Metal-catalyzed oxidation and epoxidation of alpha-hydroxy vinyl and dienyl sulfoxides

Treatment of acyclic alpha-hydroxyalkyl alpha,beta-unsaturated sulfoxides with t-BuOOH/VO(acac)2 results in rapid oxidation to the unsaturated sulfones followed by an unusual regio- and stereoselective epoxidation at the unsaturated sulfones; this methodology has been applied to the preparation of carbohydrate-like fragments.     

Synthesis of novel cross-conjugated dendritic fluorophores containing both phenylenevinylene and phenyleneethynylene moieties

New dendrons and dendrimers with dendritic arms composed of alternate phenyleneethynylene and phenylenevinylene moieties have been efficiently synthesized using orthogonal and convergent syntheses that combine Sonogashira and Horner-Wadsworth-Emmons reactions. Two different iterative routes have been developed that allow specific control over the placement of double and triple bonds within the interior of the dendrimers. A preliminary study of the UV and photoluminescence (PL) properties of the resulting compounds is also described. All of the examples prepared are blue-luminescent.     

HCCCH Experiment for Through-Bond Correlation of Thymine Resonances inC-Labeled DNA Oligonucleotides

Application of heteronuclear magnetic resonance pulse methods to¹³C,¹⁵N-labeled nucleic acids is important for the accurate structure determination of larger RNA and DNA oligonucleotides and protein–nucleic acid complexes. These methods have been applied primarily to RNA, due to the availability of labeled samples. The two major differences between DNA and RNA are at the C2′ of the ribose and deoxyribose and the additional methyl group on thymine versus uracil. We have enzymatically synthesized a¹³C,¹⁵N-labeled 32 base DNA oligonucleotide that folds to form an intramolecular triplex. We present two- and three-dimensional versions of a new HCCCH–TOCSY experiment that provides intraresidue correlation between the thymine H6 and methyl resonances via the intervening carbons (H6–C6–C5–Cme–Hme).     

Substituent effects in the migration step of the Baeyer-Villiger rearrangement. A theoretical study

Quantum mechanical calculations have been performed on the migration step of the Baeyer-Villiger (BV) rearrangements of some acetophenones, p-RC6H4COCH3 (R = CN, Cl, H, CH3, CH3O) with m-chloroperbenzoic acid. The energy barriers, charge distributions, and frontier molecular orbitals determined for the aryl migration step explain the effects of substituents on the reactivity of these ketones. A plot of the log of relative oxidation rates of the ketones versus their corresponding calculated energy barriers of the migration stage showed a downward deviation for the p-OCH3 derivative. This result is consistent with a change in the rate-determining step, from the aryl migration to the carbonyl addition, in the case of p-methoxyacetophenone, according to the suggestion that the rate-determining step of the BV oxidation can change with variations in the substituent group.     

Liquid chromatographic-mass spectrometric analysis of anthocyanin composition of dark blue bee pollen from Echium plantagineum

Dark blue bee pollen samples from pollinic type Echium plantegineum were analysed in order to identify and quantify their anthocyanin pigments. Five samples were collected from different apicultural Spanish regions and the anthocyanin composition was determined by HPLC with diode array and MS detection. Eight different pigments were identified, the principal anthocyanin being petunidin-3-O-rutinoside. The other pigments found were delphinidin, cyanidin and petunidin-3-O-glucoside; delphinidin, cyanidin, peonidin and malvidin-3-O-rutinoside and cyanidin-3-(6"-malonylglucoside). The anthocyanin content ranged from 45 to 80 mg/100 g of blue pollen, which could represent a significant source of phytochemicals. Minor variations in the anthocyanin profiles were found, which could be explained by the geographical differences between collection regions.     

Existence and stability of limit cycles in a delayed dry-friction oscillator

Periodic, chaotic, chattering, and bifurcation behavior are fundamental consequences of the nonsmooth nature of systems with dry friction. This work is concerned with the analysis of a single degree of freedom system which is additionally damped by a delayed dry friction device. We get a complete set of closed-form expressions to describe the dynamics of the delay-induced phenomena exhibited by the system. The conditions to determine the existence and stability of limit cycles are clearly defined. This analysis is addressed in the context of both classic stability theory for nonlinear systems and the qualitative theory of Piecewise Smooth Dynamical Systems. Through exhaustive numerical simulations the effectiveness of the set of closed-form expressions is confirmed. Excellent agreement was found between the numerical and analytical results.