Hydroxy acid, protected, acyl

Although oligomers of a-hydroxy acids (depsides) and mixed oligomers of cx-hydroxy and a-amino acids (depsipeptides) are found in nature (antibiotics, ion-transporters [249,250]), little solid-phase methodology has been developed for their preparation. One recent strategy is based on sequential acylation with THP-protected cx-hydroxy acids (DIC, DMAP, THF, 2 h see Entry 6, Table 13.13), followed by deprotection with TsOH/MeOH [249,251]. Under these conditions, no racemization was observed. A similar approach to the solid-phase synthesis of depsipeptides is outlined in Figure 16.24. 

Acyl-MODD (X) provided by activation of N-protected-a-amino acids and O-protected-a-hydroxy acids with COMODD have been proved to be suitable for the synthesis of p-keto ester as depicted in scheme 211 (Ref. 262). 

Liebeskind and Welker reacted 720 with lithium diisopropylamide (sec. 9.2.A) and propanal, showing the enantioselectivity in the final P-hydroxy acids (722 and 723) to be dependent on the conditions used to form the enolate. If diisobutylaluminum chloride (i-Bu2AlCl) was used, a 5.2 1 mixture of 722 and 723 was reversed (to 1 11.6) in 66% yield. Good enantioselectivity was observed in this reaction also. The acyl iron derivative behaves essentially as a protected acid, where the iron group is a chiral auxiliaryThe chiral iron moieties are useful variation of enolate condensation chemistry (sec. 9.4.B). In addition to the formation of the condensation product, the high asymmetric induction will prove valuable. 

The mesoporous silica FSM-16 has been found to catalyze oxidative photodecarboxylation of a-hydroxy acids and phenylacetic acid derivatives to yield the corresponding carbonyl compounds. It also promotes the oxidative photodecarboxylation of N-acyl-protected a-amino acids to afford the corresponding... 

Mocimycin has been chemically converted to aurodox by protection of the 4-hydroxy group at the pyridone moiety as the benzoylformate, followed by /V-methylation and hydrolytic removal of the protective group (1,55). Whereas aurodox esters are active growth promotors in animals, goldinamines that are A/-acylated by acids other than goldinonic acid, such as acetic, benzoic, or arylsulfonic acids, lack useful antimicrobial or growth-promoting activity (1). 

Hydroxy- and amino carboxylic acids can be dimerized in good to moderate yields, when the substituents are not in the a- or P-position and when they are additionally protected against oxidation by acylation (Table 2, No. 17-19). 2-Alkenoic acids cannot be dimerized but lead to more or less extensive passivation of the anode due to the formation of polymer films [136]. 3- and 4-Alkenoic acids give moderate yields when they are neutraUzed with BU3N or EtjN [136]. 3-Alkenoic acids with the structure... 

By 1960 it was clear that acetyl CoA provided its two carbon atoms to the to and co—1 positions of palmitate. All the other carbon atoms entered via malonyl CoA (Wakil and Ganguly, 1959 Brady et al. 1960). It was also known that 3H-NADPH donated tritium to palmitate. It had been shown too that fatty acid synthesis was very susceptible to inhibition by p-hydroxy mercuribenzoate, TV-ethyl maleimide, and other thiol reagents. If the system was pre-incubated with acetyl CoA, considerable protection was afforded against the mercuribenzoate. In 1961 Lynen and Tada suggested tightly bound acyl-S-enzyme complexes were intermediates in fatty acid synthesis in the yeast system. The malonyl-S-enzyme complex condensed with acyl CoA and the B-keto-product reduced by NADPH, dehydrated, and reduced again to yield the (acyl+2C)-S-enzyme complex. Lynen and Tada thought the reactions were catalyzed by a multifunctional enzyme system. 

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