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TMSBr-mediated solvent- and work-up-free synthesis of α -2-deoxyglycosides from glycals

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ABSTRACT

The thio-additions of glycals were efficiently promoted by a stoichiometric amount of trimethylsilyl bromide (TMSBr) to produce S-2-deoxyglycosides under solvent-free conditions in good to excellent yields. In addition, with triphenylphosphine oxide as an additive, the TMSBr-mediated direct glycosylations of glycals with a large range of alcohols were highly α-selective.

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Proposed mechanisms for TMSBr-mediated synthesis of 2-deoxyglycosides in the presence of TPPO.
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C2: Proposed mechanisms for TMSBr-mediated synthesis of 2-deoxyglycosides in the presence of TPPO.

Mentions: Two possible mechanisms are proposed for the α-selectivity observed here (Scheme 2). It is well-accepted that the acid-catalysed nucleophilic addition of an alcohol to a glycal is likely to proceed through the formation of an oxocarbenium ion via the protonation at C2 [6,63]. In the presence of TPPO, the oxocarbenium cation is stabilized by the ion–dipole interaction with TPPO oriented preferably at the pseudoequatorial position [78] and the ensuing SN2-like displacement by the alcohol contributes to the improvement of the α-selectivity (Scheme 2, route A). Alternatively, it is possible that a 2-deoxyglycosyl bromide is first generated mainly in the more stable α-form [61]. The glycosyl bromide intermediate then undergoes double SN2-like substitution by TPPO and the alcohol to give the α-glycoside as the major product [77] (Scheme 2, route B).


TMSBr-mediated solvent- and work-up-free synthesis of α -2-deoxyglycosides from glycals
Proposed mechanisms for TMSBr-mediated synthesis of 2-deoxyglycosides in the presence of TPPO.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4979735&req=5

C2: Proposed mechanisms for TMSBr-mediated synthesis of 2-deoxyglycosides in the presence of TPPO.
Mentions: Two possible mechanisms are proposed for the α-selectivity observed here (Scheme 2). It is well-accepted that the acid-catalysed nucleophilic addition of an alcohol to a glycal is likely to proceed through the formation of an oxocarbenium ion via the protonation at C2 [6,63]. In the presence of TPPO, the oxocarbenium cation is stabilized by the ion–dipole interaction with TPPO oriented preferably at the pseudoequatorial position [78] and the ensuing SN2-like displacement by the alcohol contributes to the improvement of the α-selectivity (Scheme 2, route A). Alternatively, it is possible that a 2-deoxyglycosyl bromide is first generated mainly in the more stable α-form [61]. The glycosyl bromide intermediate then undergoes double SN2-like substitution by TPPO and the alcohol to give the α-glycoside as the major product [77] (Scheme 2, route B).

View Article: PubMed Central - HTML - PubMed

ABSTRACT

The thio-additions of glycals were efficiently promoted by a stoichiometric amount of trimethylsilyl bromide (TMSBr) to produce S-2-deoxyglycosides under solvent-free conditions in good to excellent yields. In addition, with triphenylphosphine oxide as an additive, the TMSBr-mediated direct glycosylations of glycals with a large range of alcohols were highly α-selective.

No MeSH data available.