Abstract
A highly regio- and chemoselective reductive cleavage of benzylidene acetals derived from 1,2- and 1,3-diols was achieved under mild conditions using EtAlCl2 -Et3 SiH reagent system in good to excellent yields. Labile protecting groups such as N -Boc, N -Cbz and -OTBDMS are found to be stable under the reaction conditions.
Key words
regioselectivity - chemoselectivity - reductive cleavage - benzylidene acetal - EtAlCl2
- Et3 SiH
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Representative Experimental Procedure of Compound 14: To a stirred solution of benzylidene acetal 13 (103 mg, 0.40 mmol) in anhyd CH2 Cl2 (5 mL) was added Et3 SiH (56 mg, 0.48 mmol). The reaction mixture was cooled to -78 °C, treated with EtAlCl2 (0.49 mL of 1.8 M solution in toluene, 0.88 mmol) dropwise, and the resultant mixture was stirred at -78 °C for 30 min. After completion of the reaction, as indicated by TLC, the reaction mixture was quenched with sat. solution of NaHCO3 (10 mL) and extracted with CH2 Cl2 (3 × 10 mL). The combined organic extracts were dried over anhyd Na2 SO4 , filtered and concentrated under reduced pressure to yield the crude compound, which was purified by column chromatography on silica gel (gradient elution with 50-60% EtOAc in hexane) to afford pure compound 14 (89 mg, 86% yield) as a colorless liquid. IR (neat): 3504, 3040, 2944, 1452, 1340, 1171, 1104, 972, 924, 803, 736, 697 cm-1 . 1 H NMR (400 MHz, CDCl3 ): δ = 2.70 (br s, 1 H), 3.08 (s, 3 H), 3.87-3.69 (m, 5 H), 4.83 (d, J = 12.2 Hz, 1 H), 4.89 (d, J = 12.2 Hz, 1 H), 7.39-7.28 (m, 5 H). 13 C NMR (100 MHz, CDCl3 ): δ = 38.4, 62.3, 69.2, 73.6, 127.8, 128.1, 128.4, 137.3.
