Chiral Recognition of alpha-Amino Acid Derivatives with a Homooxaealix[3]arene: Construction of a Pseudo-C_2-Symmetrical Compound from a C_3-Symmetrical MacrocycSe

摘要

The NH_3~+ moiety of primary aikylammomum tons (RNH,_3~-) possesses C_3 symmetry, and therefore complementary hosts might best possess C_3 symmetry as well. In ehirai molecules with two complexing faces, the importance of having the two faces relatedby C_3 symmetry has been emphasized. A ehirai guest molecule interacts identically with the homotopic faces of this type of ehirai host. Therefore, for the enantioselective recognition of primary alkyiammonium ions, it is desirable for a ehirai host molecule to possess a C_2 axis perpendicular to a local C_3 symmetry element. Satisfying these requirements is quite difficult. [18]Crovvn-6 is one of the few compounds that has these two local symmetry elements--at least when it binds K~- or RNH3" ions in the solid state. In fact, several ehirai [18]crown-6 derivatives are known to bind racemic substrates containing NH_3~+ centers with high enantioselectivity. Here we describe how such a host molecule for c'niral recognition can be prepared from a homooxacalix[3]arene.