Highly Enantioselective Construction of Sterically Hindered α-Allyl-α

Research Article pubs.acs.org/acscatalysis

Highly Enantioselective Construction of Sterically Hindered α‑Allyl-αAryl Lactones via Palladium-Catalyzed Decarboxylative Asymmetric Allylic Alkylation Jinju James and Patrick J. Guiry* Centre for Synthesis and Chemical Biology, School of Chemistry, University College Dublin Belfield, Dublin 4, Ireland S Supporting Information *

ABSTRACT: Pd-catalyzed decarboxylative asymmetric allylic alkylation has been developed for sterically hindered α-aryl, βoxo-allyl ester lactone substrates. Pb-mediated α-arylation of the β-oxo-allyl ester was used as the key step to synthesize the substrates for catalysis in moderate to high yields. Optimization studies for decarboxylative asymmetric allylic alkylations (DAAA) were conducted using δ-valerolactonederived α-aryl β-oxo-allyl ester with 2,4,6-trimethoxyphenyl as the aryl substituent. Using (R,R)-ANDEN-phenyl Trost as the chiral ligand, enantioselectivities of up to >99% ee and 98% ee were achieved with the six-membered and five-membered lactone substrates, respectively. Bulky aryl groups containing di-ortho substitutions and naphthyl groups gave the highest enantioselectivities. This synthetic route allows for the simple modification of aryl groups, giving highly enantioselective access to important structural motifs. KEYWORDS: asymmetric catalysis, decarboxylation, allylic alkylation, palladium catalysis, lactones

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INTRODUCTION Lactones are important heterocycles as the lactone motif is prevalent in a wide range of natural products and bioactive compounds.1 It is also a useful functional group next to a carbon stereocenter as lactones can undergo ring-opening reactions and are easily transformed into other valuable moieties such as amides, diols, azepines, and more highly functionalized building blocks.2 The ubiquity of quaternary carbonyl stereocenters in nature has led to intensive research efforts directed toward the development of their asymmetric synthesis, particularly, quaternary carbonyl stereocenters.3 However, there are only few examples with limited substrate scope of a stereoselective route to α-substituted quaternary lactones described to date. In 1989, Fuji reported the diastereoselective nitroolefination of αquaternary δ-lactones via a Michael-type addition followed by elimination of an electron-rich chiral group (eq [1], Scheme 1).4 This process formed the α,α-dialkyl lactones in high enantioselectivities (56−96% ee). In 2002, Buchwald reported a Ni(0)-BINAP system that was used for an enantioselective α-arylation using aryl chlorides and bromides (eq [2], Scheme 1).5 High enantioselectivities were observed when meta- and para-substituted aryl halides were coupled with α-substituted γ-butyrolactones (83−98%). It was also noted that ortho-substituted aryl halides did not give the desired α-arylated products. Similarly, in a 2003 patent, Zhang described a synthetic route to enantioselectively access αmethyl-α-aryl lactones using a Pd(0)-BINAP catalytic system.6 Aryl halides were coupled with α-methyl-γ-butyrolactone to © XXXX American Chemical Society

form the desired products in modest enantioselectivities (15− 65% ee). The enantioselective step was used in the preparation of LY426965, a serotonin antagonist.7 Fu reported the catalytic enantioselective formation of αacyl-α-aryl lactones by C-acylation of silyl ketene acetals (eq [3], Scheme 1).8 Further elegant routes for the enantioselective construction of α-acyl-γ-butyrolactones possessing α-quaternary stereocenters followed in reports by Maruoka, Jacobsen, Park, and Cossy.9,10 In 2008, Fillion used the Cu-catalyzed 1,4addition of diethylzinc to 2-(2,2- dimethyl-4,6-dioxo-1,3dioxan-5-ylidene)aryl acetates as the key enantioselective step to form the precursor to α-ethyl-α-phenyl-γ-butyrolactone in 88% ee (eq [4], Scheme 1).11 Breit described the regioselective hydroformylation of 3-substituted homoallylic alcohols subsequently to produce α,α-disubstituted γ-butyrolactones and highly substituted tetrahydrofurans.12 The two examples of homoallylic alcohols with aromatic substituents at the 3position gave the α-methyl-α-phenyl-γ-butyrolactone products in modest enantioselectivities. Transition-metal-catalyzed decarboxylative asymmetric allylic alkylation (DAAA) has emerged as a powerful synthetic route for the enantioselective formation of quaternary carbon stereocenters adjacent to a carbonyl group.13 Tsuji and Saegusa independently reported the Pd-catalyzed decarboxylative allylic alkylation in 1980.14 Despite intensive focus on this transReceived: November 28, 2016 Revised: January 6, 2017

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ACS Catalysis

Scheme 2. Synthesis of α-Aryl-β-oxo-allylester Substrates (2) for Catalysis

Scheme 1. Approaches for Asymmetric Synthesis of AllCarbon α-Quaternary Lactones

Scheme 3. Synthesis of α-Aryl-β-oxo-allylester Substrates (4) for Catalysis

formation, the asymmetric variant was only reported over two decades later in 2004 by the Stoltz and Tunge groups.15 Stoltz’s work described the formation of cyclic α-allyl ketones using allyl enol carbonate substrates and a variety of chiral ligands. Screening of several ligands revealed that chelating P,N-ligands were the most effective at generating the new quaternary stereocenters. The following year, Trost reported a similar system using enol carbonate substrates for enantioselective allylic alkylation of cyclic ketones.16 Interestingly, they found that P,P-ligands, specifically the (R,R)-ANDEN-phenyl Trost ligand, gave very high levels of enantioselectivity. Previous work in our group has focused on developing methods for the formation of quaternary and tertiary α-aryl carbonyl compounds by decarboxylative asymmetric allylic alkylations and protonations, respectiveley.17 A highly enantioselective Pd-catalyzed DAAA of cyclopentanone derived α-aryl-β-keto esters was achieved using the (R,R)-ANDENphenyl Trost ligand. α-Allyl-α-aryl-cyclopentanone products were formed in high yields and enantioselectivities (83% to 99.9% ee) and the utility of this transformation was demonstrated in the asymmetric formal synthesis of the antifungal, marine natural product (+)-tanikolide.17d More recently, we reported the construction of α-allyl-α-aryl dihydrocoumarins and 3-isochromanones with very high levels of enantioselectivity (up to 96% ee) by Pd-catalyzed DAAA.17e

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RESULTS AND DISCUSSION In view of the excellent enantioselectivities obtained in our previous work and the utility of lactones as chiral building blocks, we decided to investigate whether the DAAA methodology could be successfully developed for sterically hindered α-aryl lactones (eq [5], Scheme 1). Through our study, we became aware of the absence of a general 1398

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ACS Catalysis Table 1. Optimization of DAAA Using 2a

entry

ligand

solvent

temp (°C)

time (h)

conv. (%)c

yield (%)d

ee (%)e

1a 2a 3a 4a 5a 6a 7a 8a 9a 10a 11a 12a 13a 14a 15a 16a 17b

L1 L2 L3 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4 L4

1,4-dioxane 1,4-dioxane 1,4-dioxane 1,4-dioxane toluene 2-Me-THF MTBE THF DME 1,4-dioxane:MTBE (2:1)f 1,4-dioxane (0.02 M) 1,4-dioxane (0.06 M) 1,4-dioxane 1,4-dioxane 1,4-dioxane 1,4-dioxane 1,4-dioxane

40 40 40 40 40 40 40 40 40 40 40 40 25 40 40 40 40

18 18 18 18 18 18 18 18 18 18 18 18 18 0.25 0.50 1 48

30 25 22 100 20 20 38 36 22 73 100 100 83 40 73 100 52

13 14 18 85 17 19 34 27 11 71 88 87 67 89 42

24 36 70 99.5 99 97 99 99 98 99 99 99 99 90 96 >99 99

a Reactions carried out using 5 mol % Pd2(dba)3·CHCl3, 13 mol % L4 at 0.04 M. bReaction carried out using 2.5 mol % Pd2(dba)3·CHCl3 and 5 mol % L4. cDetermined by 1H NMR spectroscopy of crude product. dIsolated yield after flash column chromatography. eDetermined by chiral SFC (Chiralcel IC-3, scCO2:Acetonitrile, 70:30, 3 mL/min): Rt = 5.13 (major) 6.00 (minor). fRatio of volume:volume.

methodology to construct quaternary α-aryl lactones with aryl groups possessing ortho-substituents. Herein, we disclose the development of a Pd-catalyzed DAAA for sterically hindered αaryl β-oxo-allyl ester lactone substrates to form α-allyl-α-aryl lactones with high levels of enantioselectivity. A Pb-mediated αarylation employing aryllead triacetates was used to generate the substrates for DAAA. Aryllead triacetates are stable and effective reagents for the electrophilic arylation of soft carbon nucleophiles.18 A variety of electron-rich and electron-poor substituents can be accommodated on the aryl ring of these reagents, making them more versatile than diaryliodonium salts for α-arylation.19 We chose to commence studies on δvalerolactone substrates, as there are only limited examples in the literature for the enantioselective construction of quaternary α-aryl δ-valerolactones.20 The synthesis of the β-oxo-allylester 1 was achieved by the αacylation of the commercially available δ-velarolactone (Scheme

2). The highest yield of 53% was obtained using LDA as the base and allyl chloroformate as the electrophile in THF at −78 °C. The α-arylation of 2-carboxycarbonyl-δ-velarolactone 1 was carried out using the aryllead triacetates in the presence of anhydrous pyridine at 40 °C in CHCl3 (Scheme 2). The substrates for catalysis, 2a−2k, were isolated in good yields ranging from 61%−94%. 2-Carboxycarbonyl-γ-butyrolactone 3 could be synthesized from cyclopropane-1,1-dicarboxylic acid when treated with allyl bromide in the presence of triethlyamine (Scheme 3).21 In contrast to the 6-membered lactones, the standard arylation conditions gave the α-arylated-β-oxo-allylester substrates 4 in lower yields. Through a temperature study, we found that carrying out the reaction at 60 °C instead of 40 °C increased the yields for arylation considerably.22 The substrates for catalysis, 4a−4h, were prepared in moderate to excellent yields 1399

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2,6-dimethoxy substitutions on the aryl ring gave the best enantioselectivites (99% ee) as observed for products 5a−c. The lack of ortho-substitution significantly reduced the enantioselectivity (8−28% ee) for substrates 2i−k. The bulky naphthyl groups in 2d and 2e also gave excellent ee’s of 97% and 94%, respectively. Interestingly, a meta-methoxy group had an adverse effect on enantioselectivity as 5h was formed in 64% ee compared to the 2,4-dimethoxyphenyl analogue 5f, which was produced in 88% ee. Another intriguing comparison of enantioselectivities is between 5a and 5g (79% ee). Here, we see that methyl substitution at the ortho-position negatively impacts the enantioselectivity. Classical steric parameters such as Winstein−Holness and Charton regard the methoxy group to be less bulky than the methyl group.23 This can be an indicator that in addition to the steric factor, there is also an electronic effect influencing the enantioselective outcome. Aryl groups without any substitution at the ortho-position (5i, 5j, 5k) gave the lowest enantioselectivities. We then investigated the DAAA of the butyrolactone-derived substrate 4a employing our previously optimized reaction conditions. It was found that the conversion of starting material to product at 40 °C was very slow, giving only ∼70% conversion even after 18 h. A temperature study was carried out to investigate the effect on conversion and enantioselectivity.24 Pleasingly, conversion of starting material to product increased at 50 °C and remained the same up to 80 °C. However, increasing the temperature decreased the enantioselectivity slightly from 97% ee at 50 °C to 96% ee at >60 °C. Substrate scope studies for DAAA of α-aryl-γ-butyrolactone substrates were thus carried out at 50 °C and the products were isolated in excellent yields ranging from 70 to 97% (Scheme 5). As

(38%−92%) using these modified reaction conditions (Scheme 3). The optimization reactions for DAAA were carried out using the 2,4,6-trimethoxyphenyl-containing substrate 2a (Table 1). In order to systemically examine the effect of different factors on the DAAA reaction, a ligand screening using (P,N)- and (P,P)-ligands was performed first, in 1,4-dioxane at 40 °C (Table 1, entries 1−4). Both PHOX-ligands, L1 and L2, gave rise to the allylated product 5a in low yields and enantioselectivities (24 and 36% ee, respectively). The (R,R)DACH-phenyl Trost L3 formed product 5a in 70% ee but in a poor yield of 18%. The best ligand for the DAAA transformation was found to be the (R,R)-ANDEN-phenyl Trost L4 giving the product in 85% yield and an excellent ee of 99.5% (Table 1, entry 4). A solvent screen was then carried to probe the effect of different solvents on enantioselectivity using 13 mol % of L4 (Table 1, entries 5−10). While the enantioselectivities were consistently high (≥97% ee), none of the solvents that were tested were better than 1,4-dioxane in terms of yield. A reaction was attempted with 2.5 mol % Pd2(dba)3·CHCl3 and 5 mol % L4 in 1,4-dioxane at 40 °C. Although excellent enantioselectivity (99% ee) was observed, only 52% conversion of the starting material was achieved after 48 h (Table 1, entry 17). Similarly, a reaction that was carried out at room temperature gave excellent enantioselectivity (99% ee) but low conversion of starting material even after 18 h (Table 1, entry 13). Through a time-dependent study on the reaction, we were pleased to find that complete conversion of starting material was achieved in 1 h, and the product was isolated in >99% ee (Table 1, entry 16). The optimized conditions (Table 1, entry 16) were applied in a substrate scope study (2b−k), and a range of aryl groups were tolerated at the α-position to form allylated products in yields ranging from 61 to 98% (Scheme 4). Aryl groups with

Scheme 5. Scope and Enantioselectivity of α-Aryl-β-oxoallylesters (4)

Scheme 4. Scope and Enantioselectivity of α-Aryl-β-oxoallylesters (2)

expected, aryl groups with 2,6-dimethoxy substitutions and the bulky naphthyl groups (4a−c and 4e) gave the best enantioselectivities (≥93% ee). Surprisingly, lower levels of enantioselectivity were observed for the majority of fivemembered lactone products lacking aryl groups with orthosubstitutions compared to the corresponding six-membered lactone products. As an example, 2,4-dimethoxy-substituted aryl group-containing product 6f was formed in only 41% ee compared to the 73% ee observed for product 6d containing a 1400

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ACS Catalysis 2-methoxy-4,6-dimethyl group. Disappointingly, substrate 4h with the 2,3,4-trimethoxy-substituted aryl ring gave the lowest enantioselectivity in the substrate scope studies (3% ee) for the DAAA. Through a reoptimization study with 4h, we found that (S,S)-L3 increased the enantioselectivity of the allylated product 6h considerably to 54% ee.25 Using (S,S)-L3 for some of the substrates that gave low enantioselectivities with (R,R-L4), we were pleased to find a general trend for increased enantioselectivities (Scheme 6). The substrate with the Scheme 6. Substrate Scope and Enantioselectivity Using Reoptimized Conditions

Figure 2. Rationalizing the stereochemical outcome.

electron-withdrawing para-CF3-substituted aryl group 2k in particular gave a much improved 80% ee (Scheme 6). These results highlight the requirement for strict matching of substrate to catalyst structure in order to induce high levels of enantioselectivity. The absolute configurations (S) of the stereocenters of lactones 5b and 5d were confirmed by X-ray crystallographic analysis (Figure 1). This stereochemical outcome is consistent

in such a way that the aryl group points away from the potential steric clash. Ortho-substitution likely prohibits coplanarity and thereby prohibits conjugation, leaving an unstabilized enolate. This may explain the high enantioselectivities observed for lactone substrates with aryl groups possessing ortho-substitutions.

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CONCLUSIONS In conclusion, we have described an enantioselective route for the construction of α-allyl-α-aryl lactones. These were not previously accessible via complementary routes constructing αsubstituted quaternary lactones. Aryl groups containing di-ortho substitution and bulky naphthyl groups were well tolerated and gave the highest enantioselectivities. (R,R)-ANDEN-phenyl Trost L4 was found to be the best ligand for DAAA and formed the product in high yields and excellent enantioselectivities (up to >99%). A limitation of the strategy is the low enantioselectivities obtained for substrates lacking orthosubstituents. However, (S,S)-DACH-phenyl Trost L3 was found to be better for these substrates, particularly for aryl groups containing the electron withdrawing −CF3 group at the para position (80% ee). An advantage of this methodology for the preparation of quaternary stereocenters is the ability to have a range of aryl groups at the α-carbon prior to the enantioselective step. In addition, prior work by the group of Trost suggests the potential for extension to the preparation of lactones possessing unsymmetrical allyl fragments at the αcarbon.13f We believe this synthetic route allows for the simple modification of the aryl groups, giving access to important lactone scaffolds in high enantioselectivities.

Figure 1. X-ray crystal structure of products 5b and 5d.

with the transition-state model described previously (Figure 2).17d,26 Mechanistic studies by Lloyd-Jones, Norrby and coworkers on Trost-type ligands, describe a cationic [allyl-PdDACH] complex, where the amide proton directs the enolate carbon to the allyl through H-bonding.26 The enantioselectivity therefore arises by the way the lactone enolate approaches the allyl group on Pd. The steric clash between the bulky aryl group and the ligand framework means that the lactone orients itself 1401

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Jacobsen, E. N. J. Am. Chem. Soc. 2011, 133, 13872−13875. (c) Ha, M. W.; Lee, H.; Yi, H. Y.; Park, Y.; Kim, S.; Hong, S.; Lee, M.; Kim, M.-h.; Kim, T.-S.; Park, H.-g. Adv. Synth. Catal. 2013, 355, 637−642. (10) During the review of this manuscript, the following report from Cossy for the preparation of α-quaternary γ-butyrolactones appeared: Nascimento de Oliveira, M.; Fournier, J.; Arseniyadis, S.; Cossy, J. Org. Lett. 2017, 19, 14. (11) Wilsily, A.; Fillion, E. Org. Lett. 2008, 10, 2801−2804. (12) Ueki, Y.; Ito, H.; Usui, I.; Breit, B. Chem.Eur. J. 2011, 17, 8555−8558. (13) (a) Burger, E. C.; Tunge, J. A. Chem. Commun. 2005, 2835− 2837. (b) Mohr, J. T.; Behenna, D. C.; Harned, A. M.; Stoltz, B. M. Angew. Chem., Int. Ed. 2005, 44, 6924−6927. (c) Trost, B. M.; Bream, R. N.; Xu, J. Angew. Chem., Int. Ed. 2006, 45, 3109−3112. (d) He, H.; Zheng, X.-J.; Li, Y.; Dai, L.-X.; You, S.-L. Org. Lett. 2007, 9, 4339− 4341. (e) Schulz, S. R.; Blechert, S. Angew. Chem., Int. Ed. 2007, 46, 3966−3970. (f) Trost, B. M.; Xu, J.; Schmidt, T. J. Am. Chem. Soc. 2009, 131, 18343−18357. (g) McDougal, N. T.; Virgil, S. C.; Stoltz, B. M. Synlett 2010, 2010 (11), 1712−1716. (h) Trost, B. M.; Lehr, K.; Michaelis, D. J.; Xu, J.; Buckl, A. K. J. Am. Chem. Soc. 2010, 132, 8915−8917. (14) (a) Shimizu, I.; Yamada, T.; Tsuji, J. Tetrahedron Lett. 1980, 21, 3199−3202. (b) Tsuda, T.; Chujo, Y.; Nishi, S.; Tawara, K.; Saegusa, T. J. Am. Chem. Soc. 1980, 102, 6381−6384. (15) (a) Behenna, D. C.; Stoltz, B. M. J. Am. Chem. Soc. 2004, 126, 15044−15045. (b) Burger, E. C.; Tunge, J. A. Org. Lett. 2004, 6, 4113−4115. (16) Trost, B. M.; Xu, J. J. Am. Chem. Soc. 2005, 127, 17180−17181. (17) (a) Carroll, M. P.; Muller-Bunz, H.; Guiry, P. J. Chem. Commun. 2012, 48, 11142−11144. (b) Doran, R.; Carroll, M. P.; Akula, R.; Hogan, B. F.; Martins, M.; Fanning, S.; Guiry, P. J. Chem. - Eur. J. 2014, 20, 15354−15359. (c) Doran, R.; Guiry, P. J. J. Org. Chem. 2014, 79, 9112−9124. (d) Akula, R.; Doran, R.; Guiry, P. J. Chem. - Eur. J. 2016, 22, 9938−9942. (e) Akula, R.; Guiry, P. J. Org. Lett. 2016, 18, 5472−5475. (18) Guiry, P. J.; McCormack, P. J. Product Class 3 (Lead Compounds), Product Subclasses 6−13. In Science of Synthesis; Moloney, M. Ed.; 2003; Vol. 5, p 673−691.10.1055/sos-SD-00500643 (19) Malmgren, J.; Santoro, S.; Jalalian, N.; Himo, F.; Olofsson, B. Chem. - Eur. J. 2013, 19, 10334−10342. (20) (a) Li, X.-H.; Wan, S.-L.; Chen, D.; Liu, Q. R.; Ding, C.-H.; Fang, P.; Hou, X.-L. Synthesis 2016, 48, 1568−1572. (b) Li, Z.; Geng, Q.; Lv, Z.; Pritchett, B. P.; Baba, K.; Numajiri, Y.; Stoltz, B. M.; Liang, G. Org. Chem. Front. 2015, 2, 236−240. For other work on enantioselective α-arylation of simple lactones forming α-tertiary stereocenters see: (c) Huang, Z.; Chen, Z.; Lim, L. H.; Quang, G. C. P.; Hirao, H.; Zhou, J. Angew. Chem., Int. Ed. 2013, 52, 5807−5812. (d) Harvey, J. S.; Simonovich, S. P.; Jamison, C. R.; MacMillan, D. W. C. J. Am. Chem. Soc. 2011, 133, 13782−13785. (21) Rao, V. S. Synth. Commun. 1989, 19, 1389−1393. (22) For temperature studies for α-arylation see Supporting Information, Table S1 (23) (a) Winstein, S.; Holness, N. J. J. Am. Chem. Soc. 1955, 77, 5562−5578. (b) Charton, M. J. Am. Chem. Soc. 1975, 97, 1552−1556. (c) Harper, K. C.; Bess, E. N.; Sigman, M. S. Nat. Chem. 2012, 4, 366− 374. (24) For temperature studies for DAAA, see Supporting Information, Table S2 (25) For temperature studies for reoptimization of DAAA, see Supporting Information, Table S3 (26) Butts, C. P.; Filali, E.; Lloyd-Jones, G. C.; Norrby, P.-O.; Sale, D. A.; Schramm, Y. J. Am. Chem. Soc. 2009, 131, 9945−9957.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b03355. Experimental procedures, compound characterization data and copies of NMR spectra for all compounds (PDF) Crystallographic data for 5b (CIF) Crystallographic data for 5d (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Patrick J. Guiry: 0000-0002-2612-8569 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.J. is grateful for the award of an Irish Research Council Enterprise Partnership Scheme Ph.D. Scholarship (EPSPG/ 2014/110) with Enterprise Partner APC Ltd. We acknowledge facilities provided by the Centre for Synthesis and Chemical Biology (CSCB), funded by the Higher Education Authority’s PRTLI. We would like to thank Dr. Helge Müller-Bunz for Xray crystal structure analysis and Dr. Yannick Ortin for help with the NMR spectroscopic studies. We acknowledge Dr. Ramulu Akula and Yuyi Wang for helpful discussions and preliminary studies, respectively.

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REFERENCES

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