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Org. Synth. 2010, 87, 143
DOI: 10.15227/orgsyn.087.0143
LITHIUM AMIDES AS HOMOCHIRAL AMMONIA EQUIVALENTS FOR CONJUGATE ADDITIONS TO α,β-UNSATURATED ESTERS: ASYMMETRIC SYNTHESIS OF (S)-β-LEUCINE
Submitted by Stephen G. Davies1, Ai M. Fletcher, and Paul M. Roberts.
Checked by David Hughes2
Discussion Addendum: Org. Synth. 2019, 96, 53
1. Procedure
A. t-Butyl (E)-4-methylpent-2-enoate (1). An oven-dried 1-L, three-necked, round-bottomed flask equipped with a 3-cm oval Teflon-coated stir bar is fitted with two septa on the outer necks and a reflux condenser on the center neck. The reflux condenser is fitted with a nitrogen gas inlet adapter connected to a nitrogen line and a gas bubbler. A thermocouple thermometer probe is inserted through one of the septa. The flask is allowed to cool under a flow of nitrogen. It is charged with t-butyl diethylphosphonoacetate (17.8 g, 70.4 mmol, uncorrected for purity, 1.0 equiv) (Note 1) followed by anhydrous tetrahydrofuran (150 mL) (Note 2) and cooled to 2 °C using an ice bath. A 2.93 M solution of methylmagnesium bromide in diethyl ether (25.3 g, 24.5 mL, 71.7 mmol, 1.02 equiv) (Note 3) is added via syringe to the stirred solution over a period of 15 min, resulting in an exotherm to 8 °C and the evolution of gas (gentle bubbling of the reaction mixture). After the addition is complete, stirring is continued at 0-5 °C for 30 min. Isobutyraldehyde (5.73 g, 79.5 mmol, 1.1 equiv) is added dropwise to the reaction mixture via syringe over 5 min, resulting in a slight exotherm from 2 °C to 6 °C. The ice-bath is replaced with a heating mantle and the mixture is heated to reflux (62 °C) for 2 h. The heating mantle is removed and the reaction flask is allowed to cool to 23 °C over 30 min (Note 4). The reflux condenser is replaced with a 250-mL pressure-equalizing dropping funnel, which is charged with saturated aqueous ammonium chloride solution (200 mL). The reaction mixture is quenched by the dropwise addition of the saturated aqueous ammonium chloride solution over a period of 10 min, with vigorous stirring being continued throughout (Note 5). After the addition is complete, diethyl ether (100 mL) is added and the mixture is transferred to a 1-L separatory funnel. The layers are separated and the aqueous layer is extracted with diethyl ether (2 × 100 mL). The combined organic layers are washed with saturated aqueous sodium chloride (100 mL), filtered through a bed of anhydrous sodium sulfate (50 g, rinsed with 2 × 75 mL diethyl ether) and concentrated by rotary evaporation (20 °C water bath, 100 mmHg, decreased to 20 mmHg) (Note 6) to give the crude reaction product 1 (20 g) as a colorless oil. The crude product is purified by chromatography on silica gel (Note 7) to afford t-butyl (E)-4-methylpent-2-enoate 1 (11.4 g, 95% yield) as a colorless oil having an E/Z ratio of 142:1 (Notes 8 and 9).
B. t-Butyl (3S,αR)-3-[N-benzyl-N-(α-methylbenzyl)amino]-4-methylpentanoate (2). An oven-dried 500-mL, three-necked, round-bottomed flask with a 3-cm oval Teflon-coated magnetic stir bar is fitted with a nitrogen gas inlet adapter connected to a nitrogen line and gas bubbler and is allowed to cool under a stream of nitrogen. The remaining two necks are fitted with septa. A thermocouple thermometer probe is inserted through one of the septa. The flask is charged with (R)-N-benzyl-N-(α-methylbenzyl)amine (20.1 g, 95 mmol, 1.6 equiv) (Note 10) and anhydrous tetrahydrofuran (200 mL) (Note 11) and cooled to -72 °C (internal temperature) using a dry-ice/acetone bath in a dish-shaped Dewar flask. A 2.37 M solution of n-butyllithium in hexanes (27.1 g, 39.1 mL, 93 mmol, 1.55 equiv) (Note 12) is added dropwise via syringe (Note 13) over 10 min. The temperature increases to -64 °C at the end of the addition (Note 14). After the addition is complete, stirring is continued at -75 to -70 °C for 30 min. t-Butyl (E)-4-methylpent-2-enoate 1 (10.1 g, 59.4 mmol, 1.0 equiv) is added dropwise by syringe (Note 13) over 15 min, keeping the internal temperature between -76 to -74 °C (Note 15). When the addition is complete, stirring is continued at -76 to -74 °C for 3 h (Note 16). The dry-ice bath is removed and the reaction mixture is allowed to warm to -50 °C over 20 min. The nitrogen gas adapter is replaced with a 100-mL pressure-equalizing dropping funnel and the gas adapter is moved to the top of the dropping funnel. The dropping funnel is charged with saturated aqueous ammonium chloride solution (60 mL). The reaction mixture is quenched by the dropwise addition of the saturated aqueous ammonium chloride solution over a period of 10 min, with vigorous stirring being continued throughout. The mixture warms to -25 °C during the quench (Note 17). The dark orange solution turns pale yellow and a white precipitate is formed. After the addition is complete, a water bath is used to warm the mixture to room temperature over 15 min. The mixture is diluted with saturated aqueous sodium chloride (60 mL) and stirred for 15 min. The white precipitate dissolves. The mixture is transferred to a 1-L separatory funnel. The layers are separated and the aqueous layer is extracted with diethyl ether (2 × 100 mL). The combined organic layers are concentrated by rotary evaporation (40 °C water bath, 150 mmHg, decreased to 20 mmHg) to provide 34 g of crude product (Note 18). The resultant pale yellow oil is dissolved in dichloromethane (300 mL) and washed sequentially with 0.5 M aqueous citric acid solution (2 × 200 mL) (Note 19), saturated aqueous sodium bicarbonate (200 mL), and saturated aqueous sodium chloride (200 mL). The organic layer is filtered through a bed of sodium sulfate (50 g, rinsed with 2 × 75 mL of dichloromethane) and the filtrate is concentrated by rotary evaporation (40 °C water bath, 150 mmHg, decreased to 20 mmHg) to give the crude reaction product 2 (22.2 g) as a viscous, pale yellow oil. The crude product is purified by chromatography on silica gel (Note 20) to afford t-butyl (3S,αR)-3-[N-benzyl-N-(α-methylbenzyl)amino]-4-methylpentanoate 2 (20.6 g, containing 2 wt% hexane by 1H NMR analysis, 89% corrected yield) as a viscous, colorless oil (Note 21).
C. t-Butyl (S)-3-amino-4-methylpentanoate (3). t-Butyl (3S,αR)-3-[N-benzyl-N-(α-methylbenzyl)amino]-4-methylpentanoate 2 (19.1 g, 50.0 mmol) and methanol (200 mL) (Note 22) are added to a 500-mL round-bottomed flask and the contents are swirled to completely dissolve the oil. The solution is transferred to a 500-mL borosilicate Parr bottle and 20% wet palladium hydroxide on carbon (5.0 g) is added. The bottle is connected to a Parr shaker hydrogenation apparatus and flushed by 3 vacuum purge/hydrogen fill cycles. The hydrogenation reaction is then carried out under 40 psi hydrogen pressure with shaking at 21-24 °C for 6 h (Note 23). The vessel is vented and 3 nitrogen fill/vacuum purge cycles are carried out to remove hydrogen from the solution. The solution is vacuum filtered through a bed of Solka-Floc (50 g) in a 350-mL medium porosity sintered glass funnel and washed with methanol (3 × 100 mL). The catalyst cake is kept wet with methanol throughout the filtration to minimize the potential for the palladium to ignite (Note 24). The hazy filtrate is transferred to a 1-L round bottomed flask and is concentrated by rotary evaporation (40 °C water bath, 20 mmHg) to give the crude reaction product 3 (9.6 g) as a pale yellow oil. The crude product is purified by chromatography on silica gel (Note 25) to afford t-butyl (S)-3-amino-4-methylpentanoate 3 (7.8 g, 83% yield) as pale yellow oil (Note 26).
D. (S)-3-Amino-4-methylpentanoic acid [(S)-β-leucine] (4). t-Butyl (S)-3-amino-4-methylpentanoate 3 (6.60 g, 35.2 mmol) and dichloromethane (15 mL) (Note 27) are added to a 250-mL round-bottomed flask equipped with a 2-cm Teflon-coated magnetic stir bar. The flask is fitted with a septum, through which a thermocouple temperature probe and an 18-gauge needle connected to a nitrogen line and gas bubbler are inserted. The solution is cooled to 5 °C (internal temperature) using an ice bath. Trifluoroacetic acid (15 mL) is added dropwise via syringe over a period of 10 min, resulting in a temperature rise to 9 °C. The cooling bath is removed and the reaction mixture is stirred at 21-23 °C for 13 h. The solution is concentrated by rotary evaporation (40 °C water bath, 20 mmHg) to give a pale orange residue. Toluene (50 mL) is added to the residue and the mixture is concentrated by rotary evaporation (60 °C water bath, 20 mmHg). A further 50 mL of toluene is added and the evaporation process is repeated to afford crude product as a gum (12 g) (Note 28). Diethyl ether (30 mL) and a 2-cm oval Teflon-coated magnetic stir bar are added to the flask. The flask is fitted with a septum through which a thermocouple temperature probe and an 18-gauge needle with an outlet to a nitrogen line and gas bubbler are inserted. A 2 M solution of hydrogen chloride in diethyl ether (30 mL) is added dropwise via syringe to the stirred solution over a 5 min period. The temperature rises to 35 °C and the mixture becomes a thick white slurry, which is stirred for 1 h at ambient temperature (35 °C initial, cooling in air to 23 °C). The stir bar is removed and the mixture is concentrated by rotary evaporation (40 °C water bath, 100 mmHg, decreased to 20 mmHg) to afford the crude hydrochloride salt of 4 (10.7 g) as a white solid (Note 29). The crude product is purified by ion-exchange resin chromatography (Note 30) to afford (S)-3-amino-4-methylpentanoic acid [(S)-β-leucine] 4 (4.06-4.30 g, 88-93% yield) as a tan crystalline solid (Notes 31-34).
2. Notes
1. t-Butyl diethylphosphonoacetate (95%) was obtained from Alfa Aesar. The submitters prepared it by heating t-butyl bromoacetate (Sigma-Aldrich, 98%) and triethylphosphite (Sigma-Aldrich, 98%) for 2 h without solvent, followed by distillation under vacuum.3
2. The following materials used in step A were obtained from Sigma-Aldrich: tetrahydrofuran (anhydrous, >99.9%, inhibitor-free), methylmagnesium bromide (3.0 M in diethyl ether), isobutyraldehyde (=99%), pentane (Chromasolv, >99%), diethyl ether (ACS reagent, anhydrous, BHT-inhibited), and silica gel (230-400 mesh, 60 Å). De-ionized tap water was used throughout.
3. Methylmagnesium bromide was titrated before use as follows: n-Butanol (350 mg, 4.72 mmol) and 1,10-phenanthroline (2 mg) are dissolved in tetrahydrofuran (7 mL) in a 25-mL round-bottomed flask equipped with a 0.5-cm oval Teflon-coated magnetic stir bar and a septum pierced with an 18-gauge nitrogen-inlet needle connected to a nitrogen line and gas bubbler. The stirred solution is cooled in an ice-bath, then 3 M methylmagnesium bromide in diethyl ether is added dropwise via a weighed syringe to a persistent pink endpoint. The syringe is weighed before and after addition (difference 1.67 g, 1.61 mL, corresponding to a solution molarity of 2.93). The submitters titrated against salicylaldehyde phenylhydrazone following the procedure described by Love and Jones.4
4. The reaction is >98% complete as determined by 1H NMR analysis as follows: A 0.1 mL aliquot of the reaction mixture is added to 1 mL of CDCl3 and 0.5 mL of saturated aqueous ammonium chloride, shaken, then the bottom organic phase is filtered through a cotton plug into an NMR tube for analysis. The doublet of the CH2 group at 2.9 ppm and the quartet of the CH2 group of the ethyl group at 4.2 ppm of the starting material compared to the olefinic resonances of the product at 5.7 and 6.8 ppm are diagnostic for reaction completion.
5. When ammonium chloride solution is first added to the reaction mixture a white precipitate is formed; this subsequently dissolves when addition of all the ammonium chloride solution is complete, giving a clear solution. The mixture warms to 30 °C during the quench with no external cooling.
6. The product 1 is volatile and the temperature of the water bath should not be increased above 20 °C while the solution is being concentrated on the rotary evaporator.
7. A 3-cm glass column is wet-packed (pentane) with 80 g of silica gel topped with 0.5 cm of sand. The crude reaction product 1 is loaded neat on the column and eluted with 500 mL of a 5:1 mixture of pentane:diethyl ether, taking 50 mL fractions. The chromatography is monitored by TLC (Rf = 0.3 in pentane:diethyl ether, 50:1). The product elutes in fractions 3-5. The eluent is concentrated by rotary evaporation (20 °C water bath, 150 mm Hg, decreased to 20 mm Hg) (Note 6) to constant weight. The pure product 1 contains 0.7 wt% residual tetrahydrofuran by 1H NMR analysis.
8. The geometric isomer purity of 1 is assessed by peak integration of the 13C-1H satellite peaks corresponding to C(2)H of the (E)-isomer (δH 5.68 ppm) against the 12C-1H peaks corresponding to C(2)H and C(3)H of the (Z)-isomer [δH 5.56 ppm (dd, J 11.5, 1.0 Hz) and 5.90 ppm (dd, J 11.5, 9.9 Hz), respectively] in the quantitative 1H NMR spectra of the crude reaction mixture and the pure product.5
9. t-Butyl (E)-4-methylpent-2-enoate 1 has the following physical and spectroscopic data: νmax (thin film) 2968, 2873, 1715, 1652, 1460, 1367, 1301, 1155 cm-1; 1H NMR pdf (400 MHz, CDCl3) δ: 1.04 (6 H, d, J = 6.8 Hz, C(4)Me2), 1.48 (9 H, s, CMe3), 2.40-2.45 (1 H, m, C(4)H), 5.68 (1 H, dd, J = 15.7, 1.5 Hz, C(2)H), 6.83 (1 H, dd, J = 15.7, 6.6 Hz, C(3)H); 13C NMR pdf (100 MHz, CDCl3) δ: 21.5 (C(4)Me2), 28.4 (CMe3), 31.0 (C(4)), 80.1 (CMe3), 120.5 (C(2)), 154.3 (C(3)), 166.6 (C(1)); MS (ESI+) m/z 193 ([M+Na]+, 100%); HRMS (ESI+) m/z calcd. for C10H18NaO2+ ([M+Na]+) 193.1199; found 193.1205.
10. (R)-N-Benzyl-N-(α-methylbenzyl)amine was obtained from Sigma-Aldrich and used without purification. The enantiomeric excess quoted by the vendor was 97.1%. The submitters prepared it from reductive alkylation of (R)-α-methylbenzylamine (Acros Organics, >99%, 99% ee).
11. The following materials used in step B were obtained from Sigma-Aldrich: tetrahydrofuran (anhydrous, >99.9%, inhibitor-free), 2.5 M butyllithium in hexanes, citric acid (ACS reagent grade, >99.5%), dichloromethane (ACS reagent grade, >99.5%), ethyl acetate (ACS reagent grade, >99.5%), and hexanes (ACS reagent grade, >98.5%). Ammonium chloride, sodium bicarbonate, and sodium sulfate were sourced from Fisher.
12. n-Butyllithium was titrated against diphenylacetic acid before use as follows: Diphenylacetic acid (0.766 g, 3.61 mmol) is dissolved in tetrahydrofuran (10 mL) in a 25-mL round-bottomed flask equipped with a 0.5 cm oval Teflon-coated magnetic stir bar and a septum pierced with an 18-gauge nitrogen-inlet needle connected to a nitrogen line and gas bubbler. The stirred solution is cooled in an ice-bath. n-Butyllithium is added dropwise via a weighed syringe until a yellow color persists. The syringe is weighed before and after addition (difference 1.05 g, 1.52 mL, corresponding to a solution molarity of 2.37).
13. The syringe is weighed before and after addition to determine the amount of n-butyllithium and t-butyl (E)-4-methylpent-2-enoate 1 added.
14. With the addition of the first few drops of n-butyllithium the reaction mixture turns from clear and colorless to clear and light pink, and darkens as further n-butyllithium solution is added.
15. With the addition of the first few drops of t-butyl (E)-4-methylpent-2-enoate 1 solution, the reaction mixture turns from dark pink to bright orange, which persists as further t-butyl (E)-4-methylpent-2-enoate 1 solution is added. The solution remains orange for the rest of the reaction.
16. The reaction is followed by 1H NMR as follows. A 0.1 mL reaction aliquot is quenched into a mixture of 1 mL of CDCl3 and 1 mL of saturated aqueous ammonium chloride. The organic layer is separated and filtered through a plug of sodium sulfate and cotton into an NMR tube. The olefin peaks at 5.7 and 6.8 ppm of the starting material are compared to the product peak at 3.2 ppm to assess reaction completion. After a 1.5 h reaction time, 13% starting material remained.
17. The submitters quenched the reaction at −78 °C after 2 h. In cases where the intermediate lithium β-amino enolate is unstable with respect to retro-conjugate addition at elevated temperatures, the quench should be performed at −78 °C.
18. 1H NMR analysis of the crude product indicated about 1% unreacted starting material.
19. The citric acid wash removes >95% of the unreacted (R)-N-benzyl-N-(α-methylbenzyl)amine.
20. A 3-cm glass column is wet-packed (hexanes) with silica gel (100 g) topped with 0.5 cm sand. The crude reaction product 2 is dissolved in dichloromethane (10 mL), loaded on the column, and eluted with 1 L of a 97:3 mixture of hexanes:ethyl acetate, taking 75 mL fractions. The chromatography is monitored by TLC (Rf = 0.5 in ethyl acetate:hexanes, 5:95). The product elutes in fractions 3-10. The eluent is concentrated by rotary evaporation (40 °C water bath, 20 mmHg) to constant weight.
21. t-Butyl (3S,αR)-3-[N-benzyl-N-(α-methylbenzyl)amino]-4-methyl-pentanoate 2 has the following physical and spectroscopic data: [α]22D -1.7 (c 2.0, chloroform); IR (thin film) νmax 3102, 3080, 2973, 1947, 1875, 1807, 1731, 1601, 1454, 1369, 950 cm-1; 1H NMR pdf (400 MHz, CDCl3) δ: 0.89 (3 H, d, J = 6.8 Hz, C(4)MeA), 1.12 (3 H, d, J = 6.8 Hz, C(4)MeB), 1.41 (3 H, d, J = 7.1 Hz, C(α)Me), 1.42 (9 H, s, CMe3), 1.67-1.73 (1 H, m, C(4)H), 1.81 (1 H, dd, J = 16.1, 2.0 Hz, C(2)HA), 1.97 (1 H, dd J = 16.1, 9.5 Hz, C(2)HB), 3.24-3.28 (1 H, m, C(3)H), 3.49 (1 H, d, J = 15.0 Hz, NCHA), 3.77 (1 H, d, J = 15.0 Hz, NCHB), 3.74-3.81 (1 H, m, C(α)H), 7.22-7.48 (10 H, m, Ph); 13C NMR pdf (100 MHz, CDCl3) δ: 19.8 (C(4)MeA), 20.4 (C(α)Me), 21.3 (C(4)MeB), 28.2 (CMe3), 33.0 (C(4)), 36.5 (C(2)), 51.4 (NCH2), 58.0 (C(3)), 58.2 (C(α)), 80.1 (CMe3), 126.8, 127.1 (p-Ph), 128.2, 128.4, 128.5 (2 degenerate peaks) (o-, m-Ph), 141.8, 142.1 (i-Ph), 172.6 (C(1)); MS (ESI+) m/z 382 ([M+H]+, 100%); HRMS (ESI+) calcd. for C25H36NO2+ ([M+H]+) 382.2741; found 382.2742. An analytically pure sample was prepared by dissolving 200 mg of the product oil in 5 mL of 50:50 pentane:diethyl ether, filtering through a 0.45 micron PTFE syringe filter, concentrating under vacuum, and vacuum drying at 50 °C for 20 h: Anal. calcd. for C25H35NO2: C, 78.70; H, 9.25; N, 3.67; found: C, 78.34; H, 9.15; N, 3.70.
22. The following materials used in step C were obtained from Sigma-Aldrich: methanol (ACS reagent grade, 99.8%), triethylamine (>99.5%, SureSeal bottle), ethyl acetate (ACS reagent grade, >99.5%), and hexanes (ACS reagent grade, >98.5%). 20% Palladium hydroxide (wet) on carbon was sourced from BASF.
23. The reaction was monitored by hydrogen pressure drop. Hydrogen uptake was complete in 1 h, but the hydrogenation was continued for 6 h to ensure complete conversion.
24. Once the cake is thoroughly rinsed with methanol to wash off all product, it is wetted with water and transferred as a water slurry to a PTFE bottle to hold the palladium waste for recycling.
25. A 3-cm glass column is wet-packed (1.5% triethylamine in hexanes) with silica gel (150 g) topped with 0.5 cm sea sand. The crude product is loaded neat to the column and the flask rinsed with dichloromethane (2 × 5 mL) to ensure complete transfer. The column is eluted as follows: (1) 400 mL of 1:3 ethyl acetate:hexanes containing 1.5% triethylamine, (2) 400 mL of 1:1 ethyl acetate:hexanes containing 1.5% triethylamine, (3) 400 mL ethyl acetate containing 2 % triethylamine, taking 50 mL fractions. The product 3 (Rf = 0.25 in 1:2 ethyl acetate:hexanes containing 1.5% triethylamine, visualized using both potassium permanganate (yellow spot on purple background) and iodine) is obtained in fractions 9-20, which are concentrated by rotary evaporation (40 °C water bath, 20 mmHg) in a 500 mL round-bottomed flask to constant weight (7.82 g, 83% yield). 1H NMR analysis indicated <1 % ethyl acetate in the product. Fractions 8 and 21-25 were combined and concentrated to constant weight (0.55 g). The purity of these fractions was assessed by 1H NMR as approx. 80% (0.44 g corrected for purity, 5% yield).
26. t-Butyl (S)-3-amino-4-methylpentanoate 3 has the following physical and spectroscopic data: [α]22D -24 (c 2.0, chloroform); IR (thin film) νmax 3387, 3321, 2964, 2933, 2874, 1727, 1597, 1466, 1392, 1367, 1152 cm-1; 1H NMR pdf (400 MHz, CDCl3) δ: 0.90 (3 H, d, J = 6.7 Hz, C(4)MeA), 0.91 (3 H, d, J = 6.7 Hz, C(4)MeB), 1.30 (2 H, br s, NH2), 1.45 (9 H, s, CMe3), 1.58-1.63 (1 H, m, C(4)H), 2.14 (1 H, dd, J = 15.3, 10.0 Hz, C(2)HA), 2.37 (1 H, dd J = 15.3, 3.5 Hz, C(2)HB), 2.98 (1 H, ddd, J = 10.0, 15.3, 3.5 Hz, C(3)H); 13C NMR pdf (100 MHz, CDCl3) δ: 17.9, 19.0 (C(4)Me2), 28.3 (CMe3), 33.5 (C(4)), 41.2 (C(2)), 53.8 (C(3)), 80.6 (CMe3), 172.6 (C(1)); MS (ESI+) m/z 210 ([M+Na]+, 100%); HRMS (ESI+) m/z calcd. for C10H21NNaO2+ ([M+Na]+) 210.1465; found 210.1469. An analytical sample is prepared by re-chromatographing a portion of the rich cut of the original chromatography, as follows: silica gel (15 g) is wet-packed (3:1 hexanes:ethyl acetate containing 1.5% triethylamine) in a 1-cm column topped with 0.5 cm sand. Compound 3 (0.53 g) is added neat to the column and is eluted sequentially with 30 mL 3:1 hexanes: ethyl acetate containing 1.5% triethylamine, 60 mL 1:1 hexanes: ethyl acetate containing 1.5% triethylamine, and 90 mL 1:2 hexanes: ethyl acetate, taking 10 mL fractions. Fractions 6 and 7 are combined, filtered through a 0.45 micron syringe filter and concentrated by rotary evaporation to constant weight (290 mg). Anal. calcd. for C10H21NO2: C, 64.13; H, 11.30; N, 7.48; found: C, 63.87; H, 11.20; N, 7.48.
27. The following materials used in step D were sourced from Sigma-Aldrich and used as received: trifluoroacetic acid (ReagentPlus, >99%), dichloromethane (ACS reagent grade, >99.5%), 2 M hydrogen chloride in diethyl ether, Dowex resin 50WX4-200, and racemic β-leucine. Ammonium hydroxide was sourced from Fisher.
28. 1H NMR analysis (CD3OD) indicated approx 8% unreacted starting material.
29. 1H NMR analysis (CD3OD) indicated approx 4% unreacted starting material and an equimolar quantity of diethyl ether.
30. A glass column (3 cm internal diameter) is wet-packed (water) with Dowex® 50WX4-200 ion-exchange resin (100 g). The column is equilibrated by sequential elution with water (200 mL), methanol (200 mL), water (200 mL), 1 M aqueous hydrogen chloride solution (200 mL) and water (500 mL). The crude reaction product 4 is diluted with distilled water (20 mL) and the resultant solution is loaded on to the column. The column is eluted with 100 mL of distilled water followed by 800 mL of 1 M aqueous ammonium hydroxide solution, taking 150 mL fractions. The fractions containing product are identified by spotting on a silica TLC plate and staining with potassium permanganate solution (yellow spot on purple background upon heating). Fractions 3-5 are concentrated by rotary evaporation (60 °C water bath, 20 mmHg) to constant weight in a 500-mL round-bottomed flask to afford 4.19-4.30 g (88-93% yield) of product 4. This material contained 0.5 wt% water by Karl Fisher titration. Fractions 2, 6, and 7 are combined and concentrated to afford 4.75 g of a white solid that contained no product by 1H NMR (peak at -75.5 ppm by 19F analysis indicates the presence of a trifluoromethyl group).
31. Due to lack of a chromophore and lack of volatility, the purity of 4 could not be assessed by HPLC or GC. Therefore, the weight percent purity of 4 recovered from the ion-exchange chromatography was determined by 1H NMR using both ethylene glycol and 1,2-dimethoxyethane as internal standards, as follows. β-Leucine (41.1 mg, 31.3 μmol) and 1,2-dimethoxyethane (67.1 mg, 74.5 μmol) are accurately weighed in a 5-mm NMR tube, then D2O (0.7 mL) is added. The 1H NMR analysis is carried out with a 5 second delay to ensure complete relaxation (10 second delay gave the same results, while a 0.1 second delay gave a 6% lower response for 1,2-dimethoxyethane). The dimethyl resonances (apparent triplet) of the product at 0.9 ppm are integrated vs. the 4 CH2 protons at 3.6 ppm for 1,2-dimethoxyethane. The average of several integrations gave a molar ratio of product:standard of 0.414 vs. the 0.420 ratio of the weighed samples, indicating a wt % of 98.6%. An additional weighing was carried out with 1,2-dimethoxyethane and 2 weighings and NMR analyses were carried out using ethylene glycol as standard, providing an average wt% assay of 98.4 ± 0.7% for material isolated directly from the ion-exchange chromatography.
32. A chiral HPLC assay was developed on the β-leucine N-p-toluenesulfonamide derivative 5. The derivatization procedure is as follows:
β-Leucine (108 mg, 0.82 mmol), p-toluenesulfonyl chloride (1.48 g, 7.8 mmol, 10 equiv), 2 N sodium hydroxide (7 mL, 14 mmol), toluene (7 mL), and a 1-cm oval Teflon-coated magnetic stir bar are added to a 50-mL round-bottomed flask sealed with a septum through which is inserted a thermocouple thermometer probe and an 18-gauge syringe needle connected to a nitrogen line and gas bubbler. The mixture is warmed to 55-60 °C using a heating mantle and vigorously stirred for 22 h at this temperature. The mixture is cooled, and transferred to a 50-mL separatory funnel along with water (5 mL) and toluene (5 mL). The layers are separated. 6 M aqueous hydrogen chloride (3 mL) is added to the aqueous layer. The mixture is transferred to a 50-mL separatory funnel and extracted with diethyl ether (2 × 15 mL). The organic layer is washed with water (10 mL), then filtered through a bed of magnesium sulfate and concentrated by rotary evaporation (40 °C water bath, 20 mmHg) to constant weight to afford (S)-3-[N-(p-toluenesulfonyl)amino]-4-methylpentanoic acid 5 (132 mg, 56% yield). This non-purified material is used to determine the optical purity of β-leucine using the chiral SFC method described below. A portion of 5 is recrystallized (90 mg dissolved in 3 mL toluene at 80 °C, cooled to ambient temperature, held for 15 h, filtered to afford 70 mg) to provide 5 with the following physical and spectroscopic data: mp 131-133 °C; 1H NMR pdf (400 MHz, CDCl3) δ: 0.87 (6 H, app t, J = 6.7 Hz, C(4)Me2), 1.88 (1 H, app octet, J = 6.8 Hz, C(4)H), 2.45 (1 H, dd, J = 16.2, 5.7 Hz, C(2)HA), 2.46 (3 H, s, CH3), 2.54 (1 H, dd, J = 16.2, 5.0 Hz, C(2)HB), 3.32-3.41 (1 H, m, C(3)H), 5.17 (1 H, d, J = 9.2 Hz, NH), 7.30-7.34 (2 H, m, Ar), 7.78-7.81 (2 H, m, Ar); 13C NMR pdf (100 MHz, CDCl3) δ: 18.6, 19.0 (C(4)Me2), 21.6 (ArMe), 31.6 (C(4)), 36.1 (C(2)), 56.0 (C(3)), 127.2, 129.7, 137.8, 143.5 (Ar), 175.8 (C(1)). The racemic derivative was similarly prepared and recrystallized from toluene, providing racemic 5 with mp 117-119 °C. A chiral supercritical fluid chromatography (SFC) method was developed for chiral analysis of the 4-toluenesulfonamide derivative (5): AD-H (250 × 4.6 mm, 5 μm) column, gradient method using methanol with 25mM i-butylamine, 4% methanol /CO2 for 4 min then ramp at 6%/min to 40% methanol /CO2, hold at 40% for 5 minutes, 3.0 mL/min, 200 bar, 35 °C, 230 nm, 15 minutes run time; minor enantiomer elutes at 7.3 min, major at 8.0 min. The enantiomeric excess of the derivatized, non-recrystallized 5 derived from β-leucine isolated directly from the resin column was 96% ee. Since the enantiomeric excess of (R)-N-benzyl-N-(α-methylbenzyl)amine used in the conjugate addition was 97%, the reaction diastereoselectivity of this step was 99%.
33. An analytically pure sample is prepared by recrystallization. β-Leucine 4 (1.04 g) is added to methanol (20 mL) in a 100-mL round bottomed flask with a 1-cm oval Teflon-coated stir bar and warmed in a 50 °C oil bath with stirring to dissolve the solids. The solution is quickly poured into a 50-mL syringe and hot filtered through a 0.45 micron PTFE syringe filter into a 100-mL round-bottomed flask. The solution is concentrated by rotary evaporation (40 °C water bath, 20 mmHg) to 5 mL. Partial crystallization occurs during this concentration step. After concentration, a 1-cm oval Teflon-coated stir bar is added to the flask, then t-butyl methyl ether (15 mL) is added at 22 °C over 20 min to the stirred mixture, resulting in further crystallization. After the addition is complete, the mixture is stirred for 1 h at 21-22 °C, then vacuum filtered through a 30-mL medium porosity sintered glass funnel, washed with t-butyl methyl ether (5 mL) and dried in a vacuum oven for 3 h at 60 °C to afford (S)-3-amino-4-methylpentanoic acid 4 (0.82 g, 79% yield) as a white crystalline powder. The ee of the recrystallized β-leucine is only marginally improved over the crude material (97 vs 96%).
34. (S)-3-Amino-4-methylpentanoic acid 4 purified by recrystallization has the following physical and spectroscopic data: ee 97%; mp 197-198 °C, lit.6a 182 °C, lit.6b 201-202.5 °C, lit.6c 202-210 °C, lit.6d 206 °C, lit.6e 212 °C; [α]D20 -53 (c 2.0, water), -52 (c 0.5, water), -40 (c 0.5, 1.N hydrochloric acid), lit.6a +40.3 [(R)-isomer, c 1, water], lit.6b +55.2 [(R)-isomer, c 1, water], lit.6c -39.2 [(S)-isomer, c 0.5, water], lit.6d +47 [(R)-isomer, c 1, water], lit.6e +52.3 [reported as (S)-isomer, but drawn as (R)-isomer, c 0.6 water], lit.6f +51.5 [reported as (S)-isomer, c 0.6, water]; IR (KBr) νmax 3397, 2965, 2936, 1649, 1621, 1467, 1326, 1262 cm-1; 1H NMR pdf (400 MHz, D2O) δ: 0.92 (6 H, app t, J = 7.0 Hz, C(4)Me2), 1.88 (1 H, app octet, J = 6.8 Hz, C(4)H), 2.33 (1 H, dd, J = 16.7, 9.3 Hz, C(2)HA), 2.50 (1 H, dd J = 16.7, 4.2 Hz, C(2)HB), 3.24-3.28 (1 H, m, C(3)H); 13C NMR pdf (100 MHz, D2O) δ: 17.3, 17.4 (C(4)Me2), 30.0 (C(4)), 36.0 (C(2)), 54.9 (C(3)), 178.5 (C(1)); m/z (ESI+) 154 ([M+Na]+, 100%); HRMS (ESI+) m/z calcd. for C6H13NNaO2+ ([M+Na]+) 154.0838; found 154.0841; Anal. calcd. for C6H13NO2: C, 54.94; H, 9.99; N, 10.68; found: C, 54.84; H, 9.95; N, 10.64.
Handling and Disposal of Hazardous Chemicals
The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.
These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.
3. Discussion
The conjugate addition reaction was first reported by Komnenos in 1883, who demonstrated the 1,4-addition of diethyl sodiomalonate to diethyl ethylidenemalonate.7 A range of carbon and heteroatom based nucleophiles have since been shown to participate in this reaction manifold. In 1991, Davies and Ichihara described the highly diastereoselective conjugate addition of lithium (R)-N-benzyl-N-(α-methylbenzyl)amide 6 to benzyl crotonate 12, which gave β-amino ester (3R,αR)-18 in 95% de. Global hydrogenolytic N-deprotection of β-amino ester mediated by Pearlman's catalyst [Pd(OH)2/C] proceeded under 5 atm of hydrogen to give the corresponding free β-amino acid, (R)-3-aminobutanoic acid, in quantitative yield and >95% ee.8 This methodology has since been developed into a generally applicable synthesis of either enantiomer of homochiral β-amino acids, via conjugate addition of homochiral lithium N-benzyl-N-(α-methylbenzyl)amide to an α,β-unsaturated t-butyl ester,9 followed by hydrogenolytic N-debenzylation and ester hydrolysis.10 A range of homochiral lithium amides that are readily derived from commercially available, homochiral α-methylbenzylamine derivatives has been developed, which allow for either differential N-deprotection or further elaboration in synthesis. All members of this family of lithium amides undergo highly diastereoselective conjugate addition to a wide range of α,β-unsaturated esters and amides to give the corresponding diastereo- and enantiomerically pure, homochiral β-amino ester or amide product (Table 1). This lithium amide conjugate addition methodology has been expanded to allow the stereoselective, in situ elaboration of the intermediate lithium (Z)-β-amino enolate to give access to homochiral α-substituted-β-amino esters; it has been employed for the synthesis of hundreds of β-amino esters, amides and acids in enantiomerically pure form, and has found utility in a plethora of synthetic applications, including total syntheses, initiation of tandem asymmetric processes, and molecular recognition phenomena. Its scope and utility was comprehensively reviewed in 2005.11
Table 1 Representative members of the lithium amide family 6-11 and α,β-unsaturated esters 12-17 for conjugate addition.
Table 1 Representative members of the lithium amide family 6-11 and α,β-unsaturated esters 12-17 for conjugate addition.
References and Notes
  1. Department of Chemistry, Chemistry Research Laboratory, University of Oxford, Mansfield Road, Oxford, OX1 3TA, UK. E-mail: .
  2. The checker would like to thank Zainab Pirzada for development of the chiral HPLC assay, Scott Hoerrner and Anthony Houck for assistance with the hydrogenation experiments, and Mirlinda Biba for determination of the specific rotations.
  3. For a representative experimental procedure see: Harwood, L. M.; Moody, C. J.; Percy, J. M. Experimental Organic Chemistry - Standard and Microscale, 1999, 2nd ed, Blackwell Publishing, p. 569-570.
  4. Love, B. E.; Jones, E. G. J. Org. Chem. 1999, 64, 3755-3756.
  5. Claridge, T. D. W.; Davies, S. G.; Polywka, M. E. C.; Roberts, P. M.; Russell, A. J.; Savory, E. D.; Smith, A. D. Org. Lett. 2008, 10, 5433-5436.
  6. (a) Enders, D.; Wahl, H.; Bettray, W. Angew. Chem. Int. Ed. 1995, 34, 455-457. Reported ee 98%. (b) Yamada, T.; Kuwata, S.; Watanabe, H. Tetrahedron Lett. 1978, 21, 1813-1816. (c) Balenovic, K.; Dvornik, D. J. Chem. Soc. 1954, 2976. (d) Callens, R.; Larcheveque, M.; Pousset, C.; Patent #FR2853315 (A1) August 10, 2004. (e) Okamoto, S.; Harada, T.; Tai, A. Bull. Chem. Soc. Jpn. 1979, 52, 2670-2673. (f) Evans, D. A.; Wu, L. D.; Wiener, J. J. M.; Johnson, J. S.; Ripin, D. H. B.; Tedrow, J. S. J. Org. Chem. 1999, 64, 6411-6417. Reported ee 98%. Designation of (S)-isomer for β-leucine having a positive rotation is inconsistent with other literature reports.
  7. Komnenos, T. Liebigs Ann. Chem. 1883, 218, 145-169.
  8. Davies, S. G.; Ichihara, O. Tetrahedron: Asymmetry 1991, 2, 183-186.
  9. Claridge, T. D. W.; Davies, S. G.; Lee, J. A.; Nicholson, R. L.; Roberts, P. M.; Russell, A. J.; Smith, A. D.; Toms, S. M. Org. Lett. 2008, 10, 5437-5440.
  10. (a) Davies, S. G.; Garrido, N. M.; Kruchinin, D.; Ichihara, O.; Kotchie, L. J.; Price, P. D.; Price Mortimer, A. J.; Russell, A. J.; Smith, A. D. Tetrahedron: Asymmetry 2006, 17, 1793-1811; (b) Davies, S. G.; Mulvaney, A. W.; Russell, A. J.; Smith, A. D. Tetrahedron: Asymmetry 2007, 18, 1554-1566.
  11. Davies, S. G.; Smith, A. D.; Price, P. D. Tetrahedron: Asymmetry 2005, 16, 2833-2891.
  12. Okamoto, S.; Iwakubo, M.; Kobayashi, K.; Sato, F. J. Am. Chem. Soc. 1997, 119, 6984-6990.
  13. Davies, S. G.; Haggitt, J. R.; Ichihara, O.; Kelly, R. J.; Leech, M. A.; Price Mortimer, A. J.; Roberts, P. M.; Smith, A. D. Org. Biomol. Chem. 2004, 2, 2630-2649.
  14. Davies, S. G.; Smyth, G. D. Tetrahedron: Asymmetry 1996, 7, 1005-1006.
  15. Bull, S. D.; Davies, S. G.; Delgado-Ballester, S.; Kelly, P. M.; Kotchie, L. J.; Gianotti, M.; Laderas, M.; Smith, A. D. J. Chem. Soc., Perkin Trans. 1 2001, 3112-3121.
  16. Ma, D.; Sun, H. Tetrahedron Lett. 2000, 41, 1947-1950.

Appendix
Chemical Abstracts Nomenclature (Collective Index Number);
(Registry Number)

t-Butyl diethylphosphonoacetate (27784-76-5)

Methylmagnesium bromide (75-16-1)

Isobutyraldehyde; 2-methylpropionaldehyde; 2-methylpropanal (78-84-2)

t-Butyl (E)-4-methylbut-2-enoate (87776-18-9)

(R)-α-Methylbenzylamine;
(R)-α-phenylethylamine;
(R)-1-phenylethylamine (3886-69-9)

(R)-N-Benzyl-N-(α-methylbenzyl)amine;
(R)-N-benzyl-α-phenylethylamine;
(R)-N-benzyl-1-phenylethylamine (38235-77-7)

Butyllithium; lithium-1-butanide (109-72-8)

t-Butyl 3-[N-benzyl-N-(α-methylbenzyl)amine]-4-methylpentanoate (911224-83-4)

Palladium hydroxide on carbon; Pearlman's catalyst (12135-22-7)

t-Butyl 3-amino-4-methylpentanoate (202072-47-7)

Trifluoroacetic acid (76-05-1)

(S)-3-Amino-4-methylpentanoic acid;
(S)-β-homovaline;
(S)-β-leucine (40469-85-0)

(S)-3-[N-(p-toluenesulfonyl)amino]-4-methylpentanoic acid (936012-07-6)

Steve Davies obtained a B.A. in Chemistry (1973) and D.Phil. in Organic Synthesis (1975) at the University of Oxford, following which he undertook postdoctoral fellowship positions with Professor M. L. H. Green at Oxford and with Professor Sir Derek Barton in France. He began his independent research career first as a member of the C.N.R.S. (1978-80) in France, followed by appointment as a Lecturer in Organic Chemistry (1980-96), Professor of Chemistry (1996-2006), and Chairman of Chemistry and Waynflete Professor of Chemistry (2006 to date) at Oxford. The development of novel and efficient methods for the production of enantiomerically pure compounds has formed the main focus of his research; highlights include stereoselective organometallic chemistry, the development of chiral relay networks, and homochiral ammonia equivalents.
Paul Roberts graduated with an M.Chem. from Jesus College, Oxford, in 2000, which was followed by a D.Phil. with Professor Steve Davies in the area of the asymmetric synthesis of piperidine alkaloids employing a ring closing metathesis approach. In 2005, he took up a post-doctoral position with Professor Davies at Oxford, where his research interests centre upon natural product synthesis and the development of new stereoselective methodologies, for example to effect the chemo- and stereoselective functionalisation of allylic amines with a range of electrophilic reagents.
After graduating from Keio University, Japan, in 2000, Ai (Matsuno) Fletcher studied for a Ph.D. at Imperial College London under supervision of Dr Chris Braddock with an O.R.S. award. During her Ph.D. she developed two novel methodologies: "one-pot" cascade catalysis of allylic isomerisation and olefin metathesis, and the cyclopropyl methyl silane terminated Prins reaction. Since completing her Ph.D. in 2004, she explored a range of chemistry, such as enantioselective synthesis of DNA analogues and palladium catalysis as a post-doctoral researcher at the University of Regensburg, and at the University of Bath. In 2007 she joined the group of Professor Steve Davies in Oxford, where she has been involved with the development of asymmetric synthetic methodology and novel ammonia-based energy technology.
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