Organocatalytic Enantioselective Cycloaddition of <i>o</i>‐Quinone Methides with Oxazolones: Asymmetric Synthesis of Dihydrocoumarins

Chiral analogues of squaramides have been fruitful in organocatalyzed asymmetric reactions over last decade. Alongside other H-bonding catalysts like ureas, thioureas: squaramides have been proved to be efficient asymmetric catalysts for the formation of acyclic and cyclic chiral molecules. A wide range of cyclic molecules bearing multiple functionalities and stereocenters have been synthesized by using several bifunctional squaramides as catalysts. These catalysts perform as base utilizing basic N atom in their chiral extension and help in stereoinduction by forming H-bonds with suitable H-bond acceptors. The present review focuses on assembling recent progresses of asymmetric synthesis of five and six membered rings promoted by squaramides. A handful of articles have been published by several research groups documenting asymmetric construction of five-and six-membered rings as part of interesting and complex molecular architectures. Different methodologies like conventional and formal cycloadditions or cascade reactions have been engineered through the assistance of chiral squaramides to fabricate the carbo-and heterocycles. This review also includes dual and cooperative catalytic system in which squaramide is one of the catalysts.

Section: Synthesis Of Six Membered Ringsmentioning

confidence: 99%

Section: Synthesis Of Oxacyclesmentioning

confidence: 99%

Squaramide Catalyzed Asymmetric Synthesis of Five‐ and Six‐Membered Rings

Biswas¹,

Ghosh

Shankhdhar

et al. 2021

“…Therefore, this reaction system involving o-hydroxynaphthalene needed to continue to optimize the reaction conditions. Subsequently, other groups including Zhang [20] (Scheme 8a), Yuan and You [21] (Scheme 8b), Deng and Shao [22] (Schemes 8c and 8d) independently published beautiful works about the same cycloaddition process of o-hydroxynaphthalene 34 by using different CPA catalysts, again forming a wide range of polysubstituted chromans (35,40) with high level of stereoselectivities. Notably, the lessexplored ortho-alkenyl 1-naphthols were also compatible in this methodology to deliver trans-trans chiral trisubstituted chromans 40.…”

Section: Chiral Phosphoric Acidmentioning

confidence: 99%

“…Very recently, Kim et. al [40] . realized the same reaction of preformed o ‐QMs 110 with azlactones 2 using a binaphthyl‐modified squaramide catalyst 111 with lower catalyst loading (5.0 mol%) (Scheme 23).…”

Section: Chiral Brønsted Acid or Base Catalysismentioning

confidence: 99%

“…al. [40] realized the same reaction of preformed o-QMs 110 with azlactones 2 using a binaphthylmodified squaramide catalyst 111 with lower catalyst loading (5.0 mol%) (Scheme 23). This catalytic strategy provided a complementary methodology to previously mentioned CPA catalyzed cycloaddition reactions of azlactones.…”

Section: Bifunctional Urea or Squaramide Catalystmentioning

confidence: 99%

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Recent Advances in Organocatalytic Asymmetric Cycloaddition Reactions Through Ortho‐Quinone Methide Scaffolds

Yang

et al. 2021

This review summarizes the very recent advances in organocatalytic asymmetric cycloaddition reactions involving ortho-quinone methides (o-QMs), a family of versatile species that have proven useful in a broad range of reactions to afford diverse chiral molecules. In the past years, different kinds of organocatalysts have been developed into a privileged class of catalytic systems in asymmetric synthesis.A number of remarkable achievements have been made by many groups around the world. Due to length limitation, this review only summarizes those works published from January 2016 to December 2020. Meanwhile, catalytic systems which combine metal catalysts and organocatalysts will not be discussed in this review.

Visible Light Photocatalytic Trifluoromethylation/SET Oxidation/Cycloaddition Sequences of 2‐Vinyl Phenols: Multicomponent Synthesis of 4H‐Chromenes

Jang

Kim

2021

Self Cite

Visible light-mediated photocatalytic trifluoromethylation, single electron transfer (SET) oxidation, and cycloaddition cascades of 2-vinyl phenols with Umemoto's reagent and malononitrile were developed. This approach provided the multicomponent synthesis of trifluoromethylated 4H-chromenes via the in situ generation of o-quinone methides, followed by sequential cyclization.Trifluoromethyl-containing compounds have attracted much attention in medicinal chemistry, agrochemistry, and materials science due to their unique lipophilic, metabolic, and electronic properties. [1] Currently, intensive efforts have been devoted to the development of practical and novel methods for the synthesis of trifluoromethyl-containing compounds. [2] Among them, the most effective approaches for the construction of CÀ CF 3 bonds is the radical trifluoromethylation of alkenes using nucleophilic and electrophilic trifluoromethylation reagents under redox conditions. [3] ortho-Quinone methides (o-QMs) are versatile synthons that are frequently used as 1,4-dipoles and Michael acceptors. [4][5][6][7] A number of recent examples for the addition reactions of o-QMs with various types of nucleophiles were reported by using transition-metal catalysis [8] or organocatalysis. [9] Highly polarized o-QMs have arisen as reactive 1,4-dipoles that react with oneor two-carbon dipolars to afford dihydrobenzofuran or chromane derivatives. Recently, Han and our groups reported the organocatalytic addition and cyclization of o-QMs with malonate to afford 4H-chromene derivatives (Scheme 1a). [10] In addition, Xiao and Chen reported the photoredox-catalyzed trifluoromethyl radical-mediated generation of o-QMs for the synthesis of 2,3-dihydrobenzofuran (Scheme 1b). [11] Although these pioneering results have been achieved by this system, the development of multicomponent reaction of the in situ generation of o-QMs with various dipoles is highly desired. Therefore, we envisioned that in situ formed o-QMs, which are generated from vinyl phenols by visible-light mediated radical trifluoromethylation and single electron transfer (SET) oxidation, could react with malononitrile as 1,2-dipole (Scheme 1c).In connection with our ongoing research program related to the synthesis of heterocyclic compounds, [12] we reported the synthesis of benzopyran derivatives via the alkylation of β-keto acids to o-QMs intermediates, followed by sequential cyclization. [13] Herein, we present the visible light photocatalytic trifluoromethyl radical-mediated generation of o-QMs from 2vinyl phenols for the multicomponent synthesis of 4H-chromenes.To determine suitable reaction conditions for the photocatalytic trifluoromethylation/SET oxidation and cycloaddition cascades of vinyl phenols, we examined the reaction of 2-vinyl phenol (1 a), Umemoto's reagent 2 a, and malononitrile 3 in dichloromethane in the presence of 2 mol% photocatalyst with blue LEDs (5 W, λ max = 455 nm). By screening photocatalysts including fac-Ir(ppy) 3 , Ir(ppy) 2 (dtbpy)PF 6, Ir[dF(CF 3 )ppy] 2 (dtbpy) PF ...