Biscavitands II: Self-Inclusion in a Back-to-Back Connected Biscavitand

A foot-to-foot or `back-to-back' connected biscavitand is prepared directly from a hexadecol resorcinarene precursor. The axial orientation of the biphenyl linker and hence the crown conformation of the hexadecol was established by an X-ray crystal study of the biscavitand. Each cavitand bowl is filled in the crystal by an alkyl `foot' from the next molecule, a self-inclusion which results in polymeric host–guest chains. The new biscavitand differs from previously prepared Z and C isomers of a bowl-to-bowl or `front-to-front' connected host, which crystallize as chains of carcerand-like, solvent-filled cages or as distinct molecules of hemicarceplex, respectively.     

Torsional Motion in (tert‐Butyl)ammonium Hemispheraplexes: Rotational Barriers and Energy of Binding

The ADPs (ADPs=atomic anisotropic displacement parameters) from the single‐crystal X‐ray studies of nine related TBA⁺ (TBA⁺=(tert‐butyl)ammonium) hemispheraplexes are analyzed, and the results compared to the free energy of binding of this guest by the nine hosts. The lipophilic hosts (Fig. 1) were synthesized over a number of years, with increasing pre‐organization for and specificity of binding. Structural studies for six of the complexes have been published, but the remaining three structures, including those of the strongest binders of TBA⁺, are disordered and have only now been completed. New area‐detector data has been analyzed for the TBA⁺ClO complexes of 5 and of 8 at two temperatures, while the original data for 9 ?TBA⁺SCN^? has been treated with a disorder model. In addition, improved models are presented for the complexes of 6 and 7 . Methods for assessing the precision of the ADP analyses are discussed. Although most of the structures are imprecise, the TBA⁺ groups do demonstrate some of the characteristics of independent motion. The general trend in calculated libration amplitudes for the TBA⁺ group suggests that the guests with the greatest free energy of binding, and the shortest distances from N⁺ to the ligand plane, are those with the highest barriers to internal rotation.     

CYTOKINE RELEASE BY PERIPHERAL BLOOD MONONUCLEAR CELLS IS AFFECTED BY 8-METHOXYPSORALEN PLUS UV-A

Psoralen plus UV-A (PUVA) is an effective therapy for psoriasis but also for other inflammatory dermatoses. The precise mechanisms of action, however, are not absolutely clear. Therefore, the effect of PUVA on the release of the proinflammatory cytokines interleukin (IL)-1, IL-6, IL-8 and tumor necrosis factor alpha (TNFα) was studied. Peripheral blood mononuclear cells (PBMC) obtained from humans were incubated with 8-methoxypsoralen (8-MOP) and exposed to UV-A (20 kJ/m²). This treatment resulted in a significant reduction of IL-6 and IL-8 amounts in the supernatants. In addition, an inhibition of IL-1β and TNFα production by lipopolysaccharide (LPS)-stimulated PBMC was observed upon PUVA treatment. Accordingly, northern blot analysis showed decreased levels of mRNA encoding for IL-1β, IL-6, IL-8 and TNFα in PUVA-treated PBMC. Finally PBMC were obtained from psoriatics undergoing oral photochemotherapy before the beginning and after completion of treatment. The PBMC collected after PUVA spontaneously produced significantly less IL-6 and IL-8 in comparison to the respective samples obtained before therapy. A similar suppression of IL-1β and TNFα by in vivo PUVA was found in LPS-stimulated PBMC. The present data demonstrate that PUVA both in vitro and in vivo suppresses the production of the proinflammatory cytokines IL-1β, IL-6, IL-8 and TNFα by PBMC. Because these cytokines are important in the mediation of inflammatory reactions, one may speculate that the inhibitory effects could contribute to the antiinflammatory activity of PUVA.     

Chiral 1,1′-Binaphthyl Molecular Clefts for the Complexation of Excitatory Amino-Acid Derivatives

The complexation of N-benzyloxycarbonyl (Cbz) derivatives of the excitatory amino acids L -aspartic acid (Asp; 1 ), L -glutamic acid (Glu; 3 ), and, for the first time, L -kainic acid ((2S,3S,3S)-2-carboxy-4-(1-methylethenyl)pyrrolidine-3-acetic acid; Kai; 5 ) was studied in CDCl₃ with a diversity of chiral receptors consisting of a 1,1′-binaphthyl spacer with (carboxamido)pyridine (CONH(py)) functionality attached to the 6,6′-positions in the major groove. Receptors of type A possess two N-(pyridin-2-yl)carboxamide H-bonding sites (e.g. 7 ), whereas type B-receptors have two N-(pyridine-6,2-diyl)acetamide residues attached (e.g. 8 and 9 ). Complexes of excitatory amino-acid derivatives and other, achiral α,β-dicarboxylic acids with these receptors are primarily stabilized by two sets of C?O···H? N and O? H ··· N H-bonds. Optically active type-A receptors such as (R)- and (S)- 7 showed a preference for the larger Glu derivative, whereas type- B receptors such as (R)- and (S)- 8 and (R)- and (S)- 9 formed more stable complexes with the smaller Cbz-Asp. To improve the poor enantioselectivity shown by 7–9 , additional functionality was introduced at the 7,7′-positions of the 1,1′-binaphthyl spacer, and the nature of the H-bonding sites in the 6,6′-positions was varied. Screening the diversity of new racemic receptors for binding affinity, which had been shown in many examples by Cram to correlate with enantioselectivity, demonstrated that (+)- 10 and (+)- 11 formed the most stable complexes with dicarboxylic acids, and these receptors were synthesized in enantiomerically pure form. Both are type- B binders and contain additional PhCH₂O ( 10 ) and MeO ( 11 ) groups in the 7,7′-positions. By ¹H-NMR binding titrations, the complexation of (R)- and (S)- 10 and (R)- and (S)- 11 with the excitatory amino-acid derivatives was studied in CDCl₃, and association constants K_a between 10³ and 2 · 10⁵ l mol^?1 were measured for the 1:1 host-guest complexes formed. Whereas both 10 and 11 formed stable complexes, enantioselective binding was limited to the PhCH₂O-substituted receptor 10 , with the (R)-enantiomer complexing Cbz-Asp by 0.7 kcal mol^?1 more tightly than the (S)-enantiomer. The structures of the diastereoisomeric complexes were analyzed in detail by experimental methods (complexation-induced changes in ¹H-NMR chemical shifts, ¹H{¹H} nuclear Overhauser effect (NOE) difference spectroscopy) and computer modeling. These studies established that an unusual variety of interesting aromatic interactions and secondary electrostatic interactions are responsible for both the high binding affinity (? ΔG° up to 7.2 kcal mol^?1) and the enantioselection observed with (R)- and (S)- 10 . In an approach to enhance the enantioselectivity by reducing the conformational flexibility of the 1,1′-binaphthyl spacer, an additional crown-ether binding site was attached to the 2,2′-positions in the minor groove of the type- B receptors (R)- and (S)- 48 . Both the binding affinity and the enantioselectivity (Δ(ΔG°) up to 0.7 kcal mol^?1) in the complexation of the excitatory amino-acid derivatives by (R)- and (S)- 48 were not altered upon complexation of Hg(CN)₂ at the crown-ether binding site, demonstrating lack of cooperativity between the minor- and major-groove recognition sites.     

Reactions of 1,3,4-triphenyl-1,2-dihydrophosphete

In contrast to previously reported reactivity of the tungsten pentacarbonyl complex of a 2-substituted 1,2-dihydrophosphete, which apparently undergoes electrocyclic ring opening to the corresponding 1-phospha-1,3-butadiene and subsequent [4+2] cycloaddition reactions with dienophiles, the reaction chemistry of 1,3,4-triphenyl-1,2-dihydrophosphete is dominated by its nucleophilic nature. Although low to modest yields of cycloadducts are obtained with some dienophiles, the reactions forming these products are apparently stepwise, as indicated by the loss of stereochemistry in the reaction of dimethyl maleate and in the competitive formation of a phosphorus-free dimer in the reaction of N-methylmaleimide. Dimethyl acetylenedicarboxylate affords three major products, each of which incorporates two equivalents of the acetylene, again apparently a result of initial nucleophilic addition of the dihydrophosphete to the “dienophile.” © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:9–19, 1998