硼氢化钠还原(4)

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Further,selectivereductionofthecarboxylicacidgroupinanole nicacidhasalsobeenachievedbyformingthecorrespondingacyloxyborohydridebeforetheadditionofI2(Eq.(27))[32].

(27)

Cyanuricchloride–NaBH4reagentsystemhasalsobeenusedtoeffectthereductionofcarboxylicacidstoalcoholsundermildconditions(Eq.(28))[33].

(28)

Facile,chemoselectivereductionofcarboxylicacidstoalcoholsusingaphosphoniumhexa uorophosphate(BOPreagent)–NaBH4reagentsystemhasbeenreported(Eqs.(29)and(30))[34].Also,thismethodisconvenient,rapidandchemoselectiveinseveralcases.Forexample,functionalgroupssuchasnitro,nitrile,azidoandesterareunaffectedundertheseconditions.

(29)

(30)

4.ReductionofaminoacidsandtheirderivativesChiralaminoalcoholsareimportantclassofcom-pounds.Theyareusefulinasymmetrictransformations,synthesisofpharmaceuticals[35],resolutionofracemicmixtures[36]andinsynthesisofinsecticides[37].Obvi-ously,severalreagentsareavailable(e.g.LiAlH4[38],DIBAL[39],H3B–THF[40])forthereductionoffreeaswellasprotectedaminoacidstothecorrespondingaminoalcohols.However,thesereagentssufferfromdisadvan-tagesofcost,in ammabilityandtediousisolationproce-dures.MeyersandcoworkersexaminedthereductionofaminoacidsusingtheNaBH4–I2reagentsystem.Theresultsindicatethatitisanexcellentreagentsystemfortheconversionofaminoacidstoaminoalcohols(Eq.(31))[41].

(31)

TheN-acylaminoacidsgivethecorrespondingN-alkylaminoalcoholsundertheseconditions(Eq.(32))[41].

(32)

However,theN-carbamateprotectinggroupisunaf-fectedundertheseconditions[41].Also,thereductionsofpentachlorophenylestersoftheBocprotectedaminoacidsandpeptidestothecorrespondingalcoholshavebeenreported(Eq.(33))[42].

(33)

TheNaBH4–I2reagentsystemissafe,simpleandinexpensive.Hence,itisuseful,especiallyinthelargescalesynthesisofchiralaminoalcohols.

AminoacidsarealsoreducedusingtheinexpensiveNaBH4–H2SO4reagentsysteminTHF(Eq.(34))[43].ItisofinteresttonotethatnoracemizationoccursinthereductionofaminoacidsusingNaBH4–I2orNaBH4–H2SO4.

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5.Reductionofcarboxylicacidesters

TheNaBH4–ZnCl2reagentsystemexhibitspowerfulreducingpropertiesinthepresenceofatertiaryamine.Thecarboxylicestersweresmoothlyreducedbythisreagenttotheircorrespondingalcohols(Eq.(35))[44].Further,thereductiondoesnottakeplacewithouttheamineundertheseconditions.

(35)