Asymmetric Diels-Alder reactions catalyzed by chiral lanthanide(III) trifluoromethanesulfonates. Unique structure of the triflate and stereoselective synthesis of both enantiomers using a single chiral source and a choice of achiral ligands
✍ Scribed by Shū Kobayashi; Haruro Ishitani; Iwao Hachiya; Mitsuharu Araki
- Publisher
- Elsevier Science
- Year
- 1994
- Tongue
- French
- Weight
- 868 KB
- Volume
- 50
- Category
- Article
- ISSN
- 0040-4020
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✦ Synopsis
Chiral lanthanide trifluoromethanesulfonates (triflates) were developed and the unique structure of the triflates was revealed.
In the presence of a catalytic amount of the triflate, acyl-2,3-oxazolidin-2-ones reacted with cyclopentadiene to afford the Diels-Alder adducts in high yields and with high enantiomeric excesses.
According to the r~ctions, both enuntiom~s of the ~iels-Alder adducts were stffeoselectiuely prepared by using a single chira~ source, R-(+)-binaphthol, and a choice of achiral ligands. When 3-acetyl-2,3-oxazolidin-2-one was combined with the original catalyst system consisting of lanthanide triflate, R-t+)binaphthol, and cis-1,2,6-trimethylpiperidine, a new catalyst was generated (catalyst A). In the presence of this catalyst, 3-acyl-2,3-oxazolidin-2-ones reacted with cyclopentadiene to afford the endo adducts in high enantiomeric excesses.
The absolute con~guration of the products was 25, 3R. On the other hand, when 3-phenylacetylacetone PAA) was mixed wifh the original catalyst system (catalyst l3), reverse enantiofacial selectivities were observed and endo adducts with the absolute configuration 2R, 35 were obtained in high enantiomeric excesses.
Recently, some efficient asymmetric
Diels-Alder reactions catalyzed by chiral Lewis acids have been reported.11 The Lewis acids employed in these reactions are generally based on traditional acids such as titanium, boron, or aluminum reagents, and they are well modified to realize high enantioselectivities. Although lanthanide compounds were expected to be Lewis acid reagents, only a few asymmetric reactions catalyzed by chiral lanthanide Lewis acids were reported. Danishefsky's pioneer work demonstrated that Eu(hf& (an NMR shift reagent) catalyzed hetero-Diels-Alder reactions of aldehydes with siloxydienes, but enantiomeric excesses were moderate.*) In our previous paper, we reported that lanthanide(III1 trifluoromethanesulfonates (lanthanide triflates, Ln(OTf)$, especially ytterbium(II9 triflate (Yb(OTf)3), were good catalysts in the Diels-Alder reaction of some dienophiles with cytlopentadiene.31 The reactions proceeded smoothly in the presence of a catalytic amount of YbfOTfl3 to give the corresponding adducts in high yields. Moreover, the catalyst was stable in water and was easily recovered