New chiral ligands based on (+)-α-pinene

A number of new chiral ligands were synthesized starting from an accessible monoterpene, (+)-α-pinene. The new ligands were used in the vanadium-catalyzed oxidation of sulfides to chiral sufoxides.     

Reaction path as a gradient line on a potential energy surface

The reaction path is shown to be always a gradient line on a potential energy surface (PES ) of a molecule. The properties of gradient lines on the PES are elucidated. Correct symmetry conservation rules along the gradient line are derived. The behavior of the gradient line on a PES with different topologies are considered. © 1994 John Wiley & Sons, Inc.     

Mechanisms of photoinduced electron transfer reactions of lappaconitine with aromatic amino acids. Time-resolved CIDNP study

CIDNP techniques were applied to the investigation of the elementary mechanism of photoinduced interaction between anti-arrhythmic drug lappaconitine and amino acids tyrosine and tryptophan. It has been shown that the reactions involve the formation of lappaconitine radical anion. Lappaconitine radical anion is unstable and rapidly eliminates N-acetyl anthranilic acid via protonation and ether bond cleavage. The rate constant of ether bond cleavage was estimated to be equal to 4 x 10(5) s(-1). The role of single electron transfer is discussed in the light of the model of drug-receptor interactions.     

Face-to-face and edge-to-face pi-pi interactions in a synthetic DNA hairpin with a stilbenediether linker

Synthetic conjugates possessing bis(2-hydroxyethyl)stilbene-4,4'-diether linkers (Sd2) form the most stable DNA hairpins reported to date. Factors that affect stability are length and flexibility of the linkers and pi-stacking of the stilbene moiety on the adjacent base pair. The crystal structure of the hairpin d(GT(4)G)-Sd2-d(CA(4)C) was determined at 1.5 A resolution. The conformations of the two molecules in the asymmetric unit differ both in the linker and the stem portions. One of them shows a planar stilbene that is stacked on the adjacent G:C base pair. The other displays considerable rotation between the phenyl rings and an unprecedented edge-to-face orientation of stilbene and base pair. The observation of considerable variations in the conformation of the Sd moiety in the crystal structure allows us to exclude restriction of motion as the reason for the absence of Sd photoisomerization in the hairpins. Conformational differences in the stem portion of the two hairpin molecules go along with different Mg(2+) binding modes. Most remarkable among them is the sequence-specific coordination of a metal ion in the narrow A-tract minor groove. The crystal structure provides unequivocal evidence that a fully hydrated Mg(2+) ion can penetrate the narrow A-tract minor groove, causing the groove to further contract. Overall, the structural data provide a better understanding of the origins of hairpin stability and their photochemical behavior in solution.     

Internal conrotation and disrotation in H2BCH2BH2 and diborylmethane 1,3 H exchange

The paths of correlated internal disrotation (barrier less than 0.4 kcal/mol) and conrotation (barrier around 1.9 kcal/mol) of the two BH₂ groups in H₂BCH₂BH₂ have been computed employing ab initio [MP2(full)/6–31G**] and density functional theory (Becke3LYP/6–311+G**) methods. Two B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative interactions stabilize the C_s symmetric H₂BCH₂BH₂ isomer ( 1 ). The B(SINGLE BOND)C(DOTTED BOND)B(p) hyperconjugative stabilization, evaluated by homodesmotic reactions and using the orbital deletion procedure (which “deactivates” the “vacant” born p orbital), is less than 6 kcal/mol in diborylmethane. The B(SINGLE BOND)C(DOTTED BOND)B(p) stabilization is shown to be remarkably large in C₄B₆H₁₀ (Td). At MP2(fu)/6–31G**, disproportionation into 1 and methane is only 5.6 kcal/mol exothermic. The 1,3 H exchange in diborylmethane is an asynchronous process and proceeds via a doubly bridged cyclic intermediate with 9.3 kcal/mol barrier. Structures with “planar tetracoordinate” carbon are stabilized considerably by BH₂ substituents, but they are still high in energy. © 1997 John Wiley & Sons, Inc. J Comput Chem 18 : 1792–1803, 1997     

The reactions of 2H-1,2,3-diazaarsoles with phenyl azide

2-Phenyl-5-methyl- and 2,5-diphenyl-2H-1,2,3-diazaarsoles 1a,b react with phenyl azide to give several crystalline products: tricyclic derivatives 2a,b and 4,4′-bis(2H-1,2,3-diazaarsoles) 5a,b formed at room temperature, and a cycloadduct 6a obtained at +4°C. Compound 6a undergoes a fast rearrangement in solutions of Et₃N or pyridine to give a stable dicoordinate arsenic compound, the 2H-1,2,3-diazaarsole 7a. Heating solutions of 2a under reflux in an inert atmosphere leads to 5a and of 2b, in the presence of water, to 4b. The structures of 2a, 4b, and 7a were characterized by X-ray crystal structure determinations. © 1996 John Wiley & Sons, Inc.     

Synthesis of Optically Active Tricyclic Ethers by Reactions of (−)‐β‐Pinene with Phenols in Organized Media

The reactions of (−)‐β‐pinene ( 1 ) with some methyl‐ and hydroxy‐substituted phenols on Hβ‐zeolite yield optically active products – tricyclic terpenyl phenyl ethers – in contrast to reactions in homogeneous media, which occur with loss of optical activity. The formation of these products is promoted by the presence of meta‐substituents in the phenol molecule.