| Popis: |
Glycosylationspromotedbytriflate-generatingreagentsarewidespreadsyntheticmethodsfortheconstruc-tion ofglycosidicscaffoldsandglycoconjugatesofbiological andchemical interest. Theseprocessesarethoughttoproceedwiththeparticipationofaplethoraofactivatedhighenergyintermediatessuchasthea-andb-glycosyltriflates,orevenincreasinglyunstableglycosyloxocarbenium-likespecies,amongwhichonlya-glycosyltriflateshavebeenwellcharacter-izedunderrepresentativereactionconditions.Interestingly,theremaininglessaccessibleintermediates,yettobeexperi-mentallydescribed,seemtobeparticularlyrelevantina-selectiveprocesses,involvingweakacceptors.Herein,we reportadetailedanalysisofseveral paradigmaticandillustrativeexamplesofsuchreactions,employingacombinationofchemical,NMR,kineticandtheoreticalapproaches,culminatingintheunprecedenteddetectionandquantificationofthetrueb-glycosyltriflateintermediateswithinactivateddonormixtures.Thisachievementwasfurtheremployedasastepping-stoneforthecharacterizationofthetriflateanomerizationdynamics,whichalongwiththeacceptorsubstitutions,governthestereochemicaloutcomeofthereaction.Theobtaineddataconclusivelyshowthat,evenforhighlydissociativereactionsinvolvingb-Closeionpair(b-CIP)species,theformationofthea-glycosideisnecessarilyprecededbyabimoleculara→btriflate interconversion,which undercertaincircumstancesdoesbecometherate-limitingstep.Overall,ourresults ruleouttheprevalenceoftheCurtin-Hammettfast-exchangeassumptionformostglycosylationsandhighlightthedistinctreactivitypropertiesofa-andb-glycosyltriflatesagainstneutralandanionicacceptors |
