Elimination reactions bromoacetals

A treatment of 2-butyltelluroaniline with an equimolar amount of bromoacetic acid results in spontaneous cyclization of the formed telluronium salt 31 to give 1-butylbenzotellurazinonium bromide 30. That the alkylation occurs at the tellurium and not at the nitrogen atom of 2-butyltelluroaniline has been proved by the isolation of the methyl ester of 31 in 60% yield when the amine was coupled with methyl bromoacetate under the same reaction conditions. Elimination of butyl bromide from 30 readily occurs on heating of its DMF solution leading to 2//-l,4-benzotellurazin-3(4//)-one 32 in 90% yield. 

In semiindustrial synthesis, to achieve better yields, it is possible to omit (A), by directly preparing the ester (B) by reaction of p-hydroxy acetophenone on ethyl 2-bromoacetate in the presence of potassium carbonate in butanone. The yield of ester is 90%, and elimination of excess of p-hydroxyacetophenone is effected by washing with sodium hydroxide. 

The same elimination strategy was used for the synthesis of the natural product (i )-(-)-dysidazirine 15 as is shown in Scheme 10 [23]. The requisite aziri-dine ester was prepared by treatment of sulfimine 19 with the lithium enolate of methyl bromoacetate. This reaction is a Darzens-type condensation leading to czs-M-sulfinylaziridine ester 20. The elimination of sulfenate was accomplished in the same manner as mentioned above (see Scheme 9). The natural product 15 (see Fig. 1) was obtained in 42% yield. Attempts to prepare azirinomycin 14 in a similar fashion all failed [23]. 

The original synthetic plan contemplated the use of the diene of the type A (Scheme 14) as the pivotal intermediate for pancratistatin by selective modification of one of the double bonds. However, the propensity of the benzylether 101 to undergo facile aromatisation to the diphenylcarboxylic acid 102, necessitated hydroxylation of the olefin 101 prior to base-induced elimination of elements of hydroiodic acid. The cis-diol 103, thus secured, was converted via the olefin 104 into tavu-a-bromoacetate 105 in an unusual reaction studied by Moffat [28], involving the use of 2-acetoxyisobutyrylbromide. The new diol 106 secured from 105 by osmylation protocol was converted into the corresponding stannylene 107 that on successive... 

Figure 12.25 shows how acetals can be brominated electrophihcally because of the (weakly) acidic reaction conditions. Proper acidity and electrophihcity is ensured by the use of pyri-dinium tribromide (B). This reagent is produced from pyridinium hydrobromide and one equivalent of bromine. Pyridinium tribromide is acidic enough to cleave the acetal A into the enol ether G. This cleavage succeeds by way of an El elimination like the one encountered in Figure 9.32 as an enol ether synthesis. The enol ether G reacts with the tribromide ion via the bromine-containing oxocarbenium ion H and the protonated acetal D to form the finally isolated neutral bromoacetal C. (The reaction can be conducted despite the unfavorable equilibrium between the acetal A and the enol ether G, since G continuously reacts and is thus eliminated from the equilibrium.)... 

Carboethoxymethylation. As in the carbonylation reaction cited above, the original two-carbon-atom homologation reaction of trialkylboranes with ethyl bromoacetate (2, 192-193) had the defect that only one alkyl group of the tri-alkylborane is utilized. This disadvantage has now been eliminated by use of 9-BBN as the hydroboration reagent. Thus the B-alkyl-9-borabicyclo[3.3.1] nonane (B-R-9-BBN, 2), prepared as above, reacts with ethyl bromoacetate under the influence of potassium f-butoxide to give the homologated ester (3) in... 

The synthetic applications of o-halobenzyl sulfones as precursors of 1,3- and 1,5-zwitter ionic synthons, have been investigated and their sulfonyl carbanions, generated by means of the phosphazene base Et-P2 reacted with different electrophiles, such as alkyl halides and aldehydes. When the alkylation was performed with ethyl bromoacetate, orf/io-substituted cinnamates were obtained after a subsequent p-elimination of sulfmic acid. When the aldol reaction was performed with paraform aldehyde, vinyl sulfones were obtained. The... 

C-metalated species is normally referred to as the Reformatsky reagent. These two species may further aggregate to form dimer by elimination of alkoxyzinc bromide, though the C-metalated species is the active one in this reaction. This assertion is supported first by the partial recovery of ketone during the complete consumption of zinc when an equivalent zinc and ketone are used and second, by the titration experiment on ethyl a-bromoisobutyrate, in which 70% active reagent and 30% dimeric substance were identified. According to these experimental results, an illustrative mechanism is proposed here using acetone and methyl a-bromoacetate as an example. 

Trimethylsilylacetate esters may he converted to the enolate by treatment with lithium dialkylamide bases (LDA in Eq. 7.28) in THF at -78°C. These will add to ketones or aldehydes quickly at -78°C, followed by elimination of MOjSiOLi and formation of a,p-unsaturated esters in high yields, uncontaminated by p,y-unsaturated isomers [47]. This is known as the Peterson reaction [48, 49]. The requisite ethyl trimethylsilylacetate is made by the reaction of cldorotrimethylsilane, ethyl bromoacetate, and zinc [50]. Esters of longer-chain acids give mostly 0-silylation under these conditions, but diphenylmethylchlorosilane gives C-silylation selectively. These diphenyl-methylsilylated esters also give the Peterson reaction (Eq. 7.29) [51]. 

Thiazolo[3,2-a]pyridines [CsNS-CsN].—3-Ethoxy-2,3-dihydrothiazolo[3,2-with acid they eliminate ethanol. Both types of compounds, (90) and its unsaturated analogue, react with... 

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