Protective groups definition

Protective group for nucleosides. Stepwise synthesis of oligonucleotides utilizing 2-cyanoethyl phosphate (1, 172-173) requires protection of the 3 -hydroxyl group which can be cleaved under essentially neutral conditions, since a methoxy-trityl ether is sensitive to acid and a /3-cyanoethyl phosphoric ester is sensitive to alkali. The /3-benzoylpropionyl group meets these requirements, since it is quantitatively cleaved by dilute solutions of hydrazine hydrate in pyridine-acetic acid. The esters are prepared by condensation with DCC (dicyclohexylcarbodiimide). Definitive paper R. L. Letsinger and P. S. Miller, Am. Soc., 91, 3356 (1969)... 

An exception to the definition for pseudo poly(amino acids) as previously established is a polymer synthesized by Chen et al. [31,32], because its backbone is comprised of lysine alternating with an isophthaloyl group and lacks nonamide linkages (Table 12.2). Hydrophobicity of the polymer is increased by conjugating ethanol or hydro-phobic amino acids, such as valine, leucine, and phenylalanine, as protecting groups on the carboxyl terminal of lysine via carbodiimide chemistry. This protected lysine is then copolymerized with isophthaloyl chloride via single-phase polymerization in the presence of a base (Table 12.2) [32]. 

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