Add time:09/27/2019         Source:infona.pl

A heterogeneous, silica‐based catalyst containing 9‐amino‐9‐deoxy‐epi‐quinine (or quinidine) and a derivative of benzoic acid was synthesized through radical thiol‐ene click reaction. The acid component allows the in situ activation of cinchona amino group, acting as a bifunctional catalyst. The heterogenized catalysts efficiently promoted the reaction of ketones with trans‐β‐nitrostyrene, with diastereo‐ and enantioselectivity comparable to those of the homogeneous counterparts (dr up to 90:10 and 90 % ee). In addition, the catalyst retained a constant activity for at least four cycles. Finally, the supported catalyst (9‐amino‐9‐deoxy‐epi‐quinine/achiral acid) was employed under continuous‐flow conditions. Two enantioselective Michael reactions were in sequence performed with the same homemade packed‐bed reactor. The addition of cyclohexanone to trans‐β‐nitrostyrene provided the evaluation of optimal residence time with high level of stereoselection (2 µL/min flow rate, 83 % ee). Furthermore, the flow reactor well performed in the preparation of warfarin (isolated yield 95 %, 78 % ee. in 16 h at room temperature). The dual (chiral amine/achiral acid) solid supported system, making an even easier work‐out, represents a valuable tool for green chemistry and is attractive for large scale applications.

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