Acidic quench

In recent years, a variety of aryl boronic acids are commercially available, albeit in some cases they may be expensive for large scale purposes. During our work in the mid-1990 s boronic acid (II) was not commercially available and so two different protocols were used to prepare this acid. The first approach involved the transmetallation with n-butyl lithium of aryl bromide (I) and trapping the lithio species generated with trialkyl borate followed by an acid quench. Aryl bromide (I) is easily prepared by reaction of o-bromobenzenesulfonyl chloride with 2-propanol in the presence of pyridine as a base. The second approach was a directed metallation of isopropyl ester of benzene sulfonic acid (VII), to generate the same lithio species and reaction with trialkyl borate. The sulfonyl ester is prepared by reaction of 2-propanol with benzenesulfonyl chloride. From a long-term strategy the latter approach is... 

Based on this information the preparation of enone was examined from the unhalogenated (VIII)(X=H). Deprotonation can be performed with n-butyl lithium in THF at 0-5 °C followed by treatment with 3-ethoxy cyclohexen-l-one, followed by an acid quench provides the same enone (XI). This deprotonation also avoids the cryogenic conditions required to prepare enone (XI) when the bromo analog is used. Pyridinium tribromide used for aromatization of enone (XI) to biaryl phenol (X) is an inexpensive reagents ( 80/kg). 

Aryloxy-l,2-diarylethanones can be cyclodehydrated to diarylbenzofurans by heating with sodium acetate and acetic anhydride in polyphosphoric acid. Quenching the hot reaction mixture with water leads to initially violent acid-catalysed hydrolysis of the excess anhydride. 

Fig. 12.2 Diagram of a fully automated system for acquiring H/D exchange MS data starting with a stock solution of the nondeuterated protein. In this system [8], the liquid handler mixes a small amount of concentrated protein solution with a selected deuterated buffer and the mixture is incubated for a programmed period of time. The exchange reaction is conducted in a temperature-controlled chamber held at 25 °C. The mixture is then transferred to an acidic quench solution held at 1 °C. After quenching the exchange reaction, the entire sample is injected onto an LC-MS system...

Nitroso compounds are formed during the addition of nitrous oxide," " dinitrogen trioxide, and nitrosyl halides to alkenes, and in some cases, from incomplete oxidation of amines with peroxyacids like peroxyacetic acid. Quenching of carbanions with nitrosyl halides is also a route to nitroso compounds. A full discussion on this subject is beyond the scope of this work and so the readers are directed to the work of Boyer. ... 

REDUCTION WITH LITHIUM ALUMINUM HYDRIDE Acidic Quenching. Reduction of Aldehydes and Ketones [ 5]... 

Biochemical Acid Quench/Cold Chase Experiments. 84... 

In classical acid quench/cold chase experiments [48] with mitochondrial Fj in unisite catalysis mode, [y- P]ATP was used as substrate and the ratio of bound Pj/total bound P, where total bound P includes both bound P, and bound [y- P]ATP, was measured at different concentrations of Fj and [y- P] ATP and at different incubation times of the reaction mixture. A kinetic scheme based on a general sequence of events leading to ATP hydrolysis which considers irreversibility of the catalysis steps, as proposed recently by some researchers [16-20,43,46,49], was developed, k and k represent the rate con-... 

The chiral enolate-imine addition methodology was examined in detail (Thiruvengadam et al., 1999). Enolate formation proceeds to completion within an hour at temperatures from — 30 to 0°C with either 1 equiv. TiCl4 or TiClaO-iPr (preformed or prepared in the presence of substrate by addition of TiCl4 and followed by a third of an equivalent Ti(0-iPr)4 and two equivalents of a tertiary amine base). Unlike the aldol process with the same titanium enolate, the nature of the tertiary amine base had no effect on the diaster-eoselectivity. The diastereoselectivity is maximized by careful control of the internal temperature to below — 20°C during the imine addition (2 equiv.) as well as during the acetic acid quench. The purity of the crude 2-amino carboxamide derivatives (17a or... 

Screening several amine racemization catalysts, we found that the SCRAM and the Shvo catalyst would both racemize the (S)-enantiomer at temperatures above 11() G. Interestingly, no dimeric products were found. The best racemization conditions were found to be using toluene or TBME at 150°C in a pressure vessel with 1 mol% SCRAM or 5 mol% Shvo catalyst over 24 h, providing quantitative conversion. In the presence of (R, R)-dibenzoyltartaric acid the racemization slowed, possibly because of unfavorable coordination of the alkylammonium substrate or acid quenching of the iridium hydride catalyst intermediate. 

TBAF-quenching calcium sulfonate resin 4 and acidic-quench resin 51 in step 4 and... 

Direct observation of the E p/t dNTP complex was obtained using pulse-chase experiments. In such experiments, incorporation of labeled nucleotide to an E p/t complex is either quenched by the addition of HC1 or allowed to proceed after the addition of a large excess of cold unlabeled dNTP (the chase step) followed by acid quench. In the HC1 quench experiments, the acid quenches all the enzyme-bound species. On the other hand, when the reaction is chased with cold dNTP, each of the enzyme-bound species is allowed to partition both in the forward and reverse directions. The amount of partitioning in the forward direction is observed as an excess of labeled product, compared with the acid quench experiment, while the dNTP that partitions in the reverse direction is diluted and remains unobservable. As an excess was observed and because the binding of dNTP to the E p/t complex is rapid, the observed flux or excess is mainly due to the E p/t dNTP complex. 

However, when a mixture of both crotyl regioisomers 60 and 61 were obtained and separated, unambiguous assignment of C-9 to an a orientation could be ascertained from NMR coupling constants (Eq. 17). After this comparison, it was clear C-9a allyl analogue 59 had been produced earlier. However, 59 underwent kinetic enolization with LDA at low temperature and subsequent acid quench afforded desired C-9P allyl analogue 57. 

If a reacting solution is acid quenched then a small amount of hydrazine, derived probably from an intermediate reduction product, is formed. The rate of ammonia formation is linear in PN2 and depends on the square of vanadium concentration, but the actual rate is a function of dioxygen pressure, metal contaminants, etc. The mechanism is believed to follow the pattern discussed above, with dinitrogen bound between two dinuclear pairs of vanadium(II) ions (232, 233), but the precise identity of the fixing species remains a mystery. 

Figure 6. Time course of change in catalytic specificity (upper panel) and Ca2+ dissociation from extracytoplasmic low affinity sites (lower panel) following phosphorylation of the SR Ca2+-ATPase with ATP. The amount of ADP-insensitive phosphoen-zyme (E2P) was measured in two ways (I) [y-32P]ATP was included in the reaction mixture and the radioactivity incorporated into the enzyme was determined after acid quenching at various time intervals. To remove the ADP-sensitive phosphoenzyme so that only ADP-insensitive phosphoenzyme was measured, ADP was added 4 sec before the quench (upper panel, right scale) (2) by the enhancement of fluorescence from a trinitrophenyl-derivative of ADP bound in the catalytic site in exchange with ADP after the phosphorylation (upper panel, left scale). The change in Ca2+ binding was measured indirectly by use of murexide as an indicator of free Ca2+ in the medium. The data show that Ca2+ dissociates simultaneously with formation of E2P. The data points were taken from Andersen et al., 1985.

Figure 17. Analysis of phosphoenzyme intermediates of SR Ca2+-ATPase mutants with alterations to carboxylate-containing residues in the transmembrane sector. Wild-type or mutant Ca2+-ATPases expressed in the endoplasmic reticulum membranes of COS-1 cells were phosphorylated with [y-32P] ATP (panel a) or [32P]P (panels b and c). Following acid-quench of the phosphorylated intermediate, the samples were subjected to SDS-polyacrylamide gel electrophoresis under acid pH conditions and the dried gels were autoradiographed to visualize the radioactivity associated with the covalently bound phosphate. Panel a shows the Ca2+-concentration dependence of phosphorylation from ATP. The Glu309- Lys mutant is unable to phosphorylate, even at 12.5 mM Ca2+. In the wild-type Ca2+-ATPase the phosphorylation reaction is fully saturated at 10 pM Ca2+. Panel b shows lack of Ca2+ inhibition of backdoor phosphorylation from P in the mutants. E indicates the presence of ECTA to chelate Ca2+ (normally a requirement for phosphorylation by the backdoor route). C indicates the...

The phosphorus-stabilized carbanion of 94 was generated at -100 °C with LDA (1.2 equiv.) in THF. Amination reactions were performed both with DTBAD and trisyl azide. Direct addition of a stoichiometric amount of DTBAD (1.1 equiv.) at -100 °C followed by acidic quench led to a mixture of hydrazino products 95 (major diastereomer indicated) in good yield and diastereoselectivity (Scheme 44). 

In the acidic quench , the gas is brought into contact with a solution of sulfuric acid of 30-40%. Recycled water is added to compensate the losses by evaporation. Precooling down to the dew point could be employed for maximum energy recovery. The entrained catalyst fines are recovered by filtration. Ammonium sulfate is separated, purified by crystallization and finally obtained as a saleable byproduct. 

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