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. 2019 Dec 26;141(51):20053-20057.
doi: 10.1021/jacs.9b12068. Epub 2019 Dec 16.

Lewis Acid Catalyzed Enantioselective Photochemical Rearrangements on the Singlet Potential Energy Surface

Malte Leverenz  1 , Christian Merten  2 , Andreas Dreuw  3 , Thorsten Bach  1
Affiliations

Lewis Acid Catalyzed Enantioselective Photochemical Rearrangements on the Singlet Potential Energy Surface

Malte Leverenz et al. J Am Chem Soc. .

Abstract

The oxadi-π-methane rearrangement of 2,4-cyclohexadienones to bicyclic ketones was found to proceed with high enantioselectivity (92-97% ee) in the presence of catalytic amounts of a chiral Lewis acid (15 examples, 52-80% yield). A notable feature of the transformation is the fact that it proceeds on the singlet hypersurface and that no triplet intermediates are involved. Rapid racemic background reactions were therefore avoided, and the catalyst loading could be kept low (10 mol %). Computational studies suggest that the enantioselectivity is determined within a Lewis acid bound singlet intermediate via a conical intersection. The utility of the method was demonstrated by a concise synthesis of the natural product trans-chrysanthemic acid.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Scheme 1
Scheme 1. Photochemical Reactivity Modes of Complexes between a Chiral Lewis Acid (L.A.*) and a Substrate
Figure 1
Figure 1
UV/vis spectra of 2,4-cyclohexadienone 1a in the absence and in the presence of different equivalents of BF3·OEt2 (c = 2 mM in CH2Cl2, rt).
Scheme 2
Scheme 2. Lewis Acid Catalyzed Photochemical Rearrangement 1a → 2a and ent-2a
Scheme 3
Scheme 3. Enantioselective Total Synthesis of trans-Chrysanthemic Acid (6)
Acac = acetylacetone; bpy = 2,2′-bipyridine; DCE = 1,2-dichloroethane, DIC = N,N′-dicarbonyldiimide; DMF = N,N-dimethylformamide; Me = methyl; NTCP = N-tetrachlorophthaloyl; THF = tetrahydrofuran.
Figure 2
Figure 2
Reaction mechanism of the photochemical rearrangement illustrated for substrate 1a.
Figure 3
Figure 3
Migration of carbon atom C5 in Lewis acid complex 1a·L.A. as the enantioselectivity-determining step leading to either enantiomer 7 or ent-7.
See this image and copyright information in PMC

References

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