Chemoenzymatic Asymmetric Total Syntheses of Antitumor Agents (3R,9R,10R)- and (3S,9R,10R)-Panaxytriol and (R)- and (S)-Falcarinol from Panax ginseng Using an Enantioconvergent Enzyme-Triggered Cascade Reaction
摘要:
Total asymmetric synthesis of two components of Panax ginseng showing antitumor activity, i.e., (3R,9R,10R)- and (3S,9R,10R)-Panaxytriol and of both enantiomers of Falcarinol was accomplished. Due to the fact that the synthetic strategy was based on enantio-convergent biotransformations, the occurrence of any undesired stereoisomer was entirely avoided. The absolute configuration of naturally occurring Panaxytriol was confirmed to be (3R,9R,10R) on the basis of optical rotation values. It was shown that enzyme-triggered cascade reactions represent a valuable tool for the synthesis of natural products.
Chemoenzymatic Asymmetric Total Syntheses of Antitumor Agents (3R,9R,10R)- and (3S,9R,10R)-Panaxytriol and (R)- and (S)-Falcarinol from Panax ginseng Using an Enantioconvergent Enzyme-Triggered Cascade Reaction
摘要:
Total asymmetric synthesis of two components of Panax ginseng showing antitumor activity, i.e., (3R,9R,10R)- and (3S,9R,10R)-Panaxytriol and of both enantiomers of Falcarinol was accomplished. Due to the fact that the synthetic strategy was based on enantio-convergent biotransformations, the occurrence of any undesired stereoisomer was entirely avoided. The absolute configuration of naturally occurring Panaxytriol was confirmed to be (3R,9R,10R) on the basis of optical rotation values. It was shown that enzyme-triggered cascade reactions represent a valuable tool for the synthesis of natural products.
An increase of α-regioselectivity was observed in the reaction of allylic barium reagents with aldehydes employing crown ether as an additive. For example, an α/γ regioselectivity was improved to 98/2 from 92/8, when an equimolar amount of 18-crown-6 was added to geranylbarium reagent in THF at -78 °C prior to the reaction with benzaldehyde.
α-Selective Allylation of Azo Compounds Using Allylic Barium Reagents
作者:Akira Yanagisawa、Takuya Jitsukawa、Kazuhiro Yoshida
DOI:10.1055/s-0032-1318306
日期:——
The addition of allylic barium reagents to azo compounds was achieved with high α-regioselectivity. The double-bond geometry of allylic barium reagents was retained throughout the reaction at –78 °C and E- or Z-enriched allylic hydrazines were selectively obtained from the corresponding allylic barium reagents. An allylic hydrazine was efficiently converted into an allylic amine by reductive N–N bond
以高α-区域选择性将烯丙基钡试剂添加到偶氮化合物中。烯丙基钡试剂的双键几何结构在 –78 °C 的整个反应过程中保持不变,并且从相应的烯丙基钡试剂中选择性地获得富含 E 或 Z 的烯丙基肼。通过还原性 N-N 键断裂,烯丙基肼被有效地转化为烯丙基胺。
Regio- and Stereoselective Synthesis of 1,5-Dienes Using Allylic Barium Reagents
The highly α,α′ selective and stereocontrolled homocoupling reaction of allylic halides was achieved using barium reagent. The double-bond geometry of the starting allylic chloride was completely retained. The α,α′ cross-coupling products were also prepared stereospecifically and regioselectively by this method.
作者:Noel P. McLaughlin、Eibhlín Butler、Paul Evans、Nigel P. Brunton、Anastasios Koidis、Dilip K. Rai
DOI:10.1016/j.tet.2010.10.049
日期:2010.12
A short, practical synthesis of the bis-acetylenic natural product falcarinol 1 is reported. This method relies on the alternate functionalisation of bis-trimethylsilylbutadiyne 10. This may be achieved in one-pot, however, better yields were obtained more conventionally. Lipase mediated enzymatic kinetic resolution of the racemic adduct in an organic solvent afforded (+)-1 in 97% enantiomeric excess
The first direct preparation of allylbarium reagents by reaction of insitu generated reactive barium with various allylic chlorides and their new and unexpected selective allylation reactions with carbonyl compounds are disclosed. Highly reactive barium was readily prepared by the reduction of barium iodide with 2 equiv of lithium biphenylide in dry THF at room temperature. A variety of carbonyl compounds reacted with barium reagents generated from (E)- or (Z)-allylic chlorides in THF at -78 degrees C. All reactions resulted in high yields with remarkable alpha-selectivities not only with aldehydes but also with ketones. The double bond geometry of the starting allylic chloride was completely retained in each case. Stereochemically homogeneous (E)- and (Z)beta,gamma-unsaturated carboxylic acids were easily prepared in good yields by highly alpha-selective carboxylation of allylic barium reagents with carbon dioxide. A selective Michael addition reaction with alpha,beta-unsaturated cycloalkanone was also achieved using an allylbarium reagent. Treatment of 2-cyclopentenone (1 equiv) with allylbarium chloride (2 equiv) in THF at -78 degrees C for 20 min afforded 3-allylcyclopentanone in 94% yield with a 1,4/1,2 ratio of >99/1. Furthermore, the in situ generated barium enolate was efficiently trapped with various kinds of electrophiles (Me(2)C=CHCH2Br, (C5H11CHO)-C-n, and CH3COCl).