Structural studies of substituted 6,7-benzomorphan compounds. I. The absolute configuration of (−)-2-cyclopropylmethyl-2'-hydroxy-5-ethyl-9,9-dimethyl-6,7-benzomorphan (gemazocine) hydrobromide

The "azido gauche effect"-implications for the conformation of azidoprolines

The "azido gauche effect" was examined both experimentally and theoretically and was found to determine the conformation of, for example, (4R)- and (4S)-azidoproline (Azp) derivatives. For (4R)Azp derivatives, the azido gauche effect induces a preferred C(4)-exo conformation of the pyrrolidine ring, which leads to stabilization of the s-trans amide conformer of, e.g., Ac-(4R)Azp-OCH(3) (5R) via an n-->pi interaction between the nonbonding electrons of the oxygen of the acetyl group and the carbonyl group of the ester. For (4S)Azp derivatives, the azido gauche effect results in a C(4)-endo conformation of the pyrrolidine ring that does not allow for this stabilizing n-->pi interaction of the s-trans conformer. Consequently, a significantly higher s-trans:s-cis amide conformer ratio is observed for (4R)Azp compared to (4S)Azp derivatives (e.g., 6.1:1 versus 2.6:1 in D(2)O for Ac-(4R)Azp-OCH(3) (5R) compared to Ac-(4S)Azp-OCH(3) (5S)). These conformational preferences are reflected in the higher tendency of (4S)Azp-containing peptides to form cyclic peptides with all-cis amide bonds compared to (4R)Azp derivatives. Ab initio calculations demonstrate that the strength of the azido gauche effect is comparable to that of the well-known "fluorine gauche effect". For azidoethane derivatives N(3)-CH(2)CH(2)-X (X = N(3), NHCOH, NHAc, or N(CH(3))Ac), the ab initio calculations revealed energy differences of 5-13 kJ mol(-)(1) between the anti and gauche conformations in favor of the gauche conformer. Calculations were also performed for the (4R)Azp and (4S)Azp derivatives 5R and 5S, supporting the experimentally observed data.

Nickel(II) Cyclidenes with Appended Ethylpyridine Receptor Centers as Molecular Tweezers for Dicarboxylic Acids

A series of 14-, 15-, and 16-membered nickel(II) cyclidene macrocycles appended with 2-aminoethyl(2-pyridine) receptors I-III, respectively, were prepared and characterized by X-ray crystallography and NMR techniques. The 14- and 15-membered macrocycles I and II exist in a planar or extended Z-configuration, whereas the 16-membered macrocycle III was saddle shaped and had two asymmetric configurations in the unit cell (IIIa in a "capped" configuration and IIIb in an "open" configuration). Variable-temperature (1)H NMR studies of III in CD(3)CN were conducted (25-65 degrees C), and at room temperature, the interconversion between capping and uncapping is slow on the NMR time scale, resulting in a broad spectrum, whereas at 65 degrees C, interconversion was fast. (1)H NMR binding studies indicated I-III bind unsaturated dicarboxylic acids in a 1:1 stoichiometry with binding constants approaching 400 M(-)(1) in CD(3)CN, and the binding strength was dependent on the shape of the macrocyclic cyclidene platforms, whereas monocarboxylic acids were not bound. Generally, the planar 14-membered cyclidene I bound diacids the weakest and the 16-membered cyclidene III bound diacids the strongest. The presence of nuclear Overhauser effect spectrometry cross peaks in a 20 mM solution of 1:1 II-maleic acid indicates that the binding mode is ditopic with the guest being encapsulated by the aminoethylpyridine arms above the macrocyclic framework.

Dinuclear copper(I) complexes with hexaaza macrocyclic dinucleating ligands: structure and dynamic properties

The synthesis and structural and spectroscopic characterization of a family of copper(I) complexes, containing a dinucleating hexaaza macrocyclic ligand, of general formula [Cu(2)(L)(X)(2)](2+) (L = Me2p, Me2m, Me3p, or Me3m; X = MeCN, n-PrCN, CO, t-BuNC, or PPh(3)) is described. This family of complexes contains ligands that differ from one another in the number of methylenic units linking the tertiary amines and in the meta or para substitution of their aromatic rings. The structural characterization in the solid-state includes a single-crystal X-ray diffraction analysis of [Cu(2)(Me2p)(CO)(2)](2+) and of [Cu(2)(Me2m)(t-BuNC)(2)](2+). In solution, those complexes are structurally characterized through NMR spectroscopy that also allows us to put forward and establish their fluxional behavior. Theoretical calculations at the DFT level have also been performed in order to further analyze the relative energy of the different potential isomers as well as to gain insight into their chemical properties. Finally, the influence of the hexaaza ligands over different structural aspects as well as on its potential chemical reactivity is discussed.

Influence of silaproline on peptide conformation and bioactivity

The analogue gamma-(dimethylsila)-proline, denoted silaproline (Sip), was synthesized in both enantiomerically pure forms by diastereoselective alkylation of a chiral glycine equivalent with use of Schöllkopf's bis-lactim ether method. The effect of replacing a proline residue in model peptides by this new proline surrogate has been examined in the crystal state by X-ray diffraction and in solution by IR absorption and NMR techniques. Silaproline and proline-containing sequences exhibit very similar conformational properties. Silaproline was also substituted for proline in a neurotensin (8-13) analogue that retained biological activity and exhibited enhanced resistance to biodegradation.

Structural and Magnetic Properties of M(mnt)(2) Salts (M = Ni, Pt, Cu) with a Ferrocene-Based Cation, [FcCH(2)N(CH(3))(3)](+). Interplay between M.M and M.S Intermolecular Interactions

A series of metal bis-mnt complexes (mnt = 1,2-dithiolatomaleonitrile) with the trimethylammonium methylferrocene cation have been synthesized and characterized using X-ray diffraction, magnetic susceptibility, and differential scanning calorimetry measurements. The complexes have the formulas (FcCH(2)NMe(3))[Ni(mnt)(2)] (2), (FcCH(2)NMe(3))[Pt(mnt)(2)] (3), and (FcCH(2)NMe(3))(2)[Cu(mnt)(2)] (4) (where Fc = ferrocene). At 300 K, the crystal structures of 1:1 complexes 2 and 3 are very similar. They consist of pairs of [M(mnt)(2)](-) in a slipped configuration packed in stacks. Each [M(mnt)(2)](-) stack is separated from adjacent stacks by two columns of cations. Within the pairs, the [M(mnt)(2)](-) anions interact via short M.S contacts, while there are no short contacts between the pairs. Complex 4, which has a 2:1 stoichiometry, exhibits a markedly different packing arrangement of the anionic units. Due to the special position of the Cu atom in the asymmetric unit cell, [Cu(mnt)(2)](2)(-) dianions are completely isolated from each other. The magnetic susceptibility behavior of the nickel complex is consistent with the presence of magnetically isolated, antiferromagnetically (AF) coupled [Ni(mnt)(2)](-) pairs with the AF exchange parameter, J = -840 cm(-)(1). The platinum complex undergoes an endothermic structural phase transition (T(p)) at 247 K. Below T(p) its structure is characterized by the formation of magnetically isolated [Pt(mnt)(2)](2)(2)(-) dimers in an eclipsed configuration with short Pt.Pt and S.S contacts between monomers. In the magnetic properties, the structural changes reveal themselves as an abrupt susceptibility drop implying a substantial increase of the AF exchange parameter. A mechanism of the phase transition in the platinum compound is proposed. For compound 4, paramagnetic behavior is observed.

X-ray molecular structures and theoretical conformational studies of narcotic analgesics alpha-(-)-N-cis-3-chloroallyl-normetazocine, ethylketazocine, and ketazocine

The X-ray molecular structures of the narcotic analgesics alpha-(-)-2-cis-3-chlorallyl-2'-hydroxy-5,9-dimethyl-6,7-benzomorp han (1) and alpha-(+-)-2-cyclopropylmethyl-2'-hydroxy-5-ethyl-9-methyl-8-oxo-6,7- benzomorphan (ethylketazocine, 2) were determined. The structures and conformations in the crystal were compared and discussed with respect to that of alpha-(+-)-2-cyclopropylmethyl-2'-hydroxy-5,9-dimethyl-8- oxo-6,7-benzomorfan (ketazocine, 3) and those of 15 analogous compounds of the 2'-hydroxy-6,7-benzomorphan series whose structures were previously determined by X-ray analysis. Molecular modeling routines for 1, 2, and 3 produced configurations (N-equatorial) and conformations (distorted chair) of the piperidine ring that were in agreement with those found in the solids. Theoretical studies of the conformations and the rotational energetics of 1, 2, and 3 as cationic species were performed by both the force field (MM2) and the semiquantitative (AM1) methods. The latter method predicted three low energy conformations about N--C(12) and C(12)--C(13) bonds, one of these being more significantly populated (60-68%). The AM1 results were not reproduced by the MM2 method, which predicted four low energy conformations. An interesting common feature of 1, 2, and 3 that was noted with both methods was the restricted interconversion route from the conformational state to another through rotations about the C(12)--C(13) bond. The conformational results were discussed in terms of a working hypothesis for regulation of relative mu and kappa analgesic activities of benzomorphans.

Comparative in vitro/in vivo autoradiography using the opiate ligand 3H-(-)-bremazocine

Autoradiograms of rat brain sections were compared obtained from animals receiving a tritiated drug through intravenous injection or from precut sections incubated in vitro. The benzomorphan analogue 3H-(-)-bremazocine was used as ligand and its distribution to all different opioid binding sites was followed. Although the general distribution of opioid binding sites visualized on 3H-LKB ultrofilms was independent of the methodological approach used, the maximal number of such sites (Bmax) was greater in brain sections incubated in vitro than after in vivo drug application. Since the number of binding sites is highly dependent on the particular incubation condition used, this finding has no further relevance.