Tertiary propargyl vinyl ethers armed with an electron-withdrawing group (amide or ester) at the tertiary propargylic position have been efficiently transformed into trisubstituted C2-chain functionalized furans. The metal-free domino transformation involves a microwave-assisted tandem [3,3]-propargyl Claisen rearrangement/5-exo-dig O-cyclization reaction. The manifold can be performed in a one-pot fashion from the primary components (1,2-ketoester/1,2-ketoamide or tertiary propargyl alcohols). © 2011 American Chemical Society.This research was supported by the Spanish MICINN and the European RDF (CTQ2008-06806-C02-02), the Spanish MSC ISCIII (RETICS RD06/0020/1046), FUNCIS (REDESFAC PI01/06). L.C. thanks Spanish MEC for a FPI grant. Authors thank technician Ms. Anna Jurado Varona (IPNA-CSIC) for her experimental assistance.Peer Reviewe

Cotos, Leandro

García-Tellado, Fernando

Tejedor, David

Digital.CSIC

A  Microwave-assisted Domino Access to C2-Chain Functionalized Furans from Tertiary Propargyl Vinyl Ethers.  David Tejedor,* Leandro Cotos and Fernando García-Tellado* Instituto de Productos Naturales y Agrobiología, CSIC, Departamento de Química Biológica y Biotecnología, Astrofísico Francisco Sánchez 3, 38206 La Laguna, Tenerife, Spain, and Instituto Canario de Investigación del Cáncer, Plaza de Sixto Machado nº 3, 38009 Santa Cruz de Tenerife, Tenerife, Spain. fgarcia@ipna.csic.es; dtejedor@ipna.csic.es Received Date (will be automatically inserted after manuscript is accepted) ABSTRACT Tertiary propargyl vinyl ethers armed with an electron withdrawing group (amide or ester) at the tertiary propargylic position have been efficiently transformed into trisubstituted C2-chain functionalized furans. The metal-free domino transformation involves a microwave-assisted tandem [3,3]-propargyl Claisen rearrangement/5-exo dig O-cyclization reaction. The manifold can be performed in a one-pot fashion from the primary components (1,2-ketoester/1,2-ketoamide or tertiary propargyl alcohols).  The [3,3]-Claisen rearrangement of propargyl vinyl ethers 1 (PVEs) constitutes a synthetically convenient route to substituted -allenals 2 (eq 1),1 which have proved to be suitable reactive units for the synthesis of important heterocycles cores.2,3 In the presence of metallic                                                      1 (a) Sherry, B. D.; Toste, F. D. J. Am. Chem. Soc. 2004, 126, 15978. (b) Hashmi, A. S. K. Synthesis of Allenes by Isomerization Reactions in Modern Allene Chemistry; Krause, N., Hashmi, A. S. K., Eds.; Wiley-VCH: Weinheim, 2004. 2 (a) Gille, A.; Rehbein, J.; Hiersemann, M., Org. Lett. 2011, 13, 2122. (b) Jiang, H.; Yao, W.; Cao, H.; Huang, H.; Cao, D., J. Org. Chem. 2010, 75, 5347. (c)  Cao, H.; Jiang, H.; Mai, R.; Zhu, S.; Qi, C., Adv. Synth. Catal. 2010, 352, 143. (d) Cao, H.; Jiang, H.; Yao, W.; Liu, X., Org. Lett. 2009, 11, 1931. (e) Suhre, M. H.; Reif, M.; Kirsch, S. F., Org. Lett. 2005, 7, 3925. (f) Binder, J. T.; Kirsch, S. F., Org. Lett. 2006, 8, 2151. (g) Menz, H.; Kirsch, S. F., Org. Lett. 2006, 8, 4795. (h) Sherry, catalysts, these intermediates have been selectively transformed into furans,2a–e pyrroles,2f 2H-pyrans,2g dihydropyrans2h or 1,2-dihydropyridines.21-j Recently, we have shown how the microwave irradiation of PVEs constitutes an alternative manner to obtain these reactive intermediates in the absence of metals,  and how these intermediates can be efficiently transformed into                                                                                       D.; Maus, L.; Laforteza, B. N. F.; Toste, D., J. Am. Chem. Soc. 2006, 128, 8132. (i) Wei, H.; Wang, Y.; Yue, B.; Xu, P-F., Adv. Synth. Catal. 2010, 352, 245. (j) Harschneck, T.; Kirsch, S. F., J. Org. Chem. 2011, 76, 2145.  3 (a) Tejedor, D.; Méndez-Abt, G.; Cotos, L.; Ramirez, M. A.; García-Tellado, F., Chem. Eur. J. 2011, 17, 3318. (b) Tejedor, D.; Méndez-Abt, G.; García-Tellado, F., Chem. Eur. J. 2010, 16, 6582. (c) Tejedor, D.; Méndez-Abt, G.; García-Tellado, F., Eur. J. Org. Chem. 2010, 6582.  substituted salicylaldehydes derivatives,3a 1,2-dihydropyridines3b or pyridines3c  via selective microwave-assisted  (MWA) domino chemistry.     Tertiary PVEs 3 (Scheme 1), armed with an electron withdrawing group (EWG) (amide or ester) at the tertiary propargylic position, constitute interesting building blocks with an unexplored synthetic potential. These units can be conveniently prepared by the triethylamine-catalyzed ABB’ three-component reaction (ABB’ 3CR)4 of alkyl propiolates and 1,2-ketoesters or 1,2-ketoamides5 or by treatment of the corresponding tertiary propargyl alcohol with alkyl propiolate in the presence of triethylamine.6 We envisioned that the [3,3]-propargyl Claisen rearrangement of these platforms should afford the mixture of the corresponding -allenals 4 and their enolic tautomers 5, which possess a highly electrophilic central allenic carbon and a reactive hydroxyl enol functionality conveniently placed to perform a 5-exo dig O-cyclization reaction (Scheme 1). The O-cyclization of 5 should afford trisubstituted furans 6 bearing a functionalized C2-chain linked to the heterocyclic ring. The synergistic activation of the central allene position by the two EWG groups should allow for the accomplishment of this   Scheme 1. Synthesis of C2-chain functionalized furans 6.                                                        4 ABB’ 3CRs refers to a three-component reaction that utilizes two different components (A and B) to give a product which incorporates into its structure one unit of component A and two chemo-differentiated units of component B (B and B’). For full details and more examples of this type of multicomponent reactions: Tejedor, D.; García-Tellado, F.; Chem. Soc. Rev. 2007, 36, 484. 5 Tejedor, D.; Santos-Expósito, A.; García-Tellado, F., Chem. Eur. J. 2007, 13, 1201. 6 (a) Tejedor, D.; Santos-Expósito, A.; Méndez-Abt, G., Ruiz-Pérez, C.; García-Tellado, F., Synlett 2009, 1223. (b) Inanaga, J.; Baba, Y.; Hanamoto, T. Chem. Lett. 1993, 241. transformation in the absence of activating metals.7 These trisubstituted furans 6 would represent, to the best of our knowledge, unprecedented hybrid structures of 3,4-disubstituted furans and C2-substituted acetic esters (or amides),8 which cannot be easily obtained by direct cyclization of simple acyclic precursors.9 Because the importance of substituted furans in organic,10  pharmaceutical11 and material chemistry,12 we decided to carry out the experimental study of this unprecedented domino transformation. We report herein the results of these studies. We undertook this work studying the MWA rearrangement of the PVE 3a, which was synthesized in 78% yield following our previously reported ABB’ 3CR protocol5 (Scheme 2). After exploring different sets of experimental conditions, we found that the MW irradiation of a solution of 3a in DMF (closed vessel, 300 W, 150 ºC, 1 h) cleanly afforded the furan derivative 6a in 99% yield. In a previous report,13 we had shown how the coupling of two domino processes in just one-pot reaction offered important advantages in terms of operational simplicity, efficiency and work economy. Thus, we assayed conditions to perform the entire process in a one-pot fashion. After several experimental trials, we found that furan 6a could be obtained in 78% yield (Scheme 2) directly from methyl 2-oxo-2-phenylacetate and methyl propiolate, without isolation of the PVE intermediate 3a.14                                                      7 Furan syntheses based on propargyl-allenyl interconversion and subsequent O-cyclization normally requieres metal-activation. See ref. 2 for selected examples. For selected reviews see: (a) Kirsch, S. F., Synthesis, 2008, 3183. (b) D’Souza, D. M.; Müller, T. J. J., Chem. Soc. Rev. 2007, 36, 1095. (c) Kirsch, S. F., Org. Biomol. Chem. 2006, 4, 2076. (d) Brown, R. C. D., Angew. Chem. Int. Ed. 2005, 44, 850. 8 We have not found precedents for these hybrid structures, featuring this substitution pattern, in our structure-driven search in SciFinder database. 9For selected examples of metal-catalyzed methodologies for the synthesis of substituted furans bearing a functionalized chain, see: (a) Rudolph M.; Hashmi A. S. K., Chem. Commun. 2011, 47, 6536 and references cited therein. (b) Albrecht, L.; Ransborg L. K.; Gschwend B.; Jorgensen K. A., J. Am. Chem.Soc. 2010, 132, 17886. (c) Hashmi A. S. K.;  Lothar Schwarz L., Choi, J.-H.; Frost T. M., Angew. Chem. Int. Ed. 2000, 39, 2285. For recent reviews of general synthetic methodologies to substituted furans, see: (d) Hou, X. L.; Yang, Z.; Yeung, K.-S.; Wong, H. N. C., in Progress in Heterocyclic Chemistry; Gribble, G. W., Joule, J. A., Eds.; Pergamon: Oxford, 2008, vol. 19, p. 176. (e) Balme, G.; Bouyssi, D.; Monteiro, N., Heterocycles 2007, 73, 87.  10 For selected reviews: (a) Hou, X. L.; Yang Z.; Wong, H. N. C. In Progress in Heterocyclic Chemistry; Gribble, G. W., Gilchrist, T. L., Eds.; Pergamon: Oxford, 2003, Vol. 15. (b) Keay, B. A.; Dibble, P.W. In Comprehensive Heterocyclic Chemistry II; Katritzky, A. R., Rees C.W., Scriven, E. F. V., Eds.; Elsevier: Oxford, 1997, Vol. 2; (c) Donnelly D.M.X.; Meegan, M. J. In Comprehensive Heterocyclic Chemistry, Katritzky A. R., Rees, C.W., Eds.; Pergamon: Oxford, 1984, Vol. 4.  11 (a) Mortensen, D. S.; Rodriguez, A. L.; Carlson, K. E.; Sun, J.; Katzenellenbogen, B. S.; Katzenellenbogen, J. A., J. Med. Chem. 2001, 44, 3838. (b) Francesconi, I.; Wilson, W. D.; Tanious, F. A.; Hall, J. E.; Bender, B. C.; Tidwell, R. R.; McCurdy, D.; Boykin, D. W., J. Med. Chem. 1999, 42, 2260. (c) Rahmathullah, S. M.; Hall, J. E.; Bender, B. C.; McCurdy, D. R.; Tidwell, R. R.; Boykin, D. W., J. Med. Chem. 1999, 42, 3994. 12 Zhang, L. Z.; Chen, C.W.; Lee, C. F.; Wu, C. C.; Luh, T. Y., Chem.Commun. 2002, 2336. 13 Tejedor, D.; González-Cruz, D.; García-Tellado, F.; Marrero-Tellado, J. J.; Rodriguez, M. L., J. Am. Chem. Soc. 2004, 126, 8390. 14 It has to be noted that although these two-step sequential processes are defined as one-pot, they require a change of reaction vessel (from a round bottom flask to a MW-special closed vessel) because the CEM MW oven requires pressure-resistant special glass flasks.   Scheme 2. Synthesis and MWA rearrangement of PVE 3a into furan 6a.   Once the experimental feasibility of our initial hypothesis was established, we explored the influence of the electron withdrawing group allocated at the tertiary propargylic position by replacing the ester group by an amide group (Scheme 3). Thus, we prepared the amide-containing PVE derivative homologous 3b. This was synthesized directly from N,N-dibenzyl-2-oxo-2-phenylacetamide and methyl propiolate following a modified version of our previously reported triethylamine-catalyzed ABB’ 3CR protocol.5 In this case, we used an excess of methyl propiolate (3 equiv) and an stoichiometry amount of triethylamine as the nucleophile activator to ensure a convenient yield of 3b (77%). The MW irradiation of a solution of this PVE in DMF (closed vessel, 300 W, 150 ºC, 1 h) delivered the furan derivative 6b in 92% yield. This result highlights that this domino   Scheme 3. Synthesis and MWA rearrangement of PVE 3b.   rearrangement/cyclization reaction is not particularly sensible to the nature of the EWG allocated on the propargylic position, supporting either amides or esters as activators. Again, the process could be performed in a one-pot format, delivering furan 6b in 73% yield. We next studied the scope of this reaction with regard to the nature of the tertiary PVE. Accordingly, we prepared the set of PVEs 3a-e spanning aromatic and aliphatic substitution at the propargylic position (Table 1). These PVEs were prepared by the corresponding ABB’ 3CR of methyl propiolate and the corresponding 1,2-ketoester or 1,2-ketoamide. PVE 3d, armed with an ester   Table 1. One-pot synthesis of trisubstituted furans 6 from methyl propiolate and 1,2-ketoesters (or 1,2-ketoamides) .a   entry EWG R  3  (yield %)b 6  (yield %)b one-pot 1 CO2Me Ph a 79 99 78 c 2 CONBn2 Ph b 77 92 73 c 3 CONBn2 Et c 60 85 50 4 CO2Et Ph(CH2)2 d 69d 93 64e 5 CONBn2 Ph(CH2)2 e 61 80 51 a See Supporting Information for experimental details. b Isolated yields. c MW was performed in xylene. d PVE was formed in two steps from methyl propiolate and methyl 2-oxo-4-phenylbutanoate. e From the corresponding tertiary propargylic alcohol.  function and an unbranched aliphatic chain at the propargylic position could not be prepared by this protocol.15 Instead, it had to be prepared by a two-step process, entailing the alkynylation16 of methyl 2-oxo-4-phenylbutanoate and the subsequent amine-catalyzed reaction of the corresponding tertiary propargylic alcohol with methyl propiolate.6 In general, furans 6a-e were obtained in excellent yields regardless if an amide or an ester were used as the second activators of the central allenic position. Remarkably, PVEs 3c-e afforded the corresponding furans 6c-e in very good yields, in spite of the tendency of the non-branched aliphatic substituents at the propargylic position to drive the domino process toward the formation of substituted salicylaldehyde derivatives (see below).3a With regard to the reaction processing, the two domino reactions could be coupled in a one-pot process with good overall yields (50-78%). The scope of the reaction was further studied using tertiary PVEs 8 armed with aliphatic or aromatic substituent at the sp-terminal alkyne position (Scheme 4, Table 2). These platforms were assembled by the two-step process aforementioned. In general, the triethylamine-catalyzed reaction of the corresponding tertiary propargyl                                                      15 The triethylamine catalyzed reaction of metyl propiolate and unbranched aliphatic 1,2-ketoesters affords the corresponding isotetronic acid derivatives via  an enolate-driven domino ABB’ 3CR. For more details see: Tejedor, D.; Santos-Expósito, A.; García-Tellado, F., Chem. Commun. 2006, 2667. 16Lithium acetylides were prepared according the standard procedure. See: Nielsen, T. E.; Tanner, D., J. Org. Chem. 2002, 67, 6366. alcohols 7 with excess of methyl propiolate afforded tertiary PVEs 8 with modest-to-good yields (see Supporting Information for details). After some experimental work, we could establish an experimental protocol for the efficient transformation of PVE 8a into the furan derivative 9a. During this experimental work, we observed that the formation of PVE 8a was always accompanied with a small amount of furan 9a (~10 %), which resulted in an added difficulty in the isolation and characterization of this PVE intermediate. For this reason, we decided to perform the process without the isolation of the intermediate PVE 8a. Thus, when the crude product of the reaction of formation of 8a was directly submitted to MW irradiation, furan 9a was generated in good yield (70%)(Table 2 entry 1). The reaction was extended to the tertiary alcohols 7b-f, featuring different combinations of aromatic/aliphatic substituents (Table 2). In general, the overall yields of these one-pot processes ranged from moderate to good, being the overall chemical efficiency compromised by the formation of the tertiary PVE intermediates. That was significant in the case of alcohol 7e, which delivered the corresponding furan 9a in 50% overall yield (entry 5). When this process was performed in a stepwise manner, the PVE 8e (57%) was transformed into furan 9e in 95% yield (data not shown). A remarkable property of these PVEs is their biased reactivity toward the 5-exo dig O-cyclization reaction even in the absence of activating substituent at the internal position of the allene (R1). Ester 8c and amide 8d constitute excellent examples of this reactivity control because they feature an aliphatic chain at the propargylic position which could steer the reaction outcome toward the formation of the salicylaldehyde  derivative,3a e.g. 10 (Scheme 4). Interestingly, this competing reactivity profile was only observed in the case of the amide-alcohol 7d, which delivered the mixture of furan 9d (58%) and salicylaldehyde 10 (18%) (entry 4).  Its homologous ester-alcohol 7c afforded the furan 9c as a sole compound and in excellent yield (85%, entry 3). These results show that the 5-exo dig cyclization pathway requires strong activation of the intermediate allenal I when a hydrogen atom is available at the homopropargylic position of the PVE. Otherwise, the formation of dienal II becomes competitive. Interestingly, when the reaction of the amide-alcohol 7d was performed in the presence of aniline2f (1.2 equiv), the imine derivative 11 was obtained in 48% yield together with 9d (38%). It is remarkable that the presence of aniline increases the formation of the salicylaldehyde derivative (from 18% to 48%) but does not afford the corresponding N-phenyl-pyrroles2f or N-phenyl-1,2-dihydropyridines.3b These facts seem to point that the aniline role in this reaction is to serve as a Lewis base catalyst for the dienal II and enol III formation.   In summary, we have shown how tertiary PVEs armed with an electron withdrawing group (amide or ester) at the tertiary propargylic position can be efficiently transformed into trisubstituted C2-chain functionalized furans in the absence of metals. This unprecedented domino transformation involves a microwave-assisted tandem [3,3]-propargyl Claisen rearrangement/5-exo dig O-cyclization reaction. The manifold is robust and it tolerates diverse patterns of substitution decorating the PVE platform. In addition, the manifold can be performed in a one-pot fashion from the primary components (1,2-ketoesters/1,2-ketoamide or  tertiary propargyl alcohols) using a simple and bench-friendly experimental protocol.  Scheme 4. Synthesis of furans 9 and salicylaldehyde 10 from alcohols 7.    Table 2. One-pot synthesis of furans 9 from alcohols 7.a entry EWG R R1  9 (yield  %)b 1 CO2Me Ph Ph a 70 2 CONBn2 Ph Ph b 74 3 CO2Me Et Ph c 85 4 CONBn2 Et Ph d 58c,d 5 CO2Me Ph Bu e 50 6 CO2Me Et Bu f 76e a See Supporting Information for experimental details. b Isolated yields. c From 8d.d 10 was also obtained in 18% yield.e From 8f.  Acknowledgment. This research was supported by the Spanish MICINN and the European RDF (CTQ2008-06806-C02-02), the Spanish MSC ISCIII (RETICS RD06/0020/1046), FUNCIS (REDESFAC PI01/06). L. C. thanks Spanish MEC for a FPI grant. Authors thank technician Ms. Anna Jurado Varona (IPNA-CSIC) for her experimental assistance. Supporting Information Available Experimental procedures and compound characterization data. This material is available free of charge via the internet at htpp://pubs.acs.org 

Microwave-assisted domino access to C2-chain functionalized furans from tertiary propargyl vinyl ethers

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