Recyclable Merrifield resin-supported thiourea organocatalysts derived from l-proline for direct asymmetric aldol reaction

Four Merrifield resin-supported thiourea organocatalysts derived from l-proline were synthesized and found to be efficient catalysts for the direct asymmetric aldol reaction between ketone and aromatic aldehydes. The catalysts exhibited high catalytic activity, diastereoselectivity and excellent enantioselectivity at room temperature with a low loading (only 2 mol %). They also retained unchanged enantioselectivities even after being reused for at least four cycles.

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(S)-2-[(R)-Hydroxy(4-nitrophenyl)methyl]cyclohexanoneC13H15NO4[α]D25=+12.8 (c 1.85, CHCl3)Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)

(S)-2-[(R)-Hydroxy(3-nitrophenyl)methyl]cyclohexanoneC13H15NO4[α]D24=+32.5 (c 1.35, CHCl3).Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)

(S)-2-[(R)-Hydroxy(2-nitrophenyl)methyl]cyclohexanoneC13H15NO4[α]D24=+19.8 (c 1.60, CHCl3).Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)

(S)-2-[(R)-Hydroxy(4-cyanophenyl)methyl]cyclohexanoneC14H15NO2[α]D22=+23.3 (c 1.55, CHCl3)Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)

(S)-2-[(R)-Hydroxy(4-fluorophenyl)methyl]cyclohexanoneC13H15FO2[α]D24=+27.5 (c 0.35, CHCl3)Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)

(S)-2-[(R)-Hydroxy(4-chlorophenyl)methyl]cyclohexanoneC13H15ClO2[α]D245=+21.7 (c 1.00, CHCl3)Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)

(S)-2-[(R)-Hydroxy(4-bromophenyl)methyl]cyclohexanoneC13H15BrO2[α]D24=+22.6 (c 0.70, CHCl3)Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)

(S)-2-[(R)-Hydroxy(4-nitrophenyl)methyl]cyclopentanoneC12H13NO4[α]D22=-30.6 (c 0.56, CHCl3)Source of chirality: asymmetric synthesisAbsolute configuration: (2S,1′R)