The reaction of [(arene)RuCl(2)](2) (arene = cymene, 1,3,5-C(6)H(3)Me(3)) and [CpRhCl(2)](2) half-sandwich complexes with tridentate heterocyclic ligands in the presence of base has been investigated. In all cases, the chloro-ligands were substituted to give metallacyclic products with ring sizes between 4 and 18 atoms. The cyclization occurs in a highly diastereoselective fashion with chiral recognition between the different metal fragments. The complexes were comprehensively characterized by elemental analysis, NMR spectroscopy, and single crystal X-ray crystallography. For 2-hydroxy-nicotinic acid and 2-amino-nicotinic acid, dinuclear structures were obtained (15-17) whereas for 2,3-dihydroxyquinoline, 2,3-dihydroxyquinoxaline, and 6-methyl-2,3-phenazinediol, trimeric assemblies were found (19-22), and for 4-imidazolecarboxylic acid, a tetrameric assembly (18) was found.

The (chloromethyl)cytidine 7 was obtained from alcohol 4 that was synthesized from the protected cytidine 3 by C(6)-formylation and reduction. Thioacetate 10 was obtained from the cytidine 2, and thioacetate 8 from a Mitsunobu reaction of alcohol 6. The thiomethylene-linked dinucleoside 11 was synthesized by thioether formation between the 6-(chloromethyl)cytidine 7 and the thiolate generated by S-deacetylating and N-debenzoylating the cytidine-5'-thioacetate 10. Dinucleoside 11 was desilylated to 12, and fully deprotected to 13. Similarly to 11, the C(6)-substituted analogue 14 was obtained from 7 and the C(6)-substituted 8. Stepwise deprotection of 14 provided 15 -17, and complete deprotection gave 18. The thioacetylated and N-benzoylated dinucleoside 21 was obtained from the methanesulfonate 9 and the thiolate that was generated from thioacetate 8. Similarly, 7 and 8 yielded 19 that was transformed into the methanesulfonate 20. The tetranucleoside 23 was synthesized from the methanesulfonate 20 and the thiol derived from 21. It was debenzoylated to 23 and completely deprotected to 24.The partially protected dinucleosides 11, 14, and 15, and the tetranucleoside 23 pair strongly in CDCl 3 . The crystal structure of 11 · MeOH shows the formation of an antiparallel cyclic duplex possessing nearly orthogonal base pairs due to MeOH acting as H-acceptor from one base pair and Hdonor to the other base pair. A large distance of ca. 6 between the base pairs of the cyclic duplexes was predicted by Maruzen modeling. It is corroborated by the absence of base stacking in CHCl 3 solution of the duplexes formed by the (self-complementary) dinucleosides 11, 14, and 15, as evidenced by a weak temperature dependence of the CD spectra. The association constants for 11, 14, 15, and 23 were calculated from the concentration dependence of the chemical shift of H 2 NÀC(4). No concentration dependence of the H 2 NÀC(4) signals was observed for solutions of 23 in CDCl 3 , (D 6 )acetone, CD 3 CN, (D 8 )THF, (D 5 )pyridine, and CDCl 3 /(D 6 )DMSO 4 : 1. As a consequence of the strong association, the association constant for 23 had to be determined in CD 3 CN/(D 6 )DMSO 4 : 1. The temperature dependence of the CD spectra of the fully deprotected 18 und 24, but not of 13, in H 2 O is rationalized by base stacking of the hydroxymethylated cytosine moieties that associate by intermolecular H-bonds of HOCH 2 ÀC(6/I) to an acceptor of unit I. The 1 H-NMR spectrum of 18 and 24, but not of 13, shows a 9 : 1 mixture of the monoplex and the base-stacked duplex.

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The protected G*[s]C*[s]U*[s]A*[s]U*[s]A*[s]G*[s]C* octanucleoside 24 was prepared by S‐alkylation of the thiolate derived from tetranucleoside 23 with the methanesulfonate 22, and transformed to the silylated and isopropylidenated 25, and further into the fully deprotected octanucleoside 26. Compound 22 was derived from the methoxytrityl‐protected tetranucleoside 21, and 21 was obtained by S‐alkylation of the thiolate derived from the dinucleoside 19 with methanesulfonate 17 derived from 16 by detritylation and mesylation. Similarly, tetranucleoside 23 resulted from S‐alkylation of the thiolate derived from 18 with the methanesulfonate 20 derived from 19. Dinucleosides 16 and 18 resulted from S‐alkylation of the thiolate derived from the known cytidine‐derived thioacetate 15 with the C(8)‐substituted guanosine‐derived methanesulfonates 12 and 14, respectively, that were synthesized from the protected precursors 4 and 7 by formylation, reduction, protection, and mesylation.The structures of the duplexes of 25 and 26 were calculated using AMBER* modelling and based on the known structure of the core tetranucleoside U*[s]A*[s]U*[s]A*. The former shows a helix with a bent helix axis and strong buckle and propeller twists, whereas the latter is a regular, right‐handed, and apparently strain‐free helix. In agreement with modelling, the silylated and isopropylidenated octanucleoside 25 in (CDCl2)2 solution led to a mixture of associated species possessing at most four WatsonCrick base pairs, while the fully deprotected octanucleoside 26 in aqueous medium forms a duplex, as evidenced by a decreasing CD absorption upon increasing the temperature and by a UV‐melting curve with a melting temperature of ca. 10° below the one of the corresponding RNA octamer, indicating cooperativity between base pairing and base‐pair stacking.

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