Chlorine selectivity, lack

As 1,2,5-thiadiazole analogues, potent HlV-1 reverse transcriptase inhibitors, some simple 1,2,5-oxadiazoles, compounds 4-6 (Fig. 9), have been synthesized using the traditional Wieland procedure as key for the heterocycle formation [121]. Such as thiadiazole parent compounds, derivative with chlorine atoms on the phenyl ring, i.e., 5, showed the best anti-viral activity. Selectivity index (ratio of cytotoxic concentration to effective concentration) ranked in the order of 5 > 6 > 4. The activity of Fz derivative 6 proved the N-oxide lack of relevance in the studied bioactivity. These products have been claimed in an invention patent [122]. On the other hand, compound 7 (Fig. 9) was evaluated for its nitric oxide (NO)-releasing property (see below) as modulator of the catalytic activity of HlV-1 reverse transcriptase. It was found that NO inhibited dose-dependently the enzyme activity, which is hkely due to oxidation of Cys residues [123]. 

On the other hand, the type of fiber was only important for the lighter PCBs, mainly for PCB-28 and PCB-52. For these compounds, the PDMS-DVB fiber is more efficient than the PDMS fiber. The effect of the fiber factor appeared negative (Figure 2) because PDMS-DVB was selected as its low level (Table 1). For the highly chlorinated PCBs, the two fibers tested seem to have similar performance, and this factor lacks statistical significance. 

Another factor that limits the usefulness of the chlorination reaction in the laboratory is the lack of selectivity exhibited by the chlorine atom when more than one type of hydrogen is available to be abstracted. As an example, the reaction of isobutane with chlorine produces l-chloro-2-methylpropane and 2-chloro-2-methylpropane in a 2 1 ra-... 

A thoughtful reader would have noticed that, while plenty of methods are available for the reductive transformation of functionalized moieties into the parent saturated fragments, we have not referred to the reverse synthetic transformations, namely oxidative transformations of the C-H bond in hydrocarbons. This is not a fortuitous omission. The point is that the introduction of functional substituents in an alkane fragment (in a real sequence, not in the course of retrosynthetic analysis) is a problem of formidable complexity. The nature of the difficulty is not the lack of appropriate reactions - they do exist, like the classical homolytic processes, chlorination, nitration, or oxidation. However, as is typical for organic molecules, there are many C-H bonds capable of participating in these reactions in an indiscriminate fashion and the result is a problem of selective functionalization at a chosen site of the saturated hydrocarbon. At the same time, it is comparatively easy to introduce, selectively, an additional functionality at the saturated center, provided some function is already present in the molecule. Examples of this type of non-isohypsic (oxidative) transformation are given by the allylic oxidation of alkenes by Se02 into respective a,/3-unsaturated aldehydes, or a-bromination of ketones or carboxylic acids, as well as allylic bromination of alkenes with NBS (Scheme 2.64). 

Because this information Is lacking and because J resembled some recently reported larvlcidal thiosemicarbazones (1 1, J ), we decided to study i in detail. Also, since 2 (13) and related chlorine derivatives (L4,l 5) are fungi-toxlc, we evaluated 1 and analogs against selected plant pathogenic fungi. 

There is increasing interest in the use of oxone (Z) for bleaching and bleach sequences such as OZEP have been examined experimentally (P stage is peroxide). It is likely that the use of oxygen and ozone in combination will be used over the next decade, with ozone replacing part or all of the chlorine. One of the disadvantages of ozone is its lack of selectivity and its tendency to attack carbohydrates as well as lignin. 

Another compound lacking the indole ring is 2,4-D, an auxin which becomes a potent selective weed-killer when used at high concentrations. By the substitution of the chlorine atom at various positions on the phenyl ring, other potent compounds have been developed including 2,4,5-T (12). 

Sohd-state sensors SoHd-state sensors, also referred to as metal oxide sensors, are best used as general survey instruments because of their lack of selectivity/specificity. This type of sensor has a rather narrow working range of 1-50 ppm. A variety of MOS sensors are available for the detection of combustible gases, chlorinated solvents, and some toxic gases, such as carbon monoxide and hydrogen sulfide. 

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