Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Rh-Catalyzed Enantioselective Allylation of N‑Tosyl- and N‑Nosylaldimines: Total Synthesis of (−)-Crispine A Pei-Fen Chiang,†,§ Wei-Sian Li,†,§ Jia-Hong Jian,† Ting-Shen Kuo,† Ping-Yu Wu,‡ and Hsyueh-Liang Wu*,† †
Department of Chemistry, National Taiwan Normal University, No. 88, Section 4, Tingzhou Road, Taipei 11677, Taiwan Oleader Technologies, Co., Ltd., 1F., No. 8, Aly. 29, Ln. 335, Chenggong Road, Hukou Township, Hsinchu 30345, Taiwan
‡
S Supporting Information *
ABSTRACT: The unprecedented development of asymmetric Rh-catalyzed 1,2-allylation of N-Ts- and N-Ns-aldimines is achieved. This protocol utilizes potassium allyltrifluoroborates and various aldimines to generate enantioenriched homoallylic amines in the presence of 3.0 mol % of Rh(I)/L1b catalyst with up to 90% yield, 98% ee (R = H), and 10:1 diastereoselectivity (R = Me or Ph), yielding the same major diastereomer when using potassium (E)- and (Z)crotyltrifluoroborate. Its synthetic utility is also illustrated in the total synthesis of (−)-crispine A.
C
imines in the presence of rhodium catalysts, we are curious if enantioselective 1,2-allylation of simple arylaldimines is possible since it has remained out of reach owing to their low reactivity.7a Hence, it is highly necessary to develop an efficient protocol to access optically active homoallylic amines hinged on enantioselective 1,2-allylation of simple aldimines. In our previous studies, Rh(I) catalysts derived from chiral bicyclo[2.2.1]heptadiene ligands L1 have demonstrated their high catalytic capability9a (TON up to 2000 with excellent enantioselectivity) in the asymmetric 1,4-addition reactions with a broad scope of substrates.9 Furthermore, this catalytic system imparted high efficiency in the synthesis of optically active diarylmethyl10 and allylic amines.6b With the prior successful 1,2-arylation and 1,2-alkenylation of N-Ts- and NNs-aldimines catalyzed by Rh/L1, herein, we report the preparation of homoallylic amines with high stereocontrol based on Rh-catalyzed 1,2-allylation of N-Ts- and N-Ns-imines. Addition reaction of N-Ts-aldimine 2a with potassium allyltrifluoroborate 3a was investigated in the initial study under our reported optimal conditions for the enantioselective 1,2-alkenylation (Table 1).6b Importantly, in the presence of 3 mol % of Rh(I)/L1a catalyst the model reaction produced the desired homoallylic amine 4aa in only 18% yield and 90% ee in toluene after 24 h using MeOH as the additive (Table 1, entry 1). The Rh(I)/L1b system, proven optimal in the asymmetric arylation of N-Ts imines, gave a slight improvement on the chemical yield and enantioselectivity (Table 1, entry 2). Replacing MeOH with H2O provided 4aa in 40% yield and 94% ee along with 5aa in 13% yield and 47% ee,11 respectively (Table 1, entry 3). Since performing the reaction with a lower mount of H2O (2.0 equiv) alleviated the formation of 5aa (Table 1, entry 4), allylation of 2a-hydrolyzed benzaldehyde
hiral homoallylic amines are commonly found in various natural products exhibiting pharmaceutical and pharmacological activities.1 In addition, enantioenriched homoallylic amines are useful and versatile building blocks, allowing further functional group transformations of the olefinic moieties to furnish natural products2 and multifunctional compounds, such as aminoalkyl epoxides, aminoalkyl cyclopropanes,3a amino alcohols, and amino acids.3b For these reasons, a highly stereoselective and efficient protocol for constructing homoallylic amines is desirable. Transition-metal-catalyzed asymmetric allylation of electrophiles containing CN functionalities is one of the direct and promising protocols to access chiral homoallylic amines.2a,4 Among the developed methods, Rh(I)-catalyzed asymmetric C−C bond-forming reactions would be an ideal selection, as it involves a straightforward addition of organoboron reagents to electron-deficient acceptor imines to yield optically pure amines.5 While arylboron reagents were often employed in this synthetic strategy, examples of 1,2-addition of alkenyl-6 and allylboron7 regents to imines were scarce. Recently, Lam et al. reported enantioselective allylation of sulfamate-derived cyclic imines catalyzed by a chiral Rh catalyst, providing the corresponding homoallylic amines in good yields with high diastereo- and enantioselectivities.7a,b Similar reports associated with stereochemical complexity can be witnessed in the addition of α,α- and γ,γ-disubstituted potassium trifluoroborates to the cyclic sulfamate imines, yielding homoallylic amines with a high degree of asymmetric control via 1,4-isomerization8 of the allyl-Rh intermediates.7c In a recent report, the allylation reaction of cyclic sulfamate imines with δ-potassium trifluoroboryl β,γ-unsaturated esters, giving rise to susceptibly isomerizable allyl-Rh(I) intermediates, furnished homoallylic amines with a (Z)-alkene moiety and two stereogenic centers was presented.7d While various potassium allyltrifluoroborates have been proven to be useful nucleophilic precursors to cyclic © XXXX American Chemical Society
Received: November 14, 2017
A
DOI: 10.1021/acs.orglett.7b03523 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 1. Rh(I)-Catalyzed Asymmetric Allylation of NTosylaldimine 2aa
entry
L1
additive (equiv)
4aa, yieldb (%)
4aa, eec (%)
5aa, yieldb (%)
1 2 3 4 5f 6 7 8 9 10 11
L1a L1b L1b L1b L1b L1a L1c L1d L1e L2 L3
MeOH (5.0) MeOH (5.0) H2O (5.0) H2O (2.0) none none none none none none none
18 40 40 68 84 73 64 72 64 50 53
90 96 94 97 96 90 72 82 83 −1 −6
0 0 13 (47)d 0 0 0 0 0 0 0 0
Table 2. Scope of Rh(I)-Catalyzed Asymmetric Allylation of Aldimines (I)a
a Reaction parameters: compound 2a (0.2 mmol), 3a (0.4 mmol), Rh(I) catalyst (3 mol %), and MeOH (40 μL) or H2O (18 or 7.2 μL) or no additive in toluene (1 mL), reaction time 24 h. bIsolated yield. c Determined by chiral HPLC. dee of 5aa. fAverage of four different batches of 3a.
entry
2 or 6, Ar
4 or 7, yieldb (%)
eec (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
2a, C6H5 2b, 2-Me-C6H4 2c, 3-Me-C6H4 2d, 4-Me-C6H4 2e, 4-MeO-C6H4 2f, 3,4-(MeO)2-C6H3 2g, 4-F-C6H4 2h, 4-Cl-C6H4 2i, 3-Cl-C6H4 2j, 3,4-Cl2-C6H3 2k, 4-NO2-C6H4 2l, 1-naphthyl 2m, 2-naphthyl 2n, PhCHCH 2o, 2-furyl 6a, C6H5 6d, 4-Me-C6H4 6e, 4-MeO-C6H4 6f, 3,4-(MeO)2-C6H3 6g, 4-F-C6H4 6m, 2-naphthyl
4aa, 84 4ba, 81 4ca, 90 4da, 78 4ea, 77 4fa, 80 4ga, 83 4ha, 75 4ia, 86 4ja, 75 4ka, 50 4la, 74 4ma, 65 4na, 51 4oa, 74 7aa, 50 7da, 51 7ea, 57 7fa, 45 7ga, 41 7ma, 45
96 95 92 91 96 98 95 96 92 95 83 87 91 84 97 91 92 95 93 91 89
a
Reaction parameters: compound 2 or 6 (0.2 mmol), 3a (0.4 mmol, 2.0 equiv with respect to 2 or 6), and Rh(I) catalyst (3 mol %) in toluene (1 mL), reaction time 24 h. bIsolated yield. cDetermined by chiral HPLC.
accounted for the homoallylic alcohol 5aa. Gratifyingly, the reaction generated 84% of 4aa in 96% ee without adding additive (Table 1, entry 5).12 X-ray single-crystal analysis unequivocally confirmed 4aa as the (S)-configurated product. While ligand L1a could also result in decent yield under additive-free conditions (Table 1, entry 6), the reaction performed with variously substituted 2,5-diarylbicyclo[2.2.1]heptadiene ligands L1c−L1e showed no superior results regarding yields and ee’s (Table 1, entries 7−9). In addition, chiral diene ligands with various structure motifs were tested for further comparison. When Rh catalysts derived from chiral bicyclo[2.2.2]octadiene ligand L2 and bicyclo[3.3.0]octadiene ligand L3 were used, 4aa was obtained in 50% and 53% yield, respectively, however, with low stereoselectivity (Table 1, entries 10 and 11). In the ensuing study, the reaction scope of this asymmetric transformation was investigated with various N-Ts-aldimines utilizing the optimal conditions identified (Table 1, entry 5) (Table 2). Aldimines derived from alkyl and electron donating group substituted aromatic aldehydes were good substrates, resulting in products with 77−90% yields and excellent ee’s of 91−98% (Table 2, entries 2−6). Similarly, aldimines bearing electron-withdrawing groups were applicable to this protocol, producing homoallylic amines in 50−86% yields and 83−96% ee’s (Table 2, entries 7−11). Good chemical yields (51−74%) and high selectivities (84−97% ee’s) were observed from the asymmetric reactions of 3a with aldimines that were prepared from 1- and 2-naphthyl aldehyde (2l and 2m) (Table 2, entries 12 and 13), cinnamaldehyde 2n (Table 2, entry 14), and 2furfural (2o) (Table 2, entry 15). Enantioselective allylation of N-Ns-aldimines 6a, 6d−g, and 6n was also successful to afford the corresponding products in moderate yields (41−57%) with
high asymmetric induction (89−95% ee) (Table 2, entries 16− 21). Observation of unreacted N-Ns-imines in the crude 1H NMR spectra hinted at their lower reactivity compared to N-Ts analogues. The enantioselective crotylation of aldimines was studied to further understand the reaction scope (Table 3). Addition reaction of potassium (E)-crotyl trifluoroborate [(E)-3b] with aldimine 2a proceeded in a diastereoselective manner (7:1) (Table 3, entry 1), affording the addition products in 87% combined yield and with 97% ee of the major diastereomer 8ab. Reactions of (E)-3b with various aldimines provided similar levels of reactivities, diastereoselectivities, and enantioselectivities (Table 3, entries 2−5). X-ray diffraction analysis on a crystal from the reaction mixture of entry 4 determined the absolute configuration of the major product 8gb as (1S,2R). Although good diastereoselectivities and excellent enantioselectivities were observed when (E)-3b was used, the reactions with the corresponding cinnamyl derivative (E)-3c witnessed a diminishing de and ee values (Table 3, entry 6). High yield and asymmetric induction were observed when asymmetric reactions of (Z)-3b with aldimines 2a and 2d were carried out, producing 8ab and 8db as the major products notably (Table 3, entries 7 and 8). Enantioselective reaction of (±)-3d with 2a afforded 8ab and 9ab (6.5:1) in a 64% combined yield and 92% ee for 8ab (Table 3, entry 10). The observed stereochemical rationale can be clarified as the observed (S)4aa adduct came from the si-face allylation of 2a, owing to the B
DOI: 10.1021/acs.orglett.7b03523 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 3. Scope of Rh(I)-Catalyzed Asymmetric Allylation of Aldimines (II)a
Figure 2. Proposed isomerization of Rh(I)-crotyl species.
entry
2, Ar
3
8 + 9 (yield, %)b
8/9c
8, eed (%)
1 2 3
2a, C6H5 2d, 4-Me-C6H4 2e, 4-MeOC6H4 2g, 4-F-C6H4 2m, 2-Naphthyl 2a, C6H5 2a, C6H5 2d, 4-Me-C6H4 2a, C6H5 2a, C6H5
(E)-3b (E)-3b (E)-3b
8ab + 9ab (87) 8db + 9db (90) 8eb + 9eb (82)
7:1 8:1 10:1
97 97 96
(E)-3b (E)-3b (E)-3c (Z)-3b (Z)-3b (Z)-3b (±)-3d
8gb 8mb 8ac 8ab 8db 8ab 8ab
7:1 8:1 6:1 4.6:1 5:1 2:1 6.5:1
98 91 67 94 97 83 92
4 5 6 7 8 9e 10
required a higher reaction temperature, which inevitably gives rise to interconversion of allyl-Rh species. Crispine A is a pyrroloisoquinoline alkaloid isolated from Carduus crispus with biological activities against KB, SKOV3, and HeLa human cell lines.14 Its asymmetric synthesis was realized to demonstrate the synthetic usefulness of this powerful method (Scheme 1). Thus, enantioselective addition Scheme 1. Total Synthesis of (−)-Crispine A
+ + + + + + +
9gb (87) 9mb (78) 9ac (58) 9ab (92) 9db (85) 9ab (32) 9ab (64)
a
Reaction parameters: compound 2 (0.2 mmol), 3 (0.4 mmol, 2.0 equiv with respect to 2), and Rh(I) catalyst (3 mol %) in toluene (1 mL), reaction time 24 h. bIsolated yield of the inseparable mixtures of 8 and 9. cDetermined by 1H NMR spectra. dDetermined by chiral HPLC. eAt 60 °C for 72 h.
steric repulsion between the 2,5-disubstituents of the ligand and 4-tolylsulfonamide from the re-face coordination of 2a to Rh(I) (Figure 1). Accordingly, the crotylation progressed via a six-
reaction of 3a with imine 2p under the optimal conditions afforded adduct 4pa in 77% yield and 99% ee. The ensuing transformation involved a protecting group switch from Ts to Boc in 87% for two steps and TBAF-mediated TBS removal to provide alcohol 12 in 85% yield. Converting the alcohol 12 into the corresponding mesylate and the subsequent cyclization gave cyclic amine 13 in 83% yield after two steps. Hydroboration− oxidation transformed 13 into 95% of alcohol 14, which was finally subjected to tosylation, Boc removal, and basic cyclization to furnish (−)-crispine A in 74% yield. In conclusion, a highly enantioselective allylation of N-Tsand N-Ns-arylaldimines catalyzed by rhodium catalysts was realized in this work. In the presence of 3 mol % of Rh(I)catalyst in situ generated from [RhCl(C2H4)2]2 and chiral bicyclo[2.2.1]heptadiene ligand L1b, addition reaction of potassium allyltrifluoroborates 3 with a variety of N-Ts- and N-Ns-arylaldimines afforded the corresponding products in 41−90% yields, up to 10:1 diastereomeric ratios, and 83−98% ee’s. The asymmetric reaction employing (E)-3b and (Z)-3b and (±)-3d gave (1S,2R)-diastereomers, supporting the involvement of a common nucleophilic Rh(I) species. This is rationalized by sequential interconversion of nucleophilic Rh(I)-species to form the E-isomer due to its thermodynamic stability. Finally, the concise and enantioselective synthesis of naturally occurring alkaloid (−)-crispine A from adduct 4pa demonstrated the usefulness of this protocol.
Figure 1. Proposed rationale for the stereochemical outcome of allylation reactions.
membered transition structure, in which the Me group was situated at the pseudoequatorial position, to account for the formation of (1S,2R) stereogenic centers. Contrary to the proposed working model, 8ab was obtained as the major product when both (Z)-3b and (±)-3d were treated with 2a, indicating the presence of a common nucleophilic crotyl-Rh(I) species. Presumably, transmetalation of (Z)-3b gave rise to the intermediate IT-(Z)-A, which underwent interconversion to offer IT-B, an intermediate that can also be obtained from transmetalation of (±)-3d (Figure 2).7b The ensuing isomerization produced the energetically more stable intermediate IT(E)-A. The lower 2:1 ratio of 8ab to 9ab from a 60 °C reaction demonstrated that this interconversion mechanism is temperature demanding (Table 3, entry 9).13 While allylation of (E)3b and (Z)-3b with more reactive sulfamate-derived cyclic imines proceeded at 55 °C to yield the corresponding anti- and syn-adducts in Lam’s studies,7a,b the crotylation of aldimines 2 C
DOI: 10.1021/acs.orglett.7b03523 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters
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7854. (g) Huo, H.-X.; Duvall, J. R.; Huang, M.-Y.; Hong, R. Org. Chem. Front. 2014, 1, 303−320. (5) For seminal reviews, see: (a) Hayashi, T.; Yamasaki, K. Chem. Rev. 2003, 103, 2829−2844. (b) Johnson, J. B.; Rovis, T. Angew. Chem., Int. Ed. 2008, 47, 840−871. (c) Defieber, C.; Grützmacher, H.; Carreira, E. M. Angew. Chem., Int. Ed. 2008, 47, 4482−4502. (d) Edwards, H. J.; Hargrave, J. D.; Penrose, S. D.; Frost, C. G. Chem. Soc. Rev. 2010, 39, 2093−2105. (e) Tian, P.; Dong, H.-Q.; Lin, G.-Q. ACS Catal. 2012, 2, 95−119. (f) Chen, D.; Xu, M.-H. Youji Huaxue 2017, 37, 1589−1612. (6) (a) Luo, Y.; Carnell, A. J.; Lam, H. W. Angew. Chem., Int. Ed. 2012, 51, 6762−6766. (b) Gopula, B.; Chiang, C.-W.; Lee, W.-Z.; Kuo, T.-S.; Wu, P.-Y.; Henschke, J. P.; Wu, H.-L. Org. Lett. 2014, 16, 632−635. (c) Cui, Z.; Chen, Y.-J.; Gao, W.-Y.; Feng, C.-G.; Lin, G.-Q. Org. Lett. 2014, 16, 1016−1019. (d) Gopula, B.; Zeng, H.-W.; Wu, P.Y.; Henschke, J. P.; Wu, H.-L. J. Chin. Chem. Soc. 2017, DOI: 10.1002/ jccs.201600888. (7) (a) Luo, Y.; Hepburn, H. B.; Chotsaeng, N.; Lam, H. M. Angew. Chem., Int. Ed. 2012, 51, 8309−8313. (b) Hepburn, H. B.; Chotsaeng, N.; Luo, Y.; Lam, H. M. Synthesis 2013, 45, 2649−2661. (c) Hepburn, H. B.; Lam, H. M. Angew. Chem., Int. Ed. 2014, 53, 11605−11610. (d) Martínez, J. I.; Smith, J. J.; Hepburn, H. B.; Lam, H. M. Angew. Chem., Int. Ed. 2016, 55, 1108−1112. For examples of highly diastereoselective allylation of imines, see: (e) Sun, X.-W.; Xu, M.-H.; Lin, G.-Q. Org. Lett. 2006, 8, 4979−4982. (f) Sun, X.-W.; Liu, M.; Xu, M.-H.; Lin, G.-Q. Org. Lett. 2008, 10, 1259−1262. (8) For 1,4-shift of Rh-species, also see: (a) Oguma, K.; Miura, M.; Satoh, T.; Nomura, M. J. Am. Chem. Soc. 2000, 122, 10464−10465. (b) Hayashi, T.; Inoue, K.; Taniguchi, N.; Ogasawara, M. J. Am. Chem. Soc. 2001, 123, 9918−9919. (9) (a) Wei, W.-T.; Yeh, J.-Y.; Kuo, T.-S.; Wu, H.-L. Chem. - Eur. J. 2011, 17, 11405−11409. (b) Huang, K.-C.; Gopula, B.; Kuo, T.-S.; Chiang, C.-W.; Wu, P.-Y.; Henschke, J. P.; Wu, H.-L. Org. Lett. 2013, 15, 5730−5733. (c) Gopula, B.; Tsai, Y.-F.; Kuo, T.-S.; Wu, P.-Y.; Henschke, J. P.; Wu, H.-L. Org. Lett. 2015, 17, 1142−1145. (d) Gopula, B.; Yang, S.-H.; Kuo, T.-S.; Hsieh, J.-C.; Wu, P.-Y.; Henschke, J. P.; Wu, H.-L. Chem. - Eur. J. 2015, 21, 11050−11055. (e) Fang, J.-H.; Jian, J.-H.; Chang, H.-C.; Kuo, T.-S.; Lee, W.-Z.; Wu, P.-Y.; Wu, H.-L. Chem. - Eur. J. 2017, 23, 1830−1838. (10) (a) Chen, C.-C.; Gopula, B.; Syu, J.-F.; Pan, J.-H.; Kuo, T.-S.; Wu, P.-Y.; Henschke, J. P.; Wu, H.-L. J. Org. Chem. 2014, 79, 8077− 8085. (b) Syu, J.-F.; Lin, H.-Y.; Cheng, Y.-Y.; Tsai, Y.-C.; Ting, Y.-C.; Kuo, T.-S.; Janmanchi, D.; Wu, P.-Y.; Henschke, J. P.; Wu, H.-L. Chem. - Eur. J. 2017, 23, 14515−14522. (11) 5aa was determined as the R-alcohol by comparing the optical rotation value ([α]23 D +26.2 (c 0.7 in CHCl3)) with the reported data ([α]25 D +60.3 (c 2.0 in CHCl3)); see: Shin, I.; Wang, G.; Krische, M. J. Chem. - Eur. J. 2014, 20, 13382−13389. (12) Similarly, the Pd-catalyzed cross-coupling of potassium aryl- and alkenyltrifluoroborates proceeded without any additive; see: Darses, S.; Genêt, J.-P.; Brayer, J.-L.; Demoute, J.-P. Tetrahedron Lett. 1997, 38, 4393−4396. (13) No reaction occurred when the temperature was lower than 60 °C. (14) (a) Zhang, Q.; Tu, G.; Zhao, Y.; Cheng, T. Tetrahedron 2002, 58, 6795−6798. (b) Louafi, F.; Moreau, J.; Shahane, S.; Golhen, S.; Roisnel, T.; Sinbandhit, S.; Hurvois, J.-P. J. Org. Chem. 2011, 76, 9720−9732.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03523. Experimental procedures and complete characterization of addition products (NMR spectra, HPLC chromatograms) (PDF) Accession Codes
CCDC 1585487 and 1585493 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hsyueh-Liang Wu: 0000-0001-7462-8536 Author Contributions §
P.-F. C. and W.-S. L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Tiow-Gan Ong of the Institute of Chemistry, Academia Sinica, for his input during the preparation of this manuscript. Financial support from the Ministry of Science and Technology of Republic of China (102-2113-M-003-006-MY2 and 104-2628-M-003-001-MY3) is gratefully acknowledged.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b03523 Org. Lett. XXXX, XXX, XXX−XXX
