References and Notes
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5 The relative configuration of the major diastereomers was assigned tentatively based on the assumption of the introduction of the nucleophile with inversion of the acetal carbon.
[4e]
[6]
6
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8 Treatment of 2-phenyl-1,3-propanediol and 1-naphthaldehyde (1.1 equiv) under the conventional reaction conditions (p-TsOH, refluxing) gave acetal 1e (trans:cis = 6:1) in quantitative yield. A pure trans isomer obtained by recrystallization from ethyl acetate and hexane was used in ring-cleavage reaction.
9 Acetal 1c with the electron-withdrawing p-chlorophenyl group showed slightly higher enantioselectivity. Electron-withdrawing character of the 1-naphthyl group relative to the phenyl group may also be responsible for the high selectivity observed for 1d.
10
Typical experimental procedure: To a solution of N-tosyl-l-(2-naphthyl)alanine (177 mg, 0.480 mmol) in CH2Cl2 (5.0 mL) at room temperature under argon was added dibromo(p-chlorophenyl)borane (72 µL, 0.48 mmol).
[4d]
After being stirred for 30 min, the mixture was concentrated in vacuo and the residue was dissolved in CH2Cl2 (0.5 mL). To the resulting solution of oxazaborolidinone 3g at -50 °C were added Et2O (0.12 mL) and a solution of acetal 1d (116 mg, 0.400 mmol) and silyl ketene acetal 2 (226 mg, 1.20 mmol) in CH2Cl2 (0.5 mL). After being stirred for 20 h at -50 °C, the mixture was quenched by the addition of aqueous NaHCO3 and filtered. The filtrate was extracted twice with ether. The organic layers were dried and concentrated in vacuo. The residue was treated with aqueous AcOH (70%, 2 mL) and THF (2 mL) at room temperature for 1 h. The mixture was diluted with water, extracted twice with ether, and washed with aqueous NaHCO3. The organic layers were dried and concentrated in vacuo. Purification of the residue by flash column chromatography (SiO2, 5-20% ethyl acetate in hexane) gave 147 mg (91% yield) of 4a: 1H NMR (500 MHz, CDCl3) δ 1.02 (3 H, s), 1.25 (3 H, s), 1.32 (3 H, t, J = 7.2 Hz), 2.20 (1 H, br), 3.12 (1 H, quint, J = 5.5 Hz), 3.58-3.64 (2 H, m), 3.84 (1 H, dd, J = 5.7 and 11.0 Hz), 4.03 (1 H, dd, J = 7.5 and 11.0 Hz), 4.20 (2 H, m), 5.70 (1 H, br), 7.15-7.30 (5 H, m), 7.47-7.56 (4 H, m), 7.83 (1 H, d, J = 8.3 Hz). 7.89 (1 H, m), 8.20 (1 H, m) [a minor diastereomer resonated at δ 3.91 (1 H, dd, J = 5.0 and 11.0 Hz)]; IR (liquid film): 3475(br), 1725 cm-1; HRMS (CI) calcd for C26H31O4 (MH+) 407.2222, found; 407.2219.
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