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Keto Esters Vincent Bizet, Valérie Lefebvre, Jérôme Baudoux, Marie-Claire Lasne, Agathe Boulangé, Stéphane Leleu, Xavier Franck, Jacques Rouden

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Keto Esters Vincent Bizet, Valérie Lefebvre, Jérôme Baudoux, Marie-Claire Lasne, Agathe Boulangé, Stéphane Leleu, Xavier Franck, Jacques Rouden Metal-Free SN2’ Decarboxylative Rearrangement of β-Keto Esters Vincent Bizet, Valérie Lefebvre, Jérôme Baudoux, Marie-Claire Lasne, Agathe Boulangé, Stéphane Leleu, Xavier Franck, Jacques Rouden To cite this version: Vincent Bizet, Valérie Lefebvre, Jérôme Baudoux, Marie-Claire Lasne, Agathe Boulangé, et al.. Metal- Free SN2’ Decarboxylative Rearrangement of β-Keto Esters. European Journal of Organic Chemistry, Wiley-VCH Verlag, 2011, 2011 (22), pp.4170-4175. 10.1002/ejoc.201100120. hal-01017341 HAL Id: hal-01017341 https://hal.archives-ouvertes.fr/hal-01017341 Submitted on 29 May 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. FULL PAPER DOI: 10.1002/ejoc.201100120 Ј Metal-Free SN2 Decarboxylative Rearrangement of β-Keto Esters Vincent Bizet,[a] Valérie Lefebvre,[a] Jérôme Baudoux,*[a] Marie-Claire Lasne,[a] Agathe Boulangé,[b] Stéphane Leleu,*[b] Xavier Franck,[b] and Jacques Rouden*[a] Keywords: Nucleophilic substitution / Reaction mechanisms / Organocatalysis / Rearrangement/ Decarboxylation Treatment of 2-methoxycarbonyl- (or 3-cyano-)allyl aceto- tested to elucidate the mechanism. The results of the study acetates with a tertiary amine or triphenylphosphane af- strongly suggest an intermolecular pathway involving a forded α-methylene γ-substituted δ-keto esters (or nitrile) in SN2Ј–decarboxylation–SN2Ј cascade. satisfactory yields. Various bases and nucleophiles were Ј[2,3] Introduction through an intramolecular SN2 reaction of the β-keto ester enolate with displacement of the acetoacetate group One of the major challenges in organic chemistry is the and further decarboxylation of the latter. It is worth noting synthesis of highly functionalized molecules in a minimum that intramolecular Michael additions of C-nucleophiles of steps with efficient control of the stereochemistry. As a under organocatalytic conditions[4–7] have been relatively part of our research on organocatalytic approaches to C–C unexplored. “Intramolecular conjugated displacements” [1] or C–H bond formation, we envisaged a metal-free syn- have been successfully applied to the synthesis of carbo- thesis of δ-keto esters 1 from easily available allylic esters 2 cycles[8] or nitrogen heterocycles.[9] In these reactions, a (Scheme 1). carbanion or an amine generated in situ reacted with an acrylate with substitution of an acetate (or a carbonate) [10] Ј group in the allylic position. SN2 reactions are also com- mon reactions for intermolecular functional modifications of Morita–Baylis–Hillman adducts.[10,11] From a slightly different point of view, the transforma- tion we envisaged could be related to a base-induced Car- roll reaction, which is a variant of the Claisen rearrange- ment (Scheme 2).[12] Although described a long time ago, applications of the Carroll reaction in organic synthesis have received little attention, probably because of the harsh thermal conditions required (typically higher than 180 °C) Scheme 1. Working hypothesis: base-catalyzed rearrangement of substituted allyl acetoacetates. We anticipated that, in the presence of a base, an elec- tron-withdrawing group (EWG = CO2R, CN) on the double bond of 2 would facilitate the C–C bond formation [a] Laboratoire de Chimie Moléculaire et Thioorganique, CNRS- ENSICAEN, Université de Caen, Institut Normand de Chimie Moléculaire, Médicinale et Macromoléculaire (INC3M), CNRS 6 Boulevard du Maréchal Juin, 14050 Caen, France Fax: +33-2-31452877 E-mail: [email protected] [email protected] [b] UMR-CNRS6014 & FR3038 COBRA, Université de Rouen, INSA de Rouen, Institut Normand de Chimie Moléculaire, Médicinale et Macromoléculaire (INC3M), 1 rue Tesnière, 76130 Mont Saint Aignan, France E-mail: [email protected] Supporting information for this article is available on the Scheme 2. Carroll rearrangement catalyzed by a strong base or a WWW under http://dx.doi.org/10.1002/ejoc.200900xxx. transition metal. 4170 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2011, 4170–4175 Ј Metal-Free SN2 Decarboxylative Rearrangement of β-Keto Esters and because of its sensitivity to the structures of the sub- and 8a could be explained by an intermolecular decarboxyl- Ј strates. Several methods have been designed to circumvent ation–SN2 sequence involving two molecules of 2a these limitations. The reaction can be accelerated by the use (Scheme 4). of strong bases[13] or a Lewis acid.[14] However, these meth- ods suffer from the need for two equivalents of base and Table 1. Tandem alkylation–decarboxylation of ester 2a. requires an additional step to carry out the decarboxyl- [a] [b] ation. Recently, decarboxylations of allyl β-keto carboxyl- Entry Base Conditions T [°C] Conv. [%] Ratio [%] 1a 7a 8a ates catalyzed by transition metals (Ru,[15] Pd[16])inthe presence of chiral ligands have been successfully developed. 1 NaH THF r.t. 100 54 14 29 2Cs2CO3,CD2Cl2/H2O (1:0.1) r.t. 37 13 0 23 Adsorption of β-keto esters on neutral alumina has also PTC[c] [d] been used for the Carroll rearrangement of a precursor of 3Et3N CH2Cl2 r.t. 100 60–66 40–34 0 [17] m zincophorin. Under organocatalytic conditions, the treat- 4Et3NCH2Cl2 (0.05or0.2 ) r.t. 100 67 33 0 ment of an allylic alcohol with diketene in the presence of 5Et3N MeCN r.t. 100 67 33 0 6EtN toluene r.t. 47 23 9 15 collidine[18] or 4-(dimethylamino)pyridine (DMAP)[19] af- 3 7Et3N THF r.t. 86 52 21 13 forded good yields of the rearranged product. 8Et3N MeOH r.t. 80 52 9 19 2 When the R group of compound 2 (Scheme 2) was an 9Et3NCH2Cl2 053202211 [20] [6] electron-withdrawing group, Craig et al. and You et al. 10 Et3NCH2Cl2 40 100 76 24 0 recently observed, under basic conditions, the formation of 11 Et3N THF 65 100 77 23 0 12 Et N MeOH 65 100 88 12 0 γ-lactone derivatives. A Michael addition of the keto enol- 3 m ate to the double bond could account for these observa- [a] Reagents and conditions: 2a (0.1 ), base (20 mol-%), r.t., 17 h. [b] Determined from the 1H NMR spectra of the crude product tions. These results prompted us to describe our first results using 1,4-bis(trichloromethyl)benzene as an internal standard. All with model substrates 2 (R = H, Me, allyl, benzyl) and 6 the products were isolated and characterized. [c] Phase-transfer (Scheme 3) and to demonstrate that their reaction in the conditions with NEt3BnBr. [d] Reactions performed with Et3N Ј loadings of 25, 50, 100, or 150 mol-% gave similar results. presence of a nucleophile catalyst involves an SN2 –decar- Ј boxylation–SN2 sequence of the allyl acetoacetate rather than a base-catalyzed rearrangement. Scheme 3. Synthesis of allyl acetoacetates 2a–d and 6. Scheme 4. Plausible reaction pathways for the transformation of 2a under basic conditions. Results and Discussion To optimize the formation of 1a we studied the reaction parameters in more detail. Phase-transfer catalysis in the Allyl acetocetates 2 were prepared by reaction of the presence of cesium carbonate and triethylbenzyl ammo- [21] Baylis–Hillman adduct 3 with 2,2,6-trimethyl-4H-1,4-di- nium bromide led to a slow reaction in a mixture of CDCl3/ [22] oxin-4-ones 5 (Scheme 3). Nitrile 6 was obtained in a H2O (Table 1, entry 2); under these conditions, triester 8a similar way from nitrile 4[23] and dioxinone 5a. Allyl ester was the major product formed, even after a prolonged reac- 2a could also be prepared in quantitative yield from alcohol tion time. In this context, homogeneous organocatalysis 3 and diketene (1 equiv.) in the presence of DMAP (2 mol- could be a valuable method to synthesize 1a from 2a. The %) in dichloromethane at room temperature for 24 h. application of organic bases were therefore tried. Triethyl- Initial attempts to study the formation of compounds 1 amine was chosen to avoid possible side reactions with the from allyl esters 2 were performed with 2a and sodium hy- Michael acceptor.[24] Catalytic, stoichiometric, or an excess dride (1 equiv.) in tetrahydrofuran (THF) at 20 °C for 17 h. amount of Et3N in dichloromethane at room temperature The expected product 1a, together with diester 7a and tri- for 17 h (Table 1, entry 3) afforded the expected compound ester 8a, were formed in 54, 14, and 29% molar ratio, 1a and the diester 7a with total conversion of the starting respectively (Table 1, entry 1). The presence of products 7a material. Regardless of the amount of amine used, the ratio Eur. J. Org. Chem. 2011, 4170–4175 © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.eurjoc.org 4171 FULL PAPER J. Baudoux, S. Leleu, J. Rouden et al. 1a/7a was in the same range (Table 1, entry 3). Poor conver- be mediated by a nucleophile. Thus, we can assume that Ј [29] sion (47 %) and the formation of a mixture of triester 8a an initial SN2 reaction of the nucleophile leads to the (9%), diester 7a (7%), and target compound 1a (16 %) was formation of cation III and acetoacetate anion II observed with a lower amount of Et3N (5 mol-%, data not (Scheme 5). Rapid decarboxylation of this latter intermedi- shown). Dilution had no effect on the ratio of 1a/7a,as ate to acetone enolate IV and recombination of IV with III Ј shown in Table 1, entry 4.
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