Acetals donors

Another observation on oxalate formation is that other a-keto acids, such as oxalosuccinic acid (74) and a-ketoglutaric acid (106) do not seem to yield oxalate directly but indirectly (123). This appears to be due to the fact that only oxaloacetic acid can function as an acetate donor. In this connection the intervention of Coenzyme A may be considered, since it is reported to function in the acetylation of sulfanilamide and choline (73) and recently was shown to take part in the enzymatic synthesis of citric acid. This concept may be illustrated as follows ... 

Representative Procedure for Lewis Acid Promoted Clycosylation with Glycosyl Acetate Donors Using SnCI4 [198]... 

W. R. Roush and C. E. Bennett, A highly stereoselective synthesis of 2-deoxy-P-glycosides using 2-deoxy-2-iodo-glucopyranosyl acetate donors, /. Am. Chem. Soc., 121 (1999)3541-3542. 

On the other hand, Roush and coworkers reported the synthesis of the functionalized C-D-E trisaccharide precursor of Olivomycin A, in which, particularly, the 2-deoxy-2-iodo-3-methyl-Q -L-mannop)Tanosyl linkage was stereoselectively introduced by using the glycosyl acetate donor (O Scheme 26) [34], This new a-glycosylation protocol was applied to the synthesis... 

Three histone-specific acetyltransferases have been partially purified and characterized from rat thymus nuclei (225). The enzymes were extracted from rat thymus nuclei by sonication in the presence of 1M ammonium sulfate and separated into two active fractions (A and B) by DEAE-cellulose chromatography. Fraction B was further separated into two active fractions (Bi and B2) by gel filtration on Sephadex G-200. Each fraction was then purified further by chromatography on hydroxyapatite. The molecular weights, determined by Sephadex G-200 and by sucrose density gradient centrifugation, were 99,000, 110,000, and 92,000 for enzymes A, Bi, and B2, respectively. All three enzymes required acetyl CoA as acetate donor, and the activity of the enzymes was inhibited by p-chloromercuribenzoate. Acetyltransferase A preferentially acetylated histone I (FI) and also poly-L-lysine. Acetyltransferase Bi and B2 preferred histone H4 (other names IV, F2al) and did not acet-ylate poly-L-lysine and histone H3 (III, F3). In addition to c-N-acetyl-lysine, two other unidentified amino acid derivatives were obtained from a digest of histone H4 acetylated by the two B enzymes. 

The longest peptide synthesized by a non-ribosomal mechanism in vitro is alamethicin which is a nonadecapeptide (9). The biosynthesis of alamethicin is initiated on the synthetase by acetylation of thiol-ester-bound aminoisobutyric acid, which remains enzyme bound. Acetvl-CoA serves as the acetate donor. Activation and elongation reactions seem to follow the biosynthesis mechanism of gramicidin S on a multienzyme complex with a molecular weight of approx. 480 000. Alamethicin biosynthesis is terminated by attachment of ohenvlalaninol. which is probably the reaction product of a separate enzyme (107i... 

Table 3 Iron triflate-catalyzed glycosylation using acetate donor 1"...

Table 4 Iron triflate-catalyzed glycosylation using acetate 1 oxazoline 2 and different 2-acetamide-p-acetate donors 83-85 ...

Studies of the synthesis of olivomycin A have been reported [183], Glycosylation of a glycal acceptor with a trichloroacetimidate and an acetate donor gave a trisaccharide which was transformed into the trichloroacetimidate and used for the glycosylation of the acyloin acceptor. The resulting p-glycoside is a model compound for the approach to the synthesis of olivomycin A. 

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