Tuning the Reactivity of O-tert-Butyldimethylsilylimidazolyl Aminals Towards Organolithium Reagents

Abstract

O-tert-Butyldimethylsilylimidazolyl aminals are N,O-acetals that form readily from aldehydes, and although they function as aldehyde stabilizing and protecting groups under various conditions, we report here that they react with organolithium reagents similarly to the parent aldehydes. The mechanism involves the intermediate formation of a 2-imidazolyl anion as is exemplified by the isolation of 2-TBDMS-imidazole. Substitution of the imidazolyl moiety at the 2-position renders these aldehyde derivatives stable to organolithium reagents, thus allowing for the tuning of their reactivity.

Key words

aldehydes - N,O-acetals - protecting groups - organometallic reagents - Brook rearrangements

References

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7

2d: Yield: 55%, white solid, mp (MeOH/H₂O): 264-265 °C. R_f (CH₂Cl₂/MeOH, 9:1): 0.34. ¹H NMR (300 MHz, CDCl₃) δ 0.07, 0.08, 0.12, 0.16 (s, 3 H each), 0.82 (s, 18 H), 1.33, 1.36 (d, 3 H each), 1.72-1.80 (m, 1 H), 2.02-2.10 (m, 1 H), 2.95 (hpt, 1 H), 4.13 (d, 1 H, J = 1.4 Hz, H-4′), 4.53 (d, 1 H, J = 4.9 Hz, H-3′), 5.74 (d, 1 H, J = 1.5 Hz, H-5′), 5.85 (dd,
1 H, J = 11.0 Hz, 4.4 Hz, H-1′), 7.14 (s, 2 H), 7.60 (s, 1 H), 7.71 (s, 1 H), 11.78 (br s, 1 H), 12.15 ppm (br s, 1 H). ¹³C NMR (125 MHz, CDCl₃) δ -5.2, -4.8, -4.4, -4.1, 18.3, 18.6, 19.1, 19.6, 25.9, 36.7, 38.7, 70.6, 81.6, 85.6, 89.8, 115.9, 123.0, 128.8, 139.3, 139.5, 148.3, 148.7, 156.1, 180.1 ppm. IR (KBr): 2922, 1708, 1680, 1610, 1548, 840 cm^-1. MS (ESI): 632.4 [M + H]⁺, 222.5 [B + H]⁺. Anal. Calcd for C₂₉H₄₉N₇O₅Si₂: C, 55.12; H, 7.82; N, 15.52; Found: C, 55.30; H, 7.81; N, 15.55. 2e: Yield 50%, white solid, R_f (CH₂Cl₂/MeOH, 9:1): 0.38. ¹H NMR (300 MHz, CDCl₃) δ -0.38, -0.28, 0.07, 0.09, 0.16, 0.18 (s, 3 H each) 0.74, 0.89, 0.91 (s, 9 H each) 1.32 (s, 3 H) 1.36 (s, 3 H) 2.92-3.00 (hpt, 1 H) 4.18 (d, 1 H, H-4′) 4.32 (d, 1 H, H-3′) 4.73 (s,1 H, H-2′) 5.46 (d, 2 H, H-5′) 5.70 (s, 1 H, H-1′) 7.18 (d, 2 H) 7.58 (s, 1 H) 7.73 (s, 1 H) 11.48 (br s, 1 H) 12.25 ppm (br s, 1 H). MS (ESI): 762.6 [M + H]⁺, 222.5 [B + H]⁺. Anal. Calcd for C₃₅H₆₃N₇O₆Si₃: C, 55.15; H, 8.33; N, 12.86; Found: C, 55.00; H, 8.30; N, 12.90.

14

3h′: Yield 27%, white foam, R_f (CHCl₃/MeOH, 9:1): 0.53. ¹H NMR (500 MHz, CDCl₃) δ 0.11 (s, 6 H), 0.93 (s, 9 H), 1.26 (s, 9 H), 1.29 (s, 3 H), 1.31 (s, 3 H), 2.18 (dd, 1 H, J = 13.2 Hz, 5.5 Hz), 2.71 (hpt, 1 H), 2.72-2.78 (m, 1 H), 3.37 (s, 1 H, H-5′), 4.23 (s, 1 H, H-4′), 4.54 (d, 1 H, J = 5.9 Hz, H-3′), 5.33 (br s, 1 H, OH), 6.18 (dd, 1 H, J = 9.1 Hz, 5.5 Hz, H-1′), 7.73 (s, 1 H), 8.70 (br s, 1 H), 12.05 ppm (br s, 1 H). ¹³C NMR (125 MHz, CDCl₃) δ -4.2, -4.1, 18.4. 19.1, 19.4, 26.2, 27.0, 35.7, 36.8, 41.1, 77.0, 80.3, 88.0, 88.5, 123.4, 138.9, 147.2, 147.7, 155.5, 178.8 ppm. MS (ESI): 508.5 [M + H]⁺, 222.5 [B + H]⁺. Anal. Calcd for C₂₄H₄₁N₅O₅Si: C, 56.78; H, 8.14; N, 13.79; Found: C, 56.68; H, 8.18; N, 13.83.
3h′′: Yield 28%, white foam, R_f (CHCl₃/MeOH, 9:1): 0.57. ¹H NMR (500 MHz, CDCl₃) δ 0.13 (s, 6 H), 0.93 (s, 9 H), 1.04 (s, 9 H), 1.26 (s, 3 H), 1.29 (s, 3 H), 2.25 (dd, 1 H, J = 13.2 Hz, 5.8 Hz), 2.70 (hpt, 1 H), 2.81-2.88 (m, 1 H), 3.57 (s, 1 H, H-5′), 4.29 (s, 1 H, H-4′), 4.76 (d, 1 H, J = 4.7 Hz, H-3′), 5.49 (br s, 1 H, OH), 6.18 (dd, H, J = 9.5 Hz, 5.9 Hz, H-1′), 7.77 (s, 1 H), 8.62 (br s, 1 H), 12.07 (br s, 1 H). ¹³C NMR (125 MHz, CDCl₃) δ -4.3, -3.6, 18.2, 19.0, 19.3, 26.1, 27.0, 34.2, 36.8, 42.1, 73.2, 80.7, 86.3, 91.4, 123.3, 139.2, 147.2, 147.7, 155.6, 178.7 ppm. MS (ESI): 508.5 [M + H]⁺, 222.5 [B + H]⁺.

22

5: Yield 82%, ¹H NMR (200 MHz, CDCl₃): δ 0.07 (s, 6 H), 1.25 (s, 9 H), 5.0 (br s, NH), 6.98 ppm (s, 2 H). ¹³C NMR (53.2 MHz, CDCl₃): δ 1.0 (CH₃), 25.9 (C), 30.3 (CH₃), 125.5 (CH), 135.7 ppm (C).

23

6b: Yield 92%, oil. R_f (CHCl₃/MeOH, 98:2): 0.36. ¹H NMR (200 MHz, CDCl₃): δ -0.13, 0.05 [s, 3 H each, Si(CH ₃)₂], 0.86 [s, 9 H, SiC(CH ₃)₃], 2.24 (s, 3 H, CH ₃), 3.72 (s, 3 H, OCH ₃), 6.45 (s, 1 H, CHO), 6.79 (d, 1 H, J = 8.7 Hz, Ar-H), 6.81 (s, 1 H, Imid-H), 6.87 (s, 1 H, Imid-H), 7.09 ppm (d, 2 H, J = 8.7 Hz, Ar-H). ¹³C NMR (53.2 MHz, CDCl₃): δ -5.0 (2 × CH₃)_, 18.2 (C), 25.8 (3 × CH₃), 36.6 (CH₃), 55.5 (CH₃), 80.6 (CH), 113.9 (C), 117.8 (C), 126.9 (2 × CH), 127.3 (2 × CH), 132.8 (C), 159.8 (C), 162.7 ppm (C). Anal. Calcd for C₁₈H₂₈N₂O₂Si: C, 65.02; H, 8.49; N, 8.42; Found: C, 65.22; H, 8.46; N, 8.45.
6c: Yield 84%, oil. ¹H NMR (200 MHz, CDCl₃): δ -0.04, 0.03 [s, 3 H each, Si(CH ₃)₂], 0.81 [s, 9 H, SiC(CH ₃)₃], 0.86 [s, 9 H, C(CH ₃)₃], 2.31 (s, 3 H, CH ₃), 5.06 (s, 1 H, CHO), 6.80 (s, 1 H, Imid-H), 6.90 ppm (s, 1 H, Imid-H). ¹³C NMR (53.2 MHz, CDCl₃): δ -3.4, -2.8 (each CH₃)_, 18.2 (C), 25.4 (3 × CH₃), 25.7 (3 × CH₃), 31.7 (C), 36.8 (CH₃), 86.4 (CH), 117.2 (CH), 121.2 (CH), 162.9 ppm (C).

24

Typical Procedure: To a solution of the protected aldehyde 2a-e (0.5 mmol) in dry THF (20 mL/mmol) was added the organolithium reagent (2 equiv, Table [1] ), drop-wise, at -78 °C. After stirring for 10 min at -78 °C the temperature was let to slowly increase to 0 °C. The reaction mixture was quenched with sat aq NaHCO₃ at 0 °C, diluted with ethyl acetate, the phases were separated and the organic phase was washed with water, dried over sodium sulfate, and concentrated under reduced pressure. The resulting crude 3a-h was purified by silica gel chromatography. Yields are reported in Table [1] .