Permanent protection

In 1956 Brown, in a series of patents(68-75), disclosed that clays could be treated with di-, tri-, or tetra-substituted ammonia derivatives. Later, McLaughlin, et al.(76,77), introduced cationic polymers as permanent clay protective chemicals. A series of published results describing laboratory and field applications soon became available(78-81). Structural details of the cationic polymers appeared in patents(82-85). In general the polymers are polyamine derivatives, mostly quaternary in nature. Theng(86,87) has discussed how the multiple cationic centers in these polymers can interact and permanently protect clays. Callaway(88) et al. has noted that cationic polymers may interfere with the performance of crosslinked fracturing fluids. 

Scheme 1.4 Solid-phase synthesis of P-(l —> 6)-linked gentiotetraose employing ester groups for permanent protection, for temporary protection, and as linkage to the support.

It appears as if one of the level K electrons of the atom disappears into the nucleus. The void created induces X-ray fluorescence from the nucleus Y. There are several known radionucleides of this type, which have sufficiently long periods that they can be used as different energy sources (see Table I3.l). These are typically used in portable instruments. The activity of these isotopic sources is generally in the order of a few mCi and they can yield a flux of 106 to 108 photons/s/steradian. Because these sources require a permit for use as well as permanent protection because they emit continuously, their use is rapidly diminishing. 

Lead is an acid-resistant material and the lead sulphate formed on the surface does not contaminate the product of nitration, merely forming a permanent protective layer against the corrosive effects of the nitrating acids. However, the cover of lead sulphate acts as a thermal insulator, which is undersirable since it lowers efficiency of operation, e.g. when leaden cooling coils are used. 

P = Temporary protecting group R = Permanent protecting group... 

Another critical consideration was the development of an effective protecting-group strategy. We chose to employ benzyl ethers as permanent protecting groups, not to be removed during the course of intermittent deprotection events, because of their stability under moderately acidic and... 

In carbohydrate chemistry, benzyl ethers are often applied as permanent protecting groups. They are robust and are stable to a wide range of basic and acidic conditions. They can also withstand hydride reducing agents... 

P = temporary protecting group p = permanent protecting group LG = leaving group... 

A branched peptide 11 [prothrombin(l-9)-Lys(Leu-enkephalin)-NH2] has been synthesized on a PAL-PEG-PSty resin in a continuous flow peptide synthesizert l by combining A -Fmoc for temporary protection, A -Aloc for semipermanent lysine protection, and Boc for permanent protection of the terminal amino group of prothrombin, as outlined in Scheme 6. Deprotection of Aloe in the continuous flow synthesizer is achieved by use of the Pd(PPh3)4/ NMM/ACOH/CHCI3 system. 

Acetates and benzoates are excellent protecting groups. They can be introduced and removed in high yields in large numbers under mild conditions. Drawbacks as permanent protecting groups include their relatively limited base stability and their tendency to migrate. 

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