An Efficient Ecofriendly Enantioselective Organocatalytic Ring‐Closing Reaction of 2‐Hydroxychalcone via Intramolecular Oxa‐Michael Reaction

Singh, Ankita; Mangawa, Shrawan Kumar; Kumar, Arvind; Dixit, A. K.; Awasthi, Satish Kumar

doi:10.1002/slct.201701047

Cited by 2 publications

(1 citation statement)

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“…A type of new chiral ammonium quaternary salt were developed by Awasthi and coworkers . By using 150 as catalyst, the 2‐hydroxychalcone 143 could be converted to chiral flavanones efficiently in an ecofriendly manner, providing flavanones 128 with up to 99% yield and 99% ee .…”

Section: Asymmetric Synthesis Of Flavanonesmentioning

confidence: 99%

Catalytic Asymmetric Syntheses of 2‐Aryl Chromenes

2019

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Chiral chromene motifs are important heterocycles in numerous natural products and bioactive molecules. Among them, flavenes, flavanes, and flavanones, which consist of the 2-aryl-chromene skeleton are observed most frequently. As subgroups of flavonoids, they often occur in various plants and show many biological activities, such as antitumor, antioxidant, antibacterial, anti-inflammatory, and anticoagulant properties. This Minireview summarizes recent reports on catalytic asymmetric synthesis of flavenes, flavanes, and flavanones. derivatives 1, giving the adduct 6. The subsequent intramolecular adol reaction and dehydration providing the desired product 3 (Scheme 2).Almost the same time, professor Córdova and coworkers presented the same domino reaction between 1 a and 2 catalyzed by 4 a, providing a series of 2H-flavenes in high yields and with 83-98% ees (Scheme 3). [5] Notably, by using 2nitrobenzoic acid as an additive, product 3 a could be obtained with 37% yield and 88% ee, comparing to 10% yield and 9% ee when adding no additives. More significantly, another additive, 4 Å molecular sieves, promoted the reaction significantly, increasing the product yield from 37% to 81% with no enantioselectivity erosion. The authors believed that the addition of an organic acid accelerated the adol condensation by activating the benzaldehyde. Also, the organic acid could stabilized the iminium intermediate, facilitating the oxa-Michael addition. The product 7 could be observed by NMR analysis, which gave an obvious evidence to the proposed reaction pathway (Scheme 2).By applying the similar approach, Wang and coworkers developed the asymmetric reaction of 1 and 2 catalyzed by organocatalyst 4 b. [6] Different from Córdova's condition, Wang found that Cl(CH 2 ) 2 Cl was the best solvent to provide the highest enantioselectivity. Lowering the reaction temperature improved ee with prolonged time for completion. The reaction had a broad scope and the functionalized 2H-flavenes were synthesized respectively high yields (53-98%) and with good to excellent levels of enantioselectivities (75-99% ee). However, the electronic nature of the α,β-unsaturated aldehydes 2 had a remarkable effect on the reactivities and ees that 2 bearing electron-withdrawing groups, such as nitro group, generally Scheme 2. Proposed mechanism of the organocatalytic asymmetric oxa-Michael addition/intramolecular adol reaction/dehydration cascade. Scheme 3. Direct organocatalytic asymmetric domino oxa-Michael/aldol condensations between 1 a and α, β-unsaturated aldehydes 2.

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Section: Asymmetric Synthesis Of Flavanonesmentioning

confidence: 99%