Calixarene-based chiral phase-transfer catalysts derived from cinchona alkaloids for enantioselective synthesis of α-amino acids

The synthesis of the first calixarene-based chiral phase-transfer catalysts derived from cinchona alkaloids has been achieved in two steps from p-tert-butylcalix[4]arene. The catalytic efficiency of the chiral calix[4]arenes 3a–c was evaluated by carrying out the phase-transfer alkylation of N-(diphenylmethylene)glycine ethyl ester with benzyl bromide. Various factors that affect the chemical yield and enantioselectivity were also examined. Benzylation of glycine imine 4 using calix[4]arene-based dimeric catalyst 3a as a chiral phase-transfer catalyst in toluene/CHCl3 mixture (7:3 v/v) at 0 °C gave the best enantioselectivities and yields in the presence of aqueous NaOH.

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25,27-Bis[(cinchonidine-yl)ethoxy]-26,28-dihydroxy-5,11,17,23-tetra(tert-butyl)calix[4]arene dibromideC86H106N4O6Br2[α]D25=-25 (c 0.2, MeOH)Source of chirality: (−)-cinchonidineAbsolute configuration: (R,R)

25,27-Bis[(cinchonidine-yl)propoxy]-26,28-dihydroxy-5,11,17,23-tetra(tert-butyl)calix[4]arene dibromideC88H110N4O6Br2[α]D25=-28 (c 0.2, MeOH)Source of chirality: (−)-cinchonidineAbsolute configuration: (R,R)

25,27-Bis[(cinchonidine-yl)butoxy]-26,28-dihydroxy-5,11,17,23-tetra(tert-butyl)calix[4]arene dibromideC90H114N4O6Br2[α]D25=-35 (c 0.2, MeOH)Source of chirality: (−)-cinchonidineAbsolute configuration: (R,R)