Mannich-Type Reactions of Aldimines and Hetero Diels-Alder Reactions of Aldehydes Catalyzed by Anion-Type Lewis Bases Derived from a Single Molecule

Kaori Ishimaru, Daiki Maeda, Kaori Ono, Yuya Tanimura

Department of Chemistry, National Defense Academy, Hashirimizu, Yokosuka, Japan.

DOI: 10.4236/ijoc.2012.23028                

Abstract

Mannich-type reactions of aldimines with silyl enolates and hetero Diels-Alder reactions of aldehydes with Danishef-sky’s diene in the presence of anion catalysts derived from proline were performed to afford the corresponding products in high yields.

Keywords

Anion Catalyst; Lewis Base; Proline; Mannich-Type Reaction; Hetero Diels-Alder Reaction

1. Introduction

Organocatalytic reactions using natural amino acids such as proline or its derivatives have recently received much attention in organic synthesis [1-9].Despite the many reports on proline-derived catalysts, the generation of iminium ions or enamine intermediates was necessary in most reactions [10-30]. Unlike these catalysts, we envisioned that anion-type Lewis base catalysts (Scheme 1) prepared from a single molecule, i.e., proline, would have a wide range of Lewis basicities and activate the various silyloxy compounds.

Yamaguchi et al. have first reported the enantioselective Michael addition of a simple malonate to enones and enals in the presence of rubidium salt of proline [31-34]. Recent reports have shown that the simple anion catalysts were useful for various reactions [35-50] including Mannich-type reactions [51-55] and hetero Diels-Alder reactions [56]. The combination of proline and amine also has been developed [57-58], however, the anioncatalyzed reactions using proline-derivedcompounds are still challenging. Here we report two different reactions (the Mannich-type reactions and hetero Diels-Alder reactions) using the anion-type Lewis base catalysts derived from a single molecule.

2. Results and Discussion

2.1. Mannich-Type Reactions of Aldimines

We first prepared various proline-derived compounds1a- 1c according to the literature [59-61]. In our initial studies, the Mannich-type reaction of aldimine 2a and silyl enolate 3a was performed in DMF solution using lithium salt of 1a as a Lewis base catalyst which was prepared from 1a and MeLi in THF [50] just before use (Table 1). The use of 25 mol% of 1a and 25 mol% of MeLi resulted in low yields of the corresponding β-amino ester 4a (entry 1 in Table 1). Since a small excess of MeLi would decompose the product, 0.5 equiv of MeLi relative to the proline-derived compound was used for the reaction (entries 2 - 6). We found that 2.5 mol% of Lewis base derived from 1b was effective to afford the corresponding β-amino ester in 74% yield (entry 5). However, attempt to use aldimine 2e having p-chlorophenyl group gave <20% yield of the product under the same conditions,

Scheme 1. Anion-type Lewis base catalysts derived from a single molecule.

Table 1. Mannich-type reaction of aldimine 2a and silyl enolate 3a in the presence of Lewis base catalysts^a.

indicating that the reaction depended on the substituent of the aldimines. After some experiments, 30 mol% of the anion catalyst 1c in the reaction of 2a with 3a also gave a high yield of the product 4a (entry 1 in Table 2). Under these optimized conditions, the Mannich-type reactions of various aldimines (2b - 2e) were carried out (entries 2 - 5). To our delight, the reaction of 2e also proceeded in high yield (entry 5). Electron-poor (entries 4 and 5) and electron-rich (entry 2) aromatic aldehydes reacted with equal facility. We also carried out the reaction with 3b having a phenyl group to afford corresponding β-amino esters in high yields (entries 6 - 8 in Table 2).

2.2. Hetero Diels-Alder Reactions of Aromatic Aldehydes

With these results in hand, we next examined the hetero Diels-Alder reaction of aromatic aldehyde and Danishefsky’s diene (Table 3) using the anion-type catalysts derived from the same molecule (proline), in which the substrates were quite different from those in Mannich-type reactions. After some experiments, the anion catalyst prepared from 1b and BuLi was suitable for the reaction. We found that 20 mol% of the catalyst promoted the reaction of benzaldehyde in 84 % yield (entry 1), however, the low yield was observed with o-tolualdehyde. Additional experiments were performed and the use of 30 mol% of the catalyst has been successfully applied for the substituted benzaldehydes (entries 2-5). The reaction of p-anisaldehyde having an electron-donating group gave moderate yield of the product

Table 2. Mannich-type reactions of aldimines with silyl enolates in the presence of Lewis base catalyst derived from 1c^a.

Table 3. Hetero diels-alder reactions of aromatic aldehydes with danishefsky’s diene^a.

(entry 6). In all cases, addition of water resulted in low yields of products with starting materials.

3. Conclusion

In conclusion, Mannich-type reactions of aldimines with silyl enolates and hetero Diels-Alder reactions of aldehydes in the presence of anion catalysts derived from proline were performed to afford the corresponding products in high yields. Further applications using the various anions of the proline are now in progress.

4. Experimental

4.1. General

All reactions were carried out under an inert atmosphere and in dried glassware. Anhydrous THF and DMF were used for the all reactions. Flash column chromatography was performed on silica gel (particle size 0.063 - 0.200 mm, Merck silica gel 60). The ¹H NMR spectra were recorded with a JEOL JNM-AL300 BK1 spectrometer at 300 MHz with chemical shift values (d) reported in ppm relative to an internal standard (TMS). High resolution mass spectra were measured with a JEOL SX-102A spectrometer.

4.2. Typical Experimental Procedure for Mannich-Type Reactions

4.2.1. General Procedure for Preparation of N-Benzylproline Lithium Salt.

To a stirred solution of 1c (0.64 g, 3.1 mmol) in THF (25 ml) was slowly added 1.55 mmol of MeLi (1.09 M diethylether solution) at 0˚C under Ar. The reaction mixture was stirred at 30˚C for 20 min and cooled to r.t. The catalyst solution was used without further purification.

4.2.2. General Procedure for Mannich-Type Reactions

The catalyst solution prepared as mentioned above (5.1 ml, 0.3 mmol) was transferred to a two-necked flask, and THF was evaporated in vacuo. Dry DMF (1 ml) was added to the flask, and 1 mmol of aldimine in dry DMF (1 ml) and silyl enolate (1.5 mmol) were added successively at r.t. The reaction mixture was stirred at r.t. for 12 h, and quenched with saturated aqueous NH₄Cl. The mixture was extracted with AcOEt, and the organic layers were dried over sodium sulfate. After filtration, the solvent was evaporated to give the crude product. The crude product was purified by flash column chromatography (hexane:EtOAc:CH₂Cl₂ = 3:1:1).

4.3. Identification of the Products

Methyl 2,2-Dimethyl-3-(2-tolyl)-3-(tosylamino) propanoate (4c). Colorless oil; ¹H NMR (300 MHz, CDCl₃) δ = 1.06 (s, 3H), 1.35 (s, 3H), 2.27 (s, 6H), 3.65 (s, 3H) 4.72 (d, 1H, J = 9.2 Hz), 6.24 (d, 1H, J = 9.2 Hz), 6.82 - 6.99 (m, 6H), 7.27 - 7.35 (m, 2H); HRMS-FAB(M + H)⁺m/z calcd for C₂₀H₂₆O₄NS 376.1599, found 376.1663.

Methyl 3-(4-Bromophenyl)-2,2-dimethyl-3-(tosylamino) propanoate (4d).

Colorless oil; ¹H NMR (300 MHz, CDCl₃) δ = 1.06 (s, 3H), 1.31 (s, 3H), 2.33 (s, 3H), 3.61(s, 3H), 4.31 (d, 1H, J = 9.5 Hz), 6.27 (d, 1H, J = 9.5 Hz), 6.77 (d, 2H, J = 8.4 Hz), 7.00 (d, 2H, J = 8.1 Hz), 7.13 - 7.16 (m, 2H), 7.38 (d, 2H, J = 8.4 Hz); HRMSFAB(M+H)⁺m/z calcd for C₁₉H₂₃O₄NSBr 440.0549, found 440.0560.

Phenyl 2,2-Dimethyl-3-phenyl-3-(tosylamino) propanoate (4f).

Colorless oil; ¹H NMR (300 MHz, CDCl₃) δ = 1.17 (s, 3H), 1.36 (s, 3H), 2.16 (s, 3H), 4.54 (d, 1H, J = 10.3 Hz), 6.18 (d, 1H, J = 10.3 Hz), 6.80 - 7.32 (m, 14H); HRMS-FAB(M+H)⁺m/z calcd for C₂₄H₂₆O₄NS 424.1599, found 424.1614.

Phenyl 3-(4-Methoxyphenyl)-2,2-dimethyl-3- (tosylamino)propanoate (4g).

Colorless oil; ¹H NMR (300 MHz, CDCl₃) δ = 1.16 (s, 3H), 1.27 (s, 3H), 2.15 (s, 3H), 3.62 (s, 3H), 4.57 (d, 1H, J = 10.4 Hz), 6.36 (d, 1H, J = 10.4 Hz), 6.44 - 6.45 (m, 2H), 6.47-6.48 (m, 4H), 6.94-6.95 (m, 2H), 7.13 - 7.17 (m, 1H), 7.23 - 7.34 (m, 4H); HRMS-FAB(M+H)⁺m/z calcd for C₂₅H₂₈O₅NS 454.1705, found 454.1723.

Phenyl 2,2-Dimethyl-3-(2-tolyl)-3-(tosylamino)propanoate (4h).

Colorless oil; ¹H NMR (300 MHz, CDCl₃) δ = 1.23 (s, 3H), 1.46 (s, 3H), 2.23 (s, 3H), 2.31 (s, 3H), 4.99 (d, 1H, J = 9.8 Hz), 6.29 (d, 1H, J = 9.8 Hz), 6.85 - 7.03 (m, 8H), 7.21 - 7.40 (m, 5H); HRMS-FAB(M+H)⁺ m/z calcd for C₂₅H₂₈O₄NS 438.1756, found 438.1764.

4.4. Typical Experimental Procedure for Hetero Diels-Alder Reactions

4.4.1. General Procedure for Preparation of Anion Catalysts for Hetero Diels-Alder Reaction

To a stirred solution of 1b (0.17 g, 0.9 mmol) in THF (2 ml) was slowly added 0.45 mmol of n-BuLi (1.6 M nhexane solution) at 0°C under Ar. The reaction mixture was stirred at r.t. for 15 min and the solvent was evaporated in vacuo. Dry DMF (3 ml) was added to the flask under Ar, and the catalyst solution was used without further purification.

4.4.2. General Procedure for Hetero Diels-Alder Reactions

The catalyst solution prepared as mentioned above (1 ml, 0.15 mmol) was transferred to a two-necked flask under Ar. To the stirred solution, DMF (2 ml), 0.5 mmol of aldehyde and Danishefsky’s diene (1 mmol) were added successively at r.t. The reaction mixture was stirred at r.t. for 12 h, and quenched with saturated aqueous NH₄Cl. The mixture was extracted with AcOEt, and the organic layers were washed with water and brine, and dried over sodium sulfate. After filtration, the solvent was evaporated. To the crude mixture, diethylether (5 ml) and trifluoroacetic acid (0.45 ml) were added. The reaction mixture was stirred at r.t. for 15 min and quenched with saturated aqueous NaHCO₃. The mixture was extracted with AcOEt and the organic layers were dried over sodium sulfate. After filtration, the solvent was evaporated to give the crude product. The crude product was purified by flash column chromatography (hexane:EtOAc = 10:1). The spectral data of the products were consistent with that in ref. [62] for 4c and 4e, ref. [63] for 4a, 4d, and 4f, and ref. [64] for 4b.

NOTES

Conflicts of Interest

The authors declare no conflicts of interest.

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