Exchange under Acidic Conditions

Anilines are well known to undergo rapid exchange under mildly acidic conditions. Labehng of desipramine (3) in dilute acid (1 MHC1/ H,0, 80 °C) gave deuterium incor- 

Another strategic approach to labeling a compound is through in situ formation of a derivative more easily exchangeable than the parent. Several related methods for isotopic labeling of C2 of a-amino acids utilize this strategy, most of them in acidic conditions. One 

Deuterium/tritium exchange-labeling at C2 of amino acids via imine/iminium 

An alternative to the use of tritiated aqueous acid or acid/organic solvent mixtures is tritiated heptafluorobutyric acid, which combines good solvating power for organic substrates, low volatility and simple preparation, from heptafluorobutyric anhydride and tritiated water, without dilution of tritium by ancillary hydrogens. The method was shown 

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H] T-2 toxin was prepared by the Wilzbach method (New England Nuclear, Boston, MA). The crude product was dissolved in acetone water (1 1) and allowed to stand for several hours. All labile tritium was removed with a rotary evaporator. The residue was dissolved in benzene and was applied to a column of silica gel 60 (14.5 X 1.5 cm, 70-230 mesh, BDH Chemicals, Toronto, Ont.) which was developed with 100 ml benzene followed by 300 ml benzene acetone (3 1). Five ml fractions were collected and a small portion of each was applied to a TLC silica gel GF plate (Fisher Scientific Co., Toronto, Ont.) which was developed in benzene acetone (63 37). Fractions containing T-2 toxin were identified by comparison with known standards. The toxin was then recrystallized by the method of Bamburg et al. (1968). Specific activity of [%] T-2 toxin was determined to be 0.172 mCi/mg with the concentration of T-2 toxin measured by gas chromatography (Mirocha et al., 1976). The in vitro method of Chi et al. (1978) was used to test for possible % exchange under acidic conditions similar to those of the stomach. 

The a hydrogens of acetaldehyde can also be exchanged under acidic conditions (Fig. 19.12). This result illustrates a general principle In carbonyl chemistry, there will usually (not quite always) be an add-catalyzed version for every base-catalyzed reaction. 

Acetals Nitroso acetals combine reactivities of both amines and carbon-centered acetals/ketals. For example, either one [134] or both [135] alkoxy group can be exchanged under acidic conditions (Scheme 16.38). The reaction is presumed to proceed via the nitrosonium ion intermediate 169. 

Figure 3.1 Structure of [ HJbleomycin A2 3.1.2 Exchange under Acidic Conditions...

Under acidic conditions, pH < 3.5, and in the presence of certain reducing agents, the permanganate ion can undergo a five-electron exchange resulting in the divalent manganese ion. 

Kinetic data exist for all these oxidants and some are given in Table 12. The important features are (i) Ce(IV) perchlorate forms 1 1 complexes with ketones with spectroscopically determined formation constants in good agreement with kinetic values (ii) only Co(III) fails to give an appreciable primary kinetic isotope effect (Ir(IV) has yet to be examined in this respect) (/ ) the acidity dependence for Co(III) oxidation is characteristic of the oxidant and iv) in some cases [Co(III) Ce(IV) perchlorate , Mn(III) sulphate ] the rate of disappearance of ketone considerably exceeds the corresponding rate of enolisation however, with Mn(ril) pyrophosphate and Ir(IV) the rates of the two processes are identical and with Ce(IV) sulphate and V(V) the rate of enolisation of ketone exceeds its rate of oxidation. (The opposite has been stated for Ce(IV) sulphate , but this was based on an erroneous value for k(enolisation) for cyclohexanone The oxidation of acetophenone by Mn(III) acetate in acetic acid is a crucial step in the Mn(II)-catalysed autoxidation of this substrate. The rate of autoxidation equals that of enolisation, determined by isotopic exchange , under these conditions, and evidently Mn(III) attacks the enolic form. 

Tartronic acid was oxidised to mesoxalic acid on 6%Pt2%Bi/C, prepared by exchange/redox, under acidic conditions (reaction f, Scheme 1) (29% yield at 53% conversion, pH=1.5). Figure 10 shows that the conversion rate of tartronic acid is high at first but decreases as the reaction proceeds, probably because the formed mesoxalic acid is more strongly adsorbed on the surface than tartronic acid. The initial high selectivity tapers off due to over-oxidation. 

Amino acids Amino acids are molecules that have an amino group (-NH2) and a carboxylic acid (-COOH) group. Under acidic conditions, this molecule donates a proton from the -COOH group to the -NH2 group. These conditions promote a cationic exchange between the -NH], cations formed in the amino acid molecules and the cations... 

Figure 10.2 Optimization of a purification protocol by LC-MS analysis. (1) LC-MS analysis of C-terminal fragment from reference molecule purified by cation-exchange chromatography (CEX) and hydroxyapatite chromatography (HA). (2) Incubation at 37°C under acidic conditions shows degradation of the purified molecule if CEX precedes HA. (3) The molecule is stabilized when the sequence of the two purification steps is swapped. 

These early studies clearly revealed the inherent problems of the chemistry for site-directed formation of unsymmetrical disulfides that has to avoid formation of homodimers. These can result from (i) slow rates of activation of the cysteine peptide, (ii) disproportionation of the activated cysteine species, (in) weak activation and thus slow thiolysis by the second cysteine component and thus its oxidation to the homodimer as well as thiol/ disulfide exchange reactions on the unsymmetrical disulfide present in the reaction media. The latter side reactions are partly controlled by operating in degassed argon-saturated buffers or in organic solvents and preferably under acidic conditions where thiol/disulfide exchange reactions on the nonactivated disulfides, i.e. on the target unsymmetrical cystine peptide occurs at slow rates. 

The interpretation of the results of tracer experiments of this sort is sometimes complicated by, sO-exchange reactions. l80 from the solvent may be incorporated into the unreacted ester as hydrolysis proceeds (see below, p. 105), or into either or both of the products. The exchange reaction is significant with alcohols, such as triphenylmethyl alcohol, which give rise to relatively stable carbonium ions under acidic conditions (see, for example, refs. 67, 85), viz-... 

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