Semisynthetic methods

The first antibiotic of the tetracycline series, chlorotetracycUne, which was isolated from a culture liquid of Streptomyces aureofaciens, was introduced into medical practice in 1948. Conseqnently, there were another six dmgs of the tetracycline series that were introdnced into medical practice between 1950 and 1972. Oxytetracycline is isolated from Streptomyces rimosus tetracycline (semisynthetic), demeclocycline is isolated from the mutant type of S. aureofaciens methacycUne (semisynthetic), doxycycUne (semisynthetic), and minocycline (semisynthetic). Methods of synthesis of the tetracycline series antibiotics have been sng-gested however, they are purely of an academic interest and do not have any practical valne. 

Semisynthesis can be defined as the use of fragments of proteins, or intact proteins themselves, as ready-made intermediates in the chemical synthesis of proteins. Many modifications to the native structure of a protein can be expected to have profound effects, and produce materials of the greatest academic or practical value, but only involve a change of one or a few atoms out of the thousands that often go to make up the complete molecule. If a semisynthetic method can be found to incorporate, at the correct site, a small synthetic fragment carrying the wanted change, there is no need to go to the trouble of constructing the rest of the molecule when it has already been preformed for us biosynthetically. 

A different semisynthetic method involves the acylation of an amino alcohol with a peptide ester and the resulting amino alcohol is subsequently oxidized to the aldehyde 40 The acylation of H-Phe[CH2OH] with the peptide ester Z-Ala-Ala-Leu-OMe is carried out in 5% DMF/MeCN with the subtilisin distributed on the surface of macroporous silica gel. The resulting peptide alcohol is oxidized under mild conditions using anhydrous dimethyl sulfoxide and 20-fold excess of acetic anhydride with purification via flash chromatography 40] Z-Phe[CH2OH] has been oxidized under these conditions and the optical rotation indicates little epimerization as compared to literature values 11 40 ... 

A similar semisynthetic method involves pronase E cleavage of leupeptin dibutyl acetal.157 Leupeptin dibutyl acetal has been prepared, cleaved by pronase E, and depro-tected as in the case of thermolysin, except pronase E cleaves the Leu-Arg bond instead of the Leu-Leu bond, thus affording H-Arg[CH(OBu)2]. This enzymatic technique was applied for the semisynthesis of Z-Val-Pro-Arg-Hj57 Again, the peptide dibutyl acetals can be treated with acid to afford the aldehydic leupeptin analogues with good yields and stereochemical conservation. 

Scheme S Semisynthetic method for the cleavage of phospholipase A2 and the reconstitution of azobenzene-modified phospholipase A2. Acm = acetamide, Amd = s-amidinated, DCC = dicyclohexylcarboxiimide, HOSu = N-hydroxysuccinimide, TFA = trifluoroacetic acid, Xaa = trans-azobenzene phenylalanine.

Hamachi, I. Nagase, T. Shinkai, S., A general semisynthetic method for fluorescent saccharide-biosensors based on a lectin, J. Am. Chem. Soc. 2000, 122, 12065-12066... 

However, from the middle of the 1980s onwards, the number of synthetic attempts increased in an almost explosive manner. Initial milestones and still the most important synthetic processes for the industrial production of taxoids are semisynthetic methods in which a suitable side chain is attached to bac-catin III (1) to give either taxol (3a) or taxo-tere (3b). [Ia-<1] Baccatin III is available from renewable parts of a variety of yews in larger amounts, [la-d, f, g]... 

Other total syntheses of cephalosporins have since been achieved. However, they cannot compete with the commercial extraction from moulds or with semisynthetic methods. There are two possible semisynthetic ways ... 

Despite small amounts of taxanes present, hazelnut by-products could have an industrial value for the production of paclitaxel [99]. The reason behind this is that the hazelnut trees grow much faster than the Pacific yew (a slow-growing plant in limited quantities in the Pacific Northwest). This could reduce the cost of the commercial drug and make it more readily available. Commercial supphes of Taxol are now manufactured by a semisynthetic method that relies on extracts from leaves of other yew species. Although pachtaxel has been synthesized artificially in the laboratory without using any yew parts, this method is currently too complex and expensive to implement commercially. 

Elucidation of the thermodynamic basis of Rab membrane targeting requires analysis of interaction between prenylated Rab proteins (GDP/GTP-bound) and REP/GDI. Such analysis is made possible by generation of fluorescent labelled prenylated Rab proteins (Scheme 10a) [30, 54]. A series of Rab7-based protein probes with one or two isoprenyl moieties and fluorophores on the lipid moiety or the lysine side chain were prepared using the EPL technique. The semisynthetic method enables precise installation of GDP/GTP into Rab proteins to generate the off and on states, yielding for the first time homogeneous preparations of functionalized prenylated proteins in a well-defined nucleotide bound state [87]. 

The Raines group published an elegant paper on the replacement of an amide bond with a 1,5-substituted [l,2,3]-triazole 22 and incorporation of this dipeptide into bovine pancreatic ribonuclease (RNase A) by semisynthetic methods and showed that the melting temperature (T ) and catalytic activity of the resulting RNase A variants were retained. The triazolyl dipeptide was prepared by a Ru(ll)-catalyzed alkyne-azide cycloaddition which afforded 1,5-substituted triazoles, selectively (see Scheme 10.6). 

In this subsection, examination is made of the fragments which are portions of well characterized proteins with exception of those which correspond to structural domains these latter are considered in Section 10.2. Several investigations have been reported concerning the folding of fragments from different proteins obtained either after proteolytic degradation or by semisynthetic methods. The most complete studies were conducted with ribo-nuclease, myoglobin, staphylococcal nuclease, and hen egg white lysozyme. 

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