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Catalytic Asymmetric Acyl Halide-Aldehyde Cyclocondensation Reactions of Substituted Ketenes

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Catalytic Asymmetric Acyl Halide-Aldehyde Cyclocondensation Reactions of Substituted Ketenes Catalytic Asymmetric Acyl Halide-Aldehyde Cyclocondensation Reactions of Substituted Ketenes Scott G. Nelson, Cheng Zhu, and Xiaoqiang Shen J. Am. Chem Soc. 2004, 126, 14-15. Michael C. Myers, Literature Presentation, January 27, 2004 The Cyclocondensation Reaction Kinetically Complex Processes: 4 Competing Reaction Pathways 1. Cyclocondensation 2. Ketene Dimerization 3. Ketene Trimerization 4. Aldehyde Homoaldol Addition Woodward-Hofmann Rules Govern Syn Selectivity: [2+2] Cyclocondensation Reaction Aluminum(III) Lewis Acid Catalysts: Catalyze Cyclocondensation / Limit Other Pathways 1st Generation 2nd Generation (Chiral) R iPr iPr N Al(SbF ) 6 3 N Al N F CO S SO CF 3 2 Me 2 3 Substituting CH2Cl2 for Benzotrifluoride (BTF): Ammonium Bromide Salts Insoluble in BTF Woodward-Hofmann Implications “The orbital requirements for concerted thermal ketene-aldehyde cycloaddition would ensure that substituted ketenes would afford the contrasteric cis-substituted 4- oxetanones, thereby providing the equivalent of a syn-selective propionate aldol.” Few Systems Meet the Geometrical Limits: Most [2π + 2π] Additions Involve Ketenes O H O R2 1 O H R or Supra/Antara Mobius System 4 Electrons O 2 H Aromatic H R Allowed HOMO-Aldehyde O O LUMO-Ketene R2 = 1 H R R1 O H Woodward, R. B.; Hoffmann, R. The conservation of Orbital Symmetry; VCH: Weinheim, 1970. Nelson, S. G.; Wan, Z. Org. Lett. 2000, 2, 1883-1886. Generation I and II Al(III) Catalysts Empirically Found that Al(SbF6)3 Catalyzed the AAC Reactions O O O 2.5-20 mol% Al(SbF6)3 O X Me H R DIEA, CH Cl , -25 oC 2 2 R 1.5 equiv. 1.0 equiv. x=Cl or Br Al(SbF6)3 Represents the AlCl3:AgSbF6 Stoichiometry (1:3) Used to Generate the Catalyst Nelson, S. G.; Wan, Z.; Peelen, T. J.; Spencer, K. L. Tet. Lett. 1999, 40, 6535-6539. 2 2 1R 1R R R1=iPr R (CF3SO2)2O Benzyl amine R2=H N H2N OH TEA MeOH 90% Tf 65% Bn Bn iPr iPr iPr Me Al or Et AlCl iPr (III) N 3 2 N Al Triamine Chiral Catalyst N Al N NH HN CH Cl 2 2 F CO S SO CF F3CO2S SO2CF3 3 2 R 2 3 R=Cl or Me Cernerud, M.; Skrinning, A.; Bergere, I.; Moberg, C. Tetrahedron: Asymmetry 1997, 8, 3437-3441. Nelson, S. G.; Peelen, T. J.; Wan, Z. J. Am. Chem. Soc. 1999, 121, 9742-9743. Aluminum(III) Catalyst Role 4-Coordinate Complex Adopting a Trigonal Monopyramidal (tmp) Geometry Balance Between Lewis Acidity of the Metal and Lewis Basicity of the Amine Distorted 4-Coordinate Geometry Access Pentacoordiate Species (KEY To Reactivity) Bn iPr iPr N N Al N F CO S SO CF 3 2 R 2 3 R=Cl or Me C2-Symmetric Al(III) Triamine Chiral Catalyst Best Performing Chiral Catalyst Nelson, S. G.; Kim, B-K.; Peelen, T. J. J. Am. Chem. Soc. 2000, 122, 9318-9319. Scott G. Nelson, Cheng Zhu, and Xiaoqiang Shen J. Am. Chem Soc. 2004, 126, 14-15. AAC Reaction Scope entry R1 R2 %ee 9a syn:anti b,c % yield 9d a Me CH2CH2Ph 90 95:5 71 b Me (CH 2)8CHCH 2 88 94:6 77 c Me CH2CH2OBn 91 86:14 75 d Me C6H5 96 >98:2 80 e e Me C CSiMe 3 95 98:2 76 CF3 f Et CH2CH2Ph 91 95:5 81 iPr iPr g Et CH2CH2OBn 91 88:12 83 N h Et CH2OBn 93 89:11 78 N Al N ArO S SO CF i Et C6H5 94 >98:2 83 2 Me 2 3 n j Pr CH2CH2OBn 91 91:9 88 n Second Generation k Pr C6H5 96 >98:2 85 Al(III) Triamine i e l Pr C CSiMe 3 94 >98:2 71 Unsymmetrical i Chiral Catalyst m Pr C6H5 96 >98:2 84 a Enantiomeric ratios determined by chiral GLC or HPLC. b Diastereomeric ratios determined by 1H NMR of crude product mixtures except for entries b and e (GLC). c Relative and absolute stereochemical assignments based on prior literature precedent; see ref 2d. d Yields for diastereomerically pure materials except entries g and j (diastereomers were inseparable). e Yield for the amide derived fr om amine -mediated ring opening of the crude ?-lactone. Scott G. Nelson, Cheng Zhu, and Xiaoqiang Shen J. Am. Chem Soc. 2004, 126, 14-15. Building Blocks Derived from β-Lactones β-Lactones are Direct Progenitors of Numerous Useful Building Blocks R3 O OH O 4 2 C R2 R O R 1 R4O R R1 Aldol Product Diverse Allenes O O R1 R2 3 O NHR3 O R 4 2 R4O R2 R O R 1 R1 R β-Amino Acids Conjugate Addition Product See References Within Presentation Aldol Bond Construction Strategies • Typical Aldol Additions Successfully Relay Enolate Geometry to the Relative Stereochemistry at the Two Stereogenic Centers • Obtain Syn Aldol Adducts Stereochemistry Relative Woodward-Hofmann Absolute Chiral Lewis Acid Relative Enolate Geometry Absolute Chiral Oxazolidinone Example O O O O OH Bu2BOTf, TEA O N ArCHO O N Ar Syn Me or TiCl4, TEA Me Bn Bn Nelson, S. G.; Wan, Z. Org. Lett. 2000, 2, 1883-1886. Evans, D. A.; Rieger, D. L.; Bilodeau, M. T.; and Urpi, F. J. Am. Chem. Soc. 1991, 113, 1047-1049. Asymmetic “Conjugate Addition” Strategies Prototypical Conjugate Addition Reactions Use an Enone Electrophile β-Lactones Opened with Cu(I) Salts Give “Conjugate Addition” Products Stereochemistry Based on Chiral Catalyst Stereochemistry Based on Stereocenter (SN2) Example Nelson, S. G., Wan, Z., Stan, M. A. J. Org. Chem. 2000, 67, 4680-4683 Degrado S. J., Mizutani H., Hoveyda A. H. J. Am. Chem. Soc. 2001, 123, 755-756. β-Amino Acid Synthesis • Amine-Mediated SN2 Ring Opening of β-Lactones O O O 10 mol% catalyst O NaN3 O N3 H R o Br Me iPr2NEt, CH2Cl2 R DMSO, 50 C HO R -50 oC 91-97% ee 80-96% yield H2, Pd/C 1. CH2N2, Et2O O NHBoc O NH3 2. H2, Pd/C, Boc2O MeO R O R R = BnOCH2, PhCH2CH2, Me2CHCH2, various alkyl chains Ser Phe-like Val-like Bn iPr iPr N AlIII Triamine Catalyst N Al N F CO S SO CF 3 2 Me 2 3 Nelson, S. G., Spencer, K. L., Angew. Chem. Int. Ed. 2000, 39 , 1323-1325. Allenes from β-Lactone Templates • Structurally Diverse Allenes are Popular Intermediates for Asymmetric Synthesis ` (I) • SN2 Ring Opening using Grignard Reagents and Cu Catalysts Nu R LG R LG SN2` Mechanism R R R R Syn Predominates R R R Nu R Syn Anti • 4-Alkynyl-β-Lactones Found to Undergo Sterospecific Anti-1,3-Substitution Reactions • Minor Amounts of SN2 Lactone Ring Opened Products When Using Unbranched Grignard Reagents ` Paquette, L. A.; Stirling, C. J. M. Tetrahedron, 1992, 48, 7383.-SN2 Review Wan, Z.; Nelson, G. S. J. Am. Chem. Soc. 2000, 122, 10470-10471. Stereocontrolled Synthesis of Malyngolide • (-)-Malyngolide is a Naturally Occurring Antibiotic • Both Stereocenters Set in the Initial Asymmetric AAC Reaction o (a) 10 mol% 1, EtCOBr, iPr2NEt, CH2Cl2, -50 C. (b) nC9H19MgBr, 10 mol % CuBr, THF, -78 oC. o (c) 10 mol%, AgNO3, 5 mol% iPr2NEt, 80 C, CH3CN. (d) H2, Pd-C. Retrosynthesis H Ag n C9H19 Step c C C C via Me OBn CO2H Four Steps / 54% Overall Yield Asymmetric Synthesis Wan, Z.; Nelson, G. S. J. Am. Chem. Soc. 2000, 122, 10470-10471. Summary for Aldehyde Cyclocondensation (AAC) Reactions Relative Stereochemistry is Set by Woodward-Hoffmann Rules: [2+2] Cycloaddition Absolute Stereochemistry is Set by a Chiral Triamine Al(III) Catalyst β-Lactones Act As Surrogates For Important Enantioenriched Building Blocks R3 O OH O 4 2 C R2 R O R Can be Used to Prepare… 1 R4O R R1 Aldol Product Diverse Allenes O 1. Aldol Addition Products O 2. Conjugate Addition Products R1 R2 3 3. β-Amino Acids O NHR3 O R 4 2 R4O R2 R O R 4. Diverse Allenes 1 R1 R β-Amino Acids Conjugate Addition Product • (AAC) Methodology Could be Used For Large Scale Preparation of Pharmaceutical Precursors if Approach is Competitive and Cost Effective Compared to Current Routes.
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