A Simple Method for the Preparation of Stainless and Highly Pure Trichloroacetimidates

Abstract

We describe a method for obtaining various allylic, benzylic, and glucosyl 2,2,2-trichloroacetimidates (TCAIs) as stainless liquids or solids at the crude stage. The general synthetic method for the preparation of TCAIs often leads to stained products, and further purification of crude TCAIs causes decomposition due to their instability. In the described method, we use a solvent that barely dissolves the reactant, providing stainless and sufficiently pure TCAIs without requiring a purification step. Furthermore, the reaction mixture is turbid at the beginning and clear at the end, allowing us to monitor the progress of the reaction visually.

Key words

preparation - 2,2,2-trichloroacetimidates - stainless - solvent optimization - visual detection

• References and Notes


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• 20 p-Methoxybenzyl 2,2,2-Trichloroacetimidate [PMB-TCAI (2)]; Typical Procedure To a suspension of PMBOH (1.00 g, 7.24 mmol) and Cl₃CCN (1.15 g, 7.97 mmol) in heptane (18 mL) was added DBU (110 mg, 723 μmol) at 0 °C. After the suspension became a solution (actual reaction time = 25 min), heptane (18 mL) and saturated aq NH₄Cl (18 mL) were added to the reaction mixture. The separated heptane layer was washed with saturated aq NH₄Cl (18 mL) and dried over Na₂SO₄. Filtration of the Na₂SO₄ and concentration of the filtrate under reduced pressure gave PMB-TCAI (2) (2.01 g, 99%) as a colorless oil. The ¹H and ¹³C NMR spectra of 2 were in good agreement with the literature data.^{22 1}H NMR (400 MHz, CDCl₃, 24 °C): δ = 8.36 (br s, 1 H), 7.37 (d, J = 8.7 Hz, 2 H), 6.91 (d, J = 8.7 Hz, 2 H), 5.28 (s, 2 H), 3.82 (s, 3 H). ¹³C NMR (101 MHz, CDCl₃, 24 °C): δ = 162.8, 159.9, 129.9 (2 C), 127.7, 114.1 (2 C), 91.6, 70.8, 55.4.
• 21 As in the syntheses of 1 and 2, the reaction mixtures for 4a–f, 5, 6 and 7 were initially suspensions, and then gradually became clear solutions (see pages S29–39 of the Supporting Information). All obtained products were stainless. We also confirmed that 4a, 4e, and 6 could also be preserved at –10 °C for more than one month.
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