Synthesis of chiral calixarenes and application as functional organic catalysts

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The racemic mixture of chiral calix [4]arenes (5-tert-butyl-11-methyl-17,23-diphenyl-25,26,27,28-tetrahydroxycalix [4]arene) with three different upper rim substituents was synthesized by the fragmentation-condensation of p-substituted phenol dimmers (2-(5-methylsalicyl)-4-tert-butylphenol and 3-(3-hydroxymethyl-5-phenylsalicyl)-5-phenyl-2-hydroxybenzyl alchol) according to K. No and coworkers' multi-step method. On the other hand, amino acid esters (Z-D(L)-Ala-PNP, Z-D(L)-Trp-PNP) were prepared from N-(benzyloxycarbonyl)-D(L)-amino acids by esterification of the COOH group with p-nitrophenol and dicyclohexylcarbodiimide. The calix[4]arenes and amino acid esters were confirmed by elemental analysis, specific rotation, IR and NMR spectroscopic measurements. With respect to the hydrolysis of Z-D(L)-Trp-PNP mediated by the calix[4]arenes in the presence of Z-L-Phe-OH, a little inhibition effect was observed on the basis of the rate constants. Particularly, the hydrolysis of D-substrate tended to be slightly slower than that of L-substrate. Furthermore, the computer modeling (molecular mechanics) studies indicated that the calix[4]arene would adopt a "cone" conformation. These results suggest that the calix[4]arenes could form inclusion complexes with Z-D(L)-Trp-PNP. Probably, the stereochemical arrangements of the chiral calix[4]arenes may induce the enantioselectivity for the hydrolysis of Z-D(L)-Trp-PNP.