Acidity continued thioesters

In contrast to oxoesters, the a-protons of thioesters are sufficiently acidic to permit continuous racemization of the substrate by base-catalyzed deprotonation at the a-carbon. Drueckhammer et al. first demonstrated the feasibility of this approach by performing DKR of a propionate thioester bearing a phenylthiogroup, which also contributes to the acidity of the a-proton (Figure 4.14) [39a]. The enzymatic hydrolysis of the thioester was coupled with a racemization catalyzed by trioctylamine. Owing to the insolubility of the substrate and base in water, they employed a biphasic system (toluene/H2O). Using P. cepacia (Amano PS-30) as the enzyme and a catalytic amount of trioctylamine, they obtained a quantitative yield of the corresponding... 

Thiol esters undergo smooth reduction to give aldehydes by the Fukuyama hydrosilylation procedure, which is an alternative way to transform carboxylic acids to aldehydes. Upon treatment with Et3SiH and 10% Pd/C, a thioester underwent smooth reduction to give an aldehyde.409,410 For example, to a stirred mixture of thioester and Pd/C in acetone may be added Et3SiH at room temperature under an Ar atmosphere. Stirring is continued until the hydrogenolysis is complete (0.5-1 h) (Scheme 4.117). 

Particularly important to the pathways of modular synthases is the incorporation of novel precursors, including nonproteinogenic amino acids in NRP systems [17] and unique CoA thioesters in PK and fatty acid synthases [18]. These building blocks expand the primary metabolism and offer practically unlimited variability applied to natural products. Noteworthy within this context is the contiguous placement of biosynthetic genes for novel precursors within the biosynthetic gene cluster in prokaryotes. Such placement has allowed relatively facile elucidation of biosynthetic pathways and rapid discovery of novel enzyme mechanisms to create such unique building blocks. These new pathways offer a continued expansion of the enzymatic toolbox available for chemical catalysis. 

CALB-catalyzed copolymerization of CL with 11-mercaptoundecanoic acid (1IMU) leads to the formation of poly(ester-c6>-thioester)s having a Mn of 13.7 kDa (PDI 1.6) after precipitation [30] (Fig. 11). The amount of incorporated IIMU (8.7mol%) was slightly less than the feed ratio (10mol%). Similar results were obtained when using 3-mercaptopropionic acid (3MP) as a comonomer (Mn 14.3 kDa, PDI 1.4). CALB-catalyzed transesterification of pCL with either 1 IMU or 3MP resulted in similar H-NMR and C-NMR spectra as the direct copolymerization of the two monomers, showing that continuous transesterification plays an important role in the microstructure of the polymer [30]. 

In contrast to oxoesters, the a-protons of thioesters are sufficiently acidic to permit continuous racemization of the substrate by base-catalyzed deprotonation at the... 

Fig. 2. Continued) chain of Cys+i (N-terminal amino acid of the C-extein) on the thioester results in the formation of a branched intermediate. Excision of the intein occurs by peptide bond cleavage coupled to succinimide formation at the C-terminal asparagine of the intein. The ligated exteins undergo a spontaneous S-N acyl rearrangement to create a stable amide bond.

Alkyl carboxylic acids, as their coenzyme A (CoA) thioesters, are metabolized by oxidation at the (3-xap(3ov to the carboxylic carbon (p-oxidation). This pathway involves the oxidative cleavage of two carbon units at a time (as acetate), beginning at the carboxyl terminus and continuing until no more acetate units can be removed. The reaction is terminated when a branch (e.g., valproic acid) or aromatic group is encountered. The metabolism of even and odd phenylalkyl acids can serve as an example ... 

Despite the fact that thioesters of optically active amino acids have long been known to racemize promptly even in the presence of rather weak bases [48], and that they are able to act as irreversible acyl donors in enzyme-mediated acyl transfers [49], it was only in 1995 that they were reported to be good substrate candidates for DKR, where they are continuously racemized by an organic base while a hydrolytic enzyme acts preferentially on one of the two enantiomers [50]. In later works, the same authors reported on a more detailed and systematic study of the same reaction system applied to a broader array of substrates. The reactions were ran in a biphasic water/toluene mixture in the presence of trioctylamine as a hydrophobic organic base, so that the substrate racemization takes place in the organic phase while the hydrolysis product is continuously extracted into the water phase. Under racemizing conditions, the authors obtained almost complete conversion and significantly better optical purity (Scheme 8.8) [51], which is attributable to the continue depletion of the unreactive substrate enantiomer. 

---