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Synlett 2004(4): 647-650  
DOI: 10.1055/s-2004-817752
LETTER
© Georg Thieme Verlag Stuttgart · New York

A Highly Regio- and Chemoselective Reductive Cleavage of Benzylidene Acetals with EtAlCl2-Et3SiH

Vijayakrishnan Balakumar, Appu Aravind, Sundarababu Baskaran*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India
Fax: +91(44)22570545; e-Mail: sbhaskar@iitm.ac.in;
Further Information

Publication History

Received 5 December 2003
Publication Date:
10 February 2004 (online)

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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-Et3SiH 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 - Et3SiH

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Compound 16 (see ref. 3c): [α]D 25 = -53.6 (c = 1.14, CHCl3). Compound 20 (see ref. 4b): [α]D 25 = +72.8 (c = 1.35, CHCl3)

20

Representative Experimental Procedure of Compound 14: To a stirred solution of benzylidene acetal 13 (103 mg, 0.40 mmol) in anhyd CH2Cl2 (5 mL) was added Et3SiH (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 CH2Cl2 (3 × 10 mL). The combined organic extracts were dried over anhyd Na2SO4, 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. 1H 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). 13C NMR (100 MHz, CDCl3): δ = 38.4, 62.3, 69.2, 73.6, 127.8, 128.1, 128.4, 137.3.