1,1,2,2-Tetraphenyl-1,2-ethanediol: A New Protecting Group for the Synthesis of Cyclopropylboronic Esters

Abstract

While enantiomerically pure auxiliaries and protecting groups for boronic acids are well established, much less attention has been paid to achiral derivatives. The commercially available benzpinacol (9) proved to yield highly stable boronic esters. The new derivatives contributed to the substrate-controlled diastereo­selective synthesis of cyclopropylboronic esters 20, 22, and 25.

asymmetric synthesis - diols - cyclopropanes - boron - protecting groups

References

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Experimental Procedure for the Synthesis of Alkenylboronic Ester 17: Under an atmosphere of dry nitrogen BH₃·SMe₂ complex (0.5 mL, 5 mmol) in THF (5 mL) was cooled to 0 °C. After addition of α-pinene (1.6 mL, 10 mmol) the reaction mixture was stirred for 1 h at 0 °C and 2 h at r.t. A colorless precipitate formed. After the mixture was cooled to -35 °C, the alkyne (650 mg, 5.2 mmol) was slowly added. The mixture was stirred at this temperature for 15 min, then allowed to warm to r.t. and continued to stir for 3 h. Acetaldehyde (5.5 mL, 100 mmol) was carefully added at 0 °C. The solution was refluxed over night and the excess aldehyde removed under reduced pressure. The residue was dissolved in THF (5 mL) and benzpinacol(9) (1.83 g, 5.0 mmol) was added. The solution was refluxed until TLC indicated no further transformation. The solvent was removed under reduced pressure and the crude product subjected to flash-column chromatography on silica gel, eluting with petroleum ether/diethyl ether (9:1). A colorless solid was isolated, yield: 1.07 g (2.1 mmol, 43%). Representative data: 17: [α]_D ²⁰ = +30.9 (c 0.97, CHCl₃), mp 71-74 °C. C₃₃H₃₁BO₄ (502.41): Calcd C, 78.89; H, 6.22. Found C, 78.83; H, 6.33. ¹H NMR (500 MHz, CDCl₃): δ(ppm) = 3.74 (dd, ² J _{5 ′′ a,5 ′′ b} = 9.3 Hz, ³ J _{4 ′′ ,5 ′′ a} = 7.8 Hz, 1 H, 5′′-H_a), 4.21 (dd, ² J _{5 ′′ a,5 ′′ b} = 9.3 Hz, ³ J _{4 ′′ ,5 ′′ b} = 6.4 Hz, 1 H, 5′′-H_b), 4.69 (ddd, ³ J _{4 ′′ ,5 ′′ a} = 7.8 Hz, ³ J _{4 ′′ ,5 ′′ b} = 6.4 Hz, ³ J _{2 ′ ,4 ′′} = 6.3 Hz, 1 H, 4′′-H), 6.15 (d, ³ J _{1 ′ ,2 ′} = 18.0 Hz, 1 H, 1′-H), 6.95 (dd, ³ J _{1 ′ ,2 ′} = 18.0 Hz, ³ J _{2 ′ ,4 ′′} = 6.3 Hz, 2′-H). ¹³C NMR (125 MHz, CDCl₃): δ(ppm) = 69.1 (C-5′′), 78.1 (C-4′′), 151.5 (C-2′). 21: [α]²⁰ _D = +7.7 (c 0.91, CHCl₃), mp
49-52 °C. C₃₆H₄₁BO₄Si (576.60): Calcd C, 74.99; H, 7.17. Found C, 74.82; H, 7.26. ¹H NMR (500 MHz, CDCl₃): δ(ppm) = 3.56 (dd, ² J _{4 ′ a,4 ′ b} = 10.0 Hz, ³ J _{3 ′ ,4 ′ a} = 7.2 Hz, 1 H, 4′-H_a), 3.79 (dd, ² J _{4 ′ a,4 ′ b} = 10.0 Hz, ³ J _{3 ′ ,4 ′ b} = 3.6 Hz, 1 H, 4′-H_b), 4.37 (m_c, 1 H, 3′-H), 6.15 (dd, ³ J _{1 ′ ,2 ′} = 18.0 Hz, ⁴ J _{1 ′ ,3 ′} = 1.7 Hz, 1 H, 1′-H), 6.95 (dd, ³ J _{1 ′ ,2 ′} = 18.0 Hz, ³ J _{2 ′ ,3 ′} = 4.6 Hz, 2′-H). ¹³C NMR (125 MHz, CDCl₃): δ(ppm) = 66.6 (C-4′), 73.7 (C-3′), 96.0 (CPh₂), ca. 118 (br, C-1′), 153.1 (C-2′). 22: [α]_D ²⁰ = -70.4 (c 1.05, CHCl₃), mp 66-70 °C. C₃₇H₄₃BO₄Si (590.63): Calcd C, 75.24; H, 7.34. Found C, 75.14; H, 7.45. ¹H NMR (500 MHz, CDCl₃): δ(ppm) = 0.23 (m_c, 1 H, 1′-H), 0.95 (m_c, 1 H, 3′-H_cis), 1.08 (ddd, ³ J _{2 ′ ,3 ′ - trans} = 7.8 Hz, ³ J _{1 ′ ,3 ′ - trans} = 6.2 Hz,
³ J _{3 ′ -cis,3 ′ - trans} = 3.6 Hz, 1 H, 3′-H _trans ), 1.38 (m_c, 1 H, 2′-H), 2.55 (d, ³ J _{OH,1 ′′} = 3.5 Hz, 1 H, OH), 3.16 (m_c, 1 H, 1′′-H), 3.43 (dd, ² J _{2 ′′ a,2 ′′ b} = 9.9 Hz, ³ J _{1 ′′ ,2 ′′ a} = 7.4 Hz, 1 H, 2′′-H_a), 3.80 (dd, ² J _{2 ′′ a,2 ′′ b} = 9.9 Hz, ³ J _{1 ′′ ,2 ′′ b} = 3.4 Hz, 1 H, 2′′-H_b).
¹³C NMR (125 MHz, CDCl₃): δ(ppm) = 9.1 (C-3′), 20.4
(C-2′), 67.0 (C-2′′), 75.9 (C-1′′), 95.8 (CPh₂).

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