Enantioselective Claisen rearrangement of difluorovinyl allyl ethers

Abstract

The enantioselective Claisen rearrangement of difluorovinyl allyl ethers was achieved, for the first time, in moderate to good enantioselectivity using a chiral boron reagent as the Lewis acid.

References

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6. Optical rotation ([α]_D) was measured in CHCl₃ at 26 °C. 3a: 41.1; 3b: 18.6; 3c: –18.3; 3d: –8.8; 3e: –20.3.
7. In the absence of Lewis acid, the E isomer 2b smoothly rearranged to 3b even at room temperature, possibly due to the presence of the phenolic hydroxy group, to form an intramolecular hydrogen bond between the ethereal oxygen.
8. The relative stereochemistry of compound 6 was determined via conversions to 10 and a NOESY experiment, as shown in Scheme 4.
9. Regarding the removal of the hydroxyphenyl moiety, we examined some conditions, i.e. oxidative degradation of the aromatic ring and cleavage of the carbon–carbon bond of the aryl ketone moiety (Baeyer–Villiger and Schmidt rearrangement). In these experiments, although the aromatic ring was absent from the ¹H NMR analysis of the crude mixture, we were unable to find a clean method for cleavage of the hydroxyphenyl moiety.