A Facile and Selective Cleavage of Prenyl Esters Catalyzed by CeCl ₃ ·7 H₂O-NaI

Abstract

A highly selective cleavage of prenyl esters has been achieved in high yields using CeCl₃·7 H₂O-NaI in refluxing acetonitrile under neutral conditions. This method is mild and compatible with a wide variety of functional groups such as BOC, Cbz, acetate, allyl, benzyl, tetrahydropyranyl, PMB ethers, allyl, methyl, and benzyl esters present in the molecule.

Key words

cerium reagents - 3-methylbut-2-enoate - α-amino acids

References

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Experimental Procedure: A mixture of 3-methylbut-2-enoate (5 mmol) CeCl₃·7 H₂O (5 mmol) and NaI (5 mmol) in acetonitrile (15 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 diluted with water (15 mL) and extracted with ethyl acetate (2 × 20 mL). The combined organic layers were dried over anhyd Na₂SO₄, concentrated in vacuo and purified by column chromatography on silica gel (Merck, 100-200 mesh, EtOAc-hexane, 2:8) to afford pure acid. Spectral data for the compounds 1e: Liquid, ¹H NMR (CDCl₃): δ = 0.87 (d, 3 H, J = 6.8 Hz), 0.98 (d, 3 H, J = 6.8 Hz), 1.73 (s, 3 H), 1.78 (s, 3 H), 2.10-2.21 (m, 1 H), 4.28 (dd, 1 H, J = 5.5, 10.3 Hz), 4.60 (d, 2 H, J = 7.0 Hz), 5.10 (s, 2 H), 5.20 (brs, NH), 5.35 (m, 1 H), 7.28-7.40 (m, 5 H). MS (EI): m/z = 319 [M⁺]. IR (KBr): ν 3347, 2965, 1723, 1511, 1220, 988, 722 cm^-1. 2e: Solid, [α]_D ²⁰ +6.3 (c 4, CHCl₃), Aldrich: [α]_D ²⁰ (c 4, CHCl₃). ¹H NMR (CDCl₃): δ = 0.85 (d, 3 H, J = 7.0 Hz), 0.99 (d, 3 H, J = 7.0 Hz), 2.10-2.23 (m, 1 H), 4.20 (m, 1 H), 5.08 (s, 2 H), 5.80 (brs, NH), 7.25-7.40 (m, 5 H). MS (EI): m/z = 251 [M⁺]. IR (KBr): ν 3339, 3037, 1695, 1531, 1229, 978, 729 cm^-1. 1f: Liquid, ¹H NMR (CDCl₃): δ = 1.40 (d, 3 H, J = 6.8 Hz), 1.78 (s, 3 H), 1.80 (s, 3 H), 4.38 (m, 1 H), 4.60 (d, 2 H, J = 7.0 Hz), 5.10 (s, 2 H), 5.37 (m, 1 H), 5.45 (brs, NH), 7.37-7.40 (m, 5 H). MS (EI): m/z = 291 [M⁺]. IR (KBr): ν 3342, 3035, 2965, 1724, 1525, 1453, 1209, 1065, 917, 735 cm^-1. 2f: Solid, [α]_D ²⁰ -13.9 (c 2, HOAc), Aldrich: [α]_D ²³ -14.2 (c 2, HOAc). 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). MS (EI): m/z = 223 [M⁺]. IR (KBr): ν 3333, 3036, 1695, 1536, 1458, 1252, 1075, 1027, 914, 739 cm^-1.

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IICT Commun. No: 01/x/09