L W q ) MezC-O-UC14(THF) 1 2-= I >uci4 + U C I~ + Licl -1 Me2C-O-UC14(THF) 1 2 0.5 1 Scheme 2. The role of 1 as an intermediate i n the formation of 2two unsuspected points of the reaction mechanism. Firstly, acetone is not involved in the transformation 1 + 2, which is induced by Li/Hg reduction and generates UCI, and LiCI. Secondly, 2 is the true precursor of the alkene, tetramethylethylene. Indeed, 2 was smoothly reduced at 20 "C by Li/Hg to give a not yet identified uranium(rr1) alkoxide, and complete formation of the alkene could be achieved after the mixture was heated at reflux for 24 h. In agreement with these observations, the reductive coupling of acetone to give pinacol or tetramethylethylene was performed in a one-pot reaction by using the proper quantities of UCl, and Li/Hg; the diol was formed by hydrolysis after 8 h at room temperature, whereas the alkene was obtained by heating for 24 h under reflux (quantitative yields by NMR). When the UCl, and Na/Hg system was used, the yield of tetramethylethylene did not exceed 10% after 48 h at 65°C. This difficulty in forming the alkene could be clearly related to the sluggish reduction of the pinacolato intermediate 4, which was quite inert towards Na/Hg at 20 "C; even after 30 h at 65 "C only about 20 % was converted into tetramethylethylene. In contrast, 4 was readily reduced by Li/Hg and transformed completely into the expected alkene after 10 h in refl uxing tetrahydrofuran.We have found that in the reductive coupling of acetone, quite distinct conditions are necessary for both the coupling process leading to the pinacolate intermediates and for the subsequent deoxygenation step giving tetramethylethylene. These differences can be used to achieve remarkable selectivity like that observed with some titanium systems." 21 The results also underline the major role of the reducing agent, which determines the structure of the intermediates and is of particular importance in the deoxygenation step. If the first intermediate, a bimetallic species with a bridging OCMe,CMe,O ligand, readily reacts with the reducing agent to give a cyclic mononuclear metallopinacol, only this intermediate is transformed under more forcing conditions into the corresponding alkene. Experimental Procedure'H NMR (TMS int.): Bruker WP60 (60 MHz). All experiments were carried out under argon ( < 5 ppm oxygen and water) using standard Schlenk-vessel and vacuum-line techniques or in a glove box. Solvents were thoroughly dried and deoxygenated by standard methods and distilled immediately before use. 1 A mixture of UCI, (354 mg. 0.93 mmol). 1 % Li/Hg (640 mg, 0.93 mmol of Li), and acetone (68 gL, 0.93 mmol) in THF (15 mL) was stirred for 3 h at 20°C. The green solution was filtered and its volume reduced to 5 mL. The green microcrystals of 1 that precipitated upon addition of pentane (10 mL) were filtered off, washed with pentane, and dried under vacuum (464 mg, 90%). 2: By using the same procedure as for 1 , green microcrystals of 2 were isolaled in 76% yield from the reaction of UCI, (3...

The novel trimethylene-bridged clips 3 and 4 have been synthesized by using repetitive stereoselective Diels-Alder reactions of the benzo- and naphthobismethylenenorbornenes 8 and 19 as dienes and norbornadiene 9 as bisdienophile, and subsequent dehydrogenation of the primary cyclobisadducts 10 and 20 by using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Clips 3 and 4 serve as receptors for a variety of electron-deficient neutral and cationic aromatic substrates, comparable to the molecular tweezers 1 and 2. The thermodynamic parameters of the complex formation, K(a) and DeltaG, were determined by (1)H NMR titration experiments and, in the case of the highly stable complex TCNB 32@4, by the use of isothermal titration microcalorimetry. The finding that clip 4 forms more stable complexes than 3 can be explained by the larger van der Waals contact surfaces of the naphthalene sidewalls in 4 compared to the corresponding benzene systems in 3. In the complexes with 4 as receptor, the plane of each aromatic substrate molecule is calculated to be oriented almost parallel to the naphthalene sidewalls. However, in the complexes of tweezers 2, the substrate is usually oriented parallel to the central naphthalene spacer unit. Due to the more open topology of 4, most complexes were calculated to consist of two or more equilibrating noncovalent conformers.

ZUSCHRIFTENwurde anhand des Abbaus eines Vahnomycin-induzierten K + - Diffusionspotentials[", ''I verfolgt und rnit Chlortetracyclin(CTC)-CaZ+-Assays['21 untersucht[' ' I. Die Ergebnisse deuten darauf hin, dafi das Dimer 1 bevorzugt Na+-und das Trimer 2 bevorzugt K+-Ionen transportiert, wahrend das Tetramer 3 fur beide Ionenarten vernachlassigbare Transporteigenschaften aufweist. Wie aus dem CTC-Ass.ay hervorgeht, konnte keine dieser drei Verbindungen Ca2 ' -1onen durch die Lipiddoppelschicht transportieren. Die Makrocyclen fuhren nicht zur Freisetzung von eingelagertem Carboxyfluorescein, was darauf hinweist, darJ die Ionenbewegung durch die Lipiddoppelschicht nicht auf der Bildung von grofien Poren oder auf einer detergensahnlichen Wirkungsweise b e r~h t~'~' , sondern auf einen Carrier-Mechanismus, ahnlich dem mit Valin~mycin['~~, zuriickzufiihren ist. Die cyclischen Cystinpeptide 1-4 gehoren zu einer neuen Klasse cyclischer, Adamantan enthaltender Peptide, die Na+und K ' -1onen durch Modellmembranen transportieren konnen. Diese Fahigkeit sowie ihre sehr einfache und kurze Synthese sollte ein Anreiz fur das Design weiterer Verbindungen dieses Typs sein. ExperimentellesEine Losung von frisch hergestelltem 1,3-Adamantandicarbonsauredichlorid (2 mmol) in 100 mL wasserfreiem CH,CI, wurde bei 0°C in 0.5 h zu einer gut geruhrten Losung von L-Cystindimethylesterhydrochlorid (2 mmol) und Triethylamin (9 mmol) in 150 mL wasserfreiem CH,CI, getropft. Das Reaktionsgemisch wurde 4-6 h bei Raumtemperatur geruhrt (diinnschichtchromatographische Kontrolle) und anschlieBend durch Waschen mit eiskalter 2 N H,SO,, Wasser und 5prOZ. NaHC0,-Losung (je ca. 20 mL) und Trocknen der organischen Phase uber wasserfreiem MgSO, aufgearbeitet. Der nach Entfernen des Losungsmittels im Vakuum verbliebene Riickstand wurde an Kieselgel mit Etbylacetat/Benzol(80/20) chromatographiert undgabdieMakrocyclen 1 (55%), 2(15%),3 (12%)und4(2%)sowie ein lineares Adm-Cyst-Polymer in geringen Mengen (ca. 5 Oh).