Abstract
We report the stereoselective synthesis of a left-handed trefoil knot from a tris(2,6-pyridinedicarboxamide) oligomer with six chiral centers using a lanthanide(III) ion template. The oligomer folds around the lanthanide ion to form an overhand knot complex of single handedness. Subsequent joining of the overhand knot end groups by ring-closing olefin metathesis affords a single enantiomer of the trefoil knot in 90% yield. The knot topology and handedness were confirmed by NMR spectroscopy, mass spectrometry, and X-ray crystallography. The pseudo-D3-symmetric knot was employed as an asymmetric catalyst in Mukaiyama aldol reactions, generating enantioselectivities of up to 83:17 er, which are significantly higher than those obtained with a comparable unknotted ligand complex.
Highlights
We report the stereoselective synthesis of a left-handed trefoil knot from a tris(2,6-pyridinedicarboxamide) oligomer with six chiral centers using a lanthanide(III) ion template
We describe the assembly of a trefoil knot of single handedness by entwining a ligand strand with six asymmetric carbon atoms around a lanthanide ion template.[8]
We find that the chiral trefoil knot is an effective catalyst for the asymmetric Mukaiyama aldol reaction
Summary
Substituting Eu(CF3SO3)[3] for Lu(CF3SO3)[3] in the reactions shown in Scheme 1 generated the corresponding europium trefoil knot complex Λ-Eu(R6)-2(CF3SO3)[3] (see the Supporting Information). The solid-state X-ray structure confirmed the molecular topology and showed that the trefoil knot is of Λ handedness Acetonitrile gave both the highest conversions and the most promising levels of enantioenrichment In comparison with both open complex Λ-Eu-6 (Figure 3 table, entry 2) and the previously reported7d trefoil knot, Λ-Eu-7 (Figure 3 table, entry 3), Λ-Eu(R6)-2 generated product 5 with improved enantiomeric enrichment (65:35 er; Figure 3 table, entry 1).
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