Convenient synthesis of nucleoside and isonucleoside analogues

[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions.     

DNase activity ofMicrococcal endonuclease insolubilized on corn cob

The endonuclease from S. aureus has been immobilized on ground maize cob, previously activated with tosyl chloride. Pretreatment of the support on acid before tosylation yielded the best insoluble enzyme derivatives. The catalytic activity has been evaluated as percent of total hydrolysis attained in a batch reactor using DNA as a model substrate. The derivatives prepared are very resistant to high temperatures under conditions of catalysis (24 h at 45 degrees C). For these long reaction times, the extent of hydrolysis in the presence of small amounts of organic solvent (dimethyl sulfoxide at 2 percent) is larger than in plain buffer (Tris). This type of derivative could be very useful for the removal of nucleic acids from single-cell protein concentrates.     

Quantum chemical predictions concerning two unknown “mesoionic” compounds

The Pariser-Parr-Pople approximation was used to predict the properties of compounds I, 3-oxo-2H-1,2,3-triazolo[3,4-a]pyridine, and II, 3-oxoisoxazolo[2,3-a]pyridine, originated by joining a pyridine ring to two sydnone-like heterocyclic systems not yet reported in the literature. A parallel computation was carried out for two known compounds of similar structure, to give the predictions a better reliability through the comparison with observed spectral data and chemical behaviour. Compound I is expected to be stable, with an absorption spectrum similar to III, 2-oxo-1,3,4-oxadiazolo[4,5-a]pyridine, and chemical properties analogous to IV, 1-methyl-3-oxo-1,2,4-triazolo-[4,3-a]pyridine. A reaction path is suggested for obtaining from I the unknown isomeric structure V, 3-oxo-1H-1,2,3-triazolo[3,4-a]pyridine. Compound II is predicted as an unstable orange-red substance which should be handled and kept at low temperatures.     

Synthesis of monobutyrylglycerol by transesterification with soluble and immobilized lipases

Transesterification between ethyl butyrate and glycerol using very different lipase preparations (native, modified, and immobilized) in a two-phase system—no extra solvent added—has been investigated. Optimal conversion was obtained with the presence of 5% water in the reaction mixture. Only monobutyrin was produced in all conditions tested. The best enzyme preparations were nativeCandida cylindracea lipase,Mucor miehei lipase immobilized on a phenol-formaldehyde exchange resin (Lipozyme?), andC. cylindracea lipase immobilized on Celite.     

Die potentiometrische Mikrobestimmung von Kupfer in organischen Verbindungen nach Verbrennung in einem modifizierten Schöniger-Kolben

Zusammenfassung Nach Verbrennung der Substanz in einem modifizierten Sauerstoffkolben werden die Verbrennungsprodukte in 2,5 ml 8M HNO₃ absorbiert; das gebildete CuO wird vom Pt-Netz durch eine spezielle Technik vollständig gelöst. Nach dem Erkalten wird die Lösung in die Titrationszelle übertragen, mit NaOH in Gegenwart von Phenolphthalein neutralisiert und das Cu(II) schließlich in gepufferter Lösung mit 0,01N Cyanoferrat(II) potentiometrisch titriert. Eine kupferselektive Elektrode und eine Single-Junction-Referenzelektrode dienen in Kombination mit einem pH-Meter mit gedehnter Skala zur Erkennung des Titrationsendpunktes. Die Resultate sind innerhalb ±0,10% genau: die Wiederfindungsrate von Kupfer liegt zwischen 99,65 und 100,10%; die Standardabweichung beträgt 0,04%. Die Bedingungen und die Charakteristiken der Titration von Cu(II) mit Cyanoferrat(II) ebenso wie die Störung einiger Anionen werden diskutiert. Der beschriebene modifizierte Verbrennungskolben eignet sich zur Mikrobestimmung auch anderer Elemente in metallorganischen Verbindungen.

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Potentiometric microdetermination of copper in organic compounds after combustion in a modified schöniger's flask

Summary After combustion of the sample in a modified oxygen flask, the combustion products are absorbed in 2,5 ml 8M HNO₃ and the CuO formed is completely dissolved from the Pt-basket by a special technique. After cooling, the solution is transferred to the titration cell and neutralized with NaOH in the presence of phenolphthalein; the copper(II) is finally titrated potentiometrically in a buffered medium with 0,01N hexacyanoferrate (II). A copper-selective electrode and a single junction reference electrode are used in combination with an expanded-scale pH-meter to detect the endpoint. The results obtained are very accurate and reproducible: the max. error does not exceed 0.10% (abs.), the recoveries of copper range from 99.65 to 100.10% and the standard deviation is 0.04%. Conditions and features of the titration of copper(II) with hexacyanoferrate(II) as well as the effect of some interfering anions are discussed. The modified oxygen flask described is also useful for the microdetermination of other elements in organometallic compounds.

     

The hydrophobic effect in aqueous solutions of nonelectrolytes. I. Self-interactions of alkylureas

In order to clarify some aspects of the hydrophobic interactions, the enthalpies of dilution of monoethylurea, 1,3-dimethylurea, and 1,3-diethylurea have been determined calorimetrically at 25°C. The calorimetric data, expressed in terms of excess enthalpy, permit the evaluation of the pair and triplet interaction coefficients. The analyses of these and of the analogous coefficientsg _xx andg _xxx, derived from osmotic data, indicate a driving force favorable to the interactions among the hydrated solute molecules. Nevertheless, the positive values of theh _xx andh _xxx coefficients seem to suggest that the source of the effect is a rearrangement of the water molecules rather than a direct association of the solute molecules. There are evidences of a strict correlation between the enthalpic and the entropic effects. Preliminary data were presented at the International Conferences on Chemical Thermodynamics at Baden (1973) and Montpellier (1975). The experimental part was carried out at the Istituto Chimico of the University of Trieste. To whom correspondence should be addressed.     

Anodic oxidation mechanism of 3-arylaminoindoles at a rotating and at a (new type of) pulsing platinum electrode in dipolar aprotic solvents

The oxidation of 2-phenyl-3-arylaminoindoles (1a–e) has been studied in acetonitrile, dimethylformamide and propylene carbonate at a new type of platinum electrode with periodical renewal of the diffusion layer and at a rotating platinum electrode by cyclic voltammetry, d.c. and a.c. voltammetry, controlled potential coulometry and spectrometry (e.s.r. and u.v.). In the examined aprotic solvents and in the presence of a base (like water or diphenylguanidine) amines 1a–e undergo a two-electron oxidation to the corresponding imines. In a protophobic medium (acetonitrile or propylene carbonate) 1a–e are oxidized in two one-electron steps, the first of which leads to the formation of a cation radical (identified by e.s.r. spectrum), oxidizable at more positive potentials (second step). The second electron transfer, however, must be simultaneous with, or be immediately followed by, a very fast deprotonation reaction. The same e.e.c. sequence explains the observed bielectronic step of 1a–e in a protophilic aprotic solvent, like dimethylformamide.