An Unexpected [3 + 2]-Cycloaddition between Spiro-Epoxyoxindoles and γ-Deconjugated Butenolides Catalyzed by Sc(OTf)₃

Area of Science:

• Organic Chemistry
• Catalysis
• Synthetic Methodology

Background:

• Spiro-oxindoles are valuable heterocyclic compounds with diverse biological activities.
• Constructing complex spiro-oxindole frameworks with multiple stereocenters remains challenging.
• Traditional methods often require harsh conditions or multiple steps.

Purpose of the Study:

• To disclose a unique [3 + 2]-cycloaddition reaction for spiro-oxindole synthesis.
• To develop a catalytic method for constructing fused bicyclic γ-lactones.
• To achieve high stereoselectivity in the formation of three consecutive stereocenters.

Main Methods:

• Employing a [3 + 2]-cycloaddition between γ-deconjugated butenolides and spiro-epoxyoxindoles.
• Utilizing scandium triflate (Sc(OTf)3) as a catalyst.
• Performing the reaction under mild conditions in a single operation.

Main Results:

• Successfully synthesized a variety of unexpected [3 + 2]-cycloaddition adducts.
• Constructed spiro-oxindoles fused with bicyclic γ-lactones.
• Achieved good yields (up to 88%) and excellent diastereoselectivities (>20:1).
• Demonstrated the formation of three consecutive stereocenters in one step.

Conclusions:

• The disclosed catalytic cycloaddition offers an efficient route to complex spiro-oxindoles.
• This method provides a stereoselective and mild approach to valuable heterocyclic scaffolds.
• The strategy overcomes limitations of traditional synthetic strategies for these compounds.