Unique solvent effect on photochemistry of ortho-alkylphenacyl benzoates

Photolysis of 2,4,6-trialkylphenacyl benzoates gives not only the corresponding indanones and benzoic acid, but also the corresponding benzocyclobutenols (CBs), which are also detected in the photolysis of mono-alkylphenacyl benzoates for the first time. The product selectivity was heavily dependent upon solvents and o-alkyl group. H-bonding acceptor solvents strongly favor the formation of the CB. As the size of the o-alkyl group increases, the relative amount of the CB increases.     

o-Formylation of electron-rich phenols with dichloromethyl methyl ether and TiCl4

o-Formylation of electron-rich phenols is accomplished with dichloromethyl methyl ether and TiCl₄. The reaction gives excellent yields, good regioselectivity, and does not leading to diformylation.     

Wavelength-selective cleavage of o-nitrobenzyl and polyheteroaromatic benzyl protecting groups

Evaluation of the wavelength-selective cleavage of five photolabile protecting groups from two different families has been performed. Alanine, as a model bifunctional target molecule was masked at the amino terminal with o-nitrobenzyl group and at the carboxylic terminal with benzyl-type nitrogen and oxygen polyheteroaromatics, namely acridine, (thioxo)benzocoumarin and a coumarin built on the julolidine nucleus. The photosensitivity of the corresponding alanine conjugates was studied at selected wavelengths with HPLC/UV and ¹H NMR monitoring. The release of the fully deprotected molecule could be achieved by sequential irradiation in variable irradiation times, which were dependent on the heteroaromatic group used.     

New deconjugation reaction of (E)-1-indanylidene methylarene brought by photolysis with protic acid

A new photodeconjugation reaction of (E) indanylidene methylarenes (1a-d) carried out by photolysis in the presence of protic acid are reported with 80% to >95% yields. The reaction mechanism is through the protonation of the less stable (Z) isomer to form stable indanyl cation followed by deprotonation.     

Regioselectivity in the reductive ring-opening reaction of 1,2-O-benzylidene sugars

Regioselectivity in the ring-opening reaction of 1,2-O-benzylidene sugars was studied. In the reductive ring-opening reaction of 1,2-O-benzylidene derivatives, only a CO1 bond was cleaved in the case of manno-type, but both the CO1 and CO2 bonds were cleaved in the case of gluco-type.     

Use of p-nitrobenzyloxycarbonyl (pNZ) as a permanent protecting group in the synthesis of Kahalalide F analogs

p-Nitrobenzyloxycarbonyl (pNZ) is used for the permanent protection of ornithine in the synthesis of derivatives of the anti-tumor cyclodepsipeptide Kahalalide F that contain acid labile residues.     

Flash photolysis/temperature jump study of the vibrational relaxation of N2O(010) by O(P) and NO

This survey begins with the photochemistry at 254 nm and 298 K in the system H₂O₂COO₂RH, the primary objective of which is to determine the rate constants for the reaction OH + RH → H₂O + R relative to the well-known rate constant for the reaction OH + CO → CO₂ + H. Inherent in the scheme is that the reaction HO₂+CO→OH+CO₂ is negligible compared with the OH reaction, and a literature consensus gives k_HO2 < 10⁻¹⁹ cm³ molecule⁻¹ s⁻¹, or some 10⁶ less than k_OH at 298 K. Theoretical calculations establish that the first stage in the HO₂ reaction is the formation of a free radical intermediate HO₂ + CO → HOOCO (perhydroxooxomethyl) which decomposes to yield the products, and that the rate of formation of the intermediate is equal to the rate of formation of the products. The structure of the intermediate and a reaction profile are shown.

High temperature rate data reported subsequent to the data in the consensus and theoretical calculations lead here to a recommendation that, in the range 250–800 K, k_HO2 = 3.45 × 10⁻¹²T^1/2 exp(1.15 × 10⁴/T) cm³ molecule⁻¹ s⁻¹, the hard-sphere-collision Arrhenius modification. This yields k_HO2(298) = 1.0 × 10⁻²⁷ cm³ molecule⁻¹ s⁻¹ or some 10¹⁴ slower than k_OH(298).     

Selective modification of oleuropein,a multifunctional bioactive natural product

Oleuropein is the major phenolic compound extracted from olive leaves and it is also present in olive fruits and virgin olive oil. It has already demonstrated a wide range of pharmacological activities, but new derivatives have to be designed in order to obtain useful Structure–Activity Relationships for this family of products, as well as new analogs with improved and/or different biological properties. However, taking into account the multiple OH groups (catechol and sugar) present in oleuropein, the selective modification of this natural product is a challenging problem. In this paper, we present an efficient strategy allowing the selective modification at the primary alcohol position of this natural product. Using appropriate protection/deprotection sequences, we prepared by hemisynthesis several triazoles linked to the oleuropein core, as representative examples for the development of such an approach.     

Temperature dependent Raman scattering study of l-ascorbic acid

Temperature dependent Raman study of l-ascorbic acid has been performed from 15 to 418 K. Changes in the wavenumber vs. temperature plots for some internal modes were interpreted as conformational molecular change and the discontinuity in the wavenumber vs. temperature plots along with the appearance of a new vibrational mode in the temperature range 200-270 K suggests that l-ascorbic acid undergoes a structural phase transition. For temperatures higher than 300 K, no relevant modification was observed on the Raman spectra thus indicating a stable structure at high temperatures. Additionally, a correlation between OH stretching wavenumber and the behavior of hydrogen bond is also made.     

Effect of temperature on hydrogen peroxide photolysis in aqueous solutions

The photolysis of hydrogen peroxide in dilute aqueous solution (1 × 10⁻⁴ M) at various temperatures (15–85°C) and pH (ph 2.5–7) was studied by flash photolysis. The rate of oxygen production under continuous photolysis conditions was measured at room temperature. The rate constants and activation parameters are reported. Evidence for the formation of complexes between hydrogen peroxide and intermediate radicals is presented. The liquid phase data are discussed and compared with those available for the gas phase.     

Semipermanent p-nitrobenzyloxycarbonyl (pNZ) protection of Orn and Lys side chains: prevention of undesired α-Fmoc removal and application to the synthesis of cyclic peptides

Semipermanent side-chain protection of Orn and Lys with p-nitrobenzyloxycarbonyl (pNZ) for Fmoc/^tBu chemistry does not result in the unwanted removal of α-Fmoc that occurs when groups such as Alloc are used for the same application. Furthermore, pNZ can be used in conjuction with p-nitrobenzyl ester (pNB) to prepare cyclic peptides.     

Detailed Temperature-dependent Study of n-Heptane Pyrolysis at High Temperature

n-Heptane is the most important straight chain paraffin in the fossil-fuel industry. In this work, pyrolysis behavior of n-heptane at high temperature is investigated by a se-ries of ReaxFF based reactive molecular dynamics simulations. Temperature effects on then-heptane pyrolysis and related products distributions have been detailedly analyzed. The simulation results indicate that the temperature effect is characterized in stages. High tem-perature can accelerate the decomposition of n-heptane, but the influence becomes small after it reaches a certain level. According to the different reaction behaviors, pyrolysis of n-heptane could be divided into three stages. The variation trends of the mass fraction evolu-tion of ethylene (C₂H₄), C3, and C4 calculated from reactive molecular dynamics simulations are in good agreement with the previous experimental results. The apparent activation en-ergy extracted from the first-order kinetic analysis is 53.96 kcal/mol and a pre-exponential factor is 55.34×10¹³ s^-1, which is reasonably consistent with the experimental results.     

Temperature effect in separation of fullerene by high-performance liquid chromatography

Summary The effect of column temperature, especially at low temperatures, on the separation of fullerenes on monomeric and polymeric octadecyl silica (ODS) bonded phases has been studied. Decreasing the column temperature induces an increase in selectivity. The best temperature for the separation of fullerenes was determined for both types of ODS phase with n-hexane eluent. The selectivity for higher fullerenes on monomeric phases becomes similar to that on polymeric phases to low temperature. It has been found that as the carbon content of monomeric phases is increased, the selectivity also becomes similar to polymeric phases.     

A Vicinal Acyloxy Group Participation SN2 Reaction of Thiol Nucleophiles in the Formation of Thioacetals

Thioacetalization of acyl protected furanosides led to products with an ethanethiol group at C‐2 and 3‐O‐acetyl‐ 1,2‐di‐O‐isopropylidene‐D‐furanoses were converted into corresponding thioacetals with two ethanethiol groups at both C‐2 and C‐3 positions under the standard thioacetalization conditions. All products were characterized by ¹H NMR, ¹³C NMR and HRMS data. X‐ray structure analysis indicates that the vicinal acyloxy group is stereoselectively substituted by ethanethiols. The supposed mechanisms for these two kinds of transformations were presented.     

Armed-disarmed effect of remote protecting groups on the glycosylation reaction of 2,3-dideoxyglycosyl donors

The armed-disarmed effect of remote protecting groups at the C-4 and/or C-6 position(s) on the glycosylation reactions of 2,3-dideoxyglycosyl donors was investigated. It was found that under various glycosylation conditions, 4- or 6-O-Bn 2,3-dideoxyglycosyl donors were much more reactive than the corresponding 4,6-di-O-Bz 2,3-dideoxyglycosyl donors. Based on these results, an effective and chemoselective glycosylation reaction using 4,6-di-O-Bn glycosyl acetate and 4-OH-6-O-Bz glycosyl acetate was realized, producing a 2,3-dideoxydisaccharide in good yield with high α-stereoselectivity.     

Chemical synthesis of RNA via 2′-O-cyanoethylated intermediates

It was found that 2′-O-cyanoethyl group could be removed from 2′-O-cyanoethylated ribonucleoside derivatives by treatment with Bu₄NF. This finding was successfully applied to the synthesis of oligoribonucleotides via their 2′-O-cyanoethylated derivatives as key intermediates where a cyanoethyl group was used as the 2′-hydroxyl protecting group. The rate of condensation using this protecting group in the presence of various activators was generally faster than that observed when a TBDMS group was used as the protecting group.     

Original and efficient synthesis of 2:1-[α/aza]-oligomer precursors

The preparation of 2:1-[α/aza]-oligomer precursors is described via Mitsunobu and exchange of protecting groups protocols in four steps in good yields starting from N-tert-butyloxycarbonylaminophtalimide. Conformational studies showed that these building blocks further led to β-turn-like folded 2:1-[α/aza]-trimer which suggests that they are good candidates to form foldamers.     

Temperature effect on the solvent extraction of some lanthanides with thenoyltrifluoroacetone

The solvent extraction of Pr, Gd and Yb with thenoyltrifluoroacetone solutions in C₆H₆, CHCl₃ and CCl₄ at 288 K, 298 K, 308 K and 318 K has been investigated. The values of the equilibrium constant have been calculated as well as the values of the thermodynamic parameters H, S and G. The temperature effect on the solvent extraction of lanthanides with thenoyltrifluoroacetone is discussed.

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Einfluß der Temperatur auf die Extraktion einiger Lanthanoide mit Thenoyltrifluoraceton

Zusammenfassung Es wurde die Extraktion von Pr, Gd und Yb mit Lösungen von Thenoyltrifluoraceton in C₆H₆, CHCl₃ und CCl₄ bei 288 K, 298 K, 308 K und 318 K untersucht. Es wurden die Gleichgewichtskonstanten für die untersucht3n Metalle, Lösungsmittel und Temperaturen sowie die Werte der thermodynamischen Parameter H, S und G berechnet. Der Einfluß der Temperatur auf die Extraktion von Lanthaniden wird diskutiert.

     

粉末导电填料／聚合物导电复合材料导电性与温度的关系

复合型导电高聚物电导率/温度关系大致可分为三类,即NTC、LPTC和HPTC。尤其是具有HPTC效应的材料实用价值很高。温度直接影响材料的导电性,且与导电机理、导电填料浓度等密切相关。     

Photochemistry synthesis. Part 1: Syntheses of xanthine derivatives by photolysis of 1-(5′-oxohexyl)-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione (pentoxifylline): an ambident chromophore

We investigated the use of photochemistry to make novel derivatives of pentoxifylline. Under conditions that favour singlet excited states, we obtained 1-allyl-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione, (R^∗,R^∗)-(±)-1-{[2-hydroxy-2-methylcyclobutyl]methyl}-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione and 1-(5-hydroxyhexyl)-3,7-dimethyl-3,7-dihydro-1H-purine-2,6-dione. Naphthalene or molecular oxygen increases the yields and triplet sensitisers (acetophenone, benzophenone and acetone) decrease the yields. Efficient intramolecular triplet energy transfer from the carbonyl to the xanthine moiety allows the carbonyl moiety to react from a singlet excited state only. In solvents with an α-hydroxyalkyl hydrogen under conditions that favour triplet excited states, we obtained 8-substituted pentoxifylline derivatives: 8-(1-hydroxy-1-methylethyl)-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1H-purine-2,6-dione in isopropanol, 8-(1-hydroxymethyl)-3,7-dimethyl-1-(5-oxo-hexyl)-3,7-dihydro-1H-purine-2,6-dione in methanol and 8-(1-hydroxyethyl)-3,7-dimethyl-1-(5-oxohexyl)-3,7-dihydro-1H-purine-2,6-dione in ethanol. The xanthine moiety reacts from a triplet state via a radical mechanism and yields are considerably improved by the addition of catalytic amounts of di-tert-butyl peroxide.