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.
Directed directing group: The C7 position of the indoline nucleus is difficult to address in CH activation. An oxidative palladium(II) catalysis that allows for cross‐dehydrogenative coupling in that position with activation of the CH bond of the arene component is disclosed here. This CH/CH cross‐coupling is applicable to various indolines acetylated at the nitrogen atom. Substitution at C2 is crucial for the CH activation to occur at C7 (see scheme).
A mild procedure for C-7-selective C-H alkenylation of various indolines under oxidative palladium(II) catalysis is reported. A fully substituted urea, formed by carbamoylation of the indoline nitrogen atom, functions as a directing group. Both α,β-unsaturated acceptors and styrenes participate in this direct C-H functionalization. With a free NH group at the urea terminus, the nitrogen atom subsequently cyclizes in a 1,4-fashion to yield a six-membered ring.
An improved method for the dehydrogenative C-H/C-H cross-coupling at the C-7 position of indolines containing a urea as a directing group is reported. The new protocol is a rare example of an aerobic palladium(II)-catalyzed cross dehydrogenative coupling (CDC) reaction that proceeds at low temperature. The use of either Cu(OAc)2 in an open flask or dioxygen (balloon) at 50 °C tolerates indolines not substituted at C-2 and C-3, thereby extending the scope of the previous method that suffers from indoline-to-indole oxidation.
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