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Catalytic Enantioselective Reactions with (Benzo)Pyrylium Salts

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Catalytic Enantioselective Reactions with (Benzo)Pyrylium Salts InnovatIve tools In organIc / organometallIc chemIstry CHIMIA 2020, 74, No. 11 857 doi:10.2533/chimia.2020.857 Chimia 74 (2020) 857–865 © S. Joseph, G. Khassenova, O. García Mancheño Catalytic Enantioselective Reactions with (Benzo)Pyrylium Salts Sumi Josepha, Gaukhar Khassenovaa, and Olga García Mancheño* Abstract: The use of (benzo)pyrylium salts as versatile synthetic building blocks has become an attractive plat- form for the preparation of valuable heterocyclic compounds. Besides other numerous direct applications of (benzo)pyryliums, the intrinsic electrophilic nature of these species or the dipole character of the related oxi- dopyrylium derivatives have been exploited towards the development of enantioselective transformations such as nucleophilic dearomatization and cycloaddition reactions. This review aims at providing an overview on the relevant catalytic enantioselective methodologies developed in the past years, which are presented considering the involved metal- and/or organic catalytic system, as well as the type of reaction. Keywords: Enantioselective catalysis · Metal catalysis · O-heterocycles · Organocatalysis · Pyrylium salts Prof. K.A. Jørgensen (University of Aarhus). She next moved to RWTH-Aachen for her postdoctoral research in the group of Prof. C. Bolm (2005–2008), followed by the start of her inde- pendent career at the University of Münster in 2008. In 2013, she was appointed as joint Chemistry Professor at the University of Regensburg and TUM-Campus Straubing, and since 2017 she is Professor for Organic Chemistry at the University of Münster. 1. Introduction (Benzo)pyrylium salts are privileged six-membered aromat- ic heterocycles containing a positively charged oxygen atom.[1] Important naturally occurring examples include the flavylium plant pigments,[2] while synthetic derivatives present fluores- cence[3] and photocatalytic activity,[4] among other properties S. Joseph, O. García Mancheño and G. Khassenova (Fig. 1, top). Moreover, the electronegativity of the oxygen atom in the aromatic ring induces a strong single perturbation, which Sumi Joseph was born in Kerala, southernmost part of India. makes it highly reactive towards nucleophilic additions leading to She earned her BSc in Chemistry from Calicut University in 2014 non-aromatic adducts, while it does not undergo typical aromatic and did her MSc (2016) studies at Mahathma Gandhi University electrophilic substitutions. with specialization in Organic Chemistry. In 2018, she was award- Several synthetic approaches have been developed for the ed a DAAD fellowship and joined the research group of Prof. Dr. preparation of (benzo)pyrylium salts (Fig. 1, bottom).[1] The Olga García Mancheño at the Westfälische Wilhelms-Universität most common method relies on the condensation of carbonyl Münster, where she is currently in her second year of the PhD compounds in the presence of a strong acid (e.g. TfOH, HBF , 4 Program in Organic Chemistry. Her research interests include etc.) as shown in Fig. 1a. However, in most cases this method non-covalent asymmetric anion-binding organocatalysis and total leads to 2,4,6-trisubstituted derivatives, in which all the nucleo- synthesis of natural products. philic sites are blocked. The second most employed synthesis of Gaukhar Khassenova was born in Karaganda, an industrial (benzo)pyrylium derivatives embraces substrates bearing a leav- city in central Kazakhstan. She completed both her Bachelor and ing group (LG) at the alpha position of the oxygen atom, which Master studies in the Department of Organic Chemistry with spe- facilitates the elimination of this group (normally RO- or RCO -) 2 cialization in Chemical Technology from the Karaganda State and formation of the corresponding salt (Fig. 1b). Besides these University. In 2019, she was awarded a DAAD fellowship and methodologies, several annulation reactions, as well as enoliza- moved to Germany to pursue her doctoral studies in the group tions of (benzo)pyranones have also been widely used to generate of Prof. Dr. Olga García Mancheño at the Institute of Organic the aromatic pyrylium core (Fig. 1c and 1d, respectively). Hence, Chemistry of the Westfälische Wilhelms-Universität Münster. multiple possibilities for the synthesis of these compounds exist, Her work primarily focuses on non-covalent asymmetric anion- which make them very interesting synthetic building blocks with binding organocatalysis. a rich chemistry as they can act as electrophiles (at the ortho and Olga García Mancheño received her PhD in 2005 from the para positions) or, in the case of oxidopyryliums, as dipoles.[1] Universidad Autónoma de Madrid under the supervision of Prof. Consequently, these salts can act as versatile synthetic interme- J.C. Carretero. During her PhD she realized two short research diates for the preparation of highly added derivatives. However, stays with Prof. M.T. Reetz (MPI für Kohlenforschung) and most of the (benzo)pyrylium compounds obtained through the *Correspondence: Prof. O. García Mancheño, E-mail: olga.garcia@uni-muenster.de Organic Chemistry Institute, Münster University, Corrensstrasse 36, D-48149 Münster, Germany aThese authors contributed equally. 858 CHIMIA 2020, 74, No. 11 InnovatIve tools In organIc / organometallIc chemIstry above-mentioned methods are quite unstable, except for the more ion intermediate (Scheme 1).[5] To achieve this, they combined a robust 2,4,6-trisubstituted or flavin-type derivatives, and there- tartaric acid amide as chiral Brønsted acid and a lanthanoid tri- fore they are normally formed in situ and reacted without prior flate complex as Lewis acid system to simultaneously activate isolation. Moreover, in the past few years, their extremely high the boronate and chromene, respectively. In this case, the formed potential for the synthesis of chiral molecules has gained more reactive dioxoborolane intermediate added to the pyrylium ion, attention. Nonetheless, due to their intrinsic high reactivity, ef- which was formed upon action of the achiral inorganic cerium or ficient catalytic enantioselective transformations are still scarce. ytterbium Lewis acid. O OH Bn N HO Bn 1 CO2H R OH Ar R1 OEt OH O (5 mol%) OH B + EtO R3 O R3 Ce(OTf) (4.5 mol%) or Et O OEt R2 4 R2 OH N O Yb(OTf)3 (4.5-13.5 mol%) HO O Ar O Ar Et R EtOAc, 4to-40 °C 68-85% yield O O N 88-98% ee Cyanidin photocatalysts tartrate-type OH Ln(OTf)n catalyst -2EtOH (Flavylium) fluorescence probes R (e.g.Ar=Ph, TPT) Condensation O Bn N R1 HO 2 O O O O R1 O O O O 2x + Ln B R R H O B O O H Bn N O R3 O (mostly to 2,4,6-trisubstitution) 3 Et EtO Ln Bn 2 R R R2 Elimination of α-LG a) Enolization Scheme 1. Enantioselective addition of boronates to in situ-formed ben- O b) O d) zopyrylium ions. or O LG O O O or electrophile dipole In 2012, the Rueping group embraced a similar approach for O c) the generation of the active chromenium intermediate from (iso) Annulations chromene acetals and described the first enantioselective addition O OR R1 of aldehydes to oxocarbenium ions for the synthesis of 2-sub- [6] LG O O stituted chiral chromenes (Scheme 2). They also used a dual or ,etc catalytic system consisted of Yb(OTf) as achiral Lewis acid and N2 3 O R2 a chiral imidazolidinone catalyst, in which the latter reacts with Fig. 1. Representative examples, synthetic approaches and reactivity of the carbonyl reagent to form the reactive enamine nucleophile. pyrylium derivatives. In order to facilitate the handling of the obtained chromene-alde- hyde products, a final reduction to the corresponding alcohol was performed. Under these conditions, the desired chiral substituted In this review, we present the state of the art on catalytic enan- chromenes were obtained in good yields and enantioselectivities tioselective reactions involving (benzo)pyrylium salts. The review up to 96% ee. is divided in metal-catalyzed and organocatalytic approaches, which provide access to valuable O-heterocycles or important 1) O synthetic building blocks towards the synthesis of bioactive prod- N (20 mol%) ucts. Thus, the methodologies discussed here are classified based Bn N H R H OH on the catalytic system involved and type of key reaction, includ- O R Yb(OTf)3 (10 mol%) O ing diverse nucleophilic dearomatization and cycloaddition reac- + R1 O OEt H H R1 tions. 2) NaBH4,MeOH 0 °Ctor.t., 2h 13 examples 72-89% yield Yb(OTf) amino 2. Metal Catalysis 3 catalyst 75-96% ee 2.1 Metal-catalyzed Nucleophilic Dearomatization O NaBH4 N Reactions H O R 2 Ongoing efforts in this area are directed towards the devel- Bn R H O O N -aminocat. O opment of effective methodologies for the synthesis of chiral R1 EtO [Yb] 1 O-heterocycles. In this regard, the stereo-controlled synthesis of H H R biologically relevant chromenes and chromones can be achieved Scheme 2. Ytterbium-amino co-catalysis for the asymmetric addition of through the nucleophilic addition to a variety of in situ gener- aldehydes to chromeniums. ated pyrylium-type electrophilic reactive species. Two main ap- proaches have been employed to date, which involve i) an achiral metallic Lewis acid catalyst for the formation of the pyrylium spe- Later on, Schaus and co-workers further investigated the ef- cies in combination with a chiral organocatalyst that activates the fect of Lewis and Brønsted acid cooperative catalysis for the ad- nucleophile, or ii) chiral organometallic nucleophiles, obtained dition reaction of diazoesters to 2H‐chromene acetals (Scheme by using a chiral metal catalyst formed with the appropriate chiral 3).[7] In particular, efficient C–C bond formation between pro- ligand. Thus, several lanthanoid-, gold- and copper-based cata- chiral oxocarbenium ions and ethyl diazoacetate were promoted lyzed reactions have been described so far and are presented in using Yb(OTf) and commercially available chiral phosphoric ac- 3 the next sections. ids to yield valuable chiral 2H-chromene-diazomethane scaffolds.
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