AcetylationMostcommonlyusedestersStirring the alcohol with acetic anhydride and a baseBasemopuptheaceticacidgenerated,andcatalysestherxnitselfNucleophiliccatalysisandbaseOHHOHO-OHOhUncatalysedrxnisveryslowatrtOMeOMeLewisacid catalysisButwithpossiblecompetingprocessofmutarotation
Acetyla,on$$ SKrring'the'alcohol'with'aceKc'anhydride'and'a'base' Most'commonly'used'esters' Nucleophilic'catalysis'and'base' Lewis'acid'catalysis' Base'mop'up'the'aceKc'acid'generated,'and' catalyses'the'rxn'itself' Uncatalysed rxn'is'very'slow'at'rt But'with'possible'compeKng'process'of'mutarotaKon
1. Acetylation with acetic anhydride/pyridineEqualproportionsassolvents.OH.OAcAc2O, pyridineOHOOHAcOOAcAcOfastOHOAcSugarsina bottleisaslowImixture of α and β/(py/fu)OHOAcAc2O,pyridineO0HCAcoHOAcOfastOHOAcOHOACRatio depends on particular sugars
1.$Acetyla,on$with$ace,c$anhydride/ pyridine$ O HO OH OH OH HO Ac2O, pyridine fast O AcO OAc OAc OAc AcO slow O HO OH OH OH HO Ac2O, pyridine fast O AcO OAc OAc OAc AcO + Sugars$in$a$boYle$is$a$$ mixture$of$α$and$β.$ /(py/fu)$$ Ra,o$depends$on$par,cular$sugars$ Equal$propor,ons$as$solvents$
2. Acetylation with acetic anhydride/sodium acetateRelatively weak base, sluggish rxn at rtCarry out at 10o°C, usually generate β-acetatesMutarotation is much faster thanthe acetylationOHOHOAcfastC0Ac2O, NaOAcHHOOOHAcoOACHOAco100°℃OHOHOAcOHslowβ Hydroxyl is more nucleophilic than the axial α one
2.$$Acetyla,on$with$ace,c$anhydride/ sodium$acetate$ O HO OH OH OH HO Ac2O, NaOAc fast O AcO OAc OAc OAc AcO slow O HO OH OH OH HO 100oC Rela,vely$weak$base,$sluggish$rxn$at$rt Carry$out$at$100°C,$usually$generate$β9acetates$ Mutarota,on$is$much$faster$than$the$acetyla,on$ β$Hydroxyl$is$more$nucleophilic$than$the$axial$α οne$
β-occupies a less hindered equatorial thanthe axial of α-repulsiveeffectbetweentheringoxygenlonepairorbitalsandthe lonepairorbitalsontheβanomericoxygenatomOH.0OO.0FKineticanomericeffectOHuliericaoicic ciccr
βF'occupies'a'less'hindered'equatorial'than' 'the'axial'of'α-$ repulsive'effect'between'the'ring'oxygen'lone' pair'orbitals'and'the'lone'pair'orbitals'on'the'βF anomeric'oxygen'atom$ KineKc'anomeric'effect
3. Acetylation with acetic anhydride/Lewis acid catalysis.OH.OAcOAcAc20ZnCl2OCAcOOAcOACCHCAcoAcoZnCl2OHOAcOAcOAcStrong Lewis acid catalyses the eguilibrium of α- andβ-, after catalyses the rxnThus equilibrium of α-and β-acetates will occur afterthe rxnAnomeric effect leads us α-acetates
3.$Acetyla,on$with$ace,c$anhydride/ Lewis$acid$catalysis$ Strong$Lewis$acid$catalyses$the$equilibrium$of$α9$and$ β9,$a^er$catalyses$the$rxn Thus$equilibrium$of$α9$and$β9acetates$will$occur$a^er$ the$rxn$$ Anomeric$effect$leads$us$α9acetates$