Abstract
7-Allylindolines are valuable synthons for designing biologically active molecular libraries. Lewis acid catalyzed amino-Claisen rearrangement provides a one pot synthetic entry to these heteroarenes. In this context, Zinc chloride (ZnCl2)–N,N-dimethylformamide system efficiently catalyzed amino-Claisen rearrangements of 1-N-allylindolines to 7-allylindolines. The rearrangement is influenced by stereoelectronic effects of substituents present in 1-N-allylindolines. The substrates containing electron donating functionalities underwent rearrangement at lower temperature than substrates with electron withdrawing functional groups. The regioselectivity of the process is governed by the substitution pattern on allyl moiety in 1-N-allylindoline as well as ZnCl2 catalyst loading in the reaction system.
Introduction
The 7-allylindoline nucleus is prevalent as structural motif for designing indole scaffolds of pharmaceutical importance [1, 2, 3, 4, 5, 6, 7, 8]. The Carbon- Carbon bond forming reactions have proven to be the most powerful tool in designing such types of molecular libraries for organic, combinatorial and medicinal chemistry [9-10]. The Claisen rearrangements of vinyl and aryl allyl ether are exploited as an expeditious approach to Carbon-Carbon bond formation in organic syntheses [11, 12, 13]. In contrast, the amino-Claisen rearrangement, the nitrogen analogue of Claisen
rearrangement has attracted little attention due to several limitations including slow rates, the need of higher temperature and low yields due to simultaneous allylic cleavage or cyclization leading to C-N bond formation. Therefore, there is the great dearth of examples exists in the literature for amino- Claisen rearrangements in aromatic and heteroaromatic series.
The well documented and classical approach of thermal and uncatalyzed amino Claisen rearrangement of N-allylindolines is suffered from severe drawbacks including thermal decomposition of the substrate and side reactions owing to the requirement of high temperature of 250-280OC, led to poor yields of 7-allylindolines [14-15]. The limitation associated with the thermal amino-Claisen rearrangement stimulated further interest of synthetic organic chemist for developing milder methods. The strategy of Lewis acid catalysis facilitates the amino Claisen rearrangement of indolines via a charge accelerated process. In this series, the amino- Claisen rearrangement of N-allylindolines has already been reported in different catalytic systems viz in ZnCl2-xylene [16], BF3.OEt2- sulfolane [17], BF3.OEt2- sulfolane for the synthesis of novel indole analogue of mycophenolic acid as potential antineoplastic agents [18] and in TFA – CH2Cl2 for the total synthesis of (+)- Okaramine J [19]. Although, these methods are advantageous over uncatalyzed rearrangements for accessing 7-allylindolines, still, some of these methods could not overcome the requirement of higher temperature, longer reaction times, costly catalyst loading in reaction systems and poor yields of targeted molecules. These limitations led us to explore the generality of this important strategic tool for C-C bond formation for targeted molecules from 1-H-N-allylindolines to 7-allylindolines. As a part of our ongoing interest, in Lewis acid catalyzed amino Claisen rearrangement [20, 21, 22], we found that ZnCl2 is an efficient catalyst for influencing the reaction. Therefore, ZnCl2–N,N-dimethylformamide catalytic system was employed for effecting amino-Claisen rearrangement of
N-allylindolines in a view to explore the versatility of the substrates containing electron withdrawing or electron donating functionalities in aryl ring with different patterns of allylic functionality to furnish 7-allylindoline derivatives. The development of reaction presented herein, takes the advantage of an efficient, one pot and regioselective synthetic entry to functionalized 7-allylindolines under very mild reaction conditions. The results emanated from the present study are described herein.
Result and discussion
A mixture of 1-H-N-allylindoline 1a-g and ZnCl2 (3.0 equiv.) in N,N-dimethylformamide as a solvent was heated at a temperature range of 120-150O C under argon for two hours to furnish 2 a-g (Scheme 1) in moderate to good yields. The use of this catalytic system reduced the formation of by products and other tarry products arising due to side reactions as compared to an uncatalyzed thermal process. The result is attributed to the use of polar and aprotic solvent DMF in which ZnCl2 catalyst showed enhanced solubility than any other nonpolar solvent. All reactions produced only a [3,3]-sigmatropic product but under similar conditions the amino Claisen rearrangement of 5-methyl-1-N-(3-methyl-2-butenyl)indoline (1h) proceeded slowly to form a mixture of 5-methyl-7-(3-methyl-2-butenyl) indoline (2h) and 1,2,5,6-tetrahydro-4,4,8-trimethyl-4H-pyrrolo[3,2,1-ij] quinoline (3). The formation of 2h is presumed to be formed from 1h, apparently by the cleavage followed by electrophilic attack of the allylic moiety on the adjacent position of heteroaryl ring without inversion of the allyl group. The failure of 1h to undergo 3,3-sigmatropic process could be assigned to the steric hindrance on the allyl moiety while passing through the concerted reaction to furnish 2h. The regioisomer 3 probably arose due to the acid catalyzed cyclization of 2hvia ionic C-C bond formation (Scheme 1) [16].
Substrate Scope in Amino Claisen rearrangement.
1-N-allylindolines (1 a-h)
1a R1= CH3, R2, R3, R4, R5, R6 =H
1b R1= OCH3, R2, R3, R4, R5, R6= H
1c R1= NO2, R2, R3, R4, R5, R6 =H
1d R1=COOEt, R2, R3, R4,R5, R6 = H
1e R1=CH3, R2, R3, R5, R6=H, R4=CH3
1f R1=CH3,R2 =CH3, R3, R4 , R5, R6= H
1g R1=CH3, R2,R3,R4, R5= H,R6 = CH3
1h R1=CH3,R2 , R3,R4= H, R5,R6 = CH3
7-Allylindolines (2 a-h)
2a R1= CH3, R2, R3, R4, R5, R6 = H
2b R1=OCH3, R2, R3, R4, R5, R6 = H
2c R1= NO2, R2, R3, R4, R5, R6 = H
2d R1=COOEt, R2, R3, R4, R5,R6 =H
2e R1=CH3, R2, R3, R5, R6=H, R4=CH3
2f R1=CH3,R2 ,R3, R4, R6= H, R5 = CH3
2g R1=CH3, R2 = CH3,R3, R4, R5,R6 = H
2h R1=CH3, R2 , R3, R4= H, R5, R6 = CH3
However, the formation of 3 was effectively inhibited by increasing the amount of ZnCl2 (4.0 equiv.) and shortening the reaction time (1.0 h) thereby providing the regioselectivity by altering the reaction centre on the substrate to give 2h in 77% yield as the sole product. In contrast, 1f, cleanly underwent the rearrangement at lowest temperature to furnish 2f in good yield (86 %). This result can be attributed to the higher reactivity of the allyl group owing to the steric hindrance in the Lewis acid mediated “charge induced” 3,3-sigmatropic rearrangement process. With ongoing similar problems, 1e reacted faster than 1g in a 3,3-sigmatropic process to give 2e (83%) and 2g (77%) respectively, again giving the evidence for inherited regioselectivity within the reaction process. All products are well characterized by elemental analysis, IR, 1H NMR, 13C NMR and Mass spectral data.
Conclusion
This rearrangement tolerated electron rich as well as electron deficient substituents on the arene ring of heteroaromatic system without involving the formation of any cyclized product thereby providing excellent regioselectivity to the process. 1-H-N-allylindoline having electron donating groups underwent rearrangement at lower temperature as compared to electron deficient ones in moderate to good yields. The reaction also tolerated acid and base sensitive functionalities on the aryl ring of indoline system under mild reaction conditions. These results are consistent to the proposed mechanism of Lewis acid catalyzed amino Claisen rearrangement via charge accelerated process [20]. Hence, this C-N activation protocol for effecting amino-Claisen rearrangement of 1-H-N-allylindolines provided a one pot, regioselective and an efficient synthetic entry to difficultly accessible 7-allylindolines and should find applicability for creating more sophisticated chemical transformations.
Experimental Details
1-H-N-allylindolines 1a-h were prepared by using the typical procedure for N-allylation of corresponding 1-H-indolines [23]. 1-H-N-allylindolines 1a-h furnished corresponding 7-allylindolines 2a-h which were characterized on the basis of elemental analysis, IR, 1H, 13CNMR and MS data as given below. IR spetra were recorded on Pye Unicam Model SP3-300 infracord in neat. 1H and 13C NMR spectra were recorded on a FT NMR Brucker AM 300L Spectrometer using CDCl3 as solvent and TMS as an internal reference at 300 MHz. and 100 MHz respectively. Mass spectra were recorded on a Jeol D-300 (EI) Spectrometer using CDCl3 as solvent
General procedure for the preparation of 7-allylindolines
A mixture of 1-H-N-allylindolines (1a-h) (2.57 g, 14.9 mmol), ZnCl2 (6.10 g 44.7 mmol, 3.0 equiv.) and DMF (25 mL) were heated under argon at the temperature 120-150OC for 2hr. The mixture was cooled and poured into ice water (50 mL) and 10% HCl solution was added dropwise to make the solution acidic (pH 1). The solution was washed with CHCl3 (2 x 40 mL), the aqueous layer was made alkaline with addition of 10 % NaOH solution and extracted with ether (3 x 50 mL). The combined ether extracts were washed with water (60 mL), dried over MgSO4 and concentrated. The residues were purified by column chromatography on silica gel (hexane : ethyl acetate; 10:1) to afford 7-allylindolines (2a-h).
Analytical data of the compounds
7-Allyl-5-methylindoline (2a): The compound was obtained from 1a at the temperature 120 -1300C in 78 % yield as pale yellow oil; IR (neat) : 3380, 2836, 1615, 1485, 1450, 1250, 995, 910 cm-1; 1H NMR: δ 3.15 (t, J = 8.2 Hz, 2H, C3-H), 3.41 (dd, J = 6.1 Hz, J = 1.2 Hz, 2H, -CH2-C=C), 3.66- 3.91(m 3H, NH and C2 H), 5.26-5.30 (m, 2H, -C=CH2), 6.04- 6.17 (m, 1H, -C-CH=C), 6.60-7.20 (m, 2H, Ar-H), 2.60 (s, 3H, Ar-CH3); 13C NMR: δ 146.6 (C8), 135.6 (-CH=CH2), 127.3 (C6), 126.3 (C9), 126.5 (C4), 125.6 (C5), 121.9 (C7), 116.1 (-C=CH2), 46.5 (C2), 38.3 (Ar-CH2), 30.4 (C3), 23.6 (Ar-CH3); MS: m/z 173 (M+); Anal. Calcd for C12H15N: C, 83.23; H, 8.67; N, 8.09. Found: C, 83.26; H, 8.98; N, 8.06.
7-Allyl-5-methoxyindoline (2b): The compound was obtained from 1b at 130 -1350C in 62 % yield as brown oil; IR (neat) : 3382, 2936, 2832, 1615, 1485, 1460, 1246, 990,
900 cm-1; 1H NMR : δ 2.99 (t, J = 8.4 Hz, 2H, C3-H), 3.33 (dd, J = 6.3 Hz, J = 1.2 Hz, 2H, -CH2-C=C), 3.57- 3.78 ( m 3H, NH and C2 H), 5.01-5.09 (m, 2H, -C=CH2), 5.97- 6.27 (m, 1H, -C-CH=C), 6.70-7.19 (m, 2H, Ar-H), 3.78 (s, 3H, Ar-OCH3); 13C NMR : δ 147.8 (C5), 141.6 (C8), 135.6 (CH=CH2), 127.7 (C9), 122.1 (C7), 116.3 (C=CH2), 115.4 (C6), 108.9 (C4), 47.2 (C2), 38.8 (Ar-CH2), 31.2 (C3), 29.6 (Ar-CH3); MS : m/z 189 (M+); Anal. Calcd for C12H15NO: C, 76.19; H, 7.94; N, 7.40. Found: C, 76.08; H, 7.98; N, 7.36.
7-Allyl-5-nitroindoline (2c): The compound was obtained from 1c at 140 -1500C in 80 % yield as yellow oil; IR (neat) : 3432, 3386, 2960, 2860, 1632, 1594, 1520, 1260, 1456, 1441, 1350, 1309, 1275, 995 910 cm-1; 1H NMR : δ 3.11 (t, J = 8.2-8.4 Hz, 2H, C3-H), 3.42 (dd, J = 6.0 Hz, J =1.2 Hz, 2H, -CH2-C=C), 3.66- 4.01 (m 3H, NH and C2 H), 5.07-5.15 (m, 2H, -C=CH2), 5.91- 6.02 (m, 1H, -C-CH=C), 7.05-7.28 (m, 2H, Ar-H); 13C NMR : δ 155.3 (C8), 135.6 (C5), 135.4 (-CH=CH2), 128.6 (C9), 122.6 (C7), 121.2 (C6), 119.6 (C4), 116.9 (-C=CH2), 46.1 (C2), 37.8 (Ar-CH2), 29.3 (C3); MS : m/z 204 (M+); Anal. Calcd for C11H12N2O2: C, 64.71; H, 5.88; N, 13.73. Found: C, 64.74; H, 5.80; N, 13.76.
7-Allyl-5-carbethoxyindoline (2d): The compound was obtained from 1d at 135 -1450C in 79 % yield as pale yellow oil; IR (neat) : 3486, 3372, 2958, 1690, 1620, 1586, 1570, 1456, 1432, 1310, 1280, 1248, 1200, 1085, 1070, 985, 900 cm-1; 1H NMR : δ 3.18 (t, J = 8.1 Hz, 2H, C3-H), 3.42 (d, J = 1.0 Hz, 2H, -CH2-C=C), 3.65- 3.86 ( m 3H, NH and C2 H), 5.08-5.10 (m, 2H, -C=CH2), 6.05- 6.75 (m, 1H, -C-CH=C), 6.20-6.89 (m, 2H, Ar-H), 2.56 (q, J = 8.1 Hz, 2H, COOCH2-C), 1.14 (t, J =7.8 Hz, 3H, COO.C-CH3); 13C NMR : δ 260 (Ar-COOCH2-C), 166 (Ar-COOC-C), 154.3(C8), 135.8 (-CH=CH2), 127.3 (C6), 126.6 (C9), 125.7 (C4), 121.4 (C7), 117.5 (C5), 116.3 (-C=CH2), 46.8 (C2), 38.9 (Ar-CH2), 29.9 (C3); MS : m/z 204 (M+); Anal. Calcd for C11H12N2O2: C, 64.71; H, 5.88; N, 13.73. Found: C, 64.74; H, 5.80; N, 13.76.
5-Methyl-7-(2-methylallyl)indoline (2e): The compound was obtained from 1e at 130 -1400C in 83 % yield as pale yellow oil; IR (neat) : 3375, 2840, 1640, 1485, 1450, 1250, 910 cm-1; 1H NMR : δ 3.17 (t, J = 8.1Hz, 2H, C3-H), 3.46 (dd, J=6.8 Hz, J=1.4 Hz, 2H, -CH2-C=C), 3.60-3.80 (m, 3H, NH and C2 H), 5.26-5.30 (dd, J = 18 Hz, J = 2 Hz, 2H, -C=CH2), 1.86 (s, 3H, -C-C(CH3)=C), 6.60-7.20 (m, 2H, Ar-H), 2.58 (s, 3H, Ar-CH3); 13C NMR : 146.8 (C8), 142.8 (CH=CH2), 127.7 (C6), 126.6 (C9), 126.4 (C4), 125.3 (C5), 121.9 (C7), 111.3 (-C=CH2), 46.8 (C2), 38.9 (Ar-CH2), 30.8 (C3), 24.9 (Ar-CH3), 22.1(CH3-CH=C); MS : m/z 187 (M+); Anal. Calcd for C11H12N2O2: C, 83.32; H, 90.91; N, 7.49. Found: C, 83.39; H, 90.87; N, 7.51.
5-Methyl-7-(trans-2-butenyl)indoline (2f): The compound was obtained from 1f at 120 -1300C in 86 % yield as pale yellow oil; IR (neat) : 3386, 3050, 2945, 2847, 1600, 1460, 1255, 970 cm-1; 1H NMR : δ : 3.18 (t, J = 8.2-8.4 Hz, 2H, C3-H), 3.48 (dd, J = 6.6 Hz, J =1.2 Hz, 2H, -CH2-C=C), 3.66-3.80 (m, 3H, NH and C2 H), 6.02- 6.19 (m, 1H, -C-CH=C), 5.10-5.30 (m, 1H, -C=CH-C), 1.64 (d, J =18 Hz, 3H, -C=C-CH3), 6.80-7.19 (m, 2H, Ar-H), 2.50 (s, 3H, Ar-CH3); 13C NMR : δ 146.6 (C8), 127.6 (C6), 126.5 (CH=CH2), 126.4 (C9), 126.1 (C4), 125.3 (-C=CH2), 125.2 (C5), 121.9 (C7), 46.9(C2), 38.8 (Ar-CH2), 30.9 (C3), 24.2(Ar-CH3), 18.3 (=CH.CH3); MS : m/z 187 (M+); Anal. Calcd for C11H12N2O2: C, 83.42; H, 9.09; N, 7.49. Found: C, 83.40; H, 9.12; N, 7.51.
5-Methyl-7-(1-methylallyl)indoline (2g): The compound was obtained from 1g at 130 -1400C in 77 % yield as pale yellow oil; IR (neat) : 3380, 2840, 1640, 1480, 1440, 1250, 990, 910 cm-1; 1H NMR : δ 3.16 (t, J = 8.2 Hz, C3-H), 2.98 (m, 1H, -CH-C=C), 1.67 (dd, J = 6.5 Hz, J =1.4 Hz 3H, -C(CH3)-C=C), 3.65- 3.86 (m 3H, NH and C2 H), 5.12-5.30 (m, 2H, -C=CH2), 6.05- 6.15 (m, 1H, -C-CH=C), 6.60-7.00 (m, 2H, Ar-H), 2.60 (s, 3H, Ar-CH3); 13C NMR : δ 144.6 (C8), 138.2 (CH=CH2), 127.9 (C7), 127.3 (C6), 126.8 (C4), 126.3 (C9), 125.4 (C5), 115.3 (-C=CH2), 46.8 (C2), 38.4 (Ar-CH2), 31.2 (C3), 24.4 (Ar-CH3), 20.9 (Ar-CH.CH3); MS : m/z 186 (M+); Anal. Calcd. for C11H12N2O2: C, 83.87; H, 8.60; N, 7.53. Found: C, 83.91; H, 8.58; N, 7.50.
5-Methyl-7-(3-methyl-2-butenyl)indoline (2h): The compound was obtained from 1h at 140 -1500C in 78 % yield as yellow oil; IR (neat) : 3380, 3045, 2970, 2850, 1670, 1625, 1455, 1260, 835 cm-1; 1H NMR : δ : 3.10 (t, J=8.2-8.4 Hz 2H, C3-H), 3.43 (dd, J = 6.1 Hz, J =1.2 Hz 2H, -CH2-C=C), 3.50- 3.60 ( m, 3H, NH and C2 H), 5.24 (t, J = 2.1 Hz 1H, -C-CH=C), 1.91 [s, 6H, -C-C=C(CH3)2], 6.61-7.16 (m, 2H, Ar-H), 2.50 (s, 3H, Ar-CH3); 13C NMR : δ 146.6 (C8), 131.8 [=C(CH3)2], 127.6 (C6), 127.4 (C9), 126.8 (C4), 125.4 (C5), 122.6 [-CH=C.(CH3)2], 121.8 (C7), 47.1 (C2), 32.6 (Ar-CH2), 30.8 (C3), 24.6 [-C-C=C(CH3)2], 23.8 (Ar-CH3), 18.8 [-C-C=C(CH3)2]; MS : m/z 201 (M+); Anal. Calcd for C11H12N2O2: C, 83.58; H, 9.45; N, 6.97. Found: C, 83.60; H, 9.44; N, 6.99.
1,2,5,6-Tetrahydro-4,4,8-trimethyl-4H-pyrrolo[3,2,1-ij]quinoline (3): The compound was obtained from 1h at 140 -1500C in 15 % yield as pale yellow oil; IR (neat) : 3380, 3050, 2875, 2860, 1670, 1630, 1460, 1250 cm-1; 1H NMR : δ 2.93 (t, J = 8.3 Hz, 2H, C3-H), 2.82-2.89 (m 2H, -CH2-C-C), 3.58 ( t, J = 8.1 Hz 2H, C2- H), 2.62 (t, J = 8 Hz 2H, -C-CH2-C), 1.07 [s, 6H, -C-C-C(CH3)2], 6.49-6.86 (m, 2H, Ar-H), 2.58 (s, 3H, Ar-CH3); 13C NMR : δ 145.7 (9b), 127.6 (9a), 127.7 (C7), 126.3 (C9), 126.8 (C5), 121.9 (6a), 47.9 (C2), 29.3 (C3), 28.9 (C6), 24.9 [-C-C=C(CH3)2], 24.9 [-C-C=C(CH3)2], 24.1 (Ar-CH3), 22.0 (C5); MS : m/z 201 (M+); Anal. Calcd for C11H12N2O2: C, 83.58; H, 9.45; N, 6.97. Found: C, 83.56; H, 9.41; N, 6.96.
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© 2019 Seema Jain, published by De Gruyter
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