Abstract:This chapter deals with the synthesis of nucleosides (e.g., the formation of
N
‐glycosides of sugars such as
D
‐ribose or 2‐deoxy‐D‐ribose with heterocyclic nitrogen bases). The methods of nucleoside synthesis have been treated in a number of reviews and monographs.
It is now generally accepted that nucleosides were among the first organic compounds formed at the start of evolution in the early history of our planet earth. To sup… Show more
“…Thus, the pyrazoline cycloadducts are initially formed, but these are unstable and spontaneous elimination of R 1 S(O)Cl gives the pyrazoline intermediate, which on 1,5-trimethylsilyl shift affords the N-trimethylsilyl pyrazole. Hydrolytic cleavage of the N-Si bond [36][37][38][39][40] (presumably when the crude product is purified by chromatography on silica gel) yields the final pyrazole product (Scheme 10).…”
Section: -Fluorophenyl-and N-benzyl-benzylthio Derivatives 19 and 28mentioning
2-Thio-3-chloroacrylamides undergo 1,3-dipolar cycloadditions with diazoalkanes leading to a series of novel pyrazolines and pyrazoles. The mechanistic and synthetic features of the cycloadditions to the 2-thio-3-chloroacrylamides at both the sulfide and sulfoxide levels of oxidation are rationalised on the basis of the nature of the substituents.
“…Thus, the pyrazoline cycloadducts are initially formed, but these are unstable and spontaneous elimination of R 1 S(O)Cl gives the pyrazoline intermediate, which on 1,5-trimethylsilyl shift affords the N-trimethylsilyl pyrazole. Hydrolytic cleavage of the N-Si bond [36][37][38][39][40] (presumably when the crude product is purified by chromatography on silica gel) yields the final pyrazole product (Scheme 10).…”
Section: -Fluorophenyl-and N-benzyl-benzylthio Derivatives 19 and 28mentioning
2-Thio-3-chloroacrylamides undergo 1,3-dipolar cycloadditions with diazoalkanes leading to a series of novel pyrazolines and pyrazoles. The mechanistic and synthetic features of the cycloadditions to the 2-thio-3-chloroacrylamides at both the sulfide and sulfoxide levels of oxidation are rationalised on the basis of the nature of the substituents.
“…Two approaches to the synthesis of purine nucleosides have been widely and extensively studied. The first, convergent approach consists in the condensation of a purine base with a suitable pentofuranose derivative and subsequent deprotection (see, e.g., [5,6,9] and the reviews [10] ). The second comprises the chemical transformation of the purine base or/and the pentofuranose fragment of the natural commercially available ribo-or 2'-deoxyribo-nucleoside in the desired compound.…”
The enzymatic transglycosylation of 2,6-dichloropurine (26DCP) and 6-chloro-2-fluoropurine (6C2FP) with uridine, thymidine and 1-(b-d-arabinofuranosyl)-uracil as the pentofuranose donors and recombinant thermostable nucleoside phosphorylases from G. thermoglucosidasius or T. thermophilus as biocatalysts was studied. Selection of 26DCP and 6C2FP as substrates is determined by their higher solubility in aqueous buffer solutions compared to most natural and modified purines and, furthermore, synthesized nucleosides are valuable precursors for the preparation of a large number of biologically important nucleosides. The substrate activity of 26DCP and 6C2FP in the synthesis of their ribo-and 2'-deoxyA C H T U N G T R E N N U N G ribo-nucleosides was closely similar to that of related 2-amino-(DAP), 2-chloro-and 2-fluoroadenines; the efficiency of the synthesis of b-d-arabinofuranosides of 26DCP and 6C2FP was lower vs. that of DAP under similar reaction conditions. For a convenient and easier recovery of the biocatalysts, the thermostable enzymes were immobilized on MagReSyn epoxide beads and the biocatalyst showed high catalytic efficiency in a number of reactions. As an example, 6-chloro-2-fluoro-(b-d-ribofuranosyl)-purine (9), a precursor of various antiviral and antitumour drugs, was synthesized by the immobilized enzymes at 60 8C under high substrate concentrations (uriA C H T U N G T R E N N U N G dine:purine ratio of 2:1, mol). The synthesis was successfully scaled-up [uridine (2.5 mmol), base (1.25 mmol); reaction mixture 50 mL] to afford 9 in 60% yield. The reaction reveals the great practical potential of this enzymatic method for the efficient production of modified purine nucleosides of pharmaceutical interest.
“…To obtain the b-oriented glycoside bond their use appears to be a convenient alternative to chemical methods which suffer from low stereoselectivity, multi-step procedures and modest total yields. [3] Several procedures based on the "transglycosylation" reaction (Scheme 1) have been employed (using both isolated enzymes and whole cells) to prepare a wide variety of nucleosides. [4][5][6][7][8][9][10][11] Structural analysis revealed that there are two distinct families of NPs: NPI and NPII.…”
Purine nucleoside phosphorylase (PNP) from Aeromonas hydrophila encoded by the deoD gene has been over-expressed in Escherichia coli, purified, characterized about its substrate specificity and used for the preparative synthesis of some 6-substituted purine-9-ribosides. Substrate specificity towards natural nucleosides showed that this PNP catalyzes the phosphorolysis of both 6-oxo-and 6-aminopurine (deoxy)ribonucleosides. A library of nucleoside analogues was synthesized and then submitted to enzymatic phosphorolysis as well. This assay revealed that 1-, 2-, 6-and 7-modified nucleosides are accepted as substrates, whereas 8-substituted nucleosides are not. A few transglycosylation reactions were carried out using 7-methylguanosine iodide (4) as a d-ribose donor and 6-substituted purines as acceptor. In particular, following this approach, 2-amino-6-chloropurine-9-riboside (2c), 6-methoxypurine-9-riboside (2d) and 2-amino-6-(methylthio)purine-9-riboside (2g) were synthesized in very high yield and purity.
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