Abstract
Metal-catalyzed enantioselective construction of the loosening axial allene chirality spreading over three carbon atoms using a chiral ligand is still a significant challenge. In the literature, steric effect of the substrates is the major strategy applied for such a purpose. Herein, we present a general palladium-catalyzed asymmetrization of readily available racemic 2,3-allenylic carbonates with different types of non-substituted and 2-substituted malonates using (R)-(−)-DTBM-SEGPHOS as the preferred ligand to afford 1,3-disubstituted chiral allenes with 90~96% ee. This protocol has been applied to the first enantioselective synthesis of natural product, (R)-traumatic lactone. Control experiments showed that in addition to the chiral ligand, conducting this transformation via Procedure C, which excludes the extensive prior coordination of the allene unit in the starting allene with Pd forming a species without the influence of the chiral ligand, is crucial for the observed high enantioselectivity.
Highlights
Metal-catalyzed enantioselective construction of the loosening axial allene chirality spreading over three carbon atoms using a chiral ligand is still a significant challenge
Unlike other unsaturated hydrocarbons such as alkenes and alkynes, allenes are unique due to the intrinsic axial chirality. 1,3-Disubstituted allenes with synthetically useful functionalities are of particular high interest due to their common existence in natural products/drug molecules (Fig. 1)[18] and their great potentials in the synthesis of natural products[19,20,21,22,23,24]
In view of the loosening nature of 1,3-substituted allene axial chirality spreading over three carbon atoms, the steric effect of the substrates is the major strategy used in such catalytic reactions of high ee (Fig. 2a)
Summary
Metal-catalyzed enantioselective construction of the loosening axial allene chirality spreading over three carbon atoms using a chiral ligand is still a significant challenge. In view of the loosening nature of 1,3-substituted allene axial chirality spreading over three carbon atoms, the steric effect of the substrates is the major strategy used in such catalytic reactions of high ee (Fig. 2a).
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