Protective groups historical development

Historically, simple Vz-alkyl ethers formed from a phenol and a halide or sulfate were cleaved under rather drastic conditions (e.g., refluxing HBr). New ether protective groups have been developed that are removed under much milder conditions (e.g., via nucleophilic displacement, hydrogenolysis of benzyl ethers, and mild acid hydrolysis of acetal-type ethers) that seldom affect other functional groups in a molecule. 

Historically, the first important cysteine protecting group was the 5-benzyl group, developed by du Vigneaud etal. and used in the classical synthesis of oxytocin,t as well as in the syntheses of a number of cysteine-containing peptides in the first years of solid-phase synthetic methodology.P l This 5-Bzl protection is nowadays used infrequently due to the fact that its removal involves harsh conditions, e.g. sodium in liquid ammoniat or anhydrous hydrogen fluoride at 20°C,t conditions which may lead to side reactions such as elimination... 

Ethlyn Gibbs, director of the non-profit Turks and Caicos National Trust, which is charged with protecting the historic and natural heritage of the islands, says her group has struggled to stay abreast of such accelerating developments. 

Unlike the employers, the employee organisations tend to be made up of members of similar trades or skills rather than from particular industries. This has its reason in the historic development of the trade unions to protect their members employment by restricting entry into a trade and so maintain a demand for those skills. In the early part of the 20th century national negotiations on behalf of groups of unions in an industry had become the practice but this is now being eroded by employment laws and a recognition that it may not always be for the benefit of all members to do so. 

The story thus commences at the beginning of the twentieth century with Emil Fischer (see the Historical Highlight at the end of Chapter 23), who prepared the first peptide and coined the term for such compounds. He developed the concepts we commonly use today for synthesizing peptides, although he was never able to develop a suitable protecting group for the amine function in amino acids. That discovery was left to a former student of his, Max Bergmann,... 

This is the most widely used class of SP linkers. Historically, the SPS of peptides (see Section 2.1) was developed using building blocks protected with acid-labile groups, thus allowing a convenient simultaneous cleavage and deprotection in the final step of the synthesis. Four commercially available acid-labile linkers are depicted in Fig. 1.7 in resin- and compound-bound forms. The preferred cleavage conditions for each linker are also provided. 

The lack of a current ASTM Standard Specification for Natural Cement, until recently, was an obstacle to the fuU realization of historically accurate and appropriate restoration work. Natural cement was used historically as a structural material, and is required to fulfill structural performance requirements in the course of some of its applications in historic repair and rebuilding. To protect the public, it is essential that materials marketed today as natural cement meet the same requirements that governed performance historically. In addition, ASTM task group Cl 2.03.03, working to develop a new standard for historic mortars, must be able to reference an ASTM standard specification for natural cement if they are to include it in an eventual standard. To exclude natural cement from any eventual ASTM standard for historic mortars would be to forsake history. 

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