Proton magnetic resonance spectra of substituted adamantane-2,4-diones. Determination of the C-6 and C-9 stereochemistry

The 220 MHz ¹H NMR spectra of 6-hydroxy-7-methyl-6-phenyladamantane-2,4-dione, 6-hydroxy-5,7-dimethyl-6-phenyladamantane-2,4-dione, and 6-hydroxy-7,9-dimethyl-6-phenyladamantane-2,4-dione have been determined and complete proton assignments carried out with the aid of multiple irradiation. The stereochemistry of the C-6 and C-9 chiral centres have been determined and some of the factors affecting the magnitude of the geminal, vicinal and W coupling constants are discussed.     

Mechanistic studies of the transfer dehydrogenation of cyclooctane catalyzed by iridium bis(phosphinite) p-XPCP pincer complexes

Reaction of bis(phosphinite) PCP iridium pincer complexes (p-XPCP)IrHCl (5a-f) [X = MeO (5a), Me (5b), H (5c), F (5d), C(6)F(5) (5e), Ar(F)(= 3,5-bis(trifluoromethyl)phenyl) (5f)] with NaOtBu in neat cyclooctane (COA) generates 1:1 mixtures of the respective (p-XPCP)IrH(2) complexes 4a-f and the cyclooctene (COE) olefin complexes (p-XPCP)Ir(COE) (6a-f) at 23 degrees C. At higher temperatures, complexes 4 and 6 are equilibrated because of the degenerate transfer dehydrogenation of COA with free COE (6 + COA right harpoon over left harpoon 4 + 2COE), as was shown by temperature-dependent equilibrium constants and spin saturation transfer experiments at 80 degrees C. At this temperature, the COE complexes 6 exchange with free COE on the NMR time scale with the more electron-deficient complexes 6 exchanging COE faster. The exchange is dissociative and zero order in [COE]. Further analysis reveals that the stoichiometric hydrogenation of COE by complex 4f, and thus the separated back reaction 4f + 2COE --> 6f + COA proceeds at temperatures as low as -100 degrees C with the intermediacy of two isomeric complexes (p-Ar(F)PCP)Ir(H)(2)(COE) (8f, 8f'). COE deuteration with the perdeuterated complex 4f-d(38) at -100 degrees C results in hydrogen incorporation into the hydridic sites of complexes 8f,8f'-d(38) but not in the hydridic sites of complex 4f-d(38), thus rendering COE migratory insertion in complexes 8f,8f' reversible and COE coordination by complex 4f rate-determining for the overall COE deuteration.     

Adjustable cavity for host-guest recognition in cofacial bis-porphyrinic tweezer

The synthesis of a cofacial bis-porphyrinic tweezer bearing a tris-anthracenic spacer is reported. Its behavior as host has been evidenced as well as the ability of its cavity to adjust to guests of various sizes.     

Cover Picture

The cover picture shows how thrombosis occurs in the deep veins of the lower limbs. Stasis, which results from slow and turbulent blood flow, combined with hypercoagulation, caused, for example, by a surgical procedure, may result in thrombus formation. The synthetic sulfated pentasaccharide shown in part is a potent antithrombotic compound that exerts its effect by activation of the plasma protein antithrombin III. Conformationally locked monosaccharides have now been synthesized to demonstrate that L-iduronic acid, one part of the pentasaccharide, must adopt an unusual distorted conformation to activate antithrombin III. Such conformational effects might be relevant in explaining the unique biological properties of glycosaminoglycans that contain L-iduronic acid. In the background of the picture, a flight of vampire bats is attracted by the pentasaccharide. Vampire was the name given to South American blood-sucking bats (Latin name: desmodus rotundus) in 1761 by the French naturalist Georges Louis Leclerc Comte de Buffon (1707-1788). These bats are known to attack cattle and, very rarely, sleeping human beings. Although their saliva has been shown to contain an anticoagulant compound, they would also be happy to benefit from the pentasaccharide mentioned above, to suck the blood out of the vein more easily. More details about this compound which would be helpful to vampire bats are reported by Petitou, Sina? et al. on p. 1670 ff.     

Synthesis of 2-anilinotropones via palladium-catalyzed amination of 2-triflatotropone

[reaction: see text] A series of 2-anilinotropones were synthesized via the palladium-catalyzed cross coupling of 2-triflatotropone and a wide variety of anilines, sterically hindered as well as electron-poor ones.     

Further correlations between fragment ion intensity and critical energy. Unimolecular covalent bond fragmentation

A correlation of fragment ion intensity with critical energy found in the collisional activation spectra of [C₄Ph]⁺˙ ions produced by electron impact can also be found in the unimolecular mass-analysed ion kinetic energy spectra of these ions. The P(E) functions of the unimolecular and collisionally activated ions should differ not only in width but also in structure and therefore, the hypothesis that P(E) functions do not have an important effect on these correlations is tested successfully.     

Determination of boron in titanium alloys by means of ion exchange

There is need for a rapid method for the determination of moderate amounts of boron in titanium alloys. In this paper a method is proposed which uses ion exchange. The method is applicable to titanium alloys containing 0.025 to 1 per cent. boron. One or two grams of the sample are dissolved in hydrochloric acid, and the titanium and boron are oxidized with nitric acid. The bulk of the titanium is removed by a cation exchanger. A calcium carbonate separation is made to remove the residual titanium and adjust the acidity. The boron is then titrated with sodium hydroxide, after the addition of mannitol. None of the elements found in commercial titanium alloys interferes with the method.