Acetals, 2-halo

Ketene di(2-melhoxyethyl) acetal has been obtained by the present method with the use of diethylene glycol dimethyl ether as solvent.3 Other methods for the preparation of ketene acetals include the dehydrohalogenation of a halo acetal with potassium t-butoxide 4 and the reaction of an a-bromo orthoester with metallic sodium.5... 

Alkynylation of halo acetals. In the presence of ZnCl2 (2 equiv.) the organotin acetylide 1 couples with chloromethyl methyl ether or chloromethyl methyl sulfide to form alkynyl ethers or sulfides, respectively (equation I). 

The reaction is very general. The aldehyde or ketone may be aliphatic, alicyclic, or aromatic (including diaryl ketones) it may contain double or triple bonds it may contain various functional groups, such as OH, OR, NR2, aromatic nitro or halo, acetal, or even ester groups.643 Double or triple bonds conjugated with the carbonyl also do not interfere, the attack being at the 0=0 carbon. 

The elimination of OR and halogen from 3-halo ethers is called the Boord reaction. It can be carried out with zinc, magnesium, sodium, or certain other reagents.35" The yields are high and the reaction is of broad scope. 3-Halo acetals readily yield vinylic ethers... 

By other reactions Iodosylbenzene, 151 Organozinc reagents, 220 Alkyl halides (see also Allylic halides, Dihalides, Halo acetals and ketals, etc., Vinyl halides)... 

Halides (see Alkyl halides) a-HALO ACETALS AND KETALS Dimethyl tartrate, 124 Tetrabutylammonium tribromide, 287 Haloamines or -imines and haloamides... 

The cyclizations of halo acetals, introduced by Stork66 and Ueno,67 are among the most popular of all radical methods because the precursors are easy to prepare, the cyclizations are very rapid, and the products can be converted to a variety of different heterocycles. The conversion of (26) to (28 Scheme 17) illustrates the basic procedure.68 Standard bromoacetalization of an alcohol provides the precursor (27). After tin hydride cyclization and acidic oxidation, lactone (28) is formed in excellent yield with 97%... 

The proposed reaction mechanism for the destruction of aqueous solutions of TCE or PCE predicts the formation of stable oxidized polar organic compounds. These compounds consist of acids, aldehydes, and possibly halo-acetic acids. Three possible mechanisms have been proposed for the formation of by-products due to the irradiation of aqueous solutions containing TCE and PCE. The first is for the formation of formaldehyde, acetaldehyde, and glyoxal, which are formed at a concentration of approximately two orders of magnitude less than the influent solute concentration. Second, the formation of formic acid decreased with increasing radiation dose. The formic acid concentration was found to be higher for PCE than TCE. These results are most probably due to the slower reaction rate constants of PCE with e and OH, compared to TCE. The third possible reaction is the formation of haloacetic acids when TCE and OH react. The mechanism of decomposition of PCE by OH is shown in Equation (12.30) to Equation... 

A number of cyclic and sugar-derived halo acetals 273 were subjected to radical 5-exo cyclizations catalyzed by a cobalt salen catalyst 274 with NaB H4 as the stoichiometric reductant but in the presence of air (entry 11) [321, 322]. Under these conditions, bicyclic oxygenated tetrahydrofurans 275a were obtained in 50-84% yield. Diastereomeric isomers 275b were isolated as the minor components. The yields were similar to those obtained with tributyltin hydride. The oxygen concentration proved to be important, since air gave better yields... 

Table 40 Reaction of a-halo acetate with 3-butenylgallium. Reproduced with permission from CSJ...

P-Halo acetals. Reaction of an a, 3-cnal or -enone with a halotrimethylsilane in the presence of ethylene glycol results in a (3-halo acetal in generally good yield. Example ... 

This synthetic sequence can also be carried out with high stereoselectivity by treating a 3-halote-tra-2/7-pyran containing an internal nucleophile, or a cyclic 2-halo acetal, with silver tetraflu-oroborate39. 

Halo acetals, amination, 669 dehydrohalogenation, 37, 79 preparation, 101, 263, 269 Halo acids, amination, 669 decarboxylation, 79 dehydrohalogenation, 38, 79 esterification, 481 in the Williamson reaction, 228 lactonization, 535 preparation, by cleavage of ethers, 93... 

The elimination of a molecule of halogen acid from halo acetals of acetaldehyde with powdered potassium hydroxide gives ketene acetals. However, the ti-bromo acetals of the homologs of acetaldehyde on similar treatment with potassium hydroxide or potassium 1-butoxide are converted into ctyjS-olefinic acetals. ... 

Halo acetals have been prepared by the action of alcohol on halo ketones and halo aldehydes.An indirect application of this reaction consists in the halogenation of enol acetates with subsequent reaction of the brominated products with alcohols to give the halo acetals. ... 

The reaction of carbonyl compounds with glycerin-a-monochlorohydrin, CHjOHCHOHCHjCl, leads to cyclic halo acetals. Treatment of a,/3-olefinic aldehydes with alcohols saturated with dry hydrogen chloride causes the addition of the halogen acid to the double bond as well as acetalization to give /3-halo acetals. ... 

Aliphatic and aryl aliphatic amino ketones are made by the amination of the halogenated carbonyl compounds, - e.g., dimethylaminoacetone (74%), l-diethylamino-2-pentanone (79%), and a-methylaminopropio-phenone (57%). It is noteworthy that this system may undergo a rearrangement, viz., ArCOCH,Br+ (C,H,),NH— ArCHjCON(C,H5)a (45%). The reaction of a-halo ketones with arylamines is even more complex. Examples of the formation of a-aminoaldehydes by this method are few. However, the same results may be achieved by the amination of the halo acetals with subsequent hydrolysis. "... 

Among various CRAs, NaH-r-pentyl-ONa-Ni(OAc)2 (NiCRA) and NaH-t- ntyl-ONa-ZnCl2 (ZnCRA) possess particularly notable reducing ability toward alkyl halides. With NiCRA, the general reactivity follows the trend RI > RBr > RCl and primary > secondary > tertiary. ZnCRA reduces only primary halides, following the trend RI > RBr RCl. Elimination is not observed in the reduction of (3-halo ethers and a-halo acetals. 

Vinylic ethers give p-halo acetals.A mixture of CI2 and... 

Cobalt-catalyzed coupling of halo acetal with a phenyl Grignard reagent proceeded smoothly, and involved radical cyclization before coupling. A variety of substrates were examined the results obtained are shown in Tab. 3.25 and Scheme 3.156 [171]. 

Halo acetals bearing a terminal alkene moiety underwent phenylative cyclization to give the corresponding benzyl-substituted tetrahydrofuran derivative in good to... 

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