Background:
The Diels-Alder reaction has been widely utilised in the syntheses of biologically important
natural products over the years and continues to greatly impact modern synthetic methodology. Recent
discovery of chiral organocatalysts, auxiliaries and ligands in organic synthesis has paved the way for their application
in Diels-Alder chemistry with the goal to improve efficiency as well as stereochemistry.
Objective:
The review focuses on asymmetric syntheses of flavonoid Diels-Alder natural products that utilize
chiral ligand-Lewis acid complexes through various illustrative examples.
Conclusion:
It is clear from the review that a significant amount of research has been done investigating
various types of catalysts and chiral ligand-Lewis acid complexes for the enantioselective synthesis of flavonoid
Diels-Alder natural products. The results have demonstrated improved yield and enantioselectivity. Much
emphasis has been placed on the synthesis but important mechanistic work aimed at understanding the
enantioselectivity has also been discussed.
The synthesis of novel l-prolinal dithioacetal and its application as an organocatalyst for the direct Michael addition of cyclic ketones and acetophenone derivatives to trans-β-nitrostyrene and related compounds is described. The prolinal dithioacetal acts as effective catalyst in the case of cyclic ketones of different ring size, in particular five- and six-membered examples, as well as larger and smaller ring systems. High enantioselectivity and diastereoselectivity is observed for different substrates and trans-β-nitrostyrenes. Also, the first asymmetric syntheses of selected 2-methyl-4-nitro-1,3-diphenylbutan-1-one derivatives by application of the obtained organocatalyst is presented.
Asymmetric Syntheses of 1-Aryl-2,2,2-trifluoroethylamines via Diastereoselective 1,2-Addition of Arylmetals to 2-Methyl-N-(2,2,2-trifluoroethylidene)propane-2-sulfinamide.
