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Multicatalytic, Asymmetric Michael/Stetter Reaction Of Multicatalytic, asymmetric Michael/Stetter reaction SPECIAL FEATURE of salicylaldehydes and activated alkynes Claire M. Filloux, Stephen P. Lathrop, and Tomislav Rovis1 Department of Chemistry, Colorado State University, Fort Collins, CO 80523 Edited by David W. C. MacMillan, Princeton University, Princeton, NJ, and accepted by the Editorial Board May 30, 2010 (received for review March 22, 2010) We report the development of a multicatalytic, one-pot, asymmetric Ar Ar Michael/Stetter reaction between salicylaldehydes and electron- N H deficient alkynes. The cascade proceeds via amine-mediated OO 3 OTMS (20 mol %) Me HO Me Michael addition followed by an N-heterocyclic carbene-promoted Me Me 1 Ar = 3,5-(CF3)2C6H3 O intramolecular Stetter reaction. A variety of salicylaldehydes, dou- + O O BF4 N bly activated alkynes, and terminal, electrophilic allenes participate 2 Me 5 Me in a one-step or two-step protocol to give a variety of benzofura- NNC F 4a 6 5 One-Pot Two - Po t none products in moderate to good yields and good to excellent (10 mol %) 93% yield 46% yield NaOAc (10 mol %) 85:15:<1:<1 dr 5:1 dr enantioselectivities. The origin of enantioselectivity in the reaction 86% ee 58% ee CHCl3, 23 C is also explored; E∕Z geometry of the reaction intermediate as well as the presence of catalytic amounts of catechol additive are O found to influence reaction enantioselectivity. O Me * O Me ∣ ∣ catalysis organic synthesis tandem catalysis Me 6 ascade catalysis has garnered significant recent attention Scheme 1. Multicatalytic Michael/Benzoin cascade. Cfrom the synthetic community as a means to swiftly assemble CHEMISTRY complex molecules from simple starting materials with minimal benzofuranones. In 2008, Jørgensen and coworkers reported time, waste, and manipulation of reaction intermediates (1–11). that 2-tert-butoxy carbonyl benzofuranone could be alkylated Especially powerful in its application to total synthesis, asym- asymmetrically with tetraethyl ethylidene-bisphosphonate to give metric tandem catalysis has enabled rapid access to enantioen- the corresponding 2,2′-disubstituted product in excellent enan- riched products with high levels of selectivity (1, 4–8, 10, 11). tioselectivity (44). In a different approach, we have shown that Although most examples exploit a single catalyst to promote mul- chiral triazolinylidene carbenes mediate the cyclization of tiple, sequential transformations (12–19), systems relying on two aldehyde-tethered, β,β-disubstituted Michael acceptors related or more catalysts have been reported (2, 3, 11, 20). Inherent in to 12 to give benzofuranone products in excellent enantioselectiv- any multiple catalyst system is the challenge of compatibility. ities (Scheme 2) (45–47). Avoidance of mutual interference often obliges stepwise addition Although the strategies described provide benzofuranone pro- of catalysts or reagents and variation of reaction conditions over ducts in good yield and exceptional selectivities, both make use of time (21–26). Nevertheless, cascades triggered by a single opera- substrates that require multiple steps to prepare (47). We ima- tion have been accomplished (20, 27, 28). gined that we could expedite the synthesis of benzofuranone Very recently, we reported the development of a one-step, products 9 by assembling intermediate aldehydes 12 in situ via asymmetric Michael/benzoin reaction of β-ketoesters 1 and α,β- a base-catalyzed conjugate addition reaction of salicylaldehydes unsaturated aldehydes 2 mediated by compatible amine 3 and 7 and electrophilic alkynes 8 (Scheme 2). Fan et al. have shown N-heterocyclic carbene (NHC) catalysts (Scheme 1) (29). Impor- that 1,4-diazabicyclo[2.2.2]octane (DABCO) (10) efficiently med- tantly, the one-pot procedure was found to give elaborated cyclo- iates the addition of amine and oxygen nucleophiles to dimethyl pentanone products 5 in higher yield and enantioselectivity than a acetylenedicarboxylate (DMAD) (8a) and alkyl propiolates stepwise protocol, wherein intermediate aldehyde 6 is isolated and (48, 49). In our envisioned sequence, a tertiary amine such as subjected to the benzoin reaction in a subsequent step (Scheme 1). quinuclidine (11) or DABCO (10) activates alkyne 8 toward This observation testifies to the power of cascade catalysis: By nucleophilic attack to give intermediate aldehyde 12 (Scheme 2). quickly relaying intermediates from one reaction to the next, cat- Subsequent chiral carbene-promoted Stetter reaction sets a alysts can work synergistically to discourage undesired pathways. quaternary stereocenter and yields product 9 asymmetrically. Encouraged by the discovery that NHCs could participate in Crucial to the success of any catalytic cascade is a compatible cascade catalysis, we were inspired to use NHCs to mediate the catalyst system. For many Stetter systems, tertiary amines per- cascade assembly of benzofuranone products 9 asymmetrically form as optimal bases for carbene generation (47, 50, 51). For (30). A range of biological activities associated with 3(2H)-benzo- furanones including antifungal (31, 32), antipsychotic (33), and anticancer (34–36) properties make these products attractive Author contributions: T.R. designed research; C.M.F. and S.P.L. performed research; and synthetic targets. Among natural products containing a 2,2′-disub- C.M.F., S.P.L. and T.R. wrote the paper. stituted benzofuranone core, rocaglamide demonstrates appreci- The authors declare no conflict of interest. able cytotoxicity in mice and human cells lines (37), vinigrol This article is a PNAS Direct Submission. D.W.M. is a guest editor invited by the displays antihypertensive properties (38), and Sch202596 shows Editorial Board. promise in Alzheimer’s therapy (39). Data deposition: The coordinates for the crystal structure of 9da have been deposited in the Cambridge Structural Database, Cambridge Crystallographic Data Centre, www.ccdc. Although a number of methods for the racemic assembly of cam.ac.uk/products/csd/, Cambridge CB2 1EZ, United Kingdom (CSD reference no. CCDC benzofuranones containing C2 quaternary centers have been re- 783384). ported, many proceed from relatively advanced starting materials 1To whom correspondence should be addressed. E-mail: [email protected]. (40, 41) or suffer from competitive reaction pathways (42, 43). This article contains supporting information online at www.pnas.org/lookup/suppl/ More rare are enantioselective preparations of 2,2′-disubstituted doi:10.1073/pnas.1002830107/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1002830107 PNAS Early Edition ∣ 1of6 Downloaded by guest on September 27, 2021 N Table 1. Solvent and temperature screen O EWG 10 or 11 O N N EWG O CO2Me O 1) (20 mol %) CO2Me OH EWG O O EWG N 11 7 8 9 OH CO2Me O CO Me solvent (0.1 M), T 2 N N 7a 8a 1 12aa N 4b BF4 Ar O EWG I N O N EWG NR3 N BF4 CO2Me 2) (20 mol %) C6F5 4b O O N 9aa CO2Me solvent (0.1 M), T2 Ar N N O Entry Solvent T1,°C T2, °C Yield, % ee, % EWG R OH 1CH2CI2 23 0 85 81 EWG O EWG O 2 THF 23 0 76 83 12 II EWG 3 t-amyl alcohol 23 0 63 78 4 PhMe/t-amyl alcohol (9∶1) 23 0 87 86 Scheme 2. Envisioned multicatalytic Michael/Stetter cascade. 5 PhMe 23 0 82 89 6 PhMe 23 23 75 86 this reason, we were encouraged that DABCO or quinuclidine 7 PhMe 23 −10 60 90 would not only serve as nucleophilic “triggers” (48) to promote 8 PhMe 23 −40 64 92 our imagined conjugate addition reaction but would also prove 9 PhMe −40 NA NR NA suitable bases to deprotonate triazolium salt precatalyst 4b and NA, not applicable; NR, no reaction. generate the active carbene species. alternative pathway could be imagined in which quinuclidine Results and Discussion deprotonates salicylaldehyde, which adds conjugately to DMAD. We first examined whether our envisioned cascade could be per- To examine the viability of a base-catalyzed pathway, we ex- formed in a one-pot, stepwise fashion. Carbenes have been shown changed quinuclidine for diisopropylethylamine. Treatment of capable of nucleophilic addition into DMAD and other activated salicylaldehyde and DMAD with this less competent nucleophile alkynes (52, 53). Tocircumvent this undesired reaction pathway, a but similarly strong base resulted in complete recovery of mixture of salicylaldehyde (7a) and DMAD (8a) was treated starting material, suggesting that the proposed nucleophilic first with quinuclidine 11 and then with triazolium salt 4b. Ben- pathway is likely at work in our developed conditions (Eq. 2). zofuranone product 9aa was isolated from this one-pot, two-step Moreover, exposure of salicylaldehyde and DMAD to the free sequence in good overall yield and enantioselectivity (Table 1, carbene derived from azolium salt 4b gave no observable product 9∶1 entry 1). In a brief solvent screen, dichloromethane and by 1H NMR spectroscopy (Eq. 3). It is highly probable that t toluene/ -amyl alcohol provided product 9aa in similarly high quinuclidine (11) rather than carbene-4b participates as the yields (Table 1, entries 1 and 4), whereas toluene gave the highest nucleophilic activator in our system. level of enantioselectivity (Table 1, entries 1–5). Lowering tem- OCO2Me O peratures of the Stetter reaction improves enantioselectivity DIPEA (20 mol %) slightly but results in longer reaction times and lower product CO2Me yields (Table 1, entries 6–8). No productive reaction is observed PhMe, 23 C [2] when the conjugate addition reaction is conducted at low tem- OH CO2Me O CO2Me peratures (Table 1, entry 9). 7a 8a 12aa Although we had developed conditions to mediate two bond- O forming events in one reaction vessel with high levels of asym- metric induction, we hoped to reduce the number of required N N synthetic manipulations to a single operation. To this end, we N O OCO2Me [3] carbene-4b Ar treated a mixture of 7a, 8a, and 4b with quinuclidine (11)at0 CO Me °C in toluene. To our delight, the cascade proceeds smoothly 2 to give 9aa in undiminished yield and enantioselectivity PhMe, 23 C O OH CO2Me CO2Me (Eq.
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