PubMed · 4897484
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O Sticher. 1969. [Iridoides].. https://pubmed.ncbi.nlm.nih.gov/4897484/
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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.
A synthesis of (-)-epicylindrospermopsin (2) was completed that establishes its absolute configuration and corroborates the corrected structural assignment previously made to this toxin by Weinreb et al. The hydroxylamine 3, prepared from 4-bromobenzyloxyacetaldehyde, was condensed with aldehyde 4, obtained in nine steps from (R)-methionine, to give nitrone 16. Intramolecular cycloaddition of 16 proceeded stereoselectively to yield the oxazabicyclo[2.2.1]heptane 17, which after reduction and deprotection afforded piperidine 18. The latter was transformed via cyclic urea 19 to the inverted C12 alcohol 20, and the derived azide 22 was cyclized to produce the guanidine moiety of 25. Final sulfation of the C12 hydroxyl group furnished (-)-2.
[reaction: see text]. The intramolecular [4 + 2]-cycloaddition of a 2-methylthio-5-amidofuran was used to create the azepinoindole skeleton present in the Stemona alkaloid stenine. The rearranged cycloadduct was converted to stenine (1) in 11 additional steps via a sequence that features a Crabtree catalyst directed hydrogenation (9-->10), iodolactonization (2-->11), and a Keck allylation (11-->12).