Acetals alkali labile

Obviously, these methods are inapplicable to esterification of free sugars and of alkali-labile glycosides and esters (including the resultant sulfonic esters), but may be employed with sugar alcohols, non-reducing di- and oligo-saccharides, and alkali-stable glycosides and acetals (e.g., certain-O-isopropylidene derivatives). 

Acidic polysaccharides (see Table IV) that contain uronic acid residues are, perhaps, the most prevalent type of exocellular polysaccharide. Often, these acidic biopolymers contain other sugars, including pentoses, hexoses, and heptoses, also found in neutral polysaccharides (see Tables V and VI). In many instances, these polymers possess alkali-labile O-acyl substituents, such as acetic, formic, ma-lonic, pyruvic, and succinic acids. Positively charged biopolymers that contain free amino sugars are rare, but have been found (see Table VII). More often, these amino sugars are N-acylated, generally with acetyl groups. 

A soln. of ethyl 6-acetoxy-3-chloro-4-coumarincarboxylate in THF treated with 0.5 eqs. N-methyl-2-dimethylaminoacetohydroxamic acid in phosphate buffer (pH 7.6), and stirred at room temp, for 1 h ethyl 3-chloro-6-hydroxy-4-coumarin-carboxylate. Y 95%. The method is especially convenient for preferential cleavage of phenol, enol, and oxime acetates in alkali-labile and/or oxygen-sensitive substrates. F.e.s. M. Ono, I. Itoh, Tetrahedron Letters 30, 207-10 (1989). 

Saponification of the extracts is generally desirable to remove unwanted lipid materials. However, this step is omitted in the isolation of carotenol esters, since these are hydrolyzed by this procedure. It is also omitted in the isolation of carotenoids such as fucoxanthin and peridinin, which are alkali-labile. If acetone has been used in the initial extraction, it is essential that all traces be removed before saponification. The general procedure used involves dissolving the total lipid fraction in an alcoholic (ethanol or methanol) solution of potassium hydroxide. The mixture is then either heated for a short period of time while kept in the dark, or left in the dark at room temperature for 12-16 h. There has been considerable discussion of the merits of these two procedures. Which method is used is dependent on the nature of the samples being analyzed and the requirements of the analysis (Davies, 1976 Liaaen-Jensen, 1971). After saponification, water is added, and neutral lipids (the unsaponifiable fraction) are extracted with diethyl ether or hexane. Acidic carotenoids remain in the alkaline phase and are extracted with diethyl ether or hexane after acidification with acetic acid. The unsaponifiable fraction usually contains sterols as well as carotenoids. If desired, sterol contaminants can be removed by precipitation from cold (- 10°C) petroleum ether or by precipitation of these compounds as their digitonides. 

As no plant could be found which did not conjugate tryptophol in vitro [14], a search for such conjugates as naturally occurring compounds could be rewarding. In fact, the presence of tryptophol esters in products of yeast fermentation was inferred by Ehrlich [6], and the acetate was recently identified in wine using GC-MS [10]. An alkali-labile tryptophol conjugate, presumably an ester, was also detected in seeds of Pinus silvestris [32]. The formation of tryptophol esters by the phylogenetically unrelated yeasts and pines may well indicate that such compounds commonly occur in plants. 

Native polysaccharides with acid groups other than the uronic type are not very common except for the sulfate esters. Total acidity may be estimated by direct titration, but erroneous results are obtained if the polysaccharide is alkali-labile as is the case with many oxidized polysaccharides. Addition of calcium acetate (7, 8) or sodium bromide 8, 9) to the polysaccharide solution increases the accuracy of the titration. Other methods for the estimation of carboxyl and other acidic groups involve determination of the amount of methylene blue absorbed, or determination of the amount of silver salt formed by exchange from a solution which contains silver in combination with a very weak acid. The sulfate content of polysaccharide sulfates, such as agar, is obtained by ordinary sulfate analysis of the completely hydrolyzed or ashed polysaccharide. 

Black crystaUine solid exists in two modifications stable black needles known as alpha form that produces ruby-red color in transmitted light, and a labile, metastable beta modification consisting of black platelets which appear brownish-red in transmitted light density of alpha form 3.86 g/cm at 0°C density of beta form 3.66 g/cm at 0°C alpha form melts at 27.3°C, vapor pressure being 28 torr at 25°C beta form melts at 13.9°C hquid iodine monochloride has bromine-hke reddish-brown color hquid density 3.10 g/mL at 29°C viscosity 1.21 centipoise at 35°C decomposes around 100°C supercools below its melting point polar solvent as a hquid it dissolves iodine, ammonium chloride and alkali metal chlorides hquid ICl also miscible with carbon tetrachloride, acetic acid and bromine the solid crystals dissolve in ethanol, ether, acetic acid and carbon disulfide solid ICl also dissolves in cone. HCl but decomposes in water or dilute HCl. 

The structural analysis of neuraminolactose was greatly stimulated by the isolation of iV-acetyl-0-acetylneuraminolactose from cow s colostrum. Sixteen liters of colostrum yielded 3.6-4.G g. of the very labile, acidic trisaccharide. Due to its own acidity, a 1 % aqueous solution of the substance has a half-life of only 48 hours and it spontaneously hydrolyzes to lactose, iV-acetylneuraminic acid, and acetic acid whereas, at pH 6, there is no appreciable hydrolysis. Dilute alkali and acid degrade the acidic trisaccharide as shown below. 

The effect of iodide and acetate on the activity and stability of rhodium catalysts for the conversion of methanol into acetic acid have been studied. Iodide salts at low water concentrations (<2 M) promote the carbonylation of methanol and stabilize the catalyst. Alkali metal iodides react with methylacetate to give methyl iodide and metal acetate the acetate may coordinate to Rh and act as an activator by forming soluble rhodium complexes and by preventing the precipitation of Rhl3. A water-gas shift process may help to increase the steady-state concentration of Rh(I). The labile phosphine oxide complex (57) is in equilibrium with the very active methanol carbonylation catalyst (58) see equation (56). 

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