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Design of Efficient Chiral Bifunctional Phase-Transfer Cata-lysts Possessing an Amino Functionality for Asymmetric Aminations Suva Paria, Qikai Kang, Miho Hatanaka, and Keiji Maruoka ACS Catal., Just Accepted Manuscript • Publication Date (Web): 27 Nov 2018 Downloaded from http://pubs.acs.org on November 27, 2018
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ACS Catalysis
Design of Efficient Chiral Bifunctional Phase-‐Transfer Catalysts Pos-‐ sessing an Amino Functionality for Asymmetric Aminations Suva Paria,† Qikai Kang,† Miho Hatanaka,‡,= and Keiji Maruoka*†Δ †
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo, Kyoto 606-8502, Japan Institute for Research Initiatives, Division for Research Strategy, Graduate School of Materials Science, Data Science Center, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan = PRESTO, Japan Science and Technology (JST) 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan ‡
Δ
School of Chemical Engineering and Light Industry, Guangdong University of Technology, No.100, West Waihuan Road, HEMC, Panyu District, Guangzhou, 510006, China ABSTRACT: An efficient, chiral bifunctional phase-transfer catalyst with an intramolecular amino functionality has been designed, and successfully applied to highly enantioselective amination of 3-aryloxindoles and 3-arylbenzofuran-2(3H)-ones with bis(adamantyl) azodicarboxylate. Optically active amination products were generally obtained in high yield with high enantioselectivity. The origin of stereoselectivity was explained by means of the DFT calculations.
KEYWORDS: phase-transfer catalyst, bifunctional catalyst, asymmetric amination, oxindole, benzofuran-2(3H)-one Asymmetric phase-transfer catalysis under basic conditions provides an extremely powerful and versatile means for inducing asymmetry in reactions of prochiral anionic nucleophiles through the formation of an ion pair with chiral ammonium cation, thereby producing high levels of enantioselectivity in a wide range of reactions including enolate alkylation, conjugate addition, cyclopropanation, oxidative cyclization, amination, etc.1 Scheme 1. Mechanistic Scenario Under Ordinary and New Bifunctional Phase-‐transfer Catalysis. (1) Ordinary amine-promoted asymmetric phase-transfer reaction H C C O
NR 3
R' 4N Br C C O
HNR 3
C C O
slow
E
+ R 3NH Br NR' 4
E
E
E * C C O 2 chiral product
C C O 1 racemic product
(2) New bifunctional phase-transfer catalysis (This work) Ar
Ar Br N R
H
C C O
N Ar H C C O
R R
fast anion exchange
Ar
C C O N R
Br R N R
H
bifunctional PTC
In the case of asymmetric nucleophilic reactions, strongly basic conditions would generally be required for the efficient generation of an ion pair with chiral ammonium cation,2 and
hence ordinary tertiary amine bases are not recommended for this purpose due to the unfavorable generation of an ammonium enolate via the slow ion-exchange reaction, thereby providing a racemic product 1 as a major product ((1) in Scheme 1). In this context, we are interested in the possibility of designing a new chiral bifunctional phase-transfer catalyst possessing an intramolecular amino linkage for affecting the deprotonation and subsequent facile anion-exchange reaction with an adjacent ammonium cation to furnish a chiral product 2 preferentially ((2) in Scheme 1). Oxindoles bearing a chiral quaternary stereocenter at the 3position are common structural motifs in various natural and bioactive products. Consequently, a significant amount of effort have been devoted to achieve this goal by the use of metal and organocatalysis.3 Chiral phase-transfer catalysis also has made appreciable contribution in this direction by means of alkylation, conjugate addition, Mannich reaction, SNAr etc.2a,e,g,i,4 Among the chiral 3,3-disubstituted oxindoles of particular interest are 3-amino-2-oxindole compounds which exhibit significant biological activities.5 Procedures describing the asymmetric synthesis of this class of oxindoles utilized bischinchona alkaloid, chiral scandium complex or Schiff base as catalyst starting from 3-substituted oxindoles.6 However, many literature procedures suffer from high catalyst loading and longer reaction times.6d,f Our aim was to design a new chiral bifunctional phase-transfer catalyst which can overcome above mentioned limitations. We began our investigation with 3-phenyloxindole derivative 3a and di-tert-butyl azodicarboxylate 4a to obtain the optimal reaction conditions (Table 1). Carrying out the reaction using 5 mol% of triethylamine without additon of any phase-transfer catalyst (PTC) gave 66% of racemic product 5aa (entry 1). Applying commercially available Maruoka catalyst® 6 with 5 mol% organic (triehtylamine) or inorganic
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(K2CO3) base also led to the formation of racemic product with moderate to high reactivity (entries 2 and 3). Maruoka catalyst® alone, in the absence of any basic additive gave the product in 6% yield as a racemic mixture (data not shown). In contrast, when catalyst 7 with an additional 2(dimethylamino)ethyl moiety was employed as PTC under similar reaction conditions, product 5aa was obtained in 70% yield and importantly 36% enantioselectivity (entry 4). With this promising result in hand, we synthesized a library of catalysts and tested them for the amination reaction. Results are delineated in Table 1. PTC 8a, bearing pyrrolidine as the additional basic moiety attached to the chiral ammonium salt scaffold improved the enantioselectivity (entry 5). Significant improvement in the enantioselectivity was obtained when the aryl substituents at 3,3´ positions of 8 were changed to 3,5(CF3)2C6H3 group. Thus, chiral bifunctional PTC 8b gave the product 5aa in 68% enantioselectivity (entry 6). PTC 9 with an additional carbon in the alkyl chain between ammonium and pyrrolidine moiety led to significant drop in enantioselectivity (entry 7), highlighting the importance of appropriate distance between ammonium and pyrrolidine base. Chiral bifunctional PTC 10 with piperidine also resulted in lower enantiocontrol (entry 8). Hence, PTC 8b was selected for further optimization studies. After some more optimization, we found that application of bis(adamantyl) azodicarboxylate 4b as electrophilic partner can improve the enantioselectivity of the product 5ab up to 82% ee (entry 9). Addition of molecular sieves and changing the solvent to mesitylene further improved the enantioselectivity (entries 10 and 11). Finally, an optimal set of reaction conditiones were obtained by adding 2 mol% of catalyst 8b at 0 ˚C, and the product was obtained in 96% yield and 95% of enantioselctivity (entry 13). Table 1. Optimization of the Reaction Conditionsa Ph
Ph CO 2R +
O N Boc 3a
entry
1d
RO 2C
N N
PTC (5 mol%) solvent (0.2 M) rt, 4 h
4a (R = t-Bu) 4b (R = 1-adamantyl)
catalyst
solvent
4
4a
HN
CO 2R
N CO 2R
O N Boc 5aa (R = t-Bu) 5ab (R = 1-adamantyl)
yield (%)b
66
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11e
8b
mesitylene
4b
89
92
12e,f
8b
mesitylene
4b
99
95
13e,f,g
8b
mesitylene
4b
99 (96)h
95
a
Reaction conditions (unless otherwise specified): 3phenyloxindole 3a (0.05 mmol), azodicarboxylate 4 (1.2 equiv) in the presence of PTC (5 mol%). bThe yield was determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as an internal standard. cDetermined by HPLC analysis using a chiral column. d With only NEt3 (5 mol%). e4 Å molecular sieves (20 mg) was added. fAt 0 °C. gWith 2 mol% of PTC 8b. hIsolated yield.
Scheme 2. Chiral Binaphthyl-‐Modified Phase-‐Transfer Cata-‐ lysts 6~10. Ar
toluene
0
2
6/NEt3
toluene
4a
72
0
3
6/K2CO3
toluene
4a
98
0
4
7
toluene
4a
70
36
Bu
toluene
4a
72
53
6
8b
toluene
4a
86
68
7
9
toluene
4a
82
33
8
10
toluene
4a
82
40
9
8b
toluene
4b
85
82
10e
8b
toluene
4b
87
88
N
Ar
6 (Ar = 3,4,5-F3-C6H 2)
Ar
7 (Ar = 3,4,5-F3-C6H 2)
Ar
8a (Ar = 3,4,5-F3-C6H 2) 8b (Ar = 3,5-(CF3) 2C6H 3)
Ar Br Me
Br Me
N
N
N
N
Ar
Ar
9 (Ar = 3,5-(CF3) 2C6H 3)
10 (Ar = 3,5-(CF3) 2C6H 3)
It should be noted that in the presence of amine 11, the reaction of oxindole 3a with azodicarboxylate 4b afforded amination product 5ab in 16% yield as a racemate, whereas chiral diamine catalyst 12 afforded 5ab in 25% yield with 16% enantioselectivity (Scheme 3). This result suggests the importance of the bifunctional groups in a suitable position on the rational design of chiral phase-transfer catalysts of type 8 possessing an amino functionality. Scheme 3. Control Experiments Ph
Ph O
3a
CO 2Ad 11 or 12 (2 mol%)
+
N Boc AdO2C
N N 4b
mesitylene (0.2 M) 4Å MS, 0 °C, 6 h
N Bu
Ar
HN CO 2Ad N CO 2Ad
* N Boc
O 5ab
11 : 16% yield, racemic 12 : 25% yield, 16% ee
Ar
11 (Ar = 3,5-(CF3) 2C6H 3)
8a
N N
Ar
Ar
5
Br Me
Br Me N
Bu
N
ee (%)c
––/NEt3
Ar
Ar Br
N
N
Ar 12 (Ar = 3,5-(CF3) 2C6H 3)
With optimal reaction conditions in hand, we studied the substrate generality for this amination reaction with respect to oxindole substrates (Table 2). To make it general, all the reactions were carried out for 6 hours before work up. Amination of 3a, when carried out in 1 mmol-scale, gave the final product 5aa in similar yield and enantioselectivity to those of small-scale reaction, thereby demonstrating the scalability of this amination protocol. Substrates with electron withdrawing groups on the 3-aryl ring proceeded with high efficiency and high (5bb) to moderate (5cb) enantioselectivity. An electron donating methoxy substituent at 4-position of the 3-aryl ring
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ACS Catalysis had no impact on reactivity but lowered the enantioselectivity of the product 5db. Aryl substituent other than phenyl also well tolerated (5eb, 5fb). Methoxy substituent at the 5-position of the oxindole ring proceded with high efficiency and selectivity (5gb). Halogen substitution at the 6-position of the oxindole ring proceeded smoothly to give 5hb in high yield with high enantioselectivity. The absolute configuration of 5hb was determined to be R by X-ray diffraction analysis.7 Table 2. Scope of 3-Aryloxindoles (3)a Ar
Ar +
O R1
3
N Boc
Ph
CO 2Ad PTC 8b (2 mol%) N N
AdO2C
mesitylene (0.2 M) 4Å MS, 0 °C, 6 h
4b
N CO 2Ad
HN CO 2Ad
HN CO 2Ad
N CO 2Ad
N CO 2Ad
O N Boc 5bb, 98% yield, 92% ee
MeO
O N Boc 5cb, 97% yield, 78% ee Ph
HN CO 2Ad N CO 2Ad O N Boc 5db, 93% yield, 84% ee[b]
HN CO 2Ad
N CO 2Ad
N CO 2Ad
N Boc 5eb, 92% yield, 92% ee
HN CO 2Ad N CO 2Ad
MeO
HN CO 2Ad O
MeO
O N Boc 5gb, 95% yield, 90% ee[b]
F
5
N Boc CF3
O N Boc 5ab, 96% yield, 95% ee
N CO 2Ad O
R1
F
HN CO 2Ad
HN CO 2Ad
Ph
Having established the substrate scope for the amination of 3-aryloxindoles 3, we sought to identify another class of nucelophile amenable to our bifunctional PTC system for this amination reaction. We selected benzofuran-2(3H)-ones 15 as pronucleophiles because benzofuranones with a chiral quaternary stereocenter at 3 position are important for their potential applications in medicinal chemistry and can be found in various natural products.8 Under the similar reaction conditions as for the amination of 3-aryloxindoles 3, a wide range of 3-arylbenzofuran-2(3H)ones 15 underwent amination to furnish 16 with high efficiency and stereo fidelity (Table 3). This methodology should also be noted for the very short reaction time (typically two hours) compared to other literature procedures for amination reactions with similar nucleophiles (24~48 hours).9 The absolute configuaration of the newly formed stereocenter was determined to be S by comparison with known benzofuranone derivative of 16ab (see supporting information). Table 3. Scope of 3-arylbenzofuran-2(3H)-ones (15)a Ar O
O
R1
N Boc 5fb, 85% yield, 89% ee[b]
AdO2C
CO 2Ad N N
PTC 8b (2 mol%) mesitylene (0.2 M) 4Å MS, 0 ˚C, 2 h
4b
O O 16ab, 98% yield, 94% ee (93% yield, 94% ee)b
S HN CO 2Ad
HN CO 2Ad N CO 2Ad O O
R1
Br HN CO 2Ad N CO 2Ad
16 F
HN CO 2Ad N CO 2Ad O O 16bb, 99% yield, 88% ee
HN CO 2Ad N CO 2Ad O O 16cb, 99% yield, 88% ee
O
N CO 2Ad O
N Boc 5ib, 96% yield, 83% ee
15
N CO 2Ad
O N Boc 5hb, 95% yield, 87% ee
O
+
O
HN CO 2Ad
Br
HN CO 2Ad Ph N CO 2Ad
Ar
HN CO 2Ad N CO 2Ad
HN CO 2Ad N CO 2Ad
N Boc 5jb, 95% yield, 93% ee
O
a
O 16db, 96% yield, 84% ee
Reaction conditions (unless otherwise specified): 3-aryloxindole (0.05 mmol), azodicarboxylate 4b (1.2 equiv) in the presence of b PTC 8b (2 mol%) with 4Å MS (20 mg). The reaction was carried out at room temperature.
O
HN CO 2Ad N CO 2Ad
O
O
O
O
16eb, 99% yield, 95% ee
16fb, 97% yield, 92% ee
HN CO 2Ad N CO 2Ad
Br
O O
Unfortunately, attempted use of 3-alkyloxindoles as pronucleophiles was unsuccessful. 3-Benzyloxindole 13, gave the product 14a in high yield with low enantioselectivity when ditert-butyl azodicarboxylate 4a was used as electrophile (Scheme 4). In case of bis(adamantly) azodicarboxylate 4b, the product 14b was obtained as racemate. The observed low or no enantioselectivity can be attributed to the lack of π−π interaction between aryl ring at the C3 carbon of oxindole and binaphthyl ring of the chiral PTC, which is believed to be crucial to achieve high enantiocontrol for such kind of amination reactions.6f Scheme 4. 3-‐Alkyl Oxindole as Pronucleophile.
Bn
Bn CO 2R O
N Boc 13
+ RO 2C
N N
PTC 8b (2 mol%) mesitylene (0.2M) 4Å MS, 0 ˚C, 16h
4a (R = t-Bu) 4b (R = 1-adamantyl)
HN
CO 2R
N CO 2R
O N Boc 14a, 88% yield, 16% ee 14b, 92% yield, racemic
16gb, 97% yield, 90% ee
a
Reaction conditions (unless otherwise specified): 3arylbenzofuran-2(3H)-one (0.05 mmol), azodicarboxylate 4b (1.2 equiv) in the presence of PTC 8b (2 mol%) with 4Å MS (20 mg). b The reaction was carried out in 1 mmol scale.
To gain more insight into the origin on the enantioselectivity of asymmetric amination reaction, we performed the density functional theory (DFT) calculations combined with the automated reaction path search strategy, called the Global Reaction Route Mapping (GRRM)20 (see Supporting Information for details). Scheme 5 shows the energetically lowest and second lowest ammonium enolate structures 17 and 18, respectively. Only the Re-face of 17 is open for the amination, whilst the Re- and Si-faces of 18 is partly blocked by the 3,5(CF3)C6H3 and pyrrolidine moieties, respectively. Then, dialkyl azodicarboxylate 4 approaches 17 from the front side to furnish amination product 5 with the desired absolute configuration.
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Scheme 5. The Structures and Gibbs Free Energy Differences ΔG (in kcal/mol) of Ammonium Enolates 17 and 18 Opti-‐ mized at the B3LYP-‐D3/6-‐31G(d) Level Including the Solva-‐ tion Effect by the Polarized Continuum Model.
fective for the enantioselective amination of 3-arylbenzofuran2(3H)-ones. Further application of this catalytic system is currently undergoing in our group.
ASSOCIATED CONTENT
Page 4 of 6
Supporting Information. The Supporting Material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] Fax: (+81) 75-753-4041
Notes
The authors declare no conflict of interest.
ACKNOWLEDGMENT This work was supported by JSPS KAKENHI Grant Numbers JP26220803 and JP17H06450 (Hybrid Catalysis).
REFERENCES A plausible reaction mechanism for asymmetric amination reaction is depicted in Scheme 6. The basic pyrrolidine moiety of the bifunctional PTC 8 deprotonates 3 to generate ion-pair 20. A rapid anion exchange between the two ammonium centers brings the enolate to the chiral environment of the bifunctional PTC, resulting in ion-pair 21. Effective shielding of one face of the enolate with PTC by the π−π interaction between Ar and binaphthyl groups implies that azocarboxylate 4b can approach the enolate by only Re-face preferentially over the Si-face leading to enantio-rich amination product 5. Scheme 6. Plausible reaction mechanism. Ar' Ar
HN CO 2Ad
Br Me
N CO Ad 2
N
Ar
O
N
N 5 Boc
Ar'
O
8 3
N Boc
CO 2Ad N N AdO2C
Ar'
4b
NBoc
ArAr'
Br Ar Me N
O N
Ar' 21
Me
H
H N
NBoc O N
Ar' 20
Br facile anion exchange
In conclusion, we have successfully developed a new class of bifunctional phase-transfer catalytic system with additional basic amino moiety attached to the chiral backbone of binaphthyl derived quaternary ammonium salt. This catalytic system was found to be superior to its parent binaphthyl ammonium scaffold for hitherto difficult enantioselective amination of 3aryloxindoles. This reliable catalyst was also found to be ef-
(1) For reviews on asymmetric phase-transfer catalysis, see: (a) O´Donnell, M. J. Aldrichimica Acta 2001, 34, 3-15. (b) Ooi, T.; Maruoka, K. Asymmetric Organocatalysis of Structurally WellDefined Chiral Quaternary Ammonium Fluorides. Acc. Chem. Res. 2004, 37, 526-533. (c) Ooi, T.; Maruoka. K. Recent Advances in Asymmetric Phase-Transfer Catalysis. Angew. Chem. Int. Ed. 2007, 46, 4222-4266. (d) Asymmetric Phase-Transfer Catalysis (Ed.: K. Maruoka), Wiley-VCH, Weinheim, 2008. (e) Jew, S.-s.; Park, H.-g. Cinchona-based Phase-transfer Catalysts for Asymmetric Synthesis. Chem. Commun. 2009, 7090-7103. (f) Werner, T. Phosphonium Salt Organocatalysis. Adv. Synth. Catal. 2009, 351, 1469-1481. (g) Enders, D.; Nguyen, T. V. Chiral Quaternary Phosphonium Salts: A New Class of Organocatalysts. Org. Biomol. Chem. 2012, 10, 5327-5331. (h) Novacek, J.; Waser, M. Bifunctional Chiral Quaternary Ammonium Salt Catalysts: A Rapidly Emerging Class of Powerful Asymmetric Catalysts. Eur. J. Org. Chem. 2013, 637-648. (i) Shirakawa, S.; Maruoka, K. Recent Developments in Asymmetric Phase-Transfer Reactions. Angew. Chem. Int. Ed. 2013, 52, 4312-4348. (j) Kaneko, S.; Kumatabara, Y.; Shirakawa, S. A New Generation of Chiral Phase-Transfer Catalysts. Org. Biomol. Chem. 2016, 14, 5367-5376. (k) Liu, S.; Kumatabara, Y., Shirakawa, S. Chiral Quaternary Phosphonium Salts as Phase-Transfer Catalysts for Environmentally Benign Asymmetric Transformations. Green Chem. 2016, 18, 331-341. (2) For base free asymmetric phase-transfer catalysis, see: (a) He, R.; Shirakawa, S.; Maruoka, K. Enantioselective Base-Free PhaseTransfer Reaction in Water-Rich Solvent. J. Am. Chem. Soc. 2009, 131, 16620-16621. (b) Wang, L.; Shirakawa, S.; Maruoka, K. Asymmetric Neutral Amination of Nitroolefins Catalyzed by Chiral Bifunctional Ammonium Salts in Water-Rich Biphasic Solvent. Angew. Chem., Int. Ed. 2011, 50, 5327-5330. (c) Shirakawa, S.; Terao, S. J.; He, R.; Maruoka. K. Diastereo- and Enantioselective Conjugate Addition of α-Substituted Nitroacetates to Maleimides under Base-Free Neutral Phase-Transfer Conditions. Chem. Commun. 2011, 1055710559. (d) Shirakawa, S.; Ota, K.; Terao, S. J.; He, R.; Maruoka, K. The Direct Catalytic Asymmetric Aldol Reaction of α-Substituted Nitroacetates with Aqueous Formaldehyde under Base-Free Neutral Phase-Transfer Conditions. Org. Biomol. Chem. 2012, 10, 5753-5755. (e) Shirakawa, S.; Kasai, A.; Tokuda, T., Maruoka, K. Efficient Approach for the Design of Effective Chiral Quaternary Phosphonium Salts in Asymmetric Conjugate Additions. Chem. Sci. 2013, 4, 22482252. (f) Shirakawa, S.; Tokuda, T.; Kasai, A.; Maruoka, K. Design of Chiral Bifunctional Quaternary Phosphonium Bromide Catalysts Possessing an Amide Moiety. Org. Lett. 2013, 15, 3350-3353. (g)
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ACS Catalysis Shirakawa, S.; Wang, L.; He, R.; Arimitsu, S.; Maruoka, K. A Base-Free Neutral Phase-Transfer Reaction System. Chem.-Asian J. 2014, 9, 1586-1593. (h) Shirakawa, S.; Wang, L.; Kasai, A.; Maruoka, K. New Neutral Reaction System with Crown Ether–KCl Complexes in Aqueous Solution. Chem.-Eur. J. 2012, 18, 8588-8590. (i) Shirakawa, S.; Makino, H.; Yoshidome, T.; Maruoka, K. Effect of Brønsted Acid Co-catalyst in Asymmetric Conjugate Addition of 3Aryloxindoles to Maleimide under Base-Free Phase-Transfer Conditions. Tetrahedron 2014, 70, 7128-7132. (j) Lee, T. B. K.; Wong, G. S. K. Asymmetric Alkylation of Oxindoles: an Approach to the Total Synthesis of (-)-Physostigmine. J. Org. Chem. 1991, 56, 872-875. (k) Wu, X.; Liu, Q.; Liu, Y.; Wang, Q.; Zhang, Y.; Chen, J.; Cao, W.; Zhao, G. Amino Acid‐Derived Phosphonium Salts‐Catalyzed Michael Addition of 3‐Substituted Oxindoles. Adv. Synth. Catal. 2013, 355, 2701-2706. (l) Deng, F.; Moteki, S. A.; Maruoka, K. Catalytic Asymmetric Alkylation of 3-Aryl Substituted Oxindoles to give 3,3 Disubstituted Oxindoles under Phase Transfer Conditions. Asian J. Org. Chem. 2014, 3, 395-398. (3) For selected recent literatures on oxindoles and their biological activities: (a) Marti, C.; Carreira, E. M. Construction of Spiro[pyrrolidine-3,3′-‐oxindoles]−Recent Applications to the Synthesis of Oxindole Alkaloids. Eur. J. Org. Chem. 2003, 2209-2219. (b) Dounay, A. B.; Overman, L. E. The Asymmetric Intramolecular Heck Reaction in Natural Product Total Synthesis. Chem. Rev. 2003, 103, 2945-2964. (c) Peddibhotla, S. 3-Substituted-3-hydroxy-2-oxindole, an Emerging New Scaffold for Drug Discovery with Potential Anticancer and Other Biological Activities. Curr. Bioact. Compd. 2009, 5, 20-38. (d) Zhou, F.; Liu, Y.-L.; Zhou, J. Catalytic Asymmetric Synthesis of Oxindoles Bearing a Tetrasubstituted Stereocenter at the C-3 Position. Adv. Synth. Catal. 2010, 352, 1381-1407. (e) Dalpozzo, R. Recent Catalytic Asymmetric Syntheses of 3,3-Disubstituted Indolin2-ones and 2,2-Disubstituted Indolin-3-ones. Adv. Synth. Catal. 2017, 359, 1772-1810. (4) For the use of 3-substituted oxindoles as pronuclophile under PTC condition, see: (a) He, R.; Ding, C.; Maruoka, K. Phosphonium Salts as Chiral Phase-Transfer Catalysts: Asymmetric Michael and Mannich Reactions of 3-Aryloxindoles. Angew. Chem. Int. Ed. 2009, 48, 4559-4561. (b) Shirakawa, S.; Koga, K.; Tokuda, T.; Yamamoto, K.; Maruoka, K. Catalytic Asymmetric Synthesis of 3,3'Diaryloxindoles as Triarylmethanes with a Chiral All-Carbon Quaternary Center: Phase-Transfer-Catalyzed SNAr Reaction. Angew. Chem. Int. Ed. 2014, 53, 6220-6223. (c) Liu, S.; Maruoka, K.; Shirakawa, S. Chiral Tertiary Sulfonium Salts as Effective Catalysts for Asymmetric Base-Free Neutral Phase-Transfer Reactions. Angew. Chem. Int. Ed. 2017, 56, 4819-4823. (5) (a) Ochi, M.; Kawasaki, K.; Kataoka, H.; Uchio, Y.; Nishi, H. AG-041R, a Gastrin/CCK-B Antagonist, Stimulates Chondrocyte Proliferation and Metabolism in Vitro. Biochem. Biophys. Res. Commun. 2001, 283, 1118-1123. (b) Bernard, K.; Bogliolo, S.; Ehrenfeld, J. Vasotocin and Vasopressin Stimulation of the Chloride Secretion in the Human Bronchial Epithelial Cell Line, 16HBE14o-. Br. J. Pharmacol. 2005, 144, 1037-1050. (c) Claudine, S. L.; Sylvain, D.; Gabrielle, B.; Maurice, M.; Jaques, S.; Rolf, G.; Guy, G.; Gilles, G. Specific Agonist and Antagonist for the V1b Receptor in Mammals. Stress 2003, 6, 199-206. (6) For the amination of 3-prochiral oxindoles, see: (a) Cheng, L.; Liu, L.; Wang, D.; Chen, Y.-J. Highly Enantioselective and Organocatalytic α-Amination of 2-Oxindoles. Org. Lett. 2009, 11, 3874-3877. (b) Qian, Z.-Q.; Zhou, F.; Du, T.-P.; Wang, B.-L.; Ding, M.; Zhao, X.-L.; Zhou, J. Asymmetric Construction of Quaternary Stereocenters by Direct Organocatalytic Amination of 3-Substituted Oxindoles. Chem. Commun. 2009, 6753-6755. (c) Bui, T.; Borregan, M.; Barbas III, C. F. Expanding the Scope of Cinchona Alkaloid-Catalyzed Enantioselective α-Aminations of Oxindoles: A Versatile Approach to Optically Active 3-Amino-2-oxindole Derivatives. J. Org. Chem. 2009, 74, 8935-8938. (d) Yang, Z.; Wang, Z.; Bai, S.; Shen, K.; Chen, D.; Liu, X.; Lin, L.; Feng, X. Highly Enantioselective Synthesis of 3-Amino-2-oxindole Derivatives: Catalytic Asymmetric α-Amination of 3-Substituted 2-Oxindoles with a Chiral Scandium Complex. Chem. Eur. J. 2010, 16, 6632-6637. (e) Mouri, S.; Chen, Z.; Mitsunuma, H.; Furutachi, M.; Matsunaga, S.; Shibasaki, M. Catalytic Asymmetric
Synthesis of 3-Aminooxindoles: Enantiofacial Selectivity Switch in Bimetallic vs Monometallic Schiff Base Catalysis. J. Am. Chem. Soc. 2010, 132, 1255-1257. (f) Bui, T.; Borregan, M.; Milite, C.; Barbas III, C. F. Highly Enantioselective Organocatalytic α-Amination Reactions of Aryl Oxindoles: Developing Designer Multifunctional Alkaloid Catalysts. Org. Lett. 2010, 12, 5696-5699. (7) CCDC 1825179 contains the supplementary crystallographic data for 5hb. This data can also be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data request/cif. (8) (a) Sontag, B.; Rϋth, M.; Spiteller, P.; Arnold, N.; Steglich, W.; Reichert, M.; Bringmann, G. Chromogenic Meroterpenoids from the Mushrooms Russula Ochroleuca and R. Viscid. Eur. J. Org. Chem. 2006, 1023-1033. (b) Wu, B.; He, S.; Wu, X.-D.; Wu, D.-K.; Pan, Y.J. Cadinane and Eudesmane Sesquiterpenoids from Chloranthus Henryi. Helv. Chim. Acta. 2007, 90, 1586-1592. (c) Kwon, Y.-J.; Sohn, M.-J.; Zheng, C.-J.; Kim, W.-G. Fumimycin: A Peptide Deformylase Inhibitor with an Unusual Skeleton Produced by Aspergillus Fumisynnematus. Org. Lett. 2007, 9, 2449-2451. (d) Ge, H. M.; Zhu, C. H.; Shi, D. H.; Zhang, L. D.; Xie, D. Q.; Yang, J.; Ng, S.W.; Tan, R. X. Hopeahainol A: An Acetylcholinesterase Inhibitor from Hopea Hainanensis. Chem. Eur. J. 2008, 14, 376-381. (9) For the amination of 3-prochiral benzofuranone, see: Zhu, C.L.; Zhang, F.-G.; Meng, W.; Nie, J.; Cahard, D.; Ma, J.-A. Enantioselective Base-Free Electrophilic Amination of Benzofuran-2(3H)-ones: Catalysis by Binol-Derived P-Spiro Quaternary Phosphonium Salts. Angew. Chem. Int. Ed. 2011, 50, 5869-5872. (10) Maeda, S.; Ohno, K.; Morokuma. K. Systematic Exploration of the Mechanism of Chemical Reactions: The Global Reaction Route Mapping (GRRM) Strategy using the ADDF and AFIR Methods. Phys. Chem. Chem. Phys. 2013, 15, 3683-3701.
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Table of Contents artwork here Ar O R1
AdO2C
HN CO 2Ad
Ar
N Boc CO 2Ad + N N
Ar
Br Me N
N CO 2Ad O
N Ar (Ar = 3,5-(CF3) 2C6H 3)
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R1
N Boc ~95% ee
6
