Vera M. Divac

Mechanistic investigation of the base-promoted cycloselenoetherification of pent-4-en-1-ol

The mechanism of phenylselenoetherification of pent-4-en-1-ol using some bases (pyridine, triethylamine, quinoline, 2,2′-bipyridine) as catalyst was examined through studies of kinetics of the cyclization, by UV-VIS spectrophotometry. It was demonstrated that the intramolecular cyclization is facilitated in the presence of bases caused by the hydrogen bond between base and alkenol’s OH-group. The obtained values for rate constants have shown that the reaction with triethylamine is the fastest one. Quantum chemical calculations (MP2(fc)/6-311+G**//B3LYP/6-311+G**) show, that the transition state of the cyclisation is SN2 like.

Cyclization of some terpenic alcohols by phenylselenoetherification reaction

Highly substituted tetrahydrofuran (THF)- and tetrahydropyran (THP)-type rings are formed through an acid- or base-catalyzed 5-exo and/or 6-endo cyclization of some natural terpenic alcohols (e.g., linalool, nerolidol, and α-terpineol) by an electrophile-mediated cyclization with PhSeCl and PhSeBr. The side chains of these cyclic ether products can be further transformed easily into a wide range of substrates owing to the versatile functionality of the double bond. Certain regioselectivity was noticed in these reactions. Nerolidol behaves like linalool in the reaction with PhSeX and affords predominantly THF derivatives, whereas α-terpineol affords THPs. Some Lewis bases (triethylamine, pyridine, 2,2′-bipyridine, and quinoline) and Lewis acids (SnCl2 and CoCl2) were used as additives, the presence of which increases the yields from 5–40xa0% to almost quantitative.Graphical Abstract

Kinetic and mechanistic studies of base-catalyzed phenylselenoetherification of (Z)- and (E)-hex-4-en-1-ols.

The mechanism of phenylselenoetherification of (Z)- and (E)-hex-4-en-1-ols using some bases (triethylamine, pyridine, quinoline, 2,2-bipyridine) as catalysts and some solvents [tetrahydrofuran (THF) and CCl4] as reaction media was examined through studies of kinetics of the cyclization by UV-vis spectrophotometry. It was demonstrated that the intramolecular cyclization is facilitated in the presence of bases as a result of the hydrogen bond between the base and the alkenols OH group. The rate constants in the base-catalyzed reactions are remarkably influenced by the bulkiness and basicity of the base used and the nature of the considered nitrogen donors. The obtained values for rate constants show that the reaction with triethylamine is the fastest one. THF with higher polarity and higher basic character is better as a solvent than CCl4. Quantum-chemical calculations [MP2(fc)/6-311+G**//B3LYP/6-311+G** + ZPE(B3LYP/6-311+G**] show that the cyclization of (Z)-hex-4-en-1-ol to a tetrahydrofuranoid five-membered ring is kinetically controlled, while the cyclization of (E)-hex-4-en-1-ol to the tetrahydropyranoid six-membered ring is thermodynamically controlled. This is in accordance with previous experimental findings.

Kinetic and mechanistic insight into Lewis base and acid-mediated phenylselenoetherification of 2,6-dimethyl-hept-5-en-2-ol

Mechanistic and kinetic aspects, as well as quantum chemical calculations, for the formation of a tetrahydrofurane ring by cyclization of the highly substituted alcohol–2,6-dimethyl-hept-5-en-2-ol with phenylselenohalides (PhSeCl and PhSeBr), in the presence of various Lewis bases (piperidine, triethylamine, pyridine and quinoline) and Lewis acids (CoCl2, SnCl2) as additives were studied. The substituted tetrahydrofuran ring is common motif in many natural products, especially in bioactive marine macrolides. Due to the presence of additives in reactions, the high regioselectivity was achieved, and the tetrahydrofuran type of ether in all studied reactions was obtained in excess in regard to the six-membered cyclic ether. In addition, kinetic aspects of reactions mediated by Lewis bases were studied by UV–Vis spectroscopy (in THF, at 288xa0K), as reactions of pseudo-first-order. Obtained values for rate constants proved catalytic role of additives and indicated that the reaction rate depends on pKa values of used base as well as on their ability for formation of hydrogen bond with OH–alcohols group.

Kinetic investigation in the formation of 2,2,5-trisubstituted tetrahydrofurans by catalyzed phenylselenoetherification of some terpenic alcohols

The mechanism of the phenylselenoetherification of some naturally occurring alcohols linalool and nerolidol in the presence of some bases (piperidine, pyridine, triethylamine, quinoline) as catalysts was examined through the kinetic study of the cyclization under pseudo-first order conditions by UV–Vis spectrophotometry. It was demonstrated that the intramolecular cyclization is facilitated in the presence of some bases caused by the hydrogen bond between base and OH-group of the alkenol. The obtained values for the rate constants show that there is a good agreement between those constants and the basicity of the catalyst. The fastest reaction is the one using the strongest base (piperidine) and the smallest rate constant corresponded to the use of the weakest base (quinoline).

Aplicyanins – brominated natural marine products with superbasic character

Abstract By using quantum chemical methods (B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d)), we investigated the structures of aplicyanin A, aplicyanin B, aplicyanin C, aplicyanin D, aplicyanin E, and aplicyanin F along with their protonated structures. The calculated gas phase proton affinities of aplicyanin A, aplicyanin C, and aplicyanin E are around –250 kcal mol−1 and therefore more than 10 kcal mol−1 higher as in typical proton sponges such as 1,8-bis(dimethylamino)naphthalene. The compounds aplicyanin B, aplicyanin D, and aplicyanin F show reduced proton affinities of approximately –240 kcal mol−1 because of the acetyl group being conjugated with the imine N=C moiety. Nucleus-independent chemical shift (NICS) calculations on the same level of theory do not show any peculiarities, and a reasonable correlation between the toxicity of aplicyanins and the gas phase proton affinity is not observed.

Synthesis, crystal and solution structures and antimicrobial screening of palladium(II) complexes with 2-(phenylselanylmethyl)oxolane and 2-(phenylselanylmethyl)oxane as ligands

Two novel Pd(II) complexes with 2-(phenylselanylmethyl)oxolane and 2-(phenylselanylmethyl)oxane as ligands were synthesized. The crystal and molecular structure of the complexes has been determined by single crystal X-ray diffraction. It turned out for both complexes that the two ligands are coordinated to Pd via Se atoms in a trans-fashion and the other two trans-positions are occupied by Cl ions. Detailed 1D- and 2D-NMR analyses revealed the existence of equilibrating trans-diastereomeric species differing in the configuration at four chiral centers (selenium and carbon) in the solution of the complexes. A computational study was also undertaken to assess the relative stabilities of the mentioned stereoisomeric species. The antimicrobial properties of the complexes were investigated against a series of human pathogenic bacterial and fungal strains. The complexes were shown to possess promising broad spectrum moderate antimicrobial activity that is more pronounced against fungal organisms. The noted activity could be completely attributed to the Pd(II) center, whereas the ligands probably mediate the transportation of a Pd(II) species across cell membranes.

Cyclization of Unsaturated Alcohols. Mild and Efficient Selenocyclization of Pent-4-en-1-ol

An innovative route for intramolecular cyclization of pent- 4-en-1-ol has been delineated through a ring closing reaction with phenylselenyl halides, in good yield. Several catalysts (triethylamine, quinoline, 2,2ʹ-bipyridine, pyridine, CoCl2 and SnCl2) enable fast, facile and efficient cyclization. Graphical Abstract Cyclization of Unsaturated Alcohols. Mild and Efficient Selenocyclization of Pent-4-en-1-ol