A-halogen acetal

The first step in the submonomer method involves the acylation of a halogenated acetic acid on the resin under standard conditions. In the second step the side chain of the peptoid is introduced by nucleophilic substitution of the halide with an excess of the primary amine. 

Keywords a-halogen acetal, a-haogen thioacetal, KOtBu, microwave irradiation, /(-elimination, ketene O,O-acetal, ketene 5,5-acetal... 

Malonic acid, ethyl malonate, and their monoalkyl derivatives can be readily halogenated in ether solution subsequent decarboxylation leads to the corresponding a-halogenated acetic acid in 55-80% yield. The reaction of the potassium salts of monoethyl alkylmalonates with bromine provides the a-bromo esters directly, although the yields ate relatively low. ... 

This preparation illustrates the Reformatsky reaction, which consists in the interaction of a carbonyl compound, an a-halogen ester (e.g., ethyl bromo-acetate) and zinc In the presence of ether or benzene, followed by hydrolysis. 

In the thiophene and selenophene series, a-halogens are preferentially removed by reduction with zinc and acetic acid, as is illustrated by the preparation of 3-bromothiophene... 

Bromobenzene. -The replacement of hydrogen by the halogens Cl and Br, in the nucleus of aromatic hydrocarbons, is assisted by the presence of a halogen carriei, the action of which lias been referred to in the Note on the piepaiations of chlor- and bioin-acetic acids, p. 252. Iodine, iion, iron and alupiinium chlorides and bromides, the aluminium-mercury... 

An aqueous solution containing 0.10 mole/liter of chloroacetic acid, ClH2CCOOH, is tested with indicators and the concentration of H (aq) is found to be 1.2 X 10 2 M. Calculate the value of Ka (if necessary, refer back to Section 11-3.2). Compare this value with KA for acetic acid—the change is caused by the substitution of a halogen atom near a carboxylic acid group. 

A number of other methods exist for the a halogenation of carboxylic acids or their derivatives. Acyl halides can be a brominated or chlorinated by use of NBS or NCS and HBr or HCl. The latter is an ionic, not a free-radical halogenation (see 14-2). Direct iodination of carboxylic acids has been achieved with I2—Cu acetate in HOAc. " ° Acyl chlorides can be a iodinated with I2 and a trace of HI. Carboxylic esters can be a halogenated by conversion to their enolate ions with lithium A-isopropylcyclohexylamide in THF and treatment of this solution at -78°C with... 

Oxidising acetaldehyde in air when cobalt acetate at -20°C is present gives rise to a detonation, if the medium is stirred. It has been put down to the formation of a very sensitive peroxidic compound. On the other hand, the presence of a halogen derivative inhibits this oxidation. 

Ketene acetal synthesis by /1-elimination of haloacids from halogenated acetals under well controlled conditions using thermal activation (A), ultrasound (US) or micro-wave irradiation [92] (MW) has been described. From a mechanistic point of view, as the TS is more charge delocalized than the GS and the polarity is enhanced during the course of the reaction, a favorable microwave effect can therefore be observed (Eqs. (37) and (38) and Scheme 3.13). 

Fig. 11 illustrates the smoke suppression effect of the zinc borate when used in conjunction with ATH in a halogen-free ethylene-vinyl acetate (EVA) system. Sumitomo Electric, in a... 

Synthesis of Five-membered Cyclic Nitronates from a-Halogen-substituted AN. The key step of this approach is presented in Scheme 3.1, path (b). Until recently, this synthetic route to nitronates (24) has been of no preparative interest, because only two examples, such as elimination of trimethylsilyl nitrite (75) and 1,2-dinitrophenylethane (85) from the corresponding nitroso acetals were documented. 

The diazonium compound itself is liberated by adding a strong organic acid, such as halogenated acetic acid, after the coupling component has been introduced into the reaction mixture. 

Before titrating, neutralise the mineral acid which has been produced with a few drops of saturated halogen-free sodium acetate solution so that the solution exhibits only a faint acetic acidity. 

The a-halogenated carboxylic adds, of which the simplest is chloro-acetic acid, are widely used in synthesis. Their conversion into hydroxy-acids (by hydrolytic elimination of the halogen) and into aminoacids (p. 275) may be mentioned here ... 

Oxidation of sulphonamides in the presence of bromide or iodide ions and sodium methoxide in methanol also leads to formation of the N-halogeno intermediate. The nitrogen-halogen bond in these intermediates is weak and will undergo themiolysis. At -10 °C, reaction proceeds by base catalysed elimination of hydrogen halide and ftirther steps lead to an a-amino acetal 20. The reaction is carried out in an undivided cell and renders a-aminoacetals readily available for the iso-... 

A similar Co-catalyzed reaction with arylmagnesium halides and alkenes 181, bearing a halogen in a suitable position, occurs via a radical intermediate, leading to cyclic acetals 182 (equation 109). ... 

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