Enantioselective Synthesis of cis-Hydroisoquinolines via Cross-[4 + 2] Cycloadditions of 2‑Pyrones and
Jun-Xiong He1, Qi-Tao Lu1, Kui Tan1
1Department of Chemistry, Research Center for Molecular Recognition and Synthesis, State Key Laboratory of Green Chemical Synthesis and Conversion, Fudan University, 220 Handan Rd., Shanghai 200433, China.
Abstract:
Chiral cis-hydroisoquinoline scaffolds are important pharmacophores and are prevalent in bioactive natural products and drug molecules. However, the straightforward construction of cis-hydroisoquinolines via asymmetric catalysis remains elusive. Herein, we report a Lewis acid-catalyzed enantioselective cross-[4 + 2] cycloaddition of 2-pyrones with dihydropyridines. This method enables the step-economical and divergent synthesis of densely functionalized cis-hydroisoquinolines with excellent yields and enantioselectivities. Density functional theory (DFT) calculations indicate that the cross-[4 + 2] cycloaddition proceeds via a stepwise reaction pathway, and reveal the origin of the chemo-, regio-, and stereoselectivity. Moreover, acid-sensitive silyloxydihydropyridine derivatives are compatible in this reaction when a ytterbium/BINOL complex is used as the catalyst, further enhancing the functional diversity of cis-hydroisoquinoline products. The synthetic utility of this method is unambiguously demonstrated by the concise synthesis of two yohimbine alkaloids (-)-alloyohimbane and (-)-rauwolscine.
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