Gold-Catalyzed Enantio- and Diastereoselective Syntheses of Left Fragments of Azadirachtin/Meliacarpin-Type Limonoids

Meliacarpin-type limonoids are an
important class of organic insecticides.
Their syntheses are challenging due to their chemical complexity.
Here, we report the highly enantio- and diastereoselective synthesis
of the left fragments of azadirachtin I and 1-cinnamoylmelianolone,
being two important family members of meliacarpin-type limonoids,
via pairwise palladium- and gold-catalyzed cascade reactions. Gold-catalyzed
reactions of 1,7-diynes were performed as model studies, and the efficient
construction of tetracyclic late-stage intermediates was achieved
on the basis of this key transformation. Our unique route gave both
of the left fragments in 23 steps from the commercially available
chiral starting material (−)-carvone. This study significantly
advances research on the synthesis of the meliacarpin-type limonoids.