Dedicated to Vludimir Prelog on the occasion of his 85th birthday (25. X. 91)The chiral tris-and pentakis(bipyridine) ligand strands 3a and 4 were synthesised, each in optically pure form with (S,S)-configuration. Ligand 3a yielded substituted double-helical metal complexes, derived from the parent trihelicate structure 2, with Cu' and Ag' ions. The spectral data, in particular the 'H-NMR spectra and the large positive Cotton effect, indicate that helicate formation occurs with very high induction of helicity. Together with consideration of the steric effects in the two possible helical diastereoisomers (P)-1 and (M)-1 that may be formed, the data favour the preferential generation of the right-handed double helicate (P)-1 from the tris(bipyridine) strand 3a of (S,S)-configuration,

The bridged bis(dihydrooxazo1e) compound (-)-2 and its silylated derivative (-)-3 have been synthesised as building blocks for chiral ligands. The complex of (-)-3 with Cu" has been obtained, and its crystal structure has been determined. The Cu" ion is bound to two deprotonated ligands in a twisted tetracoordinated geometry.Introduction. -The binding of metal ions (such as Cu', Ag') displaying a tetrahedral coordination geometry to oligo(2,2'-bipyridine) (bipy) strands leads to the self-organisation of double stranded helicate complexes [14]. These species are obtained as racemic mixtures of left-and right-handed double helices. One may imagine several ways of inducing a given helicity in helicate formation such as: a ) the introduction of permanent chirality in the backbone, b ) the use of a chiral auxiliary group [5], c) the asymmetric delivery of the metal ion in a chiral medium or from a preformed chiral complex.The purpose of' the present work was to explore option a ) by developing a chiral, bidentate ligand which, when included into an oligomeric species or into an oligobipyridine strand might be expected to form helicates with asymmetric induction.The structural requirements for such a ligand are i ) that it should have C, symmetry, ii) that it should form strong complexes with transition metals in non-planar arrangements, and iii) that it should have functionality close to the ligating site so that attachment of additional ligands would be possible at a later stage.Our attention focused on the chiral dihydrooxazoles developed in recent years [6]; two identical, suitably functionalised dihydrooxazoles, connected by a bridge, appeared to be well-suited for our purpose3). In addition, the ligand would be readily available by the condensation of a known bis(amide enol ether) dihydrochloride with an aminodiol from the chiral pool. Other possible chiral subunits could be bipy groups bearing an asymmetric centre [8] or chiral semicorrin-type units [9].We describe here the synthesis of the chiral bridged bis(dihydrooxazo1e) (-)-2 and its bis-silylated derivative (-)-3, containing four asymmetric centres, as well as as the formation and crystal structure of the 2: 1 complex of (-)-3 with Cu".

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