Five octahedral eilatin complexes of the type [M(L-L)(2)(eilatin)](2+) (M = Ru, Os; L-L = bipyridyl-type ligands) were synthesized, and their dimerization via pi-pi stacking was studied by crystallography and (1)H NMR techniques. The X-ray structures of these racemic complexes were solved and revealed that the eilatin complexes are organized as discrete dimers in which the eilatin residues of each complex are stacked in centrosymmetric packing. Chemical shift dependence on concentration in the (1)H NMR spectra support fast dimer-monomer equilibrium, and the structures of the dimers in acetonitrile solution are proposed to be analogous to their solid-state structures. Dimerization constants in acetonitrile were measured for the five racemic eilatin complexes that exhibit different structural parameters, as well as for the two enantiomeric forms of one of these complexes. They were found to be independent of the metal (Ru vs Os), strongly dependent on the steric effects introduced by the L-L ligands (2,2'-bipyridine, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, and 2,2'-biquinoline), and dependent on the optical purity of the complexes. A clear preference for heterochiral over homochiral dimer formation was demonstrated. This is the first report of chiral recognition in solution, exhibited by simple chemical systems held solely by pi-stacking interactions.

The chemical content of Sarcophyton glaucum, one of the more abundant soft corals on many coral reefs, collected from many seas, was thoroughly explored, resulting in the discovery of a large number of cembranoids, biscembranoids, sterols, and other secondary metabolites. The presently investigated Kenyan specimens of S. glaucum yielded three new metabolites, i.e., nyalolide (15), a biscembranoid, 16-oxosarcoglaucol acetate (16), a cembranoid, and the sesquiterpene guaiacophine (17). Nyalolide was also isolated from the Kenyan soft coral Sarcophyton elegans. The structures of the new compounds were elucidated by interpretation of their MS and 1D and 2D NMR experiments and, in the case of nyalolide, possessing 11 chiral centers, secured by X-ray diffraction analysis.

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Three novel compounds polycitone A (la) and polycitrins A and B (2 and 3) were isolated from the marine ascidian Polycitor sp. The structures of compounds la, 2, and 3 were established mainly on the basis of NMR spectroscopic data and, in the case of la, also by single-crystal X-ray diffraction analysis. The crystallographic analysis was performed on lb, the penta-0-methyl derivative of la, both compounds yielding poorly diffracting crystals of highly anisotropic shape. It required, in view of the dominating heavy-atom content of lb, diffraction measurements with an intense rotatinganode X-ray source. Polycitone A and polycitrins A and B represent the first examples of two new classes of marine products which might biogenetically be close to the lamellarins. The penta-0methyl derivative lb, was found to inhibit the growth of SV40 transformed fibroblast cells in a concentration of 10 pg/mL. 999Marine ascidians are known to be a rich source of unique and biologically active secondary metabo1ites.l A few bioactive examples of these metabolites are didemnin B,2 eudistomin C,3 the lissoclinamides,4 ascididemin: eilatin, and the segolins.6 The biomedical potential for ascidian secondary metabolites has resulted in focused interest in these primitive chordates.As part of our continuing interest in ascidian secondary metabolite@ we have examined the Indo Pacific ascidian Polycitor sp., collected in Sodwana Bay, South Africa.The Polycitor sp. is a newly discovered, translucent white colonial tunicate, species, often lightly dusted on the surface with minute brown or black dots. It is globular, ovoid, or lumpy in form and attached to the reef rock substratum by a short, barely apparent stalk.The lyophilized ascidian was extracted with ethyl acetate, and the residual gum, after evaporation of the solvent, was chromatographed on Sephadex LH-20 and silica gel columns to afford three compounds, polycitone A (la), (0.35%, dry weight of the ascidian), polycitrin A (2) (0.003 %), and polycitrin B (3) (0.002 %).Polycitone A (la) was obtained as yellowish needles from acetone or methanol-chloroform, mp 285 "C. Regrettably, it was not possible to obtain the molecular formula by FAB or DCI mass spectrometry. Prominent Abstract published in Advance ACS Abstracts, February 1, 1994. (1) Recent examples include: Bloor, S. J.; Schmitz, F. J.; J. Am. Chem. SOC. 1987,109,6134. Kobayashi, J.; Tsuda, M.; Tanabe, A.; Ishibashi, M.; Cheng, J.; Yamamura, S.; Saeaki, T. (2) Rinehart, K. L., Jr.; Gloer, J. B.; Renie, H. E.; McGovem, F. A,; Swynenberg, E. G.; Stringfellow, D. A.; Kuentzel, S. L.; Li, L. H. Science (Washington, DC) 1981,212,933. (3) Rinehart, K. L., Jr.; Kobayashi, J.; Harbour, G. C.; Hughes, R. G., Jr.; Mizaak, S. A.; Scahill, T. A. J. Am. Chem. SOC. 1984,106, 1524. (4) Schmitz, F. J.; Ksebati, M. B.; Chang, J. 5.; Wang, J. H.; Hossain, M. B.; van der Helm, D. J. Org. Chem. 1989,54,3463. (5) Kobayashi, J.;Chang,J.;Walchli,M.R.;Nakamura,H.;Yoehimaea, H.; Takuma, S.; Ohizumi, T.

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