Isotopic labels bromide

Syntheses and uses of isotopically labelled dienes and polyenes 829 bromide 172 in the presence of triethylamine (equation 59). 

Amino Acids and Peptides. - Wasserman s method of one-carbon homologation of carboxylic acids to give a-ketocarboxylates involves reaction with cyanomethylenetriphenyl-phosphorane followed by ozone (Scheme 24) and has been used as a key step in a chemo-enzymatic synthesis of isotopically labelled L-valine, L-isoleucine, and o/fo-isoleucine. Alkylation of the carbanion derived from the imino-substituted methylphosphonate diphenyl ester (186) with indol-3-ylmethyl bromide followed by appropriate deprotection has been used to prepare the phosphonate analogue (187) of tryptophan (Scheme 25). The deprotected analogue (188) and derived peptides show activity as inhibitors of chymotrypsin. Two approaches to solid phase Wadsworth-Enunons reactions which have applications in combinatorial chemistry have been reported. In one diethylphosphonoacetamide is bound to PEG-PAL resin via a peptide link, while... 

DL-Selenomethionine was initially synthesized by Painter" via a sodium/liquid ammonia reduction of DL-selenohomocystine followed by an alkylation of the resulting sodium selenohomocysteinate with methyl iodide. Other syntheses have since been reported including synthetic pathways for the preparation of optically active material" " and isotopically labeled material "" . DL-Selenomethionine has a solubility in water at 30° and pH 7 of 0.108 M which is considerably less than that for L-methionine (0.386 M). After seven hours of hydrolysis under anaerobic conditions in 6 N HCl at 110 °C selenomethionine is completely decomposed (under the same conditions 96% of methionine remains). Chemically, selenomethionine appears to be more reactive than methionine . For example, with cyanogen bromide, selenomethionine completely reacts in 0.1 M HCl in fifteen minutes while methionine requires twenty-four hours for the same reaction. In both cases the end product is homoserine. Although not as marked, this difference in reactivity was also confirmed in the reaction with hydrogen peroxide . 

Scheme 7.21. A representation of the addition of radiolabeled bromine (indicated as Br2> to 3-bromo-l-propene (allyl bromide, H2C=CHCH2Br). It is important to note that the initially formed bromonium ion is capable of being opened by internal (backside) attack from the bromine atom on the neighboring carbon. This is an example of the process called neighboring group participation (this chapter, Rearrangement). In this particular case, the newly formed bromonium ion is, except for the position of the label ( Br), the mirror image of the bromonium ion from which it was generated. Each of these bromonium ions is capable of being attacked by exogenous labeled bromide anion. Thus, depending on the specific bromonium ion, the final relationship of the two isotopically unique bromine atoms in the product (1,2,3-tribromopropane) will be either 1,2 or 1,3.

The use of a deuterium-labeled organosilicon hydride and location of the deuterium isotope in the reduced product shows that 1,2-hydride shifts also occur. Thus, reduction of 1-bromohexane with triethylsilane-A yields hexane with all of the deuterium at C2 (Eq. 51) similar treatment of cyclohexylmethyl bromide produces melhyIcyclohexane-1 -di (Eq. 52).186... 

Boron Bromide. Approximately 30% of BBr3 produced in the United States is consumed in the manufacture of proprietory pharmaceuticals (qv) (7). BBr3 is used in the manufacture of isotopically enriched crystalline boron, as a Friedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in semiconductor doping and etching. Examples of use of BBr3 as a catalyst include copolymerization of butadiene with olefins (112) polymerization of ethylene and propylene (113), and IV-vinylcarbazole (114) in hydroboration reactions and in tritium labeling of steroids and aryl... 

Problem 11.12 Dehydrobromination by C2HsO Na of ordinary isopropyl bromide and of labeled isopropyl bromide, (CD3)2CHBr, at 25 has been studied, and the rates found to be in the ratio 1.76 0.26. (a) What is the value of the isotope effect (b) Is this isotope effect consistent with the mechanism for dehydrohalogenation given in Sec. 5.13 (c) With the following two-step mechanism involving a carbonium ion ... 

Under conditions where reactions follow second-order kinetics, dehydro-brominalion of ordinary isopropyl bromide by sodium ethoxide takes place seven times as fast as that of the labeled compound, (CD3)2CHBr. An isotope effect of this size, we have seen (Sec. 11.15), reveals the breaking of a carbon-hydrogen bond in the transition state of the rate-determining step. 

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