Substrate-Controlled Regioselectivity Switchable [3 + 2] Annulations To Access Spirooxindole Skeletons

The additive-free [3 + 2] annulation
from isatins, amino acids
with 2-styrylbenzoxazoles, was described, providing a series of functional
and structurally complex 3,3′-pyrrolidinyl-spirooxindole derivatives
containing four contiguous and two quaternary stereogenic centers
in high yields (up to 95%) and excellent diastereoselectivities (up
to >25:1 dr). Interestingly, the reaction exhibits switchable regioselectivity
depending on the substrate of amino acids. With proline or thioproline
as the substrate, the reaction afforded α-regioselective spirooxindole
skeletons. In contrast, when piperidine acid is the substrate, the
reaction provided γ-regioselective spirooxindole skeletons.