertiarycarboncentres, with nearly complete enantio- and diastereo-selectivity.
Factors controlling the diastereoface selectivity in the complementary asymmetric alkylation of α-alkyl β-keto esters
作者:Kiyoshi Tomioka、Kaori Ando、Yutaka Takemasa、Kenji Koga
DOI:10.1016/s0040-4039(01)91410-8
日期:1984.1
Complementary asymmetricalkylation reaction of the lithioenamine derived from 2-methoxycarbonylcyclohexanone and (S)-valine tert-butyl ester was examined by employing the various electron pair donating additives in a toluene solvent, in order to clarify the factors controlling the diastereoface selectivity.
作者:Junning Lee、Jiahe Li、Shunichi Oya、John K. Snyder
DOI:10.1021/jo00046a009
日期:1992.9
The ultrasound-promoted Diels-Alder reaction of 3-methyl-4,5-benzofurandione (10) with appropriately substituted vinylcyclohexenes has led to the synthesis of tanshindiol A (9), a minor component of the Chinese traditional medicine, Dan Shen, prepared from the roots of Salvia miltiorrhiza Bunge. The regioselectivity of the cycloaddition was enhanced by the ultrasonication and was also strongly influenced by the solvent. The asymmetric synthesis of (+)-(4R)-tanshindiol A [(+)-(4R)-91, the naturally occurring enantiomer, proceeded from the optically enriched methyl 6-(benzoyloxy)-1-vinylcyclohexene-6-carboxylate [(-)-(R)-15] or 6-hydroxy-6-(hydroxymethyl)-l-vinylcyclohexene acetonide [(+)-(R)-291 (both 92% ee). Enantioselectivity in the preparation of these diene systems was accomplished by the benzoyl peroxide quench of the lithioenamine derived from methyl 1-oxocyclohexane-2-carboxylate and (S)-valine tert-butyl ester. This procedure, adapted from Koga's asymmetric alkylation of alpha-alkyl beta-keto esters, proved highly effective in preparing the necessary tertiary alcohol in protected form and with high optical purity.
TOMIOKA, KIYOSHI;ANDO, KAORI;TAKEMASA, YUTAKA;KOGA, KENJI, J. AMER. CHEM. SOC., 1984, 106, N 9, 2718-2719