Transient protection

Load-ceU mountings must be designed in such a way as to minimise the effects of extraneous forces Electrical power is required, along with power-line transient protection... 

These products are very sensitive and are best used in a controlled environment Electric power, along with suitable power line transient protection, is required Capacity range is limited, although this can be extended through the use of lever systems... 

Requires shielding and special procedures to avoid exposure of personnel to radiation Frequent recahbration is required to compensate for source decay over time Electrical power and power line transient protection required... 

Determining the severity of a transient Protection of rotating machines from switching surges Theory of surge protection (insulation coordination)... 

Antivenins are used for passive, transient protection from the toxic effects of bites by spiders (black widow and similar spiders) and snakes (rattlesnakes, copperhead and cottonmouth, and coral). The most effective response is obtained when the drug is administered within 4 hours after exposure... 

All these steps proceed to afford free or N -substituted crystalline cytidines 6 in high yields [11] (cf. the preparation of N (tetramethylene)cytidine 6b in 95.4% yield in Section 1.1.). This simple one-pot reaction is also very easy to perform on a technical scale, as are the subsequently discussed analogous silylation-aminations of purine nucleosides and other hydroxy-N-heterocycles (cf. Sections 4.2.4 and 4.2.5). The concept of silylation-activation while simultaneously protecting hydroxyl groups in alcohols, phenols, or phosphoric acids by silylation was subsequently rediscovered and appropriately termed transient protection [16-18]. 

Tetravalent silicon reagents are less suitable for transient protection of any hydroxyl groups present, because the resulting activated oligomeric or polymeric intermediates cannot be defined. It can, furthermore, be expected that any derivatives utilizing tetravalent silicon are much more polar and less lipophilic than... 

On silylation-amination of the disodium salts of inosine-5 -phosphate 238a or of guanosine-5 -phosphate 238 b with benzylamine, the phosphate moieties are also transiently protected during amination by silylation (cf also the silylation of uridine-5 -phosphate 224) to give, after transsilylation with methanol and addition of NaOH, the desired sodium salt of N -benzyladenosine-5 -phosphate 239a in 80% yield and the sodium salt of the 2-amino derivative 239 b in 78% yield [64] (Scheme 4.23). 

Thus we hope that these O-silylations-activations with the readily available HMDS (MesSiNHSiMes), TCS (MesSiCl), dimethyldichlorosilane (Me2SiCl2), hexa-methylcyclotrisilazane (HNSiMe2)s, OMCTS (HNSiMe2)4, tetra(alkoxy) silane (Si(OR)4) or sihcon tetrachloride (SiCL ), most of which can also effect the transient protection of any present hydroxyl group, and the subsequent or concomitant reaction with nucleophiles accompanied by formation of silylated water as HMDSO (MesSiOSiMes), (OSiMe2)n or Si02 will be applied more often in the fu-... 

Tertiary amine oxides can be converted into TV-hydroxy secondary amines provided that one of the TV-substituents can be selectively eliminated. This procedure has been applied to the synthesis of secondary A-hydroxy-a-amino acids 34 from the corresponding secondary a-amino acids using the /V-cyanoethyl group for transient protection of the secondary amine (Scheme 10) J40l More recently, direct oxidation with 2,2-dimethyldioxirane of a primary amine has been described for H-L-Val-OMe (82% yield) and H-L-Phe-OMe (54% yield))13 The reaction proceeds smoothly without epimerization, but no experimental details have been reported. 

The reaction of diols with alkylating agents can also be performed such that approximately the theoretical amount of monoalkylated product is obtained. Some substrates can even be monoalkylated [36, 41, 42] or monoarylated [43] with substantially higher yields than statistics would predict what must be due to the slowness of the second alkylation. This behavior, however, is not usually observed, and other strategies must usually be used to achieve clean monoalkylations. Use of excess diol will often furnish pure monoalkylated diol, but a suitable means of removal of excess diol will be required. In the examples sketched in Scheme 10.10 the products were purified by column chromatography, recrystallization, or distillation. As with acylations, the formation of monoalkylated diols can also be promoted by Ag20 [44] or by transient protection with boron [45] or tin [46] derivatives. 

A more practical approach is based on A -benzylidene protection of diamino acids by their reaction with benzaldehyde and lithium hydroxide.f " This transient protection is very convenient for the selective introduction of N -protecting groups, e.g. the Z group, with subsequent hydrolysis of the imine derivative, e.g. 12, to release the desired product, i.e. 13 (Scheme Furthermore, if the a-amino function remains protected, e.g. as the Z... 

The 5-(methoxycarbonylsulfanyl) derivatives of cysteine and cysteine peptides are prepared from 5-(acetamidomethyl)- and 5-trityl-protected precursors by reaction with SMoc-Q to produce transiently protected/activated intermediates for further chemical manipulations. According to prior experience with Nps-Cl/ even other 5-aralkyl... 

Although boronates are quite susceptible to hydrolysis, they have been useful for the protection of carbohydrates. It should be noted that as the steric demands of the diol increase, the rate of hydrolysis decreases. For example, pinacol boronates are rather difficult to hydrolyze in fact, they can be isolated from aqueous systems with no hydrolysis. The section on the protection of boronic acids should be consulted. The use of boron acids as protective agents has been reviewed. Boric acid has been used to transiently protect diols. ... 

This group was used for transient protection of histidine during its attachment to a trityl-based polymer support. It is introduced by refluxing a mixture of Me2SiCl2 and... 

Tetanus immune globulin is an immune globulin, which directly neutralizes toxin excreted by Clostridium tetani. It is indicated as a passive, transient protection against tetanus in any person that may be contaminated with tetanus spores when (1) a patient s personal history of immunization with tetanus toxoid is unknown or uncertain, (2) a person received less than 2 a prior doses of... 

A remarkably efficient method for selective N-acylation of nucleosides (given in Fig. 4 for benzoyladenosine) is the transient protection method [35], In a one-pot procedure, the func-... 

Figure 4. Transient protection method for N-acylation of nucleosides. Modified from [35].

Another approach to optimizing the coupling step in oligonucleotide synthesis has been to modify the dialkylamine or transient protecting group... 

Passive transient protection for toxic effects of venoms of coral snake in U.S... 

A phosphoramidite derivative of A -isobutryT2 -0-methylcytidine has been synthesised in order to decrease the risk of transamination reactions which occur at the C4-position of cytidine when alternatives to ammonia are used for oligonucleotide deprotection. The monomer was prepared from cytidine protected with 1,1,3,3-tetraisopropyldisiloxane. Transient protection of the 2 -hydroxyl as the trimethylsilyl derivative followed by reaction with isobutyryl chloride gave the protected nucleoside which could be reacted with methyl iodide and silver oxide after removal of the transient protection. Suitable protection and derivatisation for synthesis yield the required synthon (131). 

Transient Protection. Silyl ethers can be used for the transient protection of alcohols (eq 11). In this example the hydroxyl (4) groups were silylated to allow tritylation with concomitant desi-lylation during aqueous workup. The ease of introduction and removal of TMS groups make them well suited for temporary protection. 

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