Asymmetric Diels Alder Reactions
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1 Asymmetric Diels Alder eactios Chiral Auxiliaries (-)-8-eylmethol favorable!-stackig 3 C 3 C C 3 AlCl 3 3 C 3 C C 3 s-tras Lewis-acid complex AlCl 3 Cl JACS, 1975, 1610 frot-face approach of diee B 3 C C 3 B 3 C LiAl 4 auxiliary recovery 97%de (-)-8-pheylmethol= * *
2 Dimethyl fumarate: ' * Et 2 AlCl 3 C Et 2 Al 3 C 3 C C 3 C 3 AlEt 2 C 3 '= C 3 C 3 TL, 1986, 4507 Tetrahedro, 1963, 2333 Diee Lewis acid de% * 2 C C 2 * 99%de ibualcl Et 2 AlCl AlCl 3 96 TMS Et 2 AlCl 94
3 -acyl oxazolidioe auxiliaries Et Al Et Et 2 AlCl tolerates!-substitutio o dieophile C JACS, 1984, 4261 JACS, 1988, C Et 2 AlCl -100 C imides ad amides favor s-tras form of dieophile CX c CX c frot face attack of diee back face attack of diee chelated s-cis form of dieophile Et Al 3 C Et dieophile edo:exo isolated dr 1, = >100:1 >99:1 1, =C 3 >100:1 >99:1 1, = >100:1 >99:1 2, = >100:1 <1:99 2, =C 3 >100:1 <1:99
4 Diee Scope: Et 2 AlCl -100 C 3 C C 3 CX c =, dr=95:5 =C 3, dr=94:6 Et Al Et C 3 CX c =, dr=>100:1 =C 3, dr=95:1!-stackig ehaces stereoselectivity Asymmetric Itramolecular Diels-Alder ACIEE, 1987, X c Me 2 AlCl X c X c -30 C, 5h eatiomers diee ad dieophile i the same molecule Edo I Edo II X C Edo I : EdoII TL, 1984, :5 2 97:3 1 3:97 C :94
5 Camphor-Derived -eoyl Sultams: ppolzer s-cis coformer 3 C C 3 3 C C 3 C 3 S s-cis form of dieophile favored for amides: JACS, 1973, EtAlCl 2 (1.5 eq.) edo:exo dr 99.5: :2.5 C 3 96:4 99:1 CX c S ML Chelated Complex elv. Chim. Acta, 1989, 123. diee!-face approach Acyclic diees: 3 C C 3 S for usubstituted -acyloyl sultams EtAlCl eq. X c dr yield 98.5: C 3 97:3 64 elv. Chim. Acta. 1984, 1397 Itramolecular Diels-Alder eactios: 3 C C 3 S EtAlCl 2 X c Edo I X c Edo II EdoI:EdoII yield 1 >97.4: : TL, 1985, 5437
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7 A Early Example Catalytic Asymmetric Diels-Alder eactios 3 C (15 mol%) toluee, -78 C AlCl 2 C 3 C 72% ee exo:edo = 98:2 J. Chem. Soc., Chem. Commu. 1979, 437. C2-symmetric Diazaalumiate Catalysis: Corey JACS, 1989, 5493 F 3 C CF 3 S Al S C mol% ' ' C 2 Cl 2, -78 C ' edo:exo ee% >50:1 91 C 3 96:4 94 C 2 B 95 Proposed Trasitio-State Assembly: JACS, 1992, eyl blocks frot face Tf Al Tf C 3 o-chelated bidig mode s-tras dieophile coformer a modified versio of this catalyst expads the scope to maleimide dieophiles: JACS, 1994,
8 xazaborolidie Catalysis:!-substitutio o aldehyde required for high ee 3 (5 mol%) C 2 Cl 2, -78 C 3 (5 mol%) C 2 Cl 2, -78 C 96:4 exo:edo, 92% ee 92%ee C Ts 3, tryptopha-derived B--Bu Attractive pi-stackig iteractios betwee idole ad dieophile orgaize the T.S., resultig i reactio from the ublocked (back) face Lewis acids complex sy to the formyl proto: ACIEE, 1990, 256 S Ar B JACS, 1992, 8290 exo-ts s-cis coformer is reactive species formyl C---- hydroge bod is proposed as a additioal orgaizig elemet leadig to excellet eatioselectivities : TL, 1997, 37.
9 A modified oxazaborolidie catalyzes cycloadditios to fura: 4 (10 mol%) C 2 Cl 2, -78 C 92% ee C 3 C 4 Ts B--Bu Tetrahedro Letters, 1993, 3979 Corey has demostrated the utility of the cycloadditio products: JACS, 1991, C 2 C 2 C C 3 C 2 Et
10 Alkyl Dichloroboraes: BCl 2 Me (10 mol%) C 2 Me JACS, 1991, 7794 ee% 1 97 C C 2 Me s-tras crotoate coformatio pi-stackig of crotoate carboyl with aromatic rig Cl B Cl C 3 C 3 the apthalee substituet forces the dieophile approach from the frot face. complexatio of Lewis acid ati to ester C- bod.
11 Titaium TADDL: TiCl 2 Me 5 JACS, 1989, 5340 Chem. Lett. 1995, , (10 mol%) 2 2 edo:exo ee% 88 C 3 87: : , (10 mol%) ee% 93 C 2 Me 91 C 3 >96 C 3 C 2 Me 94 A umber of trasitio state models have bee proposed; the aalysis is complicated by the umber of coordiatio possibilities available i octahedral complexes Bull. Chem. Soc. Jp. 1991, 387.
12 Evas Bis(oxazolie) Copper Complexes 3 C C 3 Cu 2 9 2Tf - or 10 2SbF 6 ACIEE, 1995, 798 ote: catalyst 10 uiformly provides higher reativity ad higher levels of asymmetric iductio tha , (5-10 mol%) 11 X 10, (1-5 mol%) X ote: exo product 12 10, (5 mol%) Diee X edo:exo ee% 6 98:2 >98 6 C 2 Et 94: C 3 96: : Ac 85: S 98: :20 97
13 The stereochemical results are cosistet with the followig model: 3 C C SbF 6 Cu Square plaar geometry about Cu Imide bids i a bidetate fashio s-cis dieophile cofiguratio diee approaches from the back face; the frot face is blocked by the t-bu group Si-face ote: acyclic diees usubstituted at the 1-positio afforded lower eatioselectivities; these diees are proposed to approach via a exo TS, ad the exo TS is oly selective i the case of 1-substituted diees Diee edo:exo ee% yield C 3 59% 81% C C 3 C 3 Ac 27: Catalyst 10 also is effective for the itramolecular Diels-Alder reactio: mol% C 2 Cl 2, 25 C edo:exo ee yield 1 >99: >95: : JACS, 1993, 6460 JACS, 1999, 7559 JACS, 1999, 7582
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