硼氢化钠还原(10)

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Thescopeofthistransformationhasbeenillustratedusingaliphaticacyclicandcyclicketones,aliphaticandaromaticaldehydes,primaryandsecondaryaminesin-cludingavarietyofweaklybasicandnonbasicamines.Further,highlydiastereoselectivereductiveaminationofsubstitutedcyclohexanonesgiveaxialaminesusingsodiumtriacyloxyborohydridesderivedfromvariouscarboxylicacids(Eq.(80))[91].

(80)

Veryrecently,anovelone-potreductivealkylationofamines,byS-ethylthioestersmediatedbytriethylsilaneandsodiumtriacetoxyborohydrideinthepresenceofPdoncarbon,wasreported(Eq.(81))[92].

(81)

Also,thereactionofformaldehydewithprimaryandsecondaryaromaticaminesusingNaBH4–H2SO4inTHFleadtoreductiveamination(Eq.(82))[93].

(82)

Also,thereactionofthechiralO-protected-a-hy-droxyketonewithprimaryaminesbyNaBH4inthepresenceofmagnesiumperchlorate,Mg(ClO4)2ledtotheexclusiveformationoferythro(1R,2S)-O-protected-N-substitutedethanolamines(Eq.(83))[94].

(83)

Evidently,theintermediacyofmagnesiumbidendatecomplexleadstoamorerigidconformationresultinginahigherpercentageoftheerythrodiastereomer[95].Asimple,mildandef cientprocedureforreductivealkylationsofdimethylamine[96],methylaminehydro-chloride,triethylamine[97]withcarbonylcompoundsusingtitanium(IV)isopropoxideandNaBH4hasbeenreported(Eq.(84)).

(84)

Thetitanium(IV)isopropoxide[98]isamildreagentcompatiblewithavarietyofpotentiallyacidsensitivefunctionalgroupssuchasacetals,lactams,acetonideandt-butyldimethylsilylether.

TheTi(OPri)4–NaBH4hasbeenalsousedinthesynthesisofunsymmetricallydisubstitutedureasbythereductiveamidationofaldehydesandmonosubstitutedureas(Eq.(85))[99].

(85)

10.5.Reductionofepoxyketones

Thea,b-epoxyalcoholsserveasversatilesyntheticintermediatesthatcanbeef cientlyelaboratedintopolyhydroxyderivativeswithmultiplechiralcenters[100].Ahighlystereoselectivereductionofa,b-epoxyke-tonesusingNaBH4–ZnCl2reagentsystemhasbeenreported[100].Also,averysimplealternativeproce-dureforthestereoselectivereductionofa,b-epoxyke-tonesintoerythro-a,b-epoxyalcoholswithNaBH4inthepresenceofCaCl2–LaCl3inmethanolwasreported(Eq.(86))[101].

(86)

Inthesecases,useofmetalsaltfacilitatestheforma-tionoftheerythroproduct.Also,themetalchloridesthathavelargerradiiprovidedhighererythro-selectiv-ity.Forexample,thelanthanumchloridegavethehighesterythro-selectivity[102].Further,thereductionsproceededveryslowlyinetherorbenzenebecauseofinsolubilityofNaBH4andmetalchloridesinthesesolvents.

Reductionofa,b-epoxyketonesundertheLucheconditionsusingNaBH4–CeCl3inmethanolprovidesanti-(orerythro-)a,b-epoxyalcoholsinhighyieldsandwithextremelyhighstereoselectivity(Eq.(87))[102].

(87)