Synthesis of Functionalized Dihydrocoumarins by NHC-Catalyzed [3 + 3] Annulation of Enals with 2-Substituted Naphthoquinones

Abstract

The [3 + 3] annulation of α,β-unsaturated aldehydes with 2-substituted 1,4-naphthoquinones allowing the facile synthesis of functionalized dihydrocoumarins catalyzed by N-heterocyclic carbene (NHC) is reported. The initially formed NHC-homoenolates underwent an efficient Michael–isomerization–lactonization cascade to furnish the products. Preliminary studies on mechanism shed light on the homoenolate pathway over the intermediacy of the α,β-unsaturated acylazolium intermediates. Moreover, using chiral NHCs, the desired products were formed in up to 49% yield and 99:1 er.

Enantioselective Dual Catalysis of N‐Heterocyclic Carbene and Hydrogen‐Bond Donor Organocatalysts

“…The thiourea was also found to be efficient catalysts for the [3 + 3] annulation of 2-substituted 1,4-naphthoquinones 61 with α,β-unsaturated aldehydes 23. [31] Interestingly, the synthesis of functionalized dihydrocoumarins 62 by performing the reaction using 20 mol % thiourea and 4 Å MS as additives was formed in 84 % isolated yield (Scheme 14a). Furthermore, preliminary studies on the enantioselective version of the reaction were also carried out (Scheme 14b).…”

Supramolecule-Controlled Enantioselectivity for Electrochemical Asymmetric Hydrogenation of Coumarins with a Chiral Macrocyclic Compound

“…One can see that the reactivity was enhanced with the addition of AuNPs in the first stage, and the peak yield of 7-hydroxy-4-methylchroman-2-oneat reached 65% with the optimized dosage of 2.8 × 10 −2 mg of AuNPs. The ee values were kept at the level of >99% with the increase of AuNP coating, indicating that the addition of AuNPs supported the high enantioselectivity of L-Ala-MF [3,3].…”

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The supramolecular strategy was subjected to the asymmetric hydrogenation of 4-methylumbelliferone by electrochemical reduction in the presence of a chiral macrocyclic multifarane [3,3], which offered a L-7-hydroxy-4-methylchroman-2-one product with a chemical yield of 65% and enantioselectivity up to >99% ee. The high stability of the developed chiral supramolecular electrode guaranteed the recyclability and repeatability in the electrolysis, and therefore, the application was extended to more coumarin derivatives to provide satisfactory chemical yields and enantioselectivities.

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“…TEM was performed on AuNPs and L-Ala-MF [3,3]and L-Ala-MF [3,3]-coated AuNPs to observe their microstructures (Figure S11, SI). AuNPs were densely arranged as a short columnar structure (Figure S11a,b), and L-Ala-MF [3,3] was arranged as a microdispersed spherical structure (Figure S11c,d).…”

“…TEM was performed on AuNPs and L-Ala-MF [3,3]and L-Ala-MF [3,3]-coated AuNPs to observe their microstructures (Figure S11, SI). AuNPs were densely arranged as a short columnar structure (Figure S11a,b), and L-Ala-MF [3,3] was arranged as a microdispersed spherical structure (Figure S11c,d). As shown in Figure S11e, AuNPs were composited with L-Ala-MF [3,3], showing spherical structure, and the crystallinity of AuNPs did not change with the addition of chiral macrocycle compounds (Figure S11e,f).…”