Efficient and selective protection of aldehydes as geminal diacetates using LiBr †

Abstract

A variety of aldehydes react with acetic anhydride in presence of lithium bromide at room temperature to afford the corresponding 1,1-diacetates in quantitative yields within a few hours of reaction time.

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Representative data for compound 2k: Pale yellow solid, m.p. 201–203°C. ¹H NMR (CDCl₃): δ 2.2 (s, 6H), 7.45–7.65 (m, 4H), 8.0 (d, 2H, J = 8.5 Hz), 8.5 (s, 1H), 8.70 (d, 2H, J = 8.5 Hz), 9.25 (s, 1H). ¹³C (Proton decoupled, CDCl₃): d 20.50 (–CH₃), 167.80 (–CO–, 88.56 (–CH<), 125.34, 125.76, 126.35, 127.34, 127.32, 129.85, 130.30, 132.87 (aromatic). Anal. calcd for C₁₉H₁₆O₄(308.33): C, 74.10; H, 5.33. Found C, 74.25; H, 5.42.

Compound 21: Solid, mp 71–73°C. IR υ_max / (cm⁻¹) 1680 (–CO–) and 1744 (CH(OAc)2). 1H NMR (CDCl3): d 2.1 (s, 6H), 5.25 (s, 2H), 6.95 (d, 2H, J = 8.72 Hz), 7.45-7.55 (m, 5H), 7.6 (s, 1H), 8.0 (d, 2H, J = 8.7 Hz). 13C (proton decoupled, CDCl₃): d 20.5 (–CH₃), 167.9 (–CO–), 87.75 (–CH<), 75.95 (–CH₂), 191.33 (–CO-Ph), 114.55, 128.25, 129.29, 130.41, 132.78, 133.36, 188.90 (aromatic). Anal. calcd. for C₁₉H₁₈O₆ (342.35): C, 66.78; H, 5.21. Found C, 66.57; H, 5.38.