Ester basic medium

The easier elimination of pyridine compared to quinoline-4 may be related to the pK value of 4-methylthiazole, which is between those of lepidine and 2-picoline (25. 55). This reaction explains also why a neutrodimethine cyanine is obtained with such good yields when reacting together a quaternary salt, ketomethylene, and o-ester in a basic medium. As the reaction proceeds, the trimethine cyanine is attacked by the ketomethylene. The resulting 2-methyl quaternary salt is transformed into trimethine cyanine, consuming the totality of the ketomethylene (1, p. 512 661). The mesosubstituted neutrodimethine cyanine is practically pure. 

Hydrazine as Nucleophile. Reaction of hydrazine and carbon dioxide or carbon disulfide gives, respectively, hydrazinecarboxyhc acid [471-31-8], NH2NHCOOH, and hydrazinecarbodithioic acid [471-32-9], NH2NHCSSH, in the form of the hydrazinium salts. These compounds are useful starting materials for further synthesis. For example, if carbon disulfide reacts with hydrazine in basic medium with an alkyl haUde, an alkyl dithiocarbazate ester is obtained in a one-step reaction ... 

In a basic medium, it is possible to direet the ketovinylation at the a-position of the indole ring 240 to arrive at the 2-ketovinyl derivative 247 whieh furnishes the maeroeyele 248 by eondensation with eyanoaeetie ester (94CZ452). 

A number of simple reactions are now known which form 2-oxo-tetrahydro-1,3-oxazine (18). Here, also, 3-aminopropanol and some of its derivatives are frequently used, Cyclizing reagents are carbonic acid esters in strongly basic medium, " ethyl chloropropionate, trichloracetic esters, or phenyl isocyanate. An example of the first of these methods is ... 

The a-sulfo fatty acid esters are obtained by a-sulfonation of the fatty acid esters, and in basic mediums they form the corresponding salts. 

Gravimetric and volumetric methods are practicable for the quantitative determination of the a-sulfo fatty acid esters. Using gravimetric methods the surfactant is precipitated with p-toluidine or barium chloride [105]. The volumetric determination method is two-phase titration. In this technique different titrants and indicators are used. For the analysis of a-sulfo fatty acid esters the quaternary ammonium surfactant hyamine 1622 (p,f-octylphenoxyethyldimethyl-ammonium chloride) is used as the titrant [106]. The indicator depends on the pH value of the titration solution. Titration with a phenol red indicator is carried out at a pH of 9, methylene blue is used in acid medium [106], and a mixed indicator of a cationic (dimidium bromide) and an anionic (disulfine blue VN150) dye can be used in an acid and basic medium [105]. 

Base catalyzed condensation reactions of esters and ketones have an additional factor of importance in determining the product, and this is the fact that the overall reaction, as well as the intermediate steps, is highly reversible. The final product may be rate or equilibrium determined, and in the latter case the result may depend on the relative acidity of the various possible products. In a highly basic medium the product will be partly in the form of a salt and the stability of the salt is then a product-determining factor. Failure of a condensation to take place may be due either to an insufficiently high concentration of carbanions or to the instability of the product. The reactions of ethyl isobutyrate will illustrate both points.419... 

Figure 3.29 — (A) Immunosensor scheme A Cell inlet tubing B transparent PTFE tube (1.6-mm ID x 3-mm OD C immunosorbent D frit. (B) Outline of flow-injection immunoassay procedure. The assay buffer is posphate buffered saline (PBS) at pH 7, and flow-rates and times (min) are given in the figure. Immobilized anti-mouse IgG modified sample (mouse IgG) injected at T = 0 change of the flow-rate and buffer at T = 4 injection of hydrogen peroxide in a basic medium at T = 5 then, emission monitoring and regeneration step acridinium ester-labelled antibody (emitter = N-methylacridine). (Reproduced from [218] with permission of Elsevier Science Publishers).

Another interesting variant of Claisen rearrangement has been introduced by Ireland [149], and used by his group in carbohydrate chemistry. The starting compound is again an allylic alcohol that is esterified by a suitable carboxylic acid. This ester is enolized in basic medium, and quenching of the intermediate enolate at low temperature gives a ketene silyl... 

We have already discussed (p. 106) that T+ and T ionic forms can give the ester and amine products only. Thus, in acidic and neutral media which favor the formation of T+ and T1, imidate salts should always give the ester and amine products. In basic medium, which favors the formation of T , there is the possibility for the formation of both types of products, i, e., ester and amine or amide and alcohol. The cleavage of the C—N bond in the T" tetrahedral intermediate will take place only if the nitrogen electron pair can form a hydrogen bond with a solvent molecule. Thus, experimental evidence in favor of the principle of stereoelectronic control can be obtained with imidate salts, only when the hydrolysis is carried out under basic conditions. 

Take as an example the experimentally most studied reaction in micellar catalysis, ester hydrolysis in a basic medium... 

Carbonylation of aryl and vinyl iodides.11 Pd-catalyzed carbonylation in a basic medium of aryl or vinyl iodides containing enolizable carbonyl groups can result in reaction with the in situ generated enolate to form enol esters or enol lactones. 

Potassium salts of cyanoimidodithiocarbonates (291) react successively with an alkyl halide and a-halo esters, nitriles, amides or ketones (293) in basic medium to yield 2,5-disubstituted 4-aminothiazoles (294 Scheme 204) (67JPR(309)97). 

A more recent modification of the Sorensen process employs acylamino-cyanoacetic esters, RCONHCH(CN)CO,C2Hs, in place of the malonates. An alkylated acetylaminocyanoacetate may be hydrolyzed to an amino acid in acidic or basic medium. ... 

Esters of aliphatic and aromatic sulfonic acids are conveniently prepared in high yields from alcohols and sulfonyl halides. A basic medium is required. By substituting sodium butoxide for sodium hydroxide in butanol, the yield of n-butyl p-toluenesulfonate is increased from 54% to 98%. Ethyl benzenesulfonate and nuclear-substituted derivatives carrying bromo, methoxyl, and nirro groups are prepared from the corresponding sulfonyl chlorides by treatment with sodium ethoxide in absolute ethanol the yields are 74-81%. Pyridine is by far the most popular basic medium for this reaction. Alcohols (C -Cjj) react at 0-10° in 80-90% yields, and various phenols can be converted to aryl sulfonates in this base. "... 

An iterative Hantzsch reaction was applied for the synthesis of the tris-oxazole fragment of the natural product mycalolide A. The reaction between primary amide 200 and ethyl bromopyruvate in a basic medium followed by dehydration with TFAA gave oxazole 201 (Scheme 58) <1997TL5445>. The ester was converted into primary amide and the process was repeated twice to give the tris-oxazole. 

Potassium carbonate Basic Medium Medium Medium Amines, esters, ketones, bases... 

Sometimes students will optimize the source and the sink and forget that those two species cannot exist in the same medium. Since the protonated ester p Ta is -6.5 and the pATabH of hydroxide is 15.7, they span over 22 pA a units. Therefore, any medium that would be acidic enough to have some concentration of the protonated ester would have essentially no concentration of hydroxide ion. The right examples show two correct alternatives acidic medium with the protonated ester and neutral water as the nucleophile or basic medium with the neutral ester and hydroxide as the nucleophile. 

The first step is in basic medium ethoxide has a pATabH of 16. Sources The ethoxide anion. Leaving groups Only ethoxide on the ester. Sinks The ketone is a polarized multiple bond, and the ester is a polarized multiple bond with a leaving group. Acidic Hs The CH2 between the two carbonyls the most acidic, p/fa 10.7 the methyl on the carbonyl is the next most acidic, pA a 19. Resonance forms help us understand the polarization of our reactants. 

The hydrazones of thiocarbohydrazides also react in this manner (304). The 5-amino-.d4-1,3,4-thiadiazoline-2-thiones, which may be prepared from thiosemicarbazides and carbon disulfide, are listed in Table 19-The preparation of these compounds from dithiocarbazic acid esters and isothiocyanates in basic medium is discussed in Section VII.7. It must be pointed out that the numbering of the substituents in Guha s paper (146) is not always very clear so that, for the last three substances in Table 19, there is some doubt about the position of the substituents. 

---