Protection carboxyl groups, 2- ethanol

Proteins have primary, secondary, tertiary, and quaternary stmcture.The primary structure of a peptide can be determined by using a combination of enzymes and small molecule reagents. There are several analytical tools that can also be applied to this task. Peptides can be synthesized by employing protecting groups and activating reagents. The common procedures are amide formation vdth tBoc or Cbz for amine protection, esterification with ethanol for carboxylic acid protection, and acid activation with DCC. 

The synthesis of key intermediate 12, in optically active form, commences with the resolution of racemic trans-2,3-epoxybutyric acid (27), a substance readily obtained by epoxidation of crotonic acid (26) (see Scheme 5). Treatment of racemic 27 with enantio-merically pure (S)-(-)-1 -a-napthylethylamine affords a 1 1 mixture of diastereomeric ammonium salts which can be resolved by recrystallization from absolute ethanol. Acidification of the resolved diastereomeric ammonium salts with methanesulfonic acid and extraction furnishes both epoxy acid enantiomers in eantiomerically pure form. Because the optical rotation and absolute configuration of one of the antipodes was known, the identity of enantiomerically pure epoxy acid, (+)-27, with the absolute configuration required for a synthesis of erythronolide B, could be confirmed. Sequential treatment of (+)-27 with ethyl chloroformate, excess sodium boro-hydride, and 2-methoxypropene with a trace of phosphorous oxychloride affords protected intermediate 28 in an overall yield of 76%. The action of ethyl chloroformate on carboxylic acid (+)-27 affords a mixed carbonic anhydride which is subsequently reduced by sodium borohydride to a primary alcohol. Protection of the primary hydroxyl group in the form of a mixed ketal is achieved easily with 2-methoxypropene and a catalytic amount of phosphorous oxychloride. 

The allyl group, a less familiar protective group for carboxylic acids, can be easily removed from esters by treatment with sodium hydrogen telluride. In contrast to the preceding methods, ethanol has been employed as the solvent. 

Figure 3 describes the preparation of A-co-undecenoyl-L-valine CSP bonded to silica gel. The carboxylic acid group of L-valine was protected by the reaction with isobutylene using the method of Roeske [47]. The formed tert-butyl ester of L-valine was precipitated from diethyl ether as the oxalate by the dropwise addition of a solution of 10% oxalic acid in absolute ethanol. The precipitate is dried and the oxalate group is removed by the reaction of sodium hydroxide. The tert-butyl ester of L-valine was treated with undecenoic acid in tetrahydrofuran (THF), which resulted in A-co-undecenoyl-L-valine methyl ester. In another step, lOmM of monochlorosilane was dissolved in 20 mL of dry pyridine and was allowed to react with /V -to - u ndccenoyl-L-valine methyl ester. 

Fig. 14.45. Transformation of an a-phosphonylcarboxylic acid ester (B) via the related carboxylic acid azide F and its Curtius degradation in ethanol to furnish an ethoxycarbonyl-protected a-aminophosphonic acid ester E. The N- and 0-bound protective groups of the latter compounds are cleaved off under acidic conditions. In this manner a-aminophosphonic acids are synthesized. They are interesting analogs of the biologically important a-amino carboxylic acids.

The frequently employed 2-haloethyl protecting group can be removed from the carboxylic esters by means of sodium telluride in DMF or sodium hydrogen telluride in ethanol. ... 

The substituted phthalimides can be cleaved by ca. 20% hydrochloric acid at the boiling point or in a sealed tube at higher temperatures in some cases Posner s method may be used, in which the phthalimide is cleaved by potassium hydroxide solution to the phthalamic acid, whose hydrolysis is completed by hydrochloric acid.445 The phthalimides are, however, more conveniently cleaved by short boiling with a stoichiometric amount of hydrazine hydrate in ethanol 446 this gives the desired amine and phthalazone the latter may be hydrolysed by hydrochloric acid. Carbonyl groups present in the Aralkyl group of a phthalimide may be protected as acetal from the hydrazine fission.447 Phenylhydrazine is also useful for cleavage of phthalimido carboxylic acids.448... 

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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