Asymmetric electrocyclic reactions

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Asymmetricelectrocyclicreactions

SamThompson,AnthonyG.Coyne,PeterC.KnipeandMartinD.Smith*

Received21stJanuary2011DOI:10.1039/c1cs15022g

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GThiscriticalreviewoffersanoverviewofasymmetricelectrocyclicprocesses,wherediastereo-orenantioselectivityisaconsequenceoftheinfluenceofachiralcomponent(beitsubstrateorcatalyst)ontheelectrocyclicbond-formingprocess(195references).SincetheirrationalizationbyWoodwardandHoffmann,1electrocyclicreactionshavedevelopedfrommechanisticcurio-sitiesintoviableandpowerfulbond-formingprocesseswithapplicationinthegenerationofcomplexmolecules.2However,incomparisonwithionic,radicalandalternativepericyclicreactionmanifolds(suchascycloadditions)therearefewmethodsforstereocontrolinelectrocyclicprocesses,andnogeneralapproachesforthecontrolofabsolutestereochemistry.Onereasonforthismaybethatthechallengesofdesigningandengineeringsuitablesubstratesamenabletocatalysisatreason-abletemperaturescanbecomeoverriding;thismayprecludetheapplicationofthesereactionsintarget-orientedsynthesisprogrammes.Inrecentyearstherehavebeensignificantadvancesincatalyticasymmetricmethodsthathaveofferednewopportunitiesforunderexploitedreactionmanifolds.This,coupledwithanincreasedapplicationofdensityfunctionaltheorytorationalizereactivitypatterns,maybepartlyrespon-siblefornewapproachestothecatalysisofelectrocyclic

ChemistryResearchLaboratory,UniversityofOxford,12MansfieldRoad,OxfordOX13TA,UK.E-mail:martin.smith@chem.ox.ac.uk

processes.Inthisarticlewehaveattemptedtodocumentstereocontrolinelectrocyclicreactionsinwhichthediastereo-orenantioselectivityisinfluencedbyachiralcomponent(beitsubstrateorcatalyst).3Thecomplexityandmechanisticambiguityinascribingreactionsaspericyclic(ornot)canmakedeconvolutingtheintimatedetailsthatareresponsiblefortheobservedstereochemicaloutcomechallenging.Wehave,however,attemptedtorationalizethecontrollingelementsintheelectrocyclizationstepwherepossible.Itisnotintendedthatthisreviewbecomprehensive,butratherthatmethodsforcontrollingandinfluencingasymmetryinelectrocyclicprocessesbeoutlinedinthecontextoftheirutilityinsyntheticchemistry.Wehavearrangedthisaccountbyreactionmanifoldinthehopethatthismayenableeasynavigationandreference.

Photochemicalreactions

4pElectrocyclizations(p4s)

Tropoloneethersundergoa4pdisrotatoryphotocyclizationinthepresenceofachiralhosttoaffordnon-racemicbicyclicproducts.Todaetal.haveshownthattheenantioselective

SamThompsonreceivedhisMChemfromExeterCollege,Oxford(2004)spendinghisfinalyearwithBenDavisintheDysonPerrinsLaboratory.HemovedtoStEdmund’sCollege,CambridgeforaPhD(2008)withMartinSmithworkingoncascaderoutestopolycyclicalkaloids.AfterreturningtoOxfordforabriefpostdoctoralstaywithSteveDaviesandAngelaRussell,hetookuphiscurrentpositionasaJuniorResearchSamThompson

FellowatPembrokeCollege,

OxfordandpostdoctoralfellowwithAndrewHamilton,FRS.Hisinterestslieintheuseofnewsyntheticmethodstounder-standbiologicalsystemsandapplicationofthistobiomedicinalproblems.

ThisjournaliscAnthonyCoynereceivedhisBSc(1998)andPhD(2002)fromtheNationalUniversityofIreland,Galway,workingwithRichardButler.HeacceptedanindustrialpositionwithGlaxoSmithKlinebeforereturningtoacademiaasapostdoctoralfellow(2004)withPatGuiryatUniversityCollegeDublin.Afterapost-doctoralstaywithMartinSmithattheUniversityofCambridge(2006–2008)hetookuphiscurrentpositionAnthonyG.Coyne

withChrisAbellatthe

UniversityofCambridge.Hisinterestsarefocusedonfragmentbasedapproachestotargetingprotein–proteininteractions.

TheRoyalSocietyofChemistry2011Chem.Soc.Rev.,2011,40,4217–42314217View Article Online

Scheme1Enantioselectivephotoreactionofatropoloneetherinacrystallineinclusioncomplex.

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022Gphotoelectrocyclizationoftropolonederivativescanbeachievedinthesolidstatebyforminga1:1complexwithanopticallypurealcohol(Scheme1).4–7ThestereochemistryoftheproductisexplainedonthebasisoftheX-raycrystalstructureoftheinclusioncomplex.The4pdisrotatoryphotoelectrocyclizationoccursinthe‘outward’direction(toaffordthe(1S,5R)enantiomer)sincethealter-nativewouldgiverisetoanunfavourablestericinteractionbetweentheo-chlorophenylandethylgroups.Othergroupshaveexploredthephotocyclizationoftropolonederivativesusingdifferentchiralmediaforthecontrolofasymmetry.Ramamurthyetal.examinedtheinfluenceofchirally-modifiedzeolitesforthephotocyclizationoftropolone,9,10andobservedenantiomericexcessesofupto82%.b-Cyclodextrinswerelesseffective,inducingenantiomericexcessesupto33%.11Stereoinductiondirectedbyaremotechiralcentreisaconsiderablesyntheticchallengeinasymmetricphotochemistry;Ramamurthyetal.haveusedconfinementasameanstoamplifysubstrate-controlleddiastereoselectivity.12,13Host–guestcomplexesbetweensubstitutedpyridonesandanachiral‘‘cavitandoxa-acid’’wereirradiatedtoinducephotocyclization,yieldingdiastereomericratiosupto24:1,whilstintheabsenceoftheoxa-acid,thehighestdrwasjust1.0:1.1.

Kaneko,14andScheffer8havebothexploredthephoto-cyclizationof4-benzyloxy-2-pyridonesinthesolidstate,obtainingcyclizedmaterialsinhigheesbutinmoderateyields.

Scheme2Electrocyclizationof2-pyridoneswithachiralhost.

Bachetal.haveexaminedasimilarsolution-phasereactioninthepresenceofachiralhost(Scheme2).15The4pelectrocyclicringclosureofpyridonesinsolutionoccurredwith20–23%eeatÀ201C;theobservedmajorstereoisomerisconsistentwiththemodelproposedbyBachetal.16forthe6pelectrocyclizationofacrylanilides,videinfra.6pAzaelectrocyclizations(p6a)

Todaetal.17demonstratedthatthestereochemicaloutcomeofthephotocyclizationofacrylanilidesto3,4-dihydroquinolin-2(1H)-onescanbecontrolledbyinclusionwithanoptically-activeTADDOL-related18hostcompoundderivedfromtartaricacid(Scheme3).

ThedirectionofconrotatoryringclosurewithintheinclusioncomplexdeterminestheabsoluteconfigurationatC-1,andasubsequentsuprafacial1,5-hydrogenshiftdeter-minesthestereochemistrya-tothelactamcarbonyl.Thisprocessiseffectiveforarangeofsubstratesinuniformlyhighenantiomericexcess.Furtherworkfromthesamegroup19–22has

PeterKnipegraduatedwithanMSciinNaturalSciencesfromDowningCollegeintheUniversityofCambridge(2008)beforemovingtotheUniversityCollegeintheUniversityofOxfordforaDPhilwithMartinSmith.Heiscurrentlyinthethirdyearofhisdoctorateworkingonasymmetricelectrocyclizationandcascadeprocesses.

PeterC.Knipe

MartinSmithreceivedhisBA(1995)andDPhil(1999)fromtheUniversityofOxford,workingwithGeorgeW.J.Fleetonthechemistryofcarbohydrateaminoacids.HemovedtoPembrokeCollegeintheUniversityofCambridgeastheDrapersCompanyResearchFellow,toworkwithStevenV.Ley,CBEFRS.In2003hebeganhistenureasaRoyalSocietyUniversityResearchFellowintheDepartmentofChemistryinMartinD.Smith

Cambridgebeforemovingto

OxfordasaUniversityLecturerin2008.Heleadsasyntheticorganicchemistrygroupwithwide-ranginginterestsinsynthesis,structureandasymmetriccatalysis.

Thisjournalisc4218Chem.Soc.Rev.,2011,40,4217–4231TheRoyalSocietyofChemistry2011View Article Online

Thermalreactions

4pElectrocyclizations(p4a)

TheNazarovreaction.The4pelectrocyclizationofdivinylketonestogivecyclopentenones—theNazarovreaction—isprobablythemostextensivelyinvestigatedasymmetricelectrocyclictransformation.25–33Thethermalreactionpro-ceedsviatheconrotatorymode34,35andcanberenderedasymmetricinthepresenceofachiralauxiliaryorasymmetriccatalyst.Denmarketal.employedachiralsilanetotransferasymmetryinaNazarovreaction(Scheme5).36–38Treatmentofanenantiomericallyenricheddivinylketonebearingab0-stereogenicsiliconsubstituentwithFeCl3generatedapentadienylcationthatunderwentconrotatory4pringclosuretoaffordanoxyallylcation.Inthistrans-formation,theconformationoftheintermediatecationisdictatedbythepreferenceoftheC–Sis-bondtolieperpendi-culartotheplaneofthepentadienylsystem.Onlytheanti-productwasobserved,theauthorsreasoningthatsyn-conrotationwouldplacetheelectron-richSi–Cbondinanequatorialpositionorthogonaltothep*orbital,precludinghyperconjugativestabilization.Concomitantsilicongroupeliminationgavethetricyclicringsystemin58%yieldwithcompletetransmissionofchiralinformationfromthestartingmaterial.

Pridgenetal.employedtheEvanschiraloxazolidinoneauxiliarytoinfluencestereoselectivityintheNazarovcyclizationofalkylidene1,3-dicarbonylcompounds.39BothLewisandBrønstedacidscatalyzedthistransformationandsurprisinglygaveroughlyequivalentdiastereoselectivities,suggestingthatLewisacidchelationmaynotberesponsibleforthestereoinduction.Theirbestresultswereachievedusingmethanesulfonicacid,givingan85:15mixtureofC3epimers(Scheme6).

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme3Enantioselectivephotocyclizationofanacrylanilideinacrystallineinclusioncompound.

followedinwhichthestructuresoftheinclusioncomplexeshavebeenstudiedbyX-raycrystallography,therangeofsubstratesextendedwithyieldsashighas70%,withupto98%ee.

Solution-phaseenantioselective6p-photoelectrocyclizationofanacrylanilidemediatedbyachiralhostwasstudiedbyBachetal.16Theabsoluteconfigurationatcarbonatom1isestablishedinthephotocyclizationstep;amodelforthedifferentiationofenantiotopicfacesisshownbelow(Scheme4).Uponconrotatoryringclosure,thecyclohexeneringcaneitherturninthedirectionofthetetrahydro-naphthaleneoritcanmoveaway,andindoingsoopenitsSi-facetoattackfromthearylring.Thelattermovementappearstobethemoststericallyfavourable,givingtheproductbearingtheobservedstereochemistryasamajorproductboundtothehost.

PhotocyclizationintolueneatÀ551Cyieldeda2.7:1.0diastereomericmixtureofproductsinmoderateee.Relatedsolution-phasephotoelectrocyclizationswerereportedbyNinomiyaetal.23andScaianoetal.,24albeitwithlowenantioselectivities.

Scheme4Stereocontrolledconrotatoryphotocyclizationofanacrylanilideinacrystallineinclusioncomplex.Scheme5AsymmetricNazarovcyclizationwithasiliconchiralauxiliary.

ThisjournaliscTheRoyalSocietyofChemistry2011Chem.Soc.Rev.,2011,40,4217–42314219View Article Online

Scheme8Trauner’scatalyticasymmetricNazarovcyclization.

Scheme6Auxiliary-controlledNazarovcyclizations.

Flynnetal.havereportedasimilarreactionusingthechiralphenyloxazolidinonechiralauxiliaryandmethanesulfonicacidatÀ781C.40Thekineticsyn-productwasformedin73%yieldbutonsustainedexposuretoaLewisacidatroomtemperaturethethermodynamicallymorestableanti-productwasformedin78%yield.Sincebothenantiomersofthephenyloxazolidinoneauxiliaryarereadilyavailable,thisallowsaccesstoallfourstereoisomersofthecyclopentenoneproduct.Theobservedstereochemistryisjustifiedonthebasisofap-stackinginteractionbetweenthephenylandoxazol-idinonecarbamatefavouringonesenseofconrotation.41Tiusetal.havedemonstratedaxialtotetrahedralchiralitytransferthroughthecyclizationofachiralallenylketone.42Theabsolutestereochemistryofthecyclopentenoneisderivedfromaconrotatoryringclosureinvolvingcounterclockwiserotationofthealkene(Scheme7).

ThehighstereoselectivityofthistransformationisaconsequenceoftheconrotationoftheR3groupawayfromthealkene.Inadistinctapproach,allenepyranosederivatives43–46andcamphor-derivedauxiliaries47–49havebeenusedtocontrolselectivityinthecorrespondingcyclopentenylationreactions.Thescopeofthisreactionhasbeenexpandedtoawiderangeofsubstitutionpatternsandheteroatoms.Hoppeetal.haveusedsimilarmethodologyforthesynthesisof5-alkylidine-2-cyclopentenonesfromchiralallenylcarbamates.50–52Trauneretal.disclosedthefirstcatalyticasymmetricvariantoftheNazarovcyclization,employinga-oxygenateddienonesfortheirpropensityforbidentatechelation.53,54Treatmentofthesematerialswith20mol%Sc(OTf)3/PyBOXcatalystaffordedtricyclicproductsingoodee(Scheme8).

AggarwalandBelfieldhavedisclosedasimilarstoichio-metricandcatalyticasymmetricprocessusingCu–PyBOXcatalysts(Scheme9).55Treatmentofdienonessubstitutedwithesterorketonegroupswithcopper(II)bromideandbisoxazoline-derivedligandsaffordedcyclopentenoneproductsingoodtoexcellentenantiomericexcess.ThestereochemicaloutcomeofthesereactionsisconsistentwithamodelwherestericinteractionsbetweenR2andR3,andtheisopropylgroupsontheliganddistorttheplaneofthecationicdivinylketoneintermediatetofavouronemodeofconrotation.Itwasfoundthatstoichio-metricamountsofthecatalystwererequiredinordertoobtainhighyields.Frontieretal.havefoundthatiridium-basedLewisacidsoutperformothersuchcatalysts,extendingthescopeoftheNazarovreactiontocyano-andnitro-containingsubstrates,andallowingcatalystloadingtobedroppedto10mol%,althougheesofgreaterthan15%havenotyetbeenachieved.31Inrelatedwork,Maetal.havedevelopedacatalyticasymmetrictandemNazarovcyclization—electrophilicfluorination(Scheme10).56Inthisprocess,thedirectionofconrotationiscatalyst-controlled(presumablyviaamodelsimilartothatreported

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme7AxialtotetrahedralchiralitytransferinanallenylNazarovcyclization.

Scheme9Aggarwal’senantioselectiveNazarovcyclization.

4220Chem.Soc.Rev.,2011,40,4217–4231ThisjournaliscTheRoyalSocietyofChemistry2011View Article Online

Scheme10AsymmetricNazarov-fluorinationreaction.

byTraunerandAggarwal)intheelectrocyclicstep.Sub-sequenttrappingofthemetal-boundenolatewithanelectro-philicfluorinesourceaffords1-indanonesinupto95%yieldand95%ee.Thisreactionalsoproceedswithexcellentdiastereoselectivity,creatingtwonewstereocentreswithupto49:1anti:synstereochemistry.

Inadditiontofurtherworkwithscandium(III)57andcopper(II),33othermetalsincludingnickel(II),58iron(II)andcobalt(II)32havebeendemonstratedtoinduceasymmetrywhenusedincombinationwithnon-racemicC2-symmetricligands.Ruepingetal.havereportedanorganocatalyticasymmetricNazarovcyclizationusingaBINOLphosphateBrønstedacidcatalyst,59andsubstratessimilartothoseemployedbyTrauner(Scheme11).

Thehighestenantioselectivitywasobtainedinchloroformat01Cwith2mol%oftheBINOLphosphatecatalyst.Thisaffordeduptoa9.3:1.0mixtureofsyn:antidiastereomersandeesupto93%.Thecatalyticcycleforthisreactionwasenvisagedtoinvolveprotonationofthedivinylketonetoaffordanintermediatechiralionpairwhichcouldundergo4pelectrocyclicringclosure.SubsequentprotonationofthecyclicenolateresultsintheformationofcyclopentenoneandregenerationoftheBrønstedacidcatalyst.

Tiusetal.haverecentlyreportedbothstoichiometricandcatalyticexamplesofa-ketoenonecyclizations.60,61Chiralnon-racemic1,2-diaminesgiveNazarovproductsingoodyieldandexcellenteewhenusedstoichiometrically.Thedirectionofconrotationisthoughttobecontrolledbyanequilibriumbetweenenamine–iminiumions(Scheme12).Acid-catalyzedliberationofthediaminefromitsboundformisrequired,suggestingthatproductinhibitiontakesplace,preventingcatalyticturnoverandleadingtolongreactiontimes.

Non-covalentbifunctionalthioureaorganocatalystsallowedacatalyticcycletobeestablishedwithdiketoestersubstrates(Scheme13).61Themechanismappearstobecooperative,

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme12Enamine–iminiumionscontrolledNazarovcyclization.

Scheme13Thiourea-catalysedNazarovcyclization.

involvingasubtlebalancebetweenBrønstedacidactivationoftheketoneandbasicactivationoftheenolicester.Thesenseofconrotationmaybeexplainedbyselectionofthelowestenergytransitionstateofthetwopossiblehelicaldiastereo-mericcomplexes;theauthorssuggestthatthecatalystinvokesatorsionoftheC3–C4bond,stabilizingtheintermediatethatleadstotheobservedproduct.

Tiusetal.’sstudiestowardsasymmetricNazarovreactionshavealsoledtoanexampleofanunusualaza-Nazarovprocess,proceedingwithveryhighenantiomericexcessinthepresenceofadiaminetriflatecatalyst.62TheStaudingerreaction.Sincethediscoveryofpenicillinandelucidationofitsstructure,thechemistryofb-lactamshasbeenextensivelystudied;63–76theStaudingerreactionoffersaneffectiveanddirectapproachtotheirsynthesis.77Thetrans-formationgenerallyinvolvesthenucleophilicattackofanimineonaketenetogiveazwitterionicintermediate,whichundergoesa4pelectrocyclicringclosuretoaffordtheb-lactam.Mechanisticandtheoreticalstudieshaveshownthattheproductstereochemistryisdictatednotbystericeffectsbutbyrotationalpreferencesintheelectrocyclicringclosure.Itisacceptedthattheinitialnucleophilicattackgenerallyoccursexo-totheketenesubstituent,hencetheobservanceofbothcis-andtrans-lactamsisattributedtoimineisomerization,78,79bothbeforeandafterformationofthezwitterionicinter-mediate,with(E)-iminesleadingtocis-lactamsandvice-versa(Scheme14).

TheelectronicpropertiesofthesubstituentsattheR1andR2positionshavebeenshowntohaveasignificanteffecton

Chem.Soc.Rev.,2011,40,4217–42314221Scheme11Brønstedacid-catalysedasymmetricNazarovcyclization.

ThisjournaliscTheRoyalSocietyofChemistry2011View Article Online

Scheme14ProposedmechanismoftheStaudingerreaction.

thestereochemicaloutcomeofthereactionbyinfluencingtherelativeratesofisomerizationversusdirectcyclization.78,80Forexample,electron-donatinggroupspositionedatR1tendtofavourcis-productssincetheyacceleratethecyclizationoftheinitiallyformedzwitterionrelativetoimineisomerization;theoppositeisobservedwhenelectron-poorsubstituentsareused.Theoriginsofstereochemicalcontrolinthereactionhavebeenextensivelyexaminedusingtheoreticalandmecha-nisticprobes.78–87IthasalsobeenproposedthattheStaudingerreactionisnotpericyclic,andshouldberegardedasanintramolecularMannich-typeprocess.78RecentlyCossıoetal.87haveshownthatthecyclizationofthezwitterionicintermediatemaybeconsideredasaninteractionbetweentheunperturbedenolatep(HOMO)andiminiump*(LUMO)orbitals;thisnucleophilicadditionisfacilitatedbyanin-phasecouplingoftheorbitalswhichnecessarilyleadstoconrotatorycyclization.Althoughtheproblemofwhetherthereactionispericyclicornotremainsunresolved,thestereospecificconrotatorynatureofthereactionjustifiesitstreatmentinananalogousmanner.

Ingeneral,theabsolutestereochemistryoftheb-lactamsformedmaybecontrolledinthreeways:(i)theinclusionofchiralgroupsorauxiliariesontheketene;(ii)chiralgroupsorauxiliariesontheimine;or(iii)asymmetriccatalysis.Notable,exemplaryandrecentreactionsintheseareasarepresented.(i)Chiralgroupsontheketene.EvansandSjogrenused(S)-phenyloxazolidylacetylchlorideasachiralketenesynthoninanauxiliary-controlledasymmetricStaudingerreaction(Scheme15).88,Thesetransformationsproceedinexcellentyield(upto90%)andaffordexclusivelycis-productswithupto97:3dr.TheuseofEvanschiraloxazolidinone-substitutedketenesasameanstocontrolasymmetryintheStaudingerreactioniswell-established;thishasbeenexploitedbyanumberofgroups90–97andcanbehighlydiastereoselective(>99:1dr).97Panunzioetal.havereportedatrans-selectiveStaudingerreactioninvolvingthesameketeneprecursorwithtrimethyl-silylimines;98thisisconsistentwiththemodelproposedinScheme14,withthe(Z)-imineintermediateleadingexclu-sivelytothetrans-b-lactamasamixtureofdiastereomers(Scheme16).Whilstthereactiondidproceeduncatalyzedat

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme16Lewisacid-catalysedStaudingerreaction.

elevatedtemperatures(1001C),significantrateaccelerationwasobservedinthepresenceofaLewisacidcatalyst.99Thediastereoselectivityisproposedtobetheresultofacomplexseriesofinter-andintramolecularLewisacid–baseinteractions.Intheuncatalysedprocess,internalcoordinationoftheN-iminolonepairtothesiliconcausesincreasedmolecularrigidity.SubsequentconrotatorycyclizationisdirectedbythebulkoftheEvansauxiliarytogivethecorrespondingtrans-lactamasthemajorproduct.UseofanexternalLewisacidcausedareversalofthisselectivity,whichtheauthorssuggestisduetocoordinationoftheN-iminolonepairtotheboron.Thiswouldreducetheintramolecularinteraction,increasingrotationalfreedomalongtheC–Nbondandallowingformationofthealternativetrans-lactam.How-ever,theauthorsconcedethattheirproposedmechanismisnotgeneralsinceitfailstoexplainsomeanomalousresults,andconcludethatthecomplexinterplayofweakinteractionsmakesthediastereoselectivitydifficulttopredict.Althoughtheoutcomeofthisreactionhasnotbeenadequatelyexplained,itssignificanceliesintheconsiderablerateaccelerationobservedunderLewisacidcatalysis,suggestingthatasymmetricLewisacidcatalysismaybeviable.

Othercommonchiralpool-derivedauxiliarieshavealsobeenusedtoinduceasymmetry;recentexampleshaveemployedsugars100,101tartaricacid,102,103and(À)-ephedrine.104Whilstbroadlyapplicable,thesemethodsarelimitedtonon-enolizableiminesduetosidereactionsinvolvingiminium–enaminetautomerization.ThiscanbeelegantlycircumventedbyemployingN-silylimines,astheseallowisolationofasilylketenehemiaminalintermediate.99,105,106(ii)Chiralityontheimine.Enantiomerically-enrichediminesderivedfrombothchiralaldehydesandamineshavebeenshowntobeeffectiveincontrollingthediastereoselectivityoftheStaudingerreaction.Anumberofchiralamineshavebeenutilizedtothisend,includingthosederivedfrom

107108D-threonine,L-threonine,phenylethylamine,109,110111L-alanine,and(2S,5S)-2,5-hexanediol.112,113ThelastexampleisnoteworthysinceitemploysachiralhydrazoneyieldingN-aminob-lactamswithveryhighdiastereoselectivityandaninterestingtemperature-dependence(Scheme17).

ThestereoselectivityobservedatroomtemperaturemaybeexplainedbythelargeCH2OBngroupontheiminemovingtoavoidthestericclashwiththemethylgroupsontheauxiliary.Theauthorsrationalizethetrans-selectivityatelevated

ThisjournaliscScheme15Evansauxiliary-controlledStaudingerreaction.

4222Chem.Soc.Rev.,2011,40,4217–4231TheRoyalSocietyofChemistry2011View Article Online

Scheme17D-Threonine-derivediminesintheStaudingerreaction.

Scheme19TheStaudingerreactionofchromiumtricarbonyl-substitutedimines.

temperaturesonthebasisofanaddition–eliminationmecha-nismcausingimineisomerization.

TheStaudingerreactionofiminesderivedfromchiralaldehydeshasalsobeenextensivelyexamined.Particularlyhighdiastereoselectivitiesareobtainedwhenthealdehyde(andconsequentlytheimine)possessesaheteroatominthea-position.IthasbeendemonstratedthatthisisduetoastabilizinginteractionbetweentheenolateHOMOandC–Xs*orbitalwhichismorepronouncedinoneofthediastereo-merichelicaltransitionstates.85,114Themostcommonlyemployedchiralaldehydesarederivedfromsugars,115–119aminoacids85andsyntheticchiralepoxyaldehydes.120Thelevelsofstereocontrolcanbenear-perfect,asdemonstratedbyDeshmukhetal.,whoemployedanisosorbide-derivedaldehyde(Scheme18).101DelButteroetal.havedescribedadiastereoselectivesynthesisofb-lactamsusingtheStaudingerreactionofaniminederivedfromthechromiumtricarbonylcomplexofanarylaldehyde,withketenes.121Thereactiongavethecorrespondingcis-b-lactamin98%yieldasasinglediastereo-isomer,andsubsequentphotolysisofthechromiumcomplexyieldedtheopticallypurelactamin95%yield(Scheme19).

Theabsolutestereochemistryoftheproductisconsistentwithatransitionstatewherethemethoxygroupisorientedawayfromthebulkofthemolecule.Subsequentcyclizationthenoccurswiththechromium-complexedarenerotatingoutwards,leadingtotheobserveddiastereomer.

(iii)Catalyticasymmetricmethods.Therehasbeensignifi-cantinterestincatalyticasymmetricmethodstoformcis-b-lactams.Lectkaetal.andFuetal.havereportedelegantandefficientvariantsusingnucleophiliccatalysis,122,123whilstYeetal.andSmithetal.haveindependentlydevelopedNHC-catalyzedprocesses.124,125ThemechanismsforthesereactionsdifferfromthegeneralprocessoutlinedinScheme14andhencedonotinvolveelectrocyclicprocesses.

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022G6pElectrocyclizations(p6s).Notethat6pelectrocyclicreactionsasacomponentof8p–6pcascadesarediscussedseparately,videinfra.

[1,5]Aza-electrocyclicmanifold.Inoneofthefewasym-metric[1,5]electrocyclicreactions,VeenstraandSpeckamp126foundthatthebase-catalyzed1,5-electrocyclizationofaldiminesfromo-aminophenylsuccinimideinthepresenceofachiralb-aminoalcohol(suchas(–)-mentholand(–)-borneol)yieldedchiralindolinesin17to31%ee(Scheme20).

TreatmentofthealdiminewithbutyllithiuminthepresenceofN-methylephedrinegavethedihydroindolein50%yieldand>95%ee.ThiswasrationalizedonthebasisoftheZ-imineundergoingadisrotatoryelectrocyclizationviaatransitionstateorganizedthroughhydrogenbondingandchelationinvolvingtheN-methylephedrine.Thissystemdisplayssignificantsensitivitytosmallstructuralvariationsinsubstrateandcatalyst.127Smithetal.128havedevelopedthismanifoldintoanorgano-catalyticasymmetricreactiontolerantofabroadrangeofsubstituentsonboththearylringandiminegroup.Underphase-transferconditionsemployingacinchona-derivedcatalyst,functionalizedindolineswereformedinexcellentyieldandenantiomericexcess(Scheme21).Thesenseofdisrotationmaybeexplainedbytight-ionpairingofthe

Scheme18DiastereoselectiveStaudingerreactionemployinganiso-sorbide-derivedauxiliary.

Scheme20Asymmetric[1,5]-electrocyclization.

ThisjournaliscTheRoyalSocietyofChemistry2011Chem.Soc.Rev.,2011,40,4217–42314223View Article Online

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme21Catalyticasymmetric6pelectrocyclization(oneestergroupomittedforclarity).

enolateandquaternaryammoniumcation,inwhichonefaceoftheenolateisselectedandthusthedirectionoforbitalrotationinthering-closurecontrolled.MullerandListhaveshownthatchiralBrønstedacidscatalyzethecyclizationofa,b-unsaturatedhydrazonestopyrazolinesinexcellentyieldandhighenantiomericexcess.129Acid-catalyzedisomerizationofthelinearhydrazonetothereactive(Z)-s-cis-configurationisfollowedby[1,5]-electrocy-clizationofachiralhydrogen-bond-stabilizedionpair.Sub-sequentisomerizationanddeprotonationaffordedthethermodynamicallymorestablepyrazole.Alkylationoftheresultingpyrazolesinahighlydiastereoselectivefashionhasalsobeendemonstrated.130ItispossibletoreconciletheobservedenantioselectivitybyanalogytothemodelproposedbySimonandGoodmanfortheBINOL–phosphoricacid-catalyzedStreckerreaction.131ChelationofthehydrazonetotheC2-symmetricacidactivatesthesubstrateandsubsequentdisrotatoryelectrocyclizationoccurswiththelargearylgroupmovingawayfromthecatalystbulk(Scheme22).

ThesyntheticutilityoftheproceduresofSmithandListarebothextendedviaone-potpreparationoftheelectrocyclicprecursorsandcyclization.Thisallowsformationofindolinesandpyrazolesdirectlyfromarylaminesandketones,respectively.

Hexatrienemanifold.Okamuraetal.havedescribedaninterestingexampleofastereospecifictandemcentre-axis-centrechiralitytransfer,inwhicha[2,3]-sigmatropicshiftisfollowedbydisrotatarycyclization.132Uponformingapropargylicsulfenateester,asigmatropicrearrangementtakesplacetogiveanallenylsulfoxidestereospecifically(withrespecttoallenegeometry).Thesenseoforbitalrotationinthesubsequentelectrocyclizationappearstobeunidirec-tional,withcompletetransmissionofstereochemistrytotheproduct(Scheme23).

Substratecontrolofasymmetryhasalsobeenobservedinthe6pelectrocyclizationofcamphor-derivedtrienes.133,134Magomedovachievedthisbytheinsituformationoftherequisitetrieneundersoftenolizationconditions;useofthebulkyMADLewisacidwithNMPgavethedesiredcyclicenonewithcompletediastereoselectivity(Scheme24).Theobserveddiastereomericoutcomeisconsistentwithatransitionstatethatminimizesthedevelopingstericclashbetweenthephenylandbridgeheaddimethylgroups.

Otherchiralpoolmoleculeshavebeenusedtocontroltheasymmetryofelectrocyclicreactions.Akeystepintheasymmetrictotalsynthesisofpotentsodiumchannelblockertetrodotoxinwasa6pelectrocyclicreactiondirectedbytheadjacentcarbohydrate-derivedbicycle(Scheme25).135,136Theauthorsobservethatthereactionishighest-yieldingwhenX=SnBu3,postulatingthatthisisduetochelation,withtheneighbouringfreehydroxylgroupfavouringthereactivetransitionstate.SuchchelationisabsentwhenX=HandthereactiveconformationisactivelydisfavouredwhenX=SiEt3duetoallylicstrain.Thereactiondemon-stratesthatstereoselectivityinahexatrienesystemmaybecontrolledbysuitablesubstitutionatthe1-position.Hsungetal.haveexploitedthisinhighlydiastereoselective6p

Scheme23Allenyldiene6pelectrocyclization.

Scheme22Brønsted-acidcatalysed6pelectrocyclization.Scheme24Lewisacid-catalyseddiastereoselective6pelectrocyclization.

4224Chem.Soc.Rev.,2011,40,4217–4231ThisjournaliscTheRoyalSocietyofChemistry2011View Article Online

Scheme28Catalyst-controlledenantioselectivecarba-6pelectro-cyclization.

Scheme25Cyclizationofabridgedbicyclichexatrieneyieldingasinglediastereoisomer.

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GelectrocyclizationsthroughtheuseofanEvanschiraloxazo-lidinoneplacedatthehexatriene1-position(Scheme26).137,138Themajorproductobservedisconsistentwithadisrotatorypathwaywherebythelargehalogenmovesupwards(asdrawn),avoidingunfavourablestericinteractionswiththebenzylgroup.

Inthepalladium-catalyzedpolycyclizationofenediynes,TrostandShiobservedcompletediastereoselectivityinthecyclizationofaninsitugeneratedhexatriene(Scheme27).139Disrotationoccursexclusivelyinthedirectionminimizingtransitionstatetorsionalstrainbetweenthelargesilylgroupandtheadjacentring;inthefavouredcasetheOTBSgrouptwistsawayfromthemalonate-substitutedfive-memberedring.Trauneretal.havedemonstratedelegantlythatthethermalelectrocyclizationofhexatrienesystemswithacarbonylgroup

Scheme29Electrocyclizationofachromiumketenecomplex.

atthe2-positionmaybeacceleratedbytheadditionofLewisacidcatalysts.140AfterextensivescreeningofchiralcatalyststhePyBOXcatalystsystemusedsuccessfullyintheNazarovreaction53,54yieldedthehighestlevelsofasymmetricinduction,141giving57–77%eewhenusedstoichiometricallyinthepresenceof2,6-di-tert-butyl-4-methylpyridineanddimethylzirconoceneindeuteratedtetrachloroethane(Scheme28).

Wulffetal.haveinvestigatedtheelectrocyclicringclosureofmetal-complexedvinylketenes(Scheme29).142Onthebasisofearlierwork143theauthorspostulatethatformationofthechromiumketenecomplexinsituisstereospecific,andthattheobserveddiastereoselectivitiesareduetoaselective6pelectro-cyclization.Themajordiastereomerisformedfromanupwardrotationoftheinwardmethylgroup;downwardrotationisapparentlydisfavouredduetoseverecloseinteractionswiththemetalanditsligands.

[1,6]Azaelectrocyclization.Hsungetal.haveextensivelyinvestigatedstereocontrolinthep6sazaelectrocyclizationmanifoldthroughaseriesofelegantstudies.144–152Insitugenerated1-azatrienesundergoefficientelectrocyclizationenrouteto1,2-dihydropyridines,andchiralsubsituentsontheazatrienenitrogenortheC-terminusofthetrieneareeffectiveincontrollingthestereochemistryoftheelectrocyclicprocess(Scheme30).

InthecaseofachiralC-terminalsubstituent,theobservedselectivityisaresultofminimizationofallylicstrainintwopredominantconformationsbyplacingthelargestgroupperpendiculartotheplaneoftheC-terminalvinylstrand(Scheme31).153Thedisrotatorycyclizationispresumedtoproceedfromthemorestableopen-chainconformation,withselectivitydirectedbythedevelopingstericinteractionbetweentheRLandN-Bn

Chem.Soc.Rev.,2011,40,4217–42314225Scheme26Diastereoselectivehexatriene6pelectrocyclization.

Scheme27Allylicstraindirectedhexatrieneelectrocyclization.

ThisjournaliscTheRoyalSocietyofChemistry2011View Article Online

Scheme30Auxiliary-controlled6pazaelectrocyclization.

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme326pElectrocyclizationdirectedbyachiralN-substituent.

Scheme31ModelforobservedstereochemistrydirectedbyachiralgroupattheC-terminusoftheazatriene.

groups.Anincreaseinthesizeofthenitrogensubstituent,andtheuseofasix-memberedring(withagreaterinternalanglethanthepicturedg-lactoneandthusagreaterallylicstrainrelativetothegrouponnitrogen)gavethehighestdiastereo-selectivity(92:8).Reversibilityunderthesereactionconditionswasdemonstratedbythermalequilibrationexperiments150,154andhencetheobserveddiastereoselectivityispresumablyaconsequenceofbothrotationalpreferencesintheelectrocyclicstepandthermodynamicstability.

Asimilarexplanationforselectivityisofferedwhencontrolisdictatedviaachiralnitrogensubstituent(Scheme32).146,147,150,152‘Inward’rotationoftheC-terminalvinylstrandtoavoidclashingwiththearylsubstituentontheephedrine-derivedauxiliaryleadstothemajordiastereoisomer.

Katsumuraetal.155–158utilizeahighlyeffectiveStille/6p-azaelectrocyclizationapproachtogeneratefunctionalizeddihydropyridines.Thissequenceisdirectedbyanindanolauxiliaryafterasubsequenttautomerizationandnucleophilicattacktoformtherequisitehemiaminalether(Scheme33).Amodelfortheobserveddiastereoselectivityinthisprocessreliesonhydrogenbondingoftheindanetodirecttheorientationoftheauxiliary.Stericinteractionsbetweentheisopropylgrouponthearomaticringandtheformingdihydropyridinedisfavour‘upward’rotationofH(asindicated)andexplaintheobserveddiastereoselectivity.Ithasbeendemonstratedthatthestereochemicaloutcomeofthereactionisakinetic,notthermodynamiceffect.Theeffectivenessofthis

4226Chem.Soc.Rev.,2011,40,4217–4231Scheme33Highlydiastereoselectiveazaelectrocyclization.

procedureisreflectedinitssuccessfulapplicationtothesynthesisofarangeofnaturalproducts,155–163includingarecenttotalsynthesisof(À)-20-epiuleine.1[1,6]Oxaelectrocyclization.Hsunghasextendedtheworkofhisgrouponcarba-andazaelectrocyclizationtoencompasstheoxaelectrocyclizationmanifold.Ithasbeendemonstratedthatthereversibilityoftheoxaelectrocyclizationprocessusuallyplaysadominantroleindeterminingthefinaldiastereoisomericoutcomeofthesereactions,165thoughrotationalselectivitymayalsocontributeinsystemsunderkineticcontrol.Substratesforp6soxaelectrocyclizationmaybegeneratedinsituthroughthecondensationofpyronederivativeswithaldehydeoriminiumelectrophilesinwhathasbeentermedaformal[3+3]cycloadditionreaction.165,166Preformeda,b-unsaturatediminiumsaltsmaybeemployedtosolvetheproblemsofcompeting1,2-versus1,4-addition,andC-versusO-addition.Thisallowedfortheconstructionofoxo-spirocycleswithmoderatediastereoselectivities(Scheme34).167Amodeltoexplaininconsistenciesbetweencalculatedthermodynamicstabilitiesandobserveddiastereoisomericratioswasproposed,basedoncompetingallylicstrainand1,3-diaxialinteractions.ForsmallR1groups,itwasproposed

ThisjournaliscTheRoyalSocietyofChemistry2011View Article Online

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme34Moderatelydiastereoselectiveoxaelectrocyclization.

Scheme35Auxiliaryapproachtoasymmetric8pelectrocyclization.

thatequilibrationbetweenaxialandequatorialsubstituentsshouldfavortheintermediatewithminimumA1,3strain,andequatorialapproachoftheoxygenatomduringtheringclosurewouldleadtothemajorisomer.Similartrans-formationswithotherelectrophileshavebeenreported,168–171andanapproachinwhichthesourceofchiralityiscarbohydrate-derivedhasalsobeendescribed.1728pElectrocyclizations(p8a)

8p/6pElectrocyclizationcascades.8pElectrocyclizationsarecommonlyfollowedbya6pelectrocyclization,sincethe8preactionnecessarilyformsatrienewiththecis-stereochemistryrequiredforthe6pevent.Thiscascadeelectrocyclicapproachisexploitedinnumerousnaturalproductsynthesesincludingtheendiandricacids,173–179immunosuppressantpolyketides(À)-SNF4435Cand(+)-SNF4435D180–184andshimalactones.185,186Biomimeticstudiessuggestthatotherpolypropionatesareproducedfromlineartetraenesviaasimilarcascadeofelectrocyclicreactions.183,184,187Theeffectsofsubstituentsonthediastereoiomericratiosobservedintetraene–cyclooctatriene–bicyclooctadienecyclizationsarenoteasilyrationalized.182ParkerandWang188investigateddiastereoselectivityinanauxiliary-controlled8pelectrocyclizationthroughan8p–6pcascade(inwhichthe6pcomponentwasknowntocyclizeinastereoselectivemanner).183,184,187Aseriesofchiralauxiliary-bearingtetraenicesterswereassembledusingaStillecouplingandtheirinsituelectrocyclizationtoformthecoreringsystemof(À)-SNF4435Cand(+)-SNF4435Dwasinvestigated(Scheme35).ThetandemStillecoupling–6p/8pelectrocyclicsequencegavecombinedyieldsforthediastereomersofbetween30and40%andamodestdiastereoselectivityforthe8pelectrocyclizationofupto4:1(dependingontheauxiliaryemployed).

Thediastereoselectivitymayberationalizedthroughconsiderationofthehelicalconformationsofthe8pelectro-cyclizationtransitionstates.Inthiscase,theeffectoftheauxiliaryisnotsufficienttodifferentiatesignificantlybetween

Thisjournaliscthes-cis,synands-trans,synconformers,whichresultsinrelativelymodestandextremelysubstrate-sensitivediastereoselectivity.BeaudryandTrauner180havedisclosedaone-potStille/6p–8pelectrocyclizationcascade,yieldinga3:1mixtureofenantiomericallypurepolyketidesinanoverallyieldof%(Scheme36).

Theratioofdiastereomersispostulatedtoreflectthetorquoselectivityoftheconrotatory8pstep(whichisgovernedbythea-methoxy-g-pyronesubstituent),ratherthanthedisrotatory6pstage.1848p–6pCascadeshavethepotentialtoformstructurallyremarkableframeworksincludingfenestrenes,asdemonstratedbySuffertetal.1,190Cis-reductionofthealkyneusingnickelborideatroomtemperatureyieldedthecorrespondingfenestreneasasingleisomerinupto93%yield.Subsequentmechanisticinvestigationshavedeterminedthatthefenestreneformedisthekineticproduct;exposureofthealkynetothesamereducingconditionsat701C,orthefenestrenetomicrowaveirradiationledtothethermodynamicanti-cyclooctatriene(Scheme37).

Thisprocessisindicativeoftheveryhighlevelsofstereo-controlpossiblebycascadeelectrocyclization,andelegantly

Scheme36Diastereoselective8pelectrocyclizationinthetotalsynthesisofSNF4435C&D.

TheRoyalSocietyofChemistry2011Chem.Soc.Rev.,2011,40,4217–42314227View Article Online

Downloaded by Southwest University on 22/04/2013 09:02:29. Published on 12 May 2011 on http://pubs.rsc.org | doi:10.1039/C1CS15022GScheme39Chelationcontrolin8pelectrocyclization.

Scheme37Fenestrenesynthesisviacascadeelectrocyclization.

demonstratestheimpactofapplyingkineticorthermo-dynamicreactionconditions.

8pElectrocyclization.Paquetteetal.haveexaminedtheadditionoftwoalkenylanionstoasquarateester,inwhichaconrotatory4pringopeningofthecyclobutenedialkoxidegeneratesadoublycharged,octatetraenylintermediate.191Theresultantchiralhelicalpolyolefinicintermediatesequilibrate,

withthestereochemicaloutcomeofthesubsequent8pconrotatoryelectrocyclizationdeterminedbytheratesofclosureofthesematerials.Suitablesubstitutionononeofthealkenylanionscandestabilizeoneoftheseintermediatesanddiastereoselectivityisthereforeaconsequenceofprefer-entialcyclizationofthelower-energyhelix(Scheme38).

Positioningofamethoxygrouponthecyclopenteneledtominimalstereocontrol(drE1.1:1.0),howevertheauthorsfoundthatinclusionofabulkygroupattheR0positionaffordedthecyclizedproductwithcompletediastereoselectivity;theypostulatethatthemethoxygroupaloneistoodistantfromthebondingterminiofthetetraenetoexertsufficientcontrol.192Theysubsequentlyfoundthatremotestereo-inductioncouldbeachievedthroughtheuseofmultiplechelatingheteroatoms;193useofachiralaminenucleophilesufficientlyfavouredonehelicalintermediatetoyieldamodesteeof35%afterb-eliminationoftheauxiliary(Scheme39).Thismethodologyhasalsobeenutilizedinthesynthesisofseveralnaturalproductsincludinghypnophilin,coriolin,andceratopicanol.194,195Conclusionandperspectives

Thisreviewdemonstratesthatsignificantprogresshasbeenmadeincontrollingthestereochemicaloutcomeofelectro-cyclicreactions.Forsomereactions—notablytheNazarovcyclization—therearemanyelegantsubstrateorcatalyst-controlledasymmetricprocesses.Newcatalyticmethodshaveledtorecentdevelopmentsin6pelectrocyclizations,butsuccessinthesemanifoldsispredominantlydependantonspecificallyengineeredsubstrates.Assuch,trulypracticalandgeneralcatalyticasymmetricelectrocyclizationmethodsarestillelusive,butthisgoaloffersopportunitiesforthedevelopmentofnewcatalyticapproachesforthecontrolofundiscoveredorunoptimizedreactionmanifolds.

Acknowledgements

WearegratefulforsupportfromtheRoyalSociety(foraURFtoMDS),theEPSRCandGSK(foraCASEawardtoST)andPfizer(foraCASEawardtoPCK).TheEuropeanResearchCouncilhasprovidedfinancialsupportundertheEuropeanCommunity’sSeventhFrameworkProgramme(FP7/2007–2013)/ERCgrantagreementno.259056.

ThisjournaliscScheme38Octatetraene8pelectrocyclicringclosures.

4228Chem.Soc.Rev.,2011,40,4217–4231TheRoyalSocietyofChemistry2011View Article Online

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