Alternative protection

While almost any corrosion problem can be solved or avoided, it is vitally important from a commercial point of view that the economics of corrosion prevention are taken into account. This necessitates evaluation of initial protection costs together with maintenance charges throughout service life. This is very important when alternative protection schemes are available and will often reveal that an initial cheap scheme can in reality prove to be very expensive. 

Besides cholesterol efflux from arterial wall and its role in RCT, additional properties of HDL have been proposed for its protective anti-atherogenic activities. HDL protects vascular function by a number of potential alternative mechanisms, including inhibition of LDL oxidation [8,9], platelet aggregation and coagulation [10], and endothelial monocyte adhesion [11], as well as promotion of endothelial nitric oxide synthase (eNOS) [12], and prostacyclin synthesis [13-15]. The proposed alternate protective mechanisms for HDL are attractive but many of them lack validation under in vivo conditions. 

This reaction was first demonstrated using the desulfurization of /V,/V -bis(trifluoroacetyl)-L-cystine dimethyl ester which proceeded in 96% yield.121 However, mild alkaline hydrolysis to obtain the free acid resulted in significant product losses to yield only 64% of L-lanthionine (2R,6R-lanthionine). Alternative protecting strategies have been used in order to raise the versatility of the reaction for peptide synthesis, thus, A(Al -bis(benzyloxycarbonyl)-L-cystine diethyl ester could be converted into L-lanthionine in 86% yield. 

To allow for the use of the stronger sulfating reagent pyridine/S03, an alternative protection strategy has been proposed by Fujii et al. 55-99 that is based on postsynthetic protection of the amino groups as Fmoc derivatives and of the serine hydroxy groups as terf-butyldiphenylsilyl derivatives as shown in Scheme 12. The histidine side chain is not protected since model studies indicate that sulfated histidine is readily hydrolyzed by water within 60 minutes. 

One type of oligoamide that can readily be prepared on supports without the need for any partially protected monomers (which are often tedious and expensive to synthesize) are N-substituted oligoglycines (Figure 16.21). These compounds are prepared by a sequence of acylation of a support-bound amine with bromoacetic acid, displacement of the bromide with a primary aliphatic or aromatic amine, and repeated acylation with bromoacetic acid. Because primary amines are cheap and available in large number, this approach enables the cost-efficient production of large, diverse compound libraries. Alternatively, protected N-substituted glycines can also be prepared in solution and then assembled on insoluble supports (Entry 5, Table 16.2). 

With this handbook, scientists or engineers requiring hazardous operation protection can select, modify, or design a suppressive shield for their required use. This handbook is available through DDC or National Technical Information Service and provides an alternative protective method previously not available to provide increased protection to personnel Involved in hazardous operations. 

The racemization of His as discussed in Section 4.3.2.2.2 is promoted by its own internal base, the jt-nitrogen of the imidazole ring. Histidine is usually protected at the x-nitrogen, but is commercially available protected at the jt-nitrogen as Fmoc-His(Bum)-OH. However, the less-expensive alternatives protected by Boc or Trt at the x-position can be used effectively if the acidity of the acylating mixture is maintained by the addition of HOBt. 

A very useful alternative protection of indazoles is achieved using trimethylsilyl chloride in the presence of a hindered base, producing 2-trimethylsilylindazole it is the neutral indazole which is silylated by reaction at the imine nitrogen, the hindered base being present to deprotonate the initially formed salt. ... 

Alternatively, protection of the carbonyl group of 200 as a dioxolane, 207, through treatment with ethylene glycol (74% yield), was followed by debenzylation (Li-NH3) in 90% yield. The resulting lactam 208 was reduced with UAIH4 to 209 which afforded (+)-epi-dihydrotecomanine (202) in 61% yield by treatment with formaldehyde in HCOOH and acid hydrolysis of the ketal (Scheme 12) (195). 

An alternative protecting reagent to the commonly used silyl group has been reported. The fluoride labile p-nitrobenzyloxymethyl may also be used to mask the 2 -hydroxyl function in RNA synthesis. 

Protection of the oxo-group in 2a-cyano-3-oxo-steroids (420) by acetal formation (421) permits alkaline hydrolysis of the cyano-group. After esterification of the carboxylic acid (422), the 3-oxo-group could be regenerated (423). Alternatively, protection of the ketone could be achieved by forming a 3-enol ether. Other routes to esters of 2-carboxy-3-oxo-steroids and to the derived 3-alkoxy-, 3-amino-, and 3-chloro-A -unsaturated derivatives have also been devised. ... 

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