Preparation difficulties

Studies on the biosynthesis of prostaglandins revealed that a strained bicyclic bis(secondary-alkyl) peroxide was not only a key intermediate, but could survive the biological conditions long enough to be isolated, albeit in minute amounts. This was a startling discovery in that no simple bicyclic peroxides of the same type were known, clearly for reasons of preparative difficulty. The prostaglandin connection focused attention upon this missing class of peroxides and stimulated... 

Sulfated tin oxide (STO) is classified as one of the strongest solid acids (STO calcined at 550°C ranks first among sulfated metal oxides according to the Hammett function scale, Hq value = 18). However, the use of STO has been more limited than that of SZ (calcined at 650°C, Hq value = —16.1) due to preparation difficulties and poor yields. However, new preparation routes are making this catalyst more accessible, and recently its use has become more widespread. In a recent study by Furuta et al., STO was compared to SZ in the esterification of n-octanoic acid with methanol. The STO catalyst showed superior activity compared to SZ at temperatures below 150°C. For instance, STO approached a 100% ester yield at 100°C, while SZ required temperatures as high as 150°C to reached similar yields. 

The relative fragility and preparative difficulty associated with monolayer-modified electrode surfaces hampered significant analytical progress for some time, and it was not until polymer-film electrodes were developed that the utility of modified electrodes in analysis could be demonstrated. 

Because of preparative difficulties in the epoxidation of a,/3-unsaturated ketones (influenced by steric factors, e.g. CH3-group in VIII/76 [38]), a direct conversion of these compounds was developed [43],... 

Although a-aminoketones readily form imidazoles when they cyclize in the presence of formamide [48], preparative difficulties (see Sections 2,1.1, 2.2.1 and 4.1) point to alternative use of their precursors, a-oximinoketones, which can be reduced by dithionite or using catalytic methods in formamide at 70-100°C. Subsequent ring closure is achieved merely by raising the temperature to 180°C. 

The classical syntheses of peptide derivatives from easily obtainable a-amino acids correspond to converting these into suitably protected a-amino acid derivatives. They are connected and deprotected, and ultimately, after many coupling/deprotection cycles, the planned peptide derivatives are formed. Preparative difficulties increase with the number of cycles, with one of the main problems being the significant degrees of racemization that occur during some of these reactions. Consequently, chemical procedures must be carefully selected and carried out under suitable conditions [31,72,82-84]. 

Cocktail preparation difficulties and expense are also eliminated when using the detectors as opposed to standard liquid scintillation techniques in which such problems as chemiluminescence and quenching are often encountered (10). 

The simplest interpretion or explanation for a nonzero moment in Cm02 would be nonstoichiometry due to the preparation difficulties or radiation damage and lower-... 

In accordance with the preparative difficulties, the pentahydrates are structurally poorly characterized. Odent et al. (1975) showed that, as for the hexahydrates, two isostructural series, viz. La-Ce and Pr-Lu, Y, exist for the pentahydrates. They have also pubhshed the X-ray powder pattern for Yb(N03)3 5H2O and line diagrams for the isostructural Gd-Yb and Y compounds. Earlier, Mironov et al. (1969) had... 

A version of the pyrrole synthesis involving one-pot transformation of ketones into NH- and N-vinylpyrroles has been worked out. Preliminary isolation of oximes is always associated with consumption of reactants, solvents, and time. In some cases (e.g., for ketoximes having high solubility in water), this procedure presents certain preparative difficulties. To exclude the stage of ketoximes isolation, a possibility of ketone oximation in situ in DMSO and the subsequent application of the oxime solutions in the reaction with acetylene has been studied [181]. 

Trioxane oi alpha-trioxymethylene, (CHi>0)3, the cyclic trimer of formaldehyde, is a stable chemical indiiridual possessing unique, well-defined properties. Because of preparational difficulties, it has in the past been studied by veiy few iQ, estigators. Its obscurity has also been further enhanced by the incoiTect use of the term ririoxymethylene as a designation for linear formaldehyde polymers. 

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