Mannosyl iodides

Molecular iodine has also been demonstrated to adivate certain mannosyl sulfoxides albeit over extended periods of time and at higher temperatures. Mannosyl iodides may be involved as intermediates here, but O-iodyl mannosyl sulfonium salts have also been discussed [334]. The yield and selectivity of these readions vary with resped to the potency of the iodonium ion source, IBr yielding much less... 

Continuing on with their synthesis of biantennary mannans, Lam and Gervay-Hague treated diol 144 with the mannosyl iodide 145, obtained in quantitative yield from the corresponding 1,2-(ethoxyethylidene) orthoester with trimethylsilyl iodide, and silver triflate in dichloromethane at —40 °C to gain access to the 7.7-trisaccharide... 

El Badri MH, Willenbring D, Tantillo DJ, Garvay-Hague J (2007) Meehanistic studies rai the stereoselective formation of p-mannosides from mannosyl iodides using o-deuterium kinetic isotope effects. J Org Chem 72 4663-4672... 

The branched cyclodextrins (CDs, 17 a, 17 b) and their analogues with D-galactosyl and a-D-mannosyl residues (17c, 17d) have also been prepared under mild conditions by the approach depicted in Scheme 6 [24,25]. Selective in situ S-deacetylation and activation was obtained by treatment of peracetylated 1-thioglycoses (10a, 8e, 8g) by cysteamine in the presence of diAioerythritol in HMPA [26]. This method was very efficient for ffie synthesis of branched CDs (17a) (80%), (17b) (60%), and (17c) (85%) when the acceptor molecule (15b) bearing primary iodide was used. However, peracetylated 1-thioa-D-mannose (8f) failed as a donor under these conditions, but tetra-O-acetyl-l-thio-a-mannose (8 b) afforded the expected CD (17d) in high yield (83%). 

The perbenzylated a-glycosyl iodides are thought to react with oxetane via the mechanisms shown in Schemes 21 and 22 (where R=PhCH2).138 The amount of the -product increases as the reaction temperature decreases. In fact, at — 60 °C, the fi a ratio in the product from the perbenzylated derivatives of galactosyl, mannosyl, and glucosyl iodides ranges from 10 1 to 50 1, indicating that the reaction in Scheme 21 is faster than that in Scheme 22. 

Stannylene-mediated glycosylation was first applied to alkyl mannosylations. Condensation of the stannylene acetal of 3,4,6-tri-O-benzyImannopyranose (44) with methyl iodide and allyl bromide in DMF afforded the desired methyl and allyl P-mannopyranosides, respectively, in almost quantitative yields [20]. Benzyl p-mannopyranoside was obtained from the same stannylene acetal by use of BU4NI in benzene [21]. The electrophilic leaving group is responsible for the reactivity and stereoselectivity. Methylation with methyl tosylate and dimethyl sulfate gave methyl mannopyranosides as anomeric mixtures at temperatures above 75 °C [20]. 

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