RuCl3/CeCl3/NaIO4: a new bimetallic oxidation system for the mild and efficient dihydroxylation of unreactive olefins

[reaction: see text] The catalytic dihydroxylation of olefins represents a unique synthetic tool for the generation of two C,O-bonds with defined relative configuration. Whereas OsO(4) has been established as a very general dihydroxylation catalyst within the past 30 years, the less expensive and toxic isoelectronic RuO(4) has found only limited use for this type of oxygen-transfer reaction. High catalyst loading and undesired side reactions were severe drawbacks in RuO(4)-catalyzed oxidations of C,C-double bonds. Recently, we were able to improve the RuO(4)-catalyzed dihydroxylation by addition of Bronsted acids to the reaction mixture. This protocol proved to be of general applicability, however, certain limitations were observed. To address these problematic functional groups a new Lewis acid accelerated oxidation was developed. The use of only 10 mol % of CeCl(3) allowed a further decrease in the catalyst concentration down to 0.25 mol % while broadening the scope of the reaction. Silyl ethers and nitrogen containing functional groups are now tolerated in this optimized protocol. Furthermore, competing scission reactions are supressed in the presence of Lewis acid allowing longer reaction times and the successful oxidation of electron-deficient tetrasubstituted double bonds that cannot be oxidized using known dihydroxylation protocols.     

Synthesis of extended spacer-linked neooligodeoxysaccharides by metathesis olefination and evaluation of their RNA-binding properties

The preparation of linear 1,4-butanediol-linked oligodeoxysugars 50-53, 58 and 60 is described which are potential binders to polynucleotides. Various aminodeoxymonosaccharides 9, 13-18, 30, 31 and 40-42 which are either allylated at the anomeric center or at C4 were subjected to the metathesis olefination protocol. Depending on the position of allylation E/Z-mixtures of C₂-symmetric head-to-head or tail-to-tail homodimers were formed. Among them, saccharides 13, 30, 31 and 40 were transformed into the corresponding 1,4-butanediol linked disaccharides 50-53 by catalytic hydrogenation of the central olefinic double bond and exhaustive deprotection. In order to target extended spacer-linked neooligosaccharides homodimeric aminoglycoside 37 was bisallylated and subjected to cross metathesis conditions using methyl 4-O-allyl daunosamide 40 as reaction partner which yielded two desired trimeric and tetrameric linearly spacer-linked daunosamine derivatives. After hydrogenation and deprotection two additional probes 58 and 60 for nucleic acid binding studies were at hand.     

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