Montmorillonite Clay: A Novel Reagent for the Chemoselective Hydrolysis
of t-Butyl Esters

Abstract

A mild and highly selective hydrolysis of t-butyl esters has been achieved in high yields using montmorillonite KSF in refluxing acetonitrile. The method is compatible with a variety of protecting and functional groups such as BOC, Cbz, propargyl, allyl, benzyl, t-butyl ethers, allyl, methyl and benzyl esters present in the molecule.

Key words

solid acids - tert-butyl esters - α-amino acids

References

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8

Experimental procedure: A mixture of t-butyl ester
(5 mmol), KSF clay (1.0 g) in acetonitrile (10 mL) was stirred at reflux temperature for a specified time as required to complete the reaction (Table). After complete conversion, as indicated by TLC, the reaction mixture was filtered and washed with ethyl acetate (2 × 15 mL). The combined organic layers were dried over anhydrous Na₂SO₄, con-centrated in vacuo and purified by column chromatography on silica gel (Merck, 100-200 mesh, ethyl acetate-hexane, 2:8) to afford pure acid.
Spectroscopic data for the selected compounds: 1a: Liquid, ¹H NMR (CDCl₃): δ 1.45 (s, 9 H), 3.40 (s, 2 H), 3.83 (s, 3 H), 3.85 (s, 3 H), 6.75-6.78 (m, 3 H). EIMS: m/z: 252 [M⁺]; IR (KBr): 3031, 2968, 1725, 1527, 1458, 1205, 1070, 915, 727 cm^-1.
2a: Solid, mp 95-97 °C; ¹H NMR (CDCl₃): δ 3.57 (s, 2 H), 3.83 (s, 3 H), 3.85 (s, 3 H), 6.75-6.78 (m, 3 H). EIMS: m/z: 196 [M⁺]; IR (KBr): 3345, 3031, 2968, 1695, 1527, 1458, 1205, 1070, 915, 727 cm^-1. 1l: Liquid, ¹H NMR (CDCl₃): δ 1.40 (d, 3 H, J = 6.8 Hz), 1.43 (s, 9 H), 4.38 (m, 1 H), 5.10 (s, 2 H), 5.45 (brs, NH), 7.37-7.40 (m, 5 H). EIMS: m/z: 279 [M⁺]; IR (KBr): 3342, 3035, 2965, 1724, 1525, 1453, 1209, 1065, 917, 735 cm^-1. 2l: Solid, [α]_D ²⁰ -14.0 (c = 2.0, AcOH), Aldrich, [α]_D ²³ -14.2 (c = 2.0, AcOH). ¹H NMR (DMSO-d₆): δ 1.48 (d, 3 H, J = 7.0 Hz), 4.40 (m, 1 H), 5.18 (s, 2 H), 5.27 (brs, NH),
7.28-7.40 (m, 5 H), 8.15 (brs, OH). EIMS: m/z: 223 [M⁺]; IR (KBr): 3347, 3036, 1695, 1536, 1458, 1252, 1075, 1027, 914, 739 cm^-1.