Catalyst-controlled regiodivergence and stereodivergence in formal cross-[4+2] cycloadditions: The unique effect of bismuth(III)

Abstract

The [4+2] cycloaddition is crucial for constructing six-membered rings in pharmaceuticals and natural products. Cross-[4+2] cycloadditions offer greater product diversity than traditional diene-dienophile reactions due to multiple possible pathways. However, precise control over regio- and stereoselectivity for various isomers remains a great challenge. This study reports catalyst-controlled regiodivergent formal cross-cycloadditions of acyclic dienes and enones, significantly enhancing access to diverse pyrazole-fused spirooxindoles. Chiral phosphoric acid (CPA) catalysis enables endoselective [4+2] cycloadditions, while Bi(III) with a CPA ligand yields [2+4] products with high regio- and stereoselectivity. A Claisen rearrangement of the [2+4] adduct produces the exo-selective [4+2] product, further increasing stereochemical diversity and enabling the synthesis of six regio- and stereo-isomers from a single substrate set. DFT calculations reveal that Bi(III) reverses regioselectivity by repositioning reactants in the CPA pocket and stabilizing the enone oxygen's negative charge. In addition, product 3as demonstrates therapeutic potential against triple-negative breast cancer, with an IC₅₀ of 8.5 μM in MDA-MB-453 cells.

Fig. 1.. Catalyst-controlled divergence in cross-[4+2] cycloadditions: Progress, challenges, and our strategy.

(A) Catalyst-controlled diastereodivergent cross-[4+2] cycloadditions. (B) Lewis acid (LA)–enabled regiodivergence in cross-[4+2] cycloadditions of Cp with enones. (C) This work: CPA/Bi(III)-controlled regio- and diastereodivergence in cross-[4+2] cycloadditions. (D) Origin of the switchable regioselectivity and the effect of Bi(III). TS, transition state.

Fig. 2.. Substrate scope of the CPA-catalyzed cross-[4+2] cycloaddition.

Reaction conditions: 1 (0.10 mmol), 2 (0.15 mmol), and C4 (10 mol%) in 2.0 ml of toluene at −10°C for 1.5 to 4 hours; isolated yield.

Fig. 3.. Substrate scope of the Bi(III)-catalyzed cross [2+4] cycloaddition.

Reaction conditions: 1 (0.10 mmol), 2 (0.15 mmol), Bi(OTf)₃ (10 mol%), and C2 (10 mol%) in toluene (2.0 ml) at room temperature (r.t.) for 5 to 30 min; isolated yield. Asterisk (*) means at 0°C.

Fig. 4.. Synthetic transformations, scale-up reactions, and divergent synthesis of six isomers.

(A) Scale-up reactions. (B) Synthetic transformations of [4+2] and [2+4] cycloadducts. (C) Regio- and stereodivergent synthesis of six isomers from 1a and 2. THF, tetrahydrofuran. TBAT, tetrabutylammonium difluorotriphenylsilicate.

Fig. 5.. Evaluation of the antitumor activity of 3as and stereoisomers.

(A) IC₅₀ values of the 3as series in tumor cells were determined using the MTT assay. (B to D) Cell scratch assay demonstrated that 3as significantly inhibited the migration of TNBC cells, with corresponding quantitative data provided. Scale bars, 400 μm. (E and F) Colony formation assay confirmed that 3as effectively suppressed the proliferation of TNBC cells, with corresponding quantitative data provided. (G and H) Transwell analysis revealed that 3as exhibited antimigratory activity against TNBC cells, with corresponding quantitative data provided. (I and J) WB experiments indicated that 3as modulated the expression of proteins associated with tumor metastasis and apoptosis, with glyceraldehyde-3-phosphate dehydrogenase (GAPDH) serving as a loading control. (K and L) Immunofluorescence staining further validated that 3as regulated the expression of E-cadherin and MMP9, key markers of tumor metastasis. Scale bars, 20 μm. Data are presented as the means ± SEM. Results are consistent with at least three independent experiments. ns, not significant; *P < 0.05, **P < 0.01, ***P < 0.001, and ****P < 0.0001. Statistical significance was determined relative to the appropriate control groups.

Fig. 6.. Mechanistic investigations.

(A) Interconversion experiments. (B) Nonlinear effect experiment. (C) DFT calculations of the catalyst-controlled regiodivergent reactions between 1a and 2a.