Carboxyl groups, protection aldehydes

Oxidation of LLDPE starts at temperatures above 150°C. This reaction produces hydroxyl and carboxyl groups in polymer molecules as well as low molecular weight compounds such as water, aldehydes, ketones, and alcohols. Oxidation reactions can occur during LLDPE pelletization and processing to protect molten resins from oxygen attack during these operations, antioxidants (radical inhibitors) must be used. These antioxidants (qv) are added to LLDPE resins in concentrations of 0.1—0.5 wt %, and maybe naphthyl amines or phenylenediamines, substituted phenols, quinones, and alkyl phosphites (4), although inhibitors based on hindered phenols are preferred. 

In general, the methods for protection and deprotection of carboxylic acids and esters are not as convenient as for alcohols, aldehydes, and ketones. It is therefore common to carry potential carboxylic acids through synthetic schemes in the form of protected primary alcohols or aldehydes. The carboxylic acid can then be formed at a late stage in the synthesis by an appropriate oxidation. This strategy allows one to utilize the wider variety of alcohol and aldehyde protective groups indirectly for carboxylic acid protection. 

Selective protection of aldehydes vs. ketones, 218 of amino vs. carboxyl groups, 270 of a- vs. co-amino and carboxyl groups in amino acids, 373... 

Our final example is a base-labile 4-(phenylsulfonyl)methyl-l,3-dioxolane protecting group for aldehydes and ketones.4 Protection is carried out by the reaction of diol 17,1 (obtained by dihydroxylation of ally phenyl sulfone) with a carbonyl compound in the presence of pyridinium p-toluene sulfonate [Scheme 2.17], Cleavage is accomplished by treatment with DBU. /erf-Butyldimethylsilyl ethers, p-toluenesulfonate esters, tetrahydropyranyl ethers, carboxylic esters and benzoates are well tolerated. A disadvantage to the use of 17.1 is the introduc-... 

Protection of carboxyl groups. The deblocking reaction discussed above with 2 is also applicable to a halogenated protected derivative of the terminal carboxyl group of peptides. The N-protected amino acid is converted by the Passerini reaction with an o-halo aldehyde into a protected derivative such as 4. The derivative is cleaved by reiietion with 2 at 20° in acetonitrile or methanol (equation III). 

Repetitive Ugi reactions are known. This product probably arises from a reaction between the carboxylic acid, the isocyanide, and the imine formed from the aldehyde or ketone and ammonia or the primary amine. The use of an A pro-tected amino acid or peptide as the carboxylic acid component and/or the use of an isocyanide containing a C-protected carboxyl group allows the reaction to be used for peptide synthesis. 

---