trans-4-Hydroxy-l -prolinamide as an efficient catalyst for direct asymmetric aldol reaction of acetone with isatins

Tetrahedron: Asymmetry xxx (2016) xxx–xxx

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trans-4-Hydroxy-L-prolinamide as an efficient catalyst for direct asymmetric aldol reaction of acetone with isatins Geeta Devi Yadav, Surendra Singh ⇑ Department of Chemistry, University of Delhi, Delhi 110007, India

a r t i c l e

i n f o

Article history: Received 26 March 2015 Accepted 26 April 2015 Available online xxxx

a b s t r a c t Prolinamide (2S,4R)-4-hydroxy-N-((S)-1-phenylethyl)pyrrolidine-2-carboxamide was found to be an efficient organocatalyst (10 mol %) for the direct asymmetric aldol reactions of isatins with acetone at 35 °C in THF and afforded the product in 79% yield with 74% ee. We have generalised the methodology for the direct asymmetric aldol reaction between isatin derivatives and acetone, and the corresponding aldol products were obtained in high yields (up to 99%) and with moderate enantioselectivities (up to 80%). This method has been applied to the enantioselective synthesis of (S)-convolutamydine. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Enantioselective b-hydroxy ketones can be efficiently synthesised by asymmetric aldol reactions via the formation of enantioselective carbon–carbon bonds.1 Since the exploration of the proline catalysed cross-aldol reaction of ketones and aldehydes by List and Barbas,2 many amine derivatives have been applied to asymmetric cross-aldol reactions.3 Chiral amines such as pyrrolidine and primary amine based organocatalysts can often complement each other in their ability to activate different substrates4 and expand their application to a wide range of carbonyl compounds via enamine catalysis.5 The first asymmetric aldol reaction of isatin with acetone catalysed by dipeptide-based organocatalysts was reported by Tomasini et al. in 2005.6 3-Substituted-3-hydroxyindolin-2-ones have received much attention from organic and medicinal chemists because it exist as a key skeleton in natural products and drug candidates. The aldol addition of ketones to isatin and substituted isatins would appear to be a simple and suitable route towards important biological compounds.7,8 Proline,9 prolinamides,10–14 sulfonamides,15 chiral amines,16 quinidine thiourea,17 vicinal amino alcohol,18 enzymatic,19 4-hydroxydiarylprolinol20 and amino acid salts are used as organocatalysts for this reaction.21 We recently reported 4-hydroxy-(S)-prolinamide as a catalyst for asymmetric aldol reaction of ketones and aldehydes;22 N-aryl-L-prolinamides were used as catalysts for enantioselective aldol reactions of isatin and acetone.23 Herein, we extend the

⇑ Corresponding author. Tel.: +91 11 276 67794; fax: +91 11 276 66605. E-mail address: [email protected] (S. Singh).

catalytic activity of 4-hydroxy-(S)-prolinamide (Fig. 1) in enantioselective aldol reactions of isatin and acetone. 2. Results and discussion Prolinamides 1–6 were synthesised according to our previously reported procedure.22 These prolinamides were evaluated as organocatalysts for direct asymmetric aldol reactions of acetone and isatin. In our initial investigations, the catalytic activity of prolinamide 2 for aldol reactions of isatin 7a and acetone in different solvents was studied at 25 °C (Table 1). Polar solvents, such as H2O, DMF, EtOH and THF, gave excellent yields of the product 8a with 23–52% ee (Table 1, entries 1–4). We also used dichloromethane and chloroform as solvents which gave 92–94% yield but the ee was poor. The neat reaction also afforded product 8a with 84% yield but the ee was found to be at its lowest. Solvent THF was found to be the optimal choice which afforded the product in 99% yield with 52% ee after 36 h (Table 1, entry 4). The effect of the amount of acetone on the aldol reaction was investigated, and it was found that high amounts of acetone led to better yields and ee of the product 8a (Table 1, entries 4, 8 and 9). An additive can enhance the efficiency of the catalytic cycles by accelerating enamine formaton.24 We screened various acid additives, such as benzoic acid, p-nitrobenzoic acid, p-methoxybenzoic acid, p-nitrophenol, acetic acid, trifluoroacetic acid and trifluoromethanesulfonic acid (TfOH) for the reaction between acetone and isatin using a catalyst 2 at 25 °C in THF (Table 2, entries 1–7). We observed that none of them were found to be very effective in promoting the reaction in comparison to additive free conditions. Similar results have also been reported in the literature with phthalimido-prolinamide and N-arylprolinamide.13,23 The

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G. D. Yadav, S. Singh / Tetrahedron: Asymmetry xxx (2016) xxx–xxx O N H

HO

HO

O

HN

N H

1

O N H

HN

2

HO

O N H

3

O N H

HN

HO

O

HN

4

HN

N H

HN 6

5

Figure 1. Prolinamides 1–6 as organocatalysts for aldol reactions between isatin and acetone.

Table 1 Screening of the solvents and amount of acetonea O O HO

O O N H

prolinamide 2 (10 mol %) solvent, 25°C

7a

Entry

Solvent

1 2 3 4 5 6 7 8 9

H2O EtOH DMF THF DCM CHCl3 Neat THF THF

Time (h) 3 36 24 36 24 24 6 36 36

O N H 8a

Yieldb (%) 99 94 92 99 92 94 84 48 47

eec (%) 44 26 23 52 30 34 12 44d 40e

a Prolinamide 2 (10 mol %), isatin (0.3 mmol) and acetone (1 mL, 45.4 equiv) in a specified solvent (2 mL) stirred at 25 °C for the specified time. b Isolated yield after purification by column chromatography. c The ee was determined by HPLC using Chiralpak AD-H column and the absolute configuration of the product was found to be (S).6,21,23 d Acetone (0.5 mL, 22.7 equiv) was used. e Acetone (0.125 mL, 5.7 equiv) was used.

N-Phenyl-(S)-prolinamide 1 afforded product 8a in 43% yield and with 74% ee after 48 h while introduction of a hydroxyl group at the 4-poistion of prolidine ring of prolinamide 1, increased the yield up to 79% of the catalytic product 8a (Table 3, entries 1 and 2). The hydroxyl group enhanced the yield of the product with the appropriate bulkiness and absolute configuration of the amine may be due to intermolecular hydrogen bonding between molecules of trans-4-hydroxy-N-((S)-1-phenylethyl)pyrrolidine-2-carboxamide 2 and similar effect we have proposed for these catalyst in direct asymmetric aldol reaction between cyclohexanone and benzaldehydes in our earlier report.22 In order to determine the scope and limitations of this methodology, we tested prolinamide 2 for aldol reaction of a variety isatin derivatives and acetone in THF at 35 °C. The absolute configuration for all of the products was found to be (S).6,21,23 Substitution on the N-atom of the isatin molecule such as N-methyl, N-allyl and N-benzyl gave the corresponding aldol product in 72–91% yields and with lower ee than aldol product 8a. The halogen substituted on isatin at the 5-position also afforded the corresponding products in excellent yields; increasing the size of the halogens atom increased the ee (Table 4, entries 5–7). Electron withdrawing

Table 2 Effects of additive, temperature and catalyst loading on the direct aldol reaction of isatin 7a with acetonea O

effect of temperature on the reaction was investigated by varying the temperature from reflux to 35 °C. Decreasing the temperature improved the ee of product 8a, but the reaction time increased (Table 2, entries 8–10). We also varied the catalyst loading in the range of 2–20 mol % and the optimum catalyst loading was found to be 10 mol %, which is comparatively less than our previously reported N-arylprolinamide catalyst.23 Finally, we evaluated a variety of L-prolinamides 1–6 for aldol reactions between isatin 7a and acetone in THF at 35 °C. We believe that a small variation in the structural motif of the catalyst can greatly affect the enantioselectivity of the aldol product. We observed that the absolute configuration of product 8a (S) is governed by the (S)-proline while the additional enhancement in ee of the product is induced by the appropriate bulkiness and absolute configuration of the amine part of the prolinamide (Table 3, entries 1–6). The phenyl group of (2S,4R)-4-hydroxy-N-((S)-1-phenylethyl)pyrrolidine-2-carboxamide 2 was replaced by a more bulky naphthyl group, which decreased the ee and yield of product 8a (Table 3, entries 2 and 6). Prolinamide 4 derived from the less bulky amine (benzyl amine) gave the lowest ee for product 8a (Table 3, entry 4). Prolinamide 5 derived from a more bulky amine (diphenyl methyl amine) afforded the lowest yield of product 8a with 38% ee (Table 3, entry 5).

O O O N H 7a

Entry

Additive

1 2 3 4 5 6 7 8 9 10 11 12

PhCOOH p-OMePhCOOH p-NO2PhCOOH p-NO2-Phenol CH3COOH CF3COOH TfOH No additive No additive No additive No additive No additive

HO prolinamide 2 (10 mol %)

O

THF, additive, temperature

8a

Temperature (°C)

Time (h)

25 25 25 25 25 25 25 Reflux 25 35 35 35

9 9 9 6 6 4 4 12 36 48 48 36

Yieldb (%) 99 99 75 86 89 89 99 99 99 79 78 99

N H

eec (%) 29 37 19 40 34 40 31 36 52 74 50d 41e

a Prolinamide 2 (10 mol %), additive (10 mol %), isatin (0.3 mmol) and acetone (1 mL) in THF (2 mL) stirred at specified temp for specified time. b Isolated yield after purification by column chromatography. c The ee was determined by HPLC using chiralpak AD-H Column and the absolute configuration of the product was found to be (S). d Catalyst 2 mol % was used. e Catalyst 20 mol % was used.

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G. D. Yadav, S. Singh / Tetrahedron: Asymmetry xxx (2016) xxx–xxx Table 3 Evaluation of aldol reaction in different prolinamidea

3

4. Experimental O

4.1. General

O HO

O prolinamide 1-6 (10 mol %)

O N H

O

THF, -35oC, 48h

N H

7a

Entry

Catalyst

1 2 3 4 5 6

1 2 3 4 5 6

Yieldb (%)

8a

eec (%)

43 79 65 42 20 33

74 74 59 16 38 36

a Prolinamide (10 mol %), isatin (0.3 mmol), acetone (1 mL) stirred in THF (2 mL for 48 h. b Isolated yield after purification by column chromatography. c The ee was determined by HPLC using Chiralpak AD-H column and the absolute configuration of the product was found to be (S).

Table 4 Aldol reaction of derivatives of isatin with acetone catalyzed by prolinamide 2a O O R1

O prolinamide 2 (10 mol %)

O N R2

Entry

R

1 2 3 4 5 6 7 8 9 10

H H H H F Cl Br NO2 OMe Br

O

THF, -35o C

R3

1

HO

R1

N R2

R

2

H H H H H H H H H Br

R

3

H Me Bn All H H H H H H

Time 48 36 36 30 26 26 24 24 36 48

b

Yield (%) 79 91 72 72 97 97 97 99 79 92

R3

Isatin derivatives and acetone were purchased from commercial source and used as such. Proton and carbon nuclear magnetic resonance spectra (1H and 13C NMR, respectively) were recorded on 400 MHz (operating frequencies: 1H, 400.13 MHz; 13 C, 100.61 MHz) Jeol-FT-NMR spectrometers at ambient temperature. The chemical shifts (d) for all compounds are listed in parts per million (ppm) downfield from tetramethylsilane using the NMR solvent as an internal reference. The reference values used for deuterated chloroform (CDCl3) were 7.26 and 77.00 ppm for 1H and 13C NMR spectra, respectively. HRMS analysis was carried out using QSTAR XL Pro system microTOF-Q-II. Infrared spectra were recorded on a Perkin–Elmer FT-IR spectrometer. Thin layer chromatography was carried out using Merck Kieselgel 60 F254 silica gel plates. Column chromatography separations were performed using silica gel 230–400 mesh. The enantiomeric excess was determined on Shimadzu LC2010HT using chiralcel OD-H, chiralpak AD-H and IC columns. Optical rotations were taken using Rudolph digipol polarimeter. Organocatalysts 1–4 were synthesised according to our earlier report and their data also similar to the reported in literature.22 All unknown compounds were characterized by 1H, 13C NMR and HRMS and known compounds were characterized by 1H and 13C NMR. Copy of 1H NMR, 13C NMR and HPLC chromatogram are given in Supporting information. 4.2. Synthesis of catalysts 5 and 622

c

ee (%) 74 54 46 66 58 70 80 52 24 51

a Prolinamide 2 (10 mol %), isatin derivatives (0.3 mmol) and acetone (1 mL) in THF (2 mL) stirred at 35 °C for specified time. b Isolated yield after purification by column chromatography. c The ee was determined by HPLC using Chiralpak AD-H and Chiralcel OD-H and IC column and the absolute configuration of the products was found to be (S).6,21,23

groups (–NO2) at the 5-position of isatin were found to be more reactive, but the ee was lower (Table 4, entries 1 and 8). Electron donating groups (MeO–) in isatin afforded the product in 79% yield and with poor ee after 36 h (Table 4, entry 9). The aldol reaction of 5,7-di-bromoisatin afforded (S)-convolutamydines alkaloid in 92% yield and with 51% ee (Table 4, entry 10). 3. Conclusions In conclusion we have developed an organocatalyst for direct asymmetric aldol reactions between isatin derivatives and acetone. trans-4-Hydroxy-N-((S)-1-phenylethyl)pyrrolidine-2-carboxamide 2 (10 mol %) is sufficient to catalyse the reaction and afforded product 8a in 79% yield and with 74% ee. An hydroxyl group on the prolinamide enhanced the reactivity of the organocatalyst. The organocatalyst can be applied to isatin derivatives and afforded the corresponding (S)-3-hydroxy-3-(2-oxopropyl)indolin-2-one derivatives in 72–99% yields with 24–80% ee. Alkaloid (S)-convolutamydine was synthesised in excellent yields and with 51% ee.

4.2.1. (S)-N-Benzhydrylpyrrolidine-2-carboxamide 5 Compound (S)-tert-butyl 2-(benzhydrylcarbamoyl)pyrrolidine1-carboxylate (1.60 g, 4.21 mmol) was taken in dry dichloromethane (2.28 mL) and TFA (2.28 mL) was slowly added to this solution at 0 °C and stirred for 3 h at room temperature. The solution was then concentrated in vacuo, and dissolved in 10% NaOH (10 ml). The product was then extracted with ethyl acetate (3  10 ml), dried over MgSO4 and concentrated in vacuo to yield an oil, which was purified on silica gel in 10% MeOH/DCM, to give product 5 as an oil (1.04 g, 89%). [a]25 25.7 (c 0.605 in MeOH), IR D = (KBr): 3357, 2937, 1649, 1054 cm 1. 1H NMR (400 MHz, CDCl3): d = 8.44 (br s, 1H), 7.35–7.18 (m, 10H), 6.22–6.19 (m, 1H) 3.79– 3.76 (m, 1H), 3.02–2.86 (m, 2H), 2.16–1.89 (m, 2H), 1.75–1.66 (m, 2H) ppm. 13C NMR (100 MHz, CDCl3): d = 174.10, 141.86 (2C), 128.53 (2C), 128.47 (2C), 127.43 (2C), 127.25, 127.12, 127.00 (2C), 60.50, 55.92, 47.24, 30.70, 26.16 ppm. HRMS (ESI): m/z [M +H]+ calcd for C18H21N2O: 281.1653; found: 281.1660. 4.2.2. trans-4-Hydroxy-N-((S)-1-(naphthalen-1-yl)ethyl)pyrrolidine-2-carboxamide 6 Compound trans-butyl 2-((S)-1-phenylethylcarbamoyl)-4hydroxypyrrolidine-1-carboxylate (0.412 g, 1.07 mmol) was taken in dry dichloromethane (0.58 mL) with TFA (0.58 mL) using the same method as prolinamide 5 to give an oil. The oil was purified on silica gel using methanol:dichloromethane (10:90), yielding the product 6 as an oil (0.292 g, 96%). [a]25 24.0 (c 0.734 in MeOH), D = IR (KBr): 3435, 2918, 1665, 1378 cm 1. 1H NMR (400 MHz, CDCl3): d 8.40 (br s, 1H), 7.81–7.78 (m, 1H), 7.58–7.35 (m, 2H), 7.26–7.17 (m, 5H), 5.55–5.54 (m, 1H), 5.07 (br s 1H), 4.19–4.06 (m, 2H), 2.92–2.79 (m, 2H) 2.10–2.08 (m, 1H), 1.63–1.55 (m, 1H), 1.41– 1.33 (m, 3H), ppm. 13C NMR (100 MHz, CDCl3): d 170.20, 138.38, 133.18, 130.19, 128.21, 127.33, 125.73, 125.14, 124.85, 122.57, 121.89, 70.84, 58.37, 54.08, 49.26, 44.11, 20.77 ppm. HRMS (ESI): m/z [M+H]+ calcd for C17H21N2O2: 285.1603; found: 285.1610.

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4.3. General procedure for the aldol reaction of isatin with acetone Organocatalyst 2 (7 mg, 0.03 mmol) was stirred in 1 mL of acetone and 2 mL of THF for 10 min at 35 °C, after which isatin 7a (47 mg, 0.3 mmol) was added and the mixture was stirred for 48 h. The solvent was removed by rotavapor and the mixture was purified by flash column chromatography on silica gel (hexane/ethyl acetate (2:1)). 4.3.1. (S)-3-Hydroxy-3-(2-oxopropyl)indolin-2-one (Table 1, entry 1)6,21 [a]25 32.0 (c 0.39, MeOH) 74% ee. IR, (film): m = 3271, 2921, D = 1712, 1622, 1471 cm 1. 1H NMR (400 MHz, CDCl3): d = 9.50 (br s, 1H), 7.23–7.20 (m, 1H), 7.16–7.10 (m, 1H), 6.93–6.89 (m, 1H), 6.82–6.79 (m, 1H), 5.38 (br s, 1H), 3.18 (d, J = 16.78 Hz, 1H), 3.01 (d, J = 16.78 Hz, 1H), 2.06 (s, 3H) ppm. The ee was determined by HPLC (Chiralpak AD-H column, hexane/i-PrOH (80:20), flow rate 1 mL/min; tR (major) = 10.8 min; tR (minor) = 14.0 min, k = 254 nm). Acknowledgements S.S. acknowledges the financial assistance from Science and Engineering Research Board (SERB), Department of Science and Technology (DST), India, under scheme Fast Track Young Scientist (SB/FT/CS-020/2012) and University Science Instrumentation Center (USIC), University of Delhi, India for analytical data. Authors are thankful to DU-DST mass facility at USIC. G.D.Y. is thankful to CSIR for providing SRF. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetasy.2016.04. 008. References 1. (a) Mukaiyama, T. Org. React. 1982, 28, 203–331; (b) Guo, Q. S.; Bhanushali, M.; Zhao, C. G. Angew. Chem., Int. Ed. 2010, 49, 9460–9464.

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