A PCILO Study of Conformation and Internal Rotation in Mono-substituted Benzenes

The effect of Kékulé representation and hybrid function of O-atoms in the PCILO-CNDO framework of conformation and internal rotation in mono-sub stituted benzenes Ph-X (X?NH₂, OH, OCH₃, CH₃, CHO, NO₂) is studied. Three variational criteria for the choice of the appropriate third-order energy, proposed to symmetrize the PCILO results, are critically examined in relation with the height of rotational barrier in these molecules. The study shows that, in all cases, the most stable conformation is qualitatively correct predicted by the PCILO method. Since the barrier to internal rotation in the studied aromatic systems arises predominantly from delocalization effect, it is proposed to employ the arithmetic mean of the third-order energy of the two Kékulé structures. In molecules, in which the third-order energy between the two Kékulé structures is larger than 2 kcal/mol, however, the lower third-order energy representation alone seems to be appropriate. In phenol and anisole the spa-hybridization type of the O-atoms offers better values of rotational barrier, whereas in the sp³-type the delocalization is overestimated in the planar conformation.     

NMR-based methods and strategies for drug discovery

Nuclear Magnetic Resonance (NMR) spectroscopy has long been a favourite tool of chemists interested in host-guest systems because it permits access to a wealth of information about the molecular recognition reaction. NMR has evolved dramatically in the last 15 years and, in parallel with the development of NMR methods for the determination of protein structure, a variety of tools aimed at detecting protein ligand interactions have been proposed and are being now used both in industrial and academic laboratories as valuable tools for structure-based drug discovery. Very recent developments have considerably increased the fraction of therapeutic targets that can be tackled by NMR and significantly reduced the amount of sample required for analysis; in this tutorial review we outline the essential NMR-based techniques and describe some examples of their implementation as part of drug discovery programmes.     

PuPHOS: a synthetically useful chiral bidentate ligand for the intermolecular Pauson-Khand reaction

Here we describe the synthesis and use of the Pulegone-derived bidentate P,S ligands PuPHOS and CyPuPHOS in the intermolecular Pauson-Khand reaction. Ligand exchange reaction of hexacarbonyldicobalt-alkyne complexes with PuPHOS provides a diasteromeric mixture of complexes (up to 4.5:1) from which the major isomers can be conveniently separated by simple crystallization. An isomerization-crystallization sequence of the original mixture results in a dynamic resolution that allows the preparation of the pure major Co(2)(mu-TMSC(2)H)(CO)(4)-PuPHOS (15a) in a multigram scale. Pauson-Khand reaction of 15a with norbornadiene provided, for the first time, the corresponding enone 18 with up to 93% yield and 97% ee. The use of (+)-18 as a surrogate of chiral cyclopentadienone is also demonstrated. Copper-catalyzed Michael addition of a Grignard reagent followed by removal of the TMS group with TBAF were the most reliable methods to transform (+)-18 into valuable starting materials 20a-e for the enantioselective synthesis of cyclopentenoid systems.     

A straightforward synthesis of (-)-phaseolinic acid

A concise approach to (-)-phaseolinic acid starting from commercially available (S)-oct-1-yn-3-ol is disclosed. The key steps are a ring-closing metathesis reaction to prepare a C(2)-symmetrical allylic diol and its desymmetrization to a gamma-butyrolactone by using an Ireland-Claisen rearrangement. The 2S,3S,4S configuration of the levogyre natural product has been confirmed.     

Constrained systems: A unified geometric approach

A general geometric framework is devised in order to contain the presymplectic and Lagrangian formalisms as particular cases. We call these objectsconstrained dynamical systems, since their dynamics usually lead toconstraints. Their most elementary properties are studied, and several related structures, especially morphisms, are defined. In particular, a stabilization algorithm is performed. As a byproduct, the dynamics and constraints of the Lagrangian formalism (with the second-order condition) are intrinsically obtained.     

La transformation d'indolines en indoles et d'autres reactions apparentees

Phenylseleninic anhydride reacts rapidly with indolines at 0° to give, when the β-position is substituted, the corresponding indoles. When the β-position is unsubstituted β-phenylseleno- indoles are formed. These are readily reduced by nickel boride to the parent indole. Tetrahyrdroisoquinoline is dehydrogenated with comparable ease. Phenylseleninic acid is also an efficient agent for these dehydrogenations.     

Fabrication of colloidal crystals with tubular-like packings

Methods are reported for the fabrication of colloidal crystal wires with tubular packings. Both free and silica-encased wires have been prepared. Porous silicon membranes are infiltrated with silica spheres, treated with silane, and annealed. After removal of the silicon template, short annealing times were found to result in colloidal crystal wires with varied packing geometries, while repeated annealing cycles produced a thin translucent silica sheath around the wires. Packing in the wires varies with the channel diameter of the Si membrane. The channels used in this study typically produce colloidal crystal wires with six strands, though wires with four to seven strands have been observed. Both chiral and achiral packings are also possible.     

Effect of aggregation on bacteriochlorin a triplet-state formation: a laser flash photolysis study

Bacteriochlorin a (BCA) is a potential photosensitizer for photodynamic therapy of cancer. It has been shown previously that the photoefficiency of the dye is mainly dependent on singlet oxygen (1O2) generation. Nanosecond laser flash photolysis was used to produce and to investigate the excited triplet state of the dye in methanol, phosphate buffer and dimiristoyl-L-alpha-phosphatidylcholine (DMPC) liposomes. The transients were characterized in terms of their absorption spectra, decay kinetics, molar absorption coefficients and formation quantum yield of singlet-triplet intercrossing. The lifetime of the BCA triplet state was measured at room temperature. The triplet-state quantum yield is quite high in methanol (0.7) and in DMPC (0.4) but only 0.095 in phosphate buffer. In the last case, BCA is in a monomer-dimer equilibrium, and the low value of the quantum yield observed was ascribed to the fact the triplet state is only formed by the monomers.     

Stabilized and destabilized carbocations in the 1,6-methano[10]annulene series

2-Chloromethyl and 3-chloromethyl-1,6-methano[10]annulene systems solvolyze in methanol to give simple substitution products. Solvent effect studies and the special salt effect support the involvement of cationic intermediates stabilized by the 1,6-methano[10]annulene group. Rate data indicate that the degree of cation stabilization greatly exceeds that of naphthyl groups. B3LYP/6-31G computational studies also suggest that the cationic intermediates are greatly stabilized by the 1,6-methano[10]annulene. By way of contrast to these findings, solvolytic and computational studies indicate that the 11-(1,6-methano[10]annulenyl) cation is a destabilized analogue of the cycloheptatrienyl cation. There are no favorable interactions with the annulene ring. Distortions from planarity prevent charge delocalization as in the analogous aromatic cycloheptatrienyl cation.