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Dynamic Kinetic Resolution of Biaryl Lactones via a Chiral Bifunctional Amine Thiourea Catalyzed Highly atropo-Enantioselective Transesterification Chenguang Yu, He Huang, Xiangmin Li, Yueteng Zhang, and Wei Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b03609 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on May 25, 2016
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Dynamic Kinetic Resolution of Biaryl Lactones via a Chiral Bifunctional Amine Thiourea Catalyzed Highly atropoEnantioselective Transesterification Chenguang Yu,† He Huang,† Xiangmin Li,†,‡ Yueteng Zhang,† and Wei Wang†,‡,* †
Department of Chemistry & Chemical Biology, University of New Mexico, Albuquerque, NM 87131-0001, USA
‡
Shanghai Key Laboratory of New Drug Design, School of Pharmacy, and State Key Laboratory of Bioengineering Reactor, East China University of Science & Technology, Shanghai 200237, China Supporting Information Placeholder ABSTRACT: A solution to the unmet synthetic challenge in achieving highly atropo-enantioselective transesterification of Bringmann’s lactones has been realized by employing a chiral bifunctional amine thiourea as promoter. The synergistic activation mode of the lactones and alcohols/phenols by the respective thiourea and amine groups is crucial for achieving the highly enantioselective, high yielding DKR process. This protocol gives highly optically pure axially chiral biaryl compounds with a broad substrate scope under mild reaction conditions. 1
Dynamic kinetic resolution (DKR) provides an unrivaled 2 power over traditional kinetic resolution (KR) in asymmetric synthesis, as it offers the capacity of converting both enantiomers of a racemic mixture into an enantioenriched product. Impressive progress has been made for catalytic 3 DKR of compounds contained stereogenic centers. By contrast, there are only a handful of examples reported regarding the synthesis of axially chiral compounds through catalytic DKR approaches despite their prevalence and 4 5 importance in bioactive molecules and catalysis. The general strategy employs catalytic atroposelective functionalization of configurationally labile biaryls to increase restriction to rotation in biaryl products. Chiral transition metal catalyzed introduction of sterically demanding moiety at ortho-position of freely rotating, 6b,6e,6f rapidly racemizing biaryls or configurationally stable 6a,6c,6d biaryls has been elegantly developed by the groups of Hayashi, Murai, Stoltz and Virgil, Fernández and Lassaletta, 6 Colobert, and You. However, the seminal work in organocatalytic DKR of racemic biaryls was only recently published by Miller with a tripeptide-promoted 7,8 atroposelective electrophilic ortho-bromination reaction. DKR of configurationally labile biaryl lactones developed by Bringmann has proved to be a powerful approach to chiral 3a,4a,9 biaryl compounds (Eq, 1). Notably, the chiral products have found broad applications in total synthesis of a number of challenging axially chiral natural products such as 10a 10b,c korupensamine A and B, knipholone, mastigophorene 10d 10e A, and benanomicin B. Therefore, optically enriched biaryl products (> 90% ee) are highly valuable for
Scheme 1. Reported Asymmetric and Proposed Chiral Bifunctional Amine-thiourea Catalyzed Transesterification of the Bringmann’s Lactones (a) literature: chiral nucleophiles and chiral metal catalysis OMe
O
R1 R2
+K -O -70 oC
O
O
MeO
THF
(1) O OH
Me
(S)-1 48% ee fast
OMe
O
R1 R2
(R)-BINAP (24 mol%) AgBF4 (20 mol%)
O (R)-1
O
R1 R2
MeOH, THF -20 - 50 oC 50-84% ee
(2)
OMe OH
(b) Our approach: metal free organocatalysis - synergistic activation of both substrates high yields and excellent enantioselectivity (up to quantitative yields and 99% ee) broad substrate scope for both nucleophiles and electrophiles (37 examples) S R O chiral amine thiourea
R1 R2
O
N H O
match
(S)-1
O
chiral scaffold
O R
H
N
O OH
2
R
(S)-3
S R
O
OR (3)
1 fast R
R1 R2
fast
R1 R2
N H
O chiral amine thiourea
N H O
O
mismatch
(R)-1
N H
R1 R2
chiral scaffold
O R
H
N slow
OR
R1
O OH
R2
(R)-3
asymmetric total synthesis. However, achieving highly atropo-enantioselective DKR of Bringmann’s lactones presents a long-standing challenge in synthesis despite more than 20 years’s effort made by Bringmann and Yamada and 3a,4a,9,11,12 others. Diastereoselective transesterification of lactones with alcohols represents a straightforward approach to chiral biaryls but with limited success so far. Chiral (+)menthol derived potassium alcoholate as chiral nucloephile
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gave the best result with 48% ee (Scheme 1, Eq. 1, asymmetric 13 transamidation giving better enantioselectivity (86% ee)). An asymmetric catalytic version using methanol as nucleophile by a chiral BINAP silver complex delivered 14 moderate enantioselectivity (50-84% ee) (Eq 2). Clearly, a new catalytic strategy capable of promoting DKR of Bringmann’s lactones with a high level of enantioselectivity (>90% ee) and a broad scope is urgently needed to streamline the synthesis of the privileged axially chiral biaryls. Bifunctional chiral amine thioureas have demonstrated great versatility and selectivity to facilitate many transformations attributing to their capacity for synergistic 15 dual acid and base activation. The effective activation mode 16 and our experience in this area inspired us to explore them for the DKR of the challenging Bringmann’s lactones (Eq. 3). We envisioned that thiourea activated the strained lactone, while the amine interacted with the hydroxyl group of an alcohol and directed the nucleophile attack of the activated ester in a cooperative atropo-enantioselective manner. Herein, we wish to disclose the results of the investigation leading to the first example of a metal free quinine-derived thiourea promoted atropo-enantioselective transesterification of Bringmann’s lactones. Notably, this protocol is operated under very mild conditions, delivers axially chiral biaryl products in high yields and with excellent enantioselectivities (up to quantitative yields and 99% ee). Moreover, the process shows a much broader substrate scope 13, 14 than that of previous studies. A variety of alcohols including aliphatic alcohols and even phenols perform very well. Moreover, biaryl lactones with a broad range of substituent patterns are efficienty transformed to chiral biaryl products. We commenced our investigation by examining the reaction of biaryl lactone 1a with 4-nitrobenzylic alcohol 2a (Table 1). No reaction happened without a catalyst, indicative of a promoter essential for effective transesterification (entry 1). Indeed, Takemoto’s catalyst I was capable of producing desired product 3a in 95% yield and with 89% ee within 1h (entry 2). Among the commonly used amine thioureas probed, Soós’s quinine thiourea II proved to be a superior facilitator for this process giving 3a in 98% yield and with 95% ee in 3h (entry 3). The power of the synergistic activation specifically by a bifunctional amine and thiourea was demonstrated when no reaction proceeded with either triethylamine or bis(thiourea) catalyst IV (entry 4). Further examining the parameters of solvents (entries 3, and 5-9) and catalyst loading (entries 10-11) revealed the optimal reaction conditions of trifluorotoluene as medium and 5 mol% of II. With the optimized condition in hand, the scope of the process was explored (Scheme 2). Benzyl alcohols with electron-withdrawing (2a, 2b) or -donating substituents (2c, 2d) gave the desired products in excellent yields and with excellent enantioselectivities (93% to quantitative yields, 9196% ee). More sterically demanding 9-anthracenemethanol 2e and diaryl substituted methanol 2f were well tolerated. Moreover, heteroaromatic rings including furan 2g, indole 2h, pyridine 2i and quinolone 2j proved to be valid substrates. Besides benzyl alcohols, simple aliphatic alcohols such as EtOH, n-BuOH, and i-PrOH, which gave low 13 enantioselectivity in Yamada’s study, delivered high yielding and highly enantioenriched 3k-m (97, 90 and 90%
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Table 1. Exploration and Optimizationa NO2 NO2
O O + HO
Br
solvent, rt
3a
OMe
1a
OMe S
NMe2 Ar:
O OH
Br
2a
N H
O
cat. (10 mol%)
I
N H
S
N
Ar
H N
MeO
CF3
Ar NH
N H
S
N H
Ar
H N
Ar
NH
N Ar
N CF3
II
S NH
N H
III
S IV
entry
cat.
solvent
t (h)
% yieldb
% eec
1
none
CH2Cl2
12
0
-
2
I
CH2Cl2
2
95
89
3
II
CH2Cl2
3
98
95
4
III
CH2Cl2
24
24
31
5
IV
CH2Cl2
24
0
-
6
II
toluene
1.5
99
93
7
II
CF3Ph
0.5
98
95
8
II
CHCl3
1.5
99
94
9
II
xylene
1.5
91
94
10d
II
CF3Ph
6
99
96
e
II
CF3Ph
72
61
96
c
11 a
Unless specified, the reaction was carried out with 0.1 mmol of 1a and 0.12 mmol of 2a in 1.0 mL of a solvent with mol% catalyst was stirred at rt for a specified time. b Isolated yields for both isomers. c Determined by chiral HPLC analysis (Chiralcel AS-H). d 5 mol% catalyst used. e 1 mol% catalyst used.
yields, and 92, 90 and 94% ee, respectively). More acidic trifluoro- 2n and trichloro- 2o ethanols reacted much faster (within 0.5 h) but without deteriorating enantioselectivity, presumably as a result of easy deprotonation of OH group by the amine. 2-Methoxyethanol 2p was found to give higher enantioselectivity in shorter time than ethanol (95% ee, 6h vs 92% ee, 36h), which may be ascribed to the oxygen atom acting as an additional binding site with the catalyst and the increased acidity of alcohols by inductive effect of the oxygen. In contrast, 2-phenoxyethanol 2q gave lower enantioselectivity (90% ee), maybe due to weaker binding ability of the phenoxyl. Excellent yields (95-97%) and enantioselectivity (94-96%) were also attended for other functionalized alcohols 3-benzyloxy-1-propanol 2r and Nsubstituted alcohol 2s. It is noted that 1,3-dibenzyloxyl-2propanol 2t with two oxygen substituents further improved enantioselectivity (99% ee) for 3t.
Scheme 2. Alcohols as Nucleophilesa O Br
+
HO R
II (5 mol%)
2
PhCF3, rt
O
1a
O
OMe 3 NO2
3a 6h, 99%, 96% ee
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3b 2h, 98%, 96% ee
NO2
3c 8h, 100%, 93% ee
R
O OH
Br
OMe O
O 3db 2d, 93%, 91% ee
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O
3e 5h, 98%, 90% ee
3f 2d, 97%, 92% ee
N
N H 3hb 3g 24h, quantitative, 4h, 96%, 90% ee 91% ee
N
3i 3kc 3ld 3j 3h, quantitative, 36h, 97%, 92% ee 2h, 97%, 93% ee 24h, 90% yield, 90% ee 90 %ee CF3 3n 3m 0.5h, quantitative, 2d, 90%, 91% ee 93% ee d,e
OBn
OPh 3q 3h, quantitative, 90% ee
CCl3
3r 3h, 97%, 96% ee
OMe 3p 6h, quantitative, 95% ee
3o 0.5 h, quantitative, 90% ee
OBn
NHBoc
OBn 3tc 3s 16h, 95%, 94% ee 16h, 80% yield, 99% ee
a
Unless specified, see Table 1, footnote a and SI, yields referred to isolated, ee determined by chiral HPLC. b - 10°C. c 2.0 equiv. of alcohol. d 20.0 equiv. of alcohol. e 15 mol% II.
The synthesis of axially chiral phenolic esters from Bringmann’s lactones is more challenging due to the weaker nucleophilicity of phenols and their vulnerable racemization via reversible lactonization. We found that the sensitive chiral phenolic esters could be prepared by using this protocol, but have to balance selectivity and reactivity. Prolonging reaction time could increase yields but caused the racemization of the products. We managed to achieve high to excellent enantioselectivity (90-98% ee) and good yields (50-76%) for phenols bearing electron-neutral (2u), donating (2v, 2x and 2y) and -withdrawing substituents (2w) when the reaction was performed at –10 °C with controlled short reaction time (Scheme 3). It is noteworthy that although great success has been achieved for asymmetric transesterification of biaryl lactones with alcohols, disappointed results were obtained in our attempts with nitrogen-centered nucleophiles . Aniline and tosylamide didn’t deliver any transamidation product whilepiperidine reacted with 1a smoothly but gave 0% ee.
Scheme 3. Phenols as Nucleophiles
O II (5 mol%) +
O
HO Ar 2
PhCF3, -10 oC
Br
OMe 1a
Me
SMe 3u 1h, 75%, 96% ee
3v 1h, 60%, 94% ee
Br 3w 16 min,65%, 90% ee
Scheme 4. Scope of Biaryl Lactonesa
Me 3x 1h, 76%, 98% ee
2
2
R1
1
1'
+
O
HO R
PhCF3, -10 oC
1'
R2
OR
Et
O OH
O OH
Br
OMe
3aa, R = 4-NO2Bn 0.5 h, 97%, 97% ee
OR
Et Br
O OH
2'
R
O OH 3
OR
i-Pr O OH
Br
Br
OMe 3ab, R = 4-NO2Bn 1h, 94%, 96% ee O
O OH OMe
3ac, R = 4-NO2Bn 1 h, 92%, 94% ee
O
R
O OH
R
O OH
OMe 3ae, R = 4-NO2Bn OBn OBn b 3ad, R = 4-NO2Bn 1d, 79%, 62% ee 3ag , R = 3ahcd, R = OBn 1.5 h, quantitative, 3af, R = CH2CH2OMe OBn 8d, 56%, 90% ee 20h, 90%, 61% ee 2d, 84%, 96% ee 92% ee
Ar O
O OH
O
R
O OH
OMe 3 Me
3y 1h, 50%, 97% ee
O
OR
Bn
OMe
3z, R = 4-NO2Bn 1h, >99%, 94% ee
Br
1
II (5 mol%)
1
2'
OR Br
R1
O 2
R2
a
O Br
dramatic decrease in both yield and ee (3ae, 79%, 62% ee vs 3aa, 97%, 97% ee). It is believed that the methoxyl substituent in biaryl lactone may provide an additional binding site with the catalyst to increase alcohol’s differentiation in attack trajectory. We observed alcohols could affect enantioselectivity. Therefore different alcohols were probed, including 2-methoxyethanol 2p, possessing an additional oxygen atom providing an additional binding site for boosting enantioselectivity, but a similar result was attended for 3af. Pleasingly, 1,3-dibenzyloxyl-2-propanol 2t could give dramatic increase of enantioselectivity (96% ee) and in high yield (84%) for 3ag. Moreover, importantly, 2t served as a general nucleophile to give high level of enantioselectivity (90-96% ee) for 3ah, 3aj, 3al and 3am without the methoxyl group on the 2’ position. In addition, we also conducted the comparison studies with 2t and 2a. Similar enhancements were also observed (3aj, 96% ee vs 3ai, 61% ee and 3al, 96% ee vs 3ak, 61% ee).
Br 3ai, R = 4-NO2Bn 1d, quantitative, 61% ee OBn 3ajcd R = OBn 4d, 71%, 96% ee
O R
R
O OH
O OH
OBn 3ak, R = 4-NO2Bn 1d, quantitative, 61% ee OBn 3alcd R =
OBn 3am, R = OBn 4d, 79%, 91% ee,
OBn 8d, 57%, 96% ee
a
Unless specified, see Table 1, footnote a and SI, yields referred to isolated, ee determined by chiral HPLC.
Unless specified, see Table 1, footnote a and SI, yields referred to isolated, ee determined by chiral HPLC. b 2.0 equiv. of alcohol. c 10 equiv. of alcohol. d 15 mol% of II.
After probing the scope and understanding the influence of alcohols/phenols on the reaction, we next investigated the tolerance of biaryl lactones (Scheme 4). It appears that the variation of substituents on the carbonyl-containing phenyl ring does not show any influence, producing 3z, 3aa, 3ab, 3ac and 3ad in excellent yields and with excellent ee (92%quantitative yields, 92-97% ee). Nonetheless, removal of the 2’-methoxyl substituent on the phenolic parts causes
The protocol can be easily scaled up to a gram scale using a lower catalyst loading (2 mol%) at a higher concentration (1.0 M) affording 1.167 g of 3a in nearly quantitative yield and excellent ee (95%, Scheme 5, Eq. 1). Alcohol 4 can be smoothly attended by LiAlH4 reduction in 93% yield and without erosion of optical purity. Furthermore, a useful 17 chiral aminophenol ligand 6 can be prepared from product 3al via DIBAL-H mediated reduction (Eq. 2). Therefore, the
a
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absolute configuration of transesterification products 3 is confirmed by the comparison of the optical rotation of 5 with 11a the reported data.
Scheme 5. Gram Scale Synthesis and Synthetic Elaboration of the Transesterification Products HO III 2 mol%
O Br
+
O OMe 0.78 g, 1a
O O OH
NO2
PhCF3, rt, 24h Br 99% yield, 95% ee
2a
OBn-p-NO2 OH LiAlH4 O (1) OH THF, 0oC-rt Br OH 0.5h OMe OMe 1.167 g 4 3a 93%, 94% ee
DIBAL-H OBn
3al 90% ee
N(n-Bu)2
OH
OBn DCM, 0oC-rt, 2h 95%, 90% ee
ref 12 OH
5
OH
(2)
tertiary aminophenol ligand (6)
Driven by the unmet synthetic issue in atropoenantioselective transesterification of Bringmann lactones, we have developed a new chiral bifunctional amine thiourea organocatalyst promoted highly enantioselective approach to axially chiral biaryl compounds with a broad substrate scope under mild reaction conditions. The higher reaction efficiency attributes to a distinct synergistic activation mode from previous reported monoactivation methods. The new strategy will streamline the synthesis of the privileged axially chiral biaryls. Application of the axially chiral products in synthesis of natural products and new chiral molecules is underway.
ASSOCIATED CONTENT Experiment details and spectroscopic data. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION Corresponding Author
[email protected].
Author Contributions All authors have given approval to the final version of the manuscript.
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENT Financial support of this research from the National Science Foundation of China (21372073 and 21572055), and the NSF (CHE-1565085) is gratefully acknowledged.
REFERENCES (1) For selected reviews, see: (a) Noyori, R.; Tokunaga, M.; Kitamura, M. Bull. Chem. Soc. Jpn. 1995, 68, 36. (b) Huerta, F. F.; inidis, . B. . B ckvall, J. E. Chem. Soc. Rev. 2001, 30, 321. (c) Pellissier, H. Adv. Synth. Catal. 2011, 353, 659. (2) For selected reviews on kinetic resolution, see: (a) Kagan, H. B.; Fiaud, J. C. Top. Stereochem. 1988, 18, 249. (b) Keith, J. M.; Larrow, J. F.; Jacobsen, E. N. Adv. Synth. Catal. 2001, 343, 5. (c) Vedejs, E.; Jure, M. Angew. Chem., Int. Ed. 2005, 44, 3974. (d) Müller, C. E.; Schreiner, P. R. Angew. Chem. Int. Ed. 2011, 50, 6012. (3) For recent reviews of DKR and KR (a) Bringmann, G.; Price Mortimer, A. J.; Keller, P. A.; Gresser, M. J.; Garner, J.; Breuning, M. Angew. Chem., Int. Ed. 2005, 44, 5384. (b) Ma, G.; Sibi, M. P. Chem.
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Journal of the American Chemical Society
TOC: R1 O R2
R1
O +
cat (5 mol%)
HO R
cat
F3C N
MeO
H N
PhCF3, -10 C or rt CF3
NH S
R
O OH
o
R: alkyl and aryl
O
R2 37 examples 90-99% ee up to quantitative yields
N
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