Stereospecific Reaction of α-Carbamoyloxy-2-alkenylboronates and α-Carbamoyloxy-alkylboronates with Grignard Reagents - Synthesis of Highly Enantioenriched Secondary Alcohols

Abstract

Highly enantioenriched secondary alcohols were synthesized by treatment of α-carbamoyloxy-2-alkenylboronates and α-carbamoyloxy-alkylboronates with Grignard reagents. An intermediary boronate complex was transformed stereospecifically to the corresponding secondary 2-alkenyl- and alkylboronates by migration of an introduced residue. Oxidative workup furnished the enantioenriched secondary alcohols.

boron - asymmetric synthesis - alcohols - chirality - stereo­selectivity

References

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4

Beckmann, E.; Hoppe, D. Synthesis, submitted.

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6

Experimental Procedure: The amount of 1.315 g (5.0 mmol, 1.0 equiv) of 3 and 1.490 g (6.0 mmol, 1.2 equiv) of (-)-sparteine were dissolved in 10 mL of anhyd Et₂O and the mixture was cooled to -78 °C. To this mixture 5.0 mL (6.0 mmol, 1.2 equiv) of a 1.2 M solution of sec-butyllithium in cyclohexane were added slowly within 15 min with a syringe pump. After a deprotonation time of 5 h 1.410 g (7.5 mmol, 1.5 equiv) triisopropyl borate were added at -78 °C and the reaction mixture was stirred for an additional hour. The reaction was quenched with 5 mL of 2 M HCl at -78 °C and the layers were separated. The aqueous layer was extracted with Et₂O (3 × 5 mL), the combined organic extracts were dried with MgSO₄ and the solvent was removed in vacuum. The crude product was dissolved in 10 mL of CH₂Cl₂ and added to a flask charged with 885 mg (7.5 mmol, 1.5 equiv) of pinacol, 0.050 g of p-toluenesulfonic acid and MgSO₄. This mixture was stirred at r.t. for 24 h, the solid material was filtered off and the solvent was removed. After purification of the crude product by flash column chromatography (SiO₂, cyclohexane-EtOAc = 5:1) we obtained 1.788 g (4.5 mmol, 90%) of compound 5 as a colorless oil. The ee was determined by ¹H NMR shift experiment using 30 mol% of Eu(hfc)₃. R_f = 0.24 (Et₂O-pentane = 1:1); [α]_D ²⁰ +36.7 (c 0.97, CH₂Cl₂). ¹H NMR (300 MHz, CDCl₃): δ = 1.27 [s, 12 H, CH₃ (pinacol)], 1.29-1.35 [m, 12 H, CH₃ (Cb)], 1.94-2.19 (m, 2 H, CH₂), 2.75-2.98 (m, 2 H, CH₂), 3.85, 4.14 [sep, 2 H, CH(Cb), ³ J = 7.0 Hz)], 3.92 (dd, 1 H, CH, ³ J = 4.2 Hz, ³ J = 10.3 Hz), 7.21-7.38 [m, 5 H, CH(Ar)] ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 20.2, 20.3, 20.6 [CH_{3 (}pinacol) + CH₃(Cb)], 26.9 (CH₂), 33.3 (CH₂), 46.7, 48.4 [CH(Cb)], 79.8 (CH), 125.6 [C_q(pinacol)], 128.2, 128.5 [CH(Ar)], 142.5 [C_q(Ar)], 162.7 [C=O(Cb)] ppm. Anal. Calcd for C₂₂H₃₆BNO₄ (389.27): C, 67.87; H, 9.32; N, 3.60. Found: C, 67.76; H, 9.36; N, 3.85.

7 Matteson DS. Stereodirected Synthesis with Organoboranes   Springer; Berlin: 1995. 

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11

Experimental Procedure: The amount of 163 mg (0.5 mmol, 1.0 equiv) of 2 was dissolved in 5 mL of anhyd Et₂O and cooled to -78 °C. To this mixture 1.0 mmol (2.0 equiv) of the Grignard reagent as solution in Et₂O was slowly added. The mixture was stirred for 1 h at -78 °C and then warmed to r.t. for an additional hour. The solution was quickly filtered over ca. 5 g of silica gel with pentane and the solvent was removed carefully at 800 mbar to furnish the crude product 14, which was subjected to the subsequent reactions without further purification.

12

The crude product 14 was dissolved in 5.00 mL of THF. At r.t., 1.20 mL (0.6 mmol, 1.2 equiv) of 0.5 M NaOH was added dropwise and after 5 min, 0.07 mL (0.7 mmol, 1.4 equiv) H₂O₂ (35%) was added. The mixture was stirred for 30 min at r.t., then diluted with 5.00 mL of H₂O and the layers were separated. The aqueous layer was extracted with Et₂O (3 × 5 mL), the combined organic extracts were washed with sat. FeSO₄ solution to destroy the peroxides and then dried with MgSO₄. After removal of the solvent in vacuum the crude product was purified by flash column chromatography (SiO₂, Et₂O-pentane = 1:6) to furnish the alcohols 15 as colorless liquids. Compound 15a: [21] R_f = 0.44 (Et₂O-pentane = 1:1); [α]_D ²⁰ -8.3 (c 0.85, CHCl₃). The ee was determined by ¹H NMR shift experiment with 40 mol% of Eu(hfc)₃. Compound 15b: [22] R_f = 0.46 (Et₂O-pentane = 1:1); [α]_D ²⁰ -7.4 (c 0.76, CHCl₃). The ee was determined by chiral HPLC (column: chiragrom 2, 60 × 2 mm; solvent: i-PrOH-hexane = 1:600). Compound 15c: R_f = 0.52 (Et₂O-pentane = 1:1); [α]_D ²⁰ -11.8 (c 2.4, EtOH). The ee was determined by comparison of optical rotation. [14] Compound 15d: R_f = 0.59 (Et₂O-pentane = 1:1); [α]_D ²⁰ -3.3 (c 1.1, CHCl₃). ¹H NMR (300 MHz, CDCl₃): δ = 1.19-1.71 [m, 19 H, CH₂ (oct), CH(oct), CH₃, OH], 3.81 [t, 1 H, CH(OH), ³ J = 6.2 Hz], 5.46 (ddq, 1 H, CH, ⁴ J = 1.0 Hz, ³ J = 7.0 Hz, ³ J = 15.2 Hz), 5.63 (ddq, 1 H, CH) ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 19.5 (CH₃), 25.7, 29.6, 30.0, 37.8 [CH₂ (oct)], 42.3 [CH(oct)], 75.2 [CH(OH)], 127.0, 133.8 (CH=CH) ppm. Anal. Calcd for C₁₂H₂₂O (182.17): C, 79.06; H, 12.16. Found: C, 78.67; H, 12.44. The ee was determined by ¹H NMR shift experiment with 40 mol% of Eu(hfc)₃.

13a Hoffmann RW. Hölzer B. Knopff O. Org. Lett.  2001,  12:  1945 

13b Hoffmann RW. Hölzer B. Knopff O. Harms K. Angew. Chem. Int. Ed.  2000,  39:  3072 ; Angew. Chem. 2000, 112, 3206

14 Belelie JL. Chong JM. J. Org. Chem.  2001,  66:  5552 

15

The crude product 14 was dissolved in 5 mL of anhyd toluene and 106 mg (1.0 mmol, 2.0 equiv) of benzaldehyde were added. The mixture was heated to 60 °C for 12 h and then cooled to r.t. The toluene was removed in vacuo, the residue was dissolved in 5 mL of Et₂O, and 90 mg (0.6 mmol, 1.2 equiv) of triethanolamine were added. After one hour of stirring at r.t. the precipitate was filtered off and the solvent was removed. Flash column chromatography of the crude product (SiO₂, Et₂O-pentane = 1:5) furnished the homoallyl alcohols 17 as colorless liquids. The E:Z-ratios of the compounds were determined by ¹H NMR of the crude products. The ee were determined by HPLC using a chiral column (chiragrom 2, 250 × 2 mm) and the solvent mixture i-PrOH-hexane = 1:400. Compound 17a: E:Z = 80:20; R_f = 0.54 (Et₂O-pentane = 1:1); [α]_D ²⁰ +24.3 (c 0.67, CHCl₃). Compound 17b: E:Z = 25:75; R_f = 0.57 (Et₂O-pentane = 1:1); [α]_D ²⁰ +4.7 (c 0.50, CHCl₃). Compound 17c: E:Z = 33:67; R_f = 0.69 (Et₂O-pentane = 1:1); [α]_D ²⁰ +26.5 (c 1.12, CHCl₃). Compound 17d: E:Z = 89:11; R_f = 0.58 (Et₂O-pentane = 1:1); [α]_D ²⁰ +30.1 (c 1.00, CHCl₃). Compound 17e: E:Z = 28:72; R_f = 0.69 (Et₂O-pentane = 1:1); [α]_D ²⁰ +74.2 (c 1.21, CHCl₃). Compound (E)-17e: ¹H NMR (300 MHz, CDCl₃): δ = 0.83 (d, 3 H, CH₃, ³ J = 6.9 Hz), 1.01 (s, 9 H, CH₃), 2.17 (d, 1 H, OH, ³ J = 2.4 Hz), 3.36-3.49 [m, 1 H, CH(CH₃)], 4.27 [dd, 1 H, CH(OH), ³ J = 2.4 Hz, ³ J = 7.9 Hz], 5.25 (dd, 1 H, CH, ³ J = 8.6 Hz, ³ J = 15.9 Hz), 5.63 (d, 1 H, CH, ³ J = 15.9 Hz), 7.23-7.35 [m, 5H, CH(Ar)] ppm. Compound (Z)-17e: ¹H NMR (300 MHz, CDCl₃): δ = 0.77 (d, 3 H, CH₃, ³ J = 6.8 Hz), 1.14 (s, 9 H, CH₃), 2.14 (d, 1 H, OH, ³ J = 1.7 Hz), 2.97-3.10 [m, 1 H, CH(CH₃)], 4.23 [dd, 1 H, CH(OH), ³ J = 1.7 Hz, ³ J = 8.4 Hz], 5.09 (dd, 1 H, CH, ³ J = 11.0 Hz, ³ J = 11.9 Hz), 5.59 (d, 1 H, CH, ³ J = 11.9 Hz), 7.23-7.35 [m, 5 H, CH(Ar)] ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 17.6 [CH(CH₃)], 29.8, 31.6, 33.5 (CH₃), 41.2 [CH(CH₃)], 79.0 [CH(OH)], 127.0, 127.1, 127.7, 128.2, 130.5 [CH(Ar), CH], 142.4 (CH), 143.1 [C_q(Ar)] ppm. Anal. Calcd for C₁₅H₂₂O (218.17): C, 82.52; H, 10.16. Found: C, 82.17; H, 10.22.

16 Andersen MW. Hildebrandt B. Köster G. Hoffmann RW. Chem. Ber.  1989,  122:  1777 

17

Experimental Procedure: The amount of 195 mg (0.5 mmol, 1.0 equiv) of 5 was dissolved in 5.00 mL of anhyd Et₂O and the solution was cooled to -78 °C. To this mixture 1.0 mmol (2.0 equiv) of the Grignard reagent as solution in Et₂O was slowly added. The mixture was stirred for 1 h at -78 °C and then warmed to r.t. for an additional hour. The solution was quickly filtered over ca. 5 g of silica gel with pentane and the solvent was removed carefully at 800 mbar. The residue was dissolved in 5.00 mL of THF. At r.t. 1.20 mL (0.6 mmol, 1.2 equiv) of 0.5 M NaOH were added dropwise and after 5 min 0.07 mL (0.7 mmol, 1.4 equiv) H₂O₂ (35%) were added. The mixture was stirred for 30 min at r.t., then diluted with 5.00 mL of H₂O and the layers were separated. The aqueous layer was extracted with Et₂O (3 × 5 mL), the combined organic extracts were washed with sat. FeSO₄ solution to destroy the peroxides and then dried with MgSO₄. After removal of the solvent in vacuum the crude product was purified by flash column chromatography (SiO₂, Et₂O-pentane = 1:6) to furnish the alcohols 20. The ee were determined by HPLC using a chiral column (chiragrom 2, 250 × 2 mm) and the solvent mixture i-PrOH-hexane = 1:400. Compound 20a: [23] colorless liquid; R_f = 0.43 (Et₂O-pentane = 1:1); [α]_D ²⁰ +11.9 (c 1.00, CHCl₃). Compound 20b: [24] white solid; mp 96 °C; R_f = 0.53 (Et₂O-pentane = 1:1); [α]_D ²⁰ +28.7 (c 0.91, CHCl₃). Compound 20c: colorless liquid; R_f = 0.55 (Et₂O-pentane = 1:1); [α]_D ²⁰ +15.8 (c 0.92, CHCl₃). ¹H NMR (300 MHz, CDCl₃): δ = 1.29-1.82 [m, 16 H, CH_{2 (}oct), CH(oct), OH], 2.67 (ddd, 2 H, CH₂, ³ J = 7.0 Hz, ³ J = 9.4 Hz, ³ J = 13.5 Hz), 2.86 (ddd, 2 H, CH₂, ³ J = 5.7 Hz, ³ J = 9.4 Hz, ³ J = 13.5 Hz), 3.48 [m, 1 H, CH(OH)], 7.17-7.34 [m, 5 H, CH(Ar)] ppm. ¹³C NMR (75 MHz, CDCl₃): δ = 27.1, 27.5, 26.0, 30.0, 32.7 [CH₂(oct), CH₂], 36.47 (CH₂), 48.1 [CH(oct)], 75.0 [CH(OH)], 127.9, 128.4, 130.0 [CH(Ar)], 141.1 [C_q(Ar)] ppm. Anal. Calcd for C₁₇H₂₆O (246.20): C, 82.87; H, 10.64. Found: C, 82.51; H, 10.86. Compound 20d: [25] colorless liquid; R_f = 0.54 (Et₂O-pentane = 1:1); [α]_D ²⁰ +27.5 (c 0.85, CHCl₃). Compound 20e: [26] colorless liquid; R_f = 0.55 (Et₂O-pentane = 1:1); [α]_D ²⁰ +50.0 (c 1.15, CHCl₃).

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