[reaction: see text] The epoxide hydrolase (EH) from Aspergillus niger, which shows a selectivity factor of only E = 4.6 in the hydrolytic kinetic resolution of glycidyl phenyl ether, has been subjected to directed evolution for the purpose of enhancing enantioselectivity. After only one round of error-prone polymerase chain reaction (epPCR), enantioselectivity was more than doubled (E = 10.8). The improved mutant enzyme contains three amino acid exchanges, two of which are spatially far from the catalytically active center.

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positions situated spatially nearby, thereby leading to synergistic effects.To test this concept experimentally we synthesized the boron-containing crown ethers ['] The corresponding ammonium ions RNH? are bound more strongly by suitable crown ethers, but generally the degree of selectivity here is also quite low.['] The latter is hardly surprising because the RNHf/crown ether complexes, which are held together by hydrogen bonds, are structurally so arranged that the groups R are too remote from the framework of the host m~l e c u l e . [~.~~ The formation of adducts with Lewis acids,[" for example with trialkyl boranes leading to reversible formation of adducts R,N. BR3,16] is a quite different type of amine complexation. The dissociation constants of these equilibria depend, among other factors, on the number and size of the groups R in the amine.We describe here a new concept in molecular recognition in which host molecules containing a Lewis acid metal centerrzb. 4 9 '1 and at least two donor sites selectively bind primary amines via three-point binding (Scheme l). If the amine RNH, forms a reversible dative bond with the metal center, it takes on the character of an ammonium ion. This should permit the formation of hydrogen bonds with the donor