Development of α-Borylmethyl-(Z)-crotylboronate Reagent and Enantioselective Syntheses of (E)-δ-Hydroxymethyl-syn-homoallylic Alcohols via Highly Stereoselective Allylboration
Jiaming Liu, Shang Gao and Ming Chen*
*Department of Chemistry and Biochemistry, Auburn University, Auburn, Alabama 36849, United States, Email: mzc0102auburn.edu
J. Liu, S. Gao, M. Chen, Org. Lett., 2021, 23, 9451-9456.
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A chiral α-borylmethyl-(Z)-crotylboronate reagent enables highly stereo- and enantioselective syntheses of (E)-δ-hydroxymethyl-syn-homoallylic alcohols via aldehyde allylboration and a subsequent oxidative workup.
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proposed transition states
General procedure for the syntheses of homoallylic alcohols:
To a reaction flask containing a Teflon-coated magnetic stir bar were added freshly distilled aldehyde (0.1 mmol, 1.0 equiv, if it is a liquid) and toluene (0.3mL). After stirring at ambient temperature for 5 min, allylboronate 64 mg, 0.2 mmol, 2.0 equiv) was added to the reaction mixture via a microliter syringe. The mixture was kept stirring at ambient temperature for 12h. After complete consumption of the aldehyde, 3N NaOH (0.5 mL) was added to the reaction mixture followed by slow addition of 30% H2O2 (0.5 mL) at 0 °C. The reaction was vigorously stirred at 0°C for 3 h. Then ethyl acetate (1mL) and brine (1 mL) were added. The organic layer was separated and the aqueous layer was extracted with ethyl acetate (2 mL x 3). The combined organic extracts were concentrated under reduced pressure. The crude reaction product was dissolved in Et2O (1.0 mL); water (1.0 mL) and NaIO4 (171 mg, 0.8 mmol) were added. The resulting mixture was stirred at ambient temperature for 2 h; then Brine (1 mL) and ethyl acetate (1.0 mL) were added. The organic layer was separated and the aqueous layer was extracted with ethyl acetate (1 mLx 3). The combined organic layers were dried over anhydrous sodium sulfate, filtered, and concentrated under reduced pressure. Purification of the crude product was performed by flash column chromatography to give diol.
Syntheses of Unsymmetrical 1,4-Bifunctional Allylboron Reagents via Cu-catalyzed Highly Regio- and Stereoselective 1,4-Protoboration of Dienylboronates and Analysis of the Origin of Chemoselective Aldehyde syn-(Hydroxymethyl)allylation
S. Gao, M. Wang, M. Chen, Org. Lett., 2018, 20, 7921-7925.