Letter pubs.acs.org/OrgLett
Asymmetric Synthesis of Cyclopenta[3,4]pyrroloindolones via NHeterocyclic Carbene-Catalyzed Michael/Aldol/Lactamization Cascade Reaction Yu-Jie Yang, Yuanyuan Ji, Liangliang Qi, Guanjun Wang, and Xin-Ping Hui* State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China S Supporting Information *
ABSTRACT: The N-heterocyclic carbene-catalyzed asymmetric Michael/aldol/lactamization cascade reaction of enals and indole-derived enones for the synthesis of functionalized cyclopenta[3,4]pyrroloindolones with four consecutive stereogenic centers has been achieved. The products were obtained in good yield with high diastereoselectivity and excellent enantioselectivity.
P
via an intermediate azolium enolate. Stanley and co-workers3f reported a chiral rhodium-catalyzed intramolecular hydroacylation of N-vinylindole-2-carboxaldehydes to afford pyrrolo[1,2-a]indolones. Recently, Schneider3g described chiral BINOL-derived phosphoric acid catalyzed, formal [3 + 2] cycloaddition reaction to afford indolo[1,2-a]-indoles. Although some methods have been developed for asymmetric synthesis of pyrroloindolone derivatives, most of the methods suffer from low efficiency and need a high amount of the chiral sources. Thus, the development of efficient synthetic methodology for constructing chiral pyrroloindolones is still in great demand. In recent decades, N-heterocyclic carbenes (NHCs) have become powerful organocatalysts for organic synthesis.4,5 The cascade reactions catalyzed by NHCs have received much attention in recent years.6−8 The important active mode of NHCs with respect to α,β-unsaturated aldehydes is the formation of homoenolate equivalents9 via NHC-catalyzed umpolung, which has captured much attention in the chemical community. As our group is dedicated to the development of NHC-catalyzed cascade reactions,7h,k herein, we report an efficient Michael/aldol/lactamization cascade reaction of enals and indole-derived enones for the asymmetric synthesis of functionalized cyclopenta[3,4]pyrroloindolones (Scheme 1). The products, which possess four consecutive stereogenic centers, were obtained in good yields and diastereoselectivities and excellent enantioselectivities. We started our studies with indole-derived α,β-unsaturated ketone 1a and cinnamaldehyde (2a) in the presence of 10 mol % triazolium catalyst 3a and 1.5 equiv of 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU) in dichloromethane at 30 °C. To our delight, the desired product 4a was isolated in 11% yield, >25:1 dr, and 54% ee (Table 1, entry 1). Subsequently, some other N-heterocyclic carbene catalysts were evaluated, and a
yrroloindolone derivatives have garnered extensive attention in synthetic and medicinal chemistry because of their presence as structural motifs in numerous natural products and biologically active molecules (Figure 1).1 Given this, tremen-
Figure 1. Representative biological active natural products bearing a pyrroloindolone moiety.
dous efforts have been focused on developing efficient methods to access such structural motifs.2,3 In the asymmetric synthesis of pyrroloindolones, the group of Yang3a reported the first Pd(II)-catalyzed enantioselective oxidative tandem cyclization reaction of nitrogen-atom-based nucleophiles using (−)-sparteine as the chiral ligand and molecular oxygen as the oxidant. Wang and co-workers3b developed a highly useful method for the construction of pyrrolo[1,2-a]indole-2-carbaldehydes through an asymmetric cascade aza-Michael/aldol reaction of indole-2-carbaldehydes with α,β-unsaturated aldehydes catalyzed by the diphenylprolinol silylether. The group of Enders3c also reported the reaction between indole-2-methylene malononitriles and α,β-unsaturated aromatic aldehydes to give tetracyclic double-annulated indole derivatives through a threecomponent quadruple cascade reaction catalyzed by (S)diphenylprolinol silylether. Xiao, Chen and co-workers3d disclosed the copper-catalyzed, enantioselective Friedel−Crafts alkylation/N-hemiacetalization cascade reaction to functionalized pyrrolo[1,2-a]-indoles. In 2013, Enders and co-workers3e reported the [2 + 3] annulation of nitrovinylindoles with αchloroaldehydes to furnish trans-disubstituted pyrroloindolones © 2017 American Chemical Society
Received: May 10, 2017 Published: June 2, 2017 3271
DOI: 10.1021/acs.orglett.7b01411 Org. Lett. 2017, 19, 3271−3274
Letter
Organic Letters
slightly lower diastereo- and enantioselectivities were afforded (entry 17). When the optimal reaction conditions were established, we evaluated the scope with respect to enals. As shown in Table 2, a variety of enals 2 reacted smoothly with indole-derived enone 1a in moderate to good yields and excellent enantioselectivities. Enals with both electron-donating and -withdrawing groups in the para-positions of the phenyl ring gave high diastereoselectivities (entries 1, 4, 6, 8, and 11) compared to metapositions (entries 3, 5, 7, and 10). It is worth noting that the enals with electron-rich aromatic rings (entries 7−12, 14) gave higher yields than those with electron-withdrawing substituents (entries 2−6). Steric hindrance has been proven obviously, and 2-chlorophenyl substituted enal 2b resulted in the lowest yield and dr value (entry 2). In addition, the 2-naphthyl substituted enal 2m could also undergo the stereoselective cascade reaction (entry 13). However, when alkyl-substituted enals were used, the cascade reaction did not occur. Next, we studied the generality of this asymmetric cascade reaction by investigating various indole-derived α,β-unsaturated ketones. The substituents in the para-position of the phenyl, irrespective of halogen (1b−1c) and methyl (1d) groups, all afforded the corresponding products in good yields, high diastereoselectivities, and excellent enantioselectivities (entries 15−17). The absolute configuration of the products was determined to be (2S, 3R, 3aS, 10bS) by X-ray crystallographic analysis of compound 4j (Figure 2). To demonstrate the practicality of this stereoselective cascade reaction, we carried out a gram scale of indole-derived enone 1c with cinnamaldehyde (2a) (Scheme
Scheme 1. NHC-Catalyzed Stereoselective Cascade Reaction
higher yield and excellent diastereo- and enantioselectivities were observed by using catalyst 3c (entry 3). When chiral NHC-catalysts 3d were used, this cascade reaction failed. After screening several bases, inferior results were obtained compared with DBU (entries 3−5). Further optimization of solvents led to DCM being identified as the best choice (entries 3, 6−9). To increase the yield of this cascade reaction, the molar ratio of indole-derived enone 1a and cinnamaldehyde (2a) was optimized (entries 10−12). When 1.75 equiv of the cinnamaldehyde (2a) was used, the product 4a was obtained in 48% yield, >25:1 dr, and with 94% ee (entry 12). It is worth noting that alcohol as an additive was necessary for this stereoselective cascade reaction. The addition of 0.2 equiv of methanol or t-BuOH led to a slight improvement in yield (entries 13−14). Further addition of 0.5 equiv of t-BuOH increased the yield to 61% with 20:1 dr and 93% ee (entry 15). When the amount of t-BuOH was increased to 1.0 equiv, the product 4a was obtained in 68% yield, 16:1 dr, and with 94% ee (entry 16). Upon further increasing the t-BuOH to 2.0 equiv, Table 1. Optimization of Reaction Conditionsa
entry
catalyst
solvent
base
additive
time (h)
yield (%)b
dr (%)c
ee (%)d
1 2 3 4 5 6 7 8 9 10e 11f 12g 13g 14g 15g 16g 17g
3a 3b 3c 3c 3c 3c 3c 3c 3c 3c 3c 3c 3c 3c 3c 3c 3c
DCM DCM DCM DCM DCM DCE THF PhCH3 CHCl3 DCM DCM DCM DCM DCM DCM DCM DCM
DBU DBU DBU Cs2CO3 NEt3 DBU DBU DBU DBU DBU DBU DBU DBU DBU DBU DBU DBU
none none none none none none none none none none none none MeOH (0.2 equiv) t-BuOH (0.2 equiv) t-BuOH (0.5 equiv) t-BuOH (1 equiv) t-BuOH (2 equiv)
3 1 5 2 3 2 1 1 2 5 6 6 5 5 5 5 5
11 24 37 26 13 13 13 10 26 37 40 48 50 54 61 68 68
>25:1 >25:1 >25:1 >25:1 >25:1 >25:1 >25:1 >25:1 >25:1 >25:1 >25:1 >25:1 20:1 20:1 20:1 16:1 14:1
54 86 90 90 96 96 96 96 96 95 94 94 93 93 93 94 88
a Reaction conditions: 1a/2a/base = 1:1:1.5 (molar ratio). To a solution of enals 2a, DBU and catalyst 3c in DCM, a solution of indole-derived enones 1a in DCM and t-BuOH were added at 30 °C under a nitrogen atmosphere. bIsolated yield. cDetermined by 1H NMR (400 MHz) of the crude product. dDetermined by chiral HPLC analysis. e1a/2a = 1:1.25 (molar ratio). f1a/2a = 1:1.5 (molar ratio). g1a/2a = 1:1.75 (molar ratio).
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DOI: 10.1021/acs.orglett.7b01411 Org. Lett. 2017, 19, 3271−3274
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Organic Letters Table 2. Substrate Scope of the NHC-Catalyzed Reactiona
entry
R1 (1), R2 (2)
time (h)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Ph (1a), Ph (2a) Ph (1a), 2-ClPh (2b) Ph (1a), 3-ClPh (2c) Ph (1a), 4-ClPh (2d) Ph (1a), 3-BrPh (2e) Ph (1a), 4-BrPh (2f) Ph (1a), 3-MePh (2g) Ph (1a), 4-MePh (2h) Ph (1a), 3,4-Me2Ph (2i) Ph (1a), 3-MeOPh (2j) Ph (1a), 4-MeOPh (2k) Ph (1a), 3,4-(MeO)2Ph (2l) Ph (1a), 2-C10H7 (2m) Ph (1a), 2-thienyl (2n) 4-ClPh (1b), Ph (2a) 4-BrPh (1c), Ph (2a) 4-MePh (1d), Ph (2a)
5 6 8 5 8 5 5 5 5 5 5 5 12 5 5 5 5
yield (%)b 68 47 53 53 51 56 63 75 66 62 72 71 53 60 69 66 59
(4a) (4b) (4c) (4d) (4e) (4f) (4g) (4h) (4i) (4j) (4k) (4l) (4m) (4n) (4o) (4p) (4q)
drc
ee (%)d
16:1 5:1 8:1 15:1 8:1 13:1 10:1 16:1 12:1 11:1 16:1 12:1 7:1 20:1 10:1 10:1 20:1
93.8 98.9 97.8 97.9 93.9 98.4 92.8 96.2 95.2 96.1 96.4 93.7 95.5 97.3 98.4 96.9 99.9
Figure 3. Possible catalytic cycle.
intramolecular aldol reaction to afford intermediate IV. Finally, the intermediate IV participates in intramolecular lactamization to give the desired product 4 with the release of the NHC catalyst. In conclusion, we have developed an NHC-catalyzed stereoselective Michael/aldol/lactamization cascade reaction of indole-derived enones with enals. The functionalized cyclopenta[3,4]pyrrolo[1,2-a]indol-4-ones emerge with four consecutive stereogenic centers in good yield, high diastereoselectivities, and excellent enantioselectivities. The reaction features gram-scale production capability and mild reaction conditions. Further studies and applications of this cascade reaction are ongoing in our laboratory.
a Reaction conditions: 1/2/catalyst 3c/DBU = 1:1.75:0.1:1.5 (molar ratio). To a solution of enals 2, DBU and catalyst 3c in DCM, a solution of indole-derived enones 1 in DCM and t-BuOH were added at 30 °C under a nitrogen atmosphere. bIsolated yield. cDetermined by 1 H NMR (400 MHz) of the crude product. dDetermined by chiral HPLC analysis.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b01411. Experimental procedures, analytical data, and copies of the 1H NMR, 13C NMR, and HPLC charts for all new products (PDF) Crystallographic data for 4j (CIF)
Figure 2. X-ray crystallography of compound 4j.
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2). It is worthy of note that the cascade reaction proceeded smoothly and the adduct 4p was obtained in 69% yield, 10:1 dr, and 97% ee. The possible mechanism for the cascade reaction is proposed as shown in Figure 3. Initially, addition of the carbene to the α,β-unsaturated aldehyde 2 generates Breslow intermediate I. Next, the Michael addition of I to the Si face of indole-derived α,β-unsaturated ketone 1 forms intermediate II. Subsequent proton transfer of II affords the enolate III which undergoes an
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Xin-Ping Hui: 0000-0001-9108-7209 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support of the National Natural Science Foundation (21372106) and Program “111”.
Scheme 2. Gram Scale Synthesis of the Compound 4p
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DOI: 10.1021/acs.orglett.7b01411 Org. Lett. 2017, 19, 3271−3274
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