The existence and nature of C-H‚‚‚F-C interactions in crystalline fluorobenzenes 1-3 and 7-10 are discussed. These compounds were chosen because they contain only C, H, and F atoms; this is necessary in the evaluation of the weak acceptor capabilities of the C-F group. All of these compounds are liquids at room temperature, and single crystals for X-ray diffraction were grown in situ. The analysis of the C-H‚‚‚F interactions that are found in all of these crystal structures takes the form of comparisons with related C-H‚‚‚O/C-H‚‚‚N analogues. Fluorobenzene, 1, bears a close relationship to pyridinium fluoride, pyridine 1-oxide, and benzonitrile at the level of individual interactions, showing that the character of the structuredetermining intermolecular interactions in these four crystal structures are the same. Similarly, 1,4-difluorobenzene, 3, and 1,4-benzoquinone are related, the C-H‚‚‚F interactions in the former playing the same structural role as the C-H‚‚‚O interactions in the latter. A comparison of 3 with the unsymmetrical 1,4-dihalogenated benzenes, 4-6 indicates the importance of C-H‚‚‚F interactions in these structures. With an increase in the F content of the molecules, the C-H acidity also increases and the C-H‚‚‚F interactions in 1,3,5-trifluorobenzene, 7, and 1,2,4,5-tetrafluorobenzene, 8, become stronger and more important. Compounds 7 and 8 are structurally very similar to 1,3,5-triazine and 1,2,4,5-tetrazine, and this similarity further strengthens the argument that C-H‚‚‚F interactions resemble C-H‚‚‚N interactions and provides evidence for their description as weak hydrogen bonds. 1,2,3,4-Tetrafluorobenzene, 9, is polymorphic but the role of the C-H‚‚‚F interactions in the two forms is similar. A comparison of the C-H‚‚‚F geometries in compounds 1-10 with other C-, H-, and F-containing compounds in the Cambridge Structural Database reveals that the hydrogen bond properties are more pronounced in 1-10. It is concluded that only when the carbon acidity is enhanced to the levels of the compounds in the present study, is the hydrogen-bond nature of the C-H‚‚‚F interaction even revealed. This study also demonstrates that the C-F group prefers to form C-H‚‚‚F interactions rather than F‚‚‚F contacts. The behavior of organic fluorine in crystal packing is therefore quite different from the heavier halogens.

A less dense packing is observed in the odd-numbered n-alkanes compared to the even-numbered members, which consequently lowers melting temperatures. The reason for this is that the even-numbered n-alkanes have optimal intermolecular interactions at both ends (see the picture on the left), while the odd-numbered ones possess these only at one end-at the other end the intermolecular distances are longer (right).

Four cucurbitane glycosides, momordicosides Q, R, S, and T, and stereochemistry-established karaviloside XI, were isolated from the vegetable bitter melon (Momordica charantia). These compounds and their aglycones exhibited a number of biologic effects beneficial to diabetes and obesity. In both L6 myotubes and 3T3-L1 adipocytes, they stimulated GLUT4 translocation to the cell membrane--an essential step for inducible glucose entry into cells. This was associated with increased activity of AMP-activated protein kinase (AMPK), a key pathway mediating glucose uptake and fatty acid oxidation. Furthermore, momordicoside(s) enhanced fatty acid oxidation and glucose disposal during glucose tolerance tests in both insulin-sensitive and insulin-resistant mice. These findings indicate that cucurbitane triterpenoids, the characteristic constituents of M. charantia, may provide leads as a class of therapeutics for diabetes and obesity.

Dedicated to Professor Paul Rademacher on the occasion of his 60th birthdayHydrogen bonding and hydrophobic interactions are ubiquitous in biological structures, be they lipids, proteins, or nucleic acids. [1] The interference between these two kinds of interactions in such natural systems is obvious, but hard to study and difficult to perceive owing to their inherent complexity. An understanding of such interference has important implications in biological and material phenom-Under the best conditions (2.75 mol % 5, 2.5 mol % Cu(OTf) 2 , toluene/hexane 80/20, 5 h), 5bhr was identified as the best ligand for cyclohexenone 9 and cycloheptenone 10 OH N HN S Cl Cl O O OH N HN S tBu O O 5bhr 5chqtBu (at À 20 8C), giving 12 in 90 % ee and 13 in 85 % ee, with 100 % conversion in both cases and 93 ± 95 % yield (isolated product). Compound 5chq (Figure 1) was recognized as the best ligand for cyclopentenone (11) (at 0 8C), giving 14 in 80 % ee albeit in a low yield (25 %).In conclusion, we have developed a parallel library of new Schiff base chiral ligands 5 and optimized their use in the enantioselective conjugate addition of Et 2 Zn to cyclic enones by a high-throughput screening approach. Work is in progress to extend the scope of ligands 5 in other enantioselective reactions.

The principles of crystal engineering have been used to design a family of structures with potential as octupolar nonlinear optical (NLO) materials. The major aim in such an exercise, a carry-over of molecular symmetry into the crystal, is possible with a retrosynthetic approach. An appropriate choice of precursor trigonal molecules leads from the concept of the dimeric Piedfort unit. The crystal structures and NLO properties of a series of 2,4,6-triaryloxy-1,3,5-triazines, 1−6, are reported. These compounds consistently form quasi-trigonal or trigonal networks that are two-dimensionally noncentrosymmetric. Substitutional variations on the phenyl moieties that were expected to maintain or to perturb this trigonal network have been explored. Molecular nonlinearities have been measured by Harmonic Light Scattering (HLS) experiments. Among the compounds studied, 2,4,6-triphenoxy-1,3,5-triazine, 1 adopts a noncentrosymmetric crystal structure with a measurable SHG powder signal. All these crystal structures are stabilized by weak intermolecular interactions such as herringbone, π···π, C−H···O, and C−H···N hydrogen bonding. These octupolar molecules are more isotropic than the classical p-nitroaniline based dipolar NLO molecules, and this is advantageous from the viewpoint of potential electrooptic applications.