Misassigned Natural Products and the Role of Chemical Synthesis in Modern Structure Ilucidation : Total Synthes

1. Misassigned Natural Products and the Role of ChemicalSynthesis in Modern Structure Ilucidation: Total Synthesis of (+)-Hexacyclinol Jolaine Savoie, 18 Mars 2008 Department of Chemistry, Université de Montréal, C.P. 6128 Station Downtown, Montréal, Québec, H3C 3J7, Canada

2. Summary • Introduction • The state of modern structure elucidation • Total synthesis of (+)-hexacyclinol by La Clair • Formation of endoperoxides • Stability endoperoxides • Using the endoperoxide as a synthetic intermediate • Total synthesis of (+)-hexacyclinol by Porco & Rychnovsky • Conclusion

3. Molecules of Mistaken Identity & Total Synthesis Structural Hypotheses • Today • Multidimensional NMR Spectroscopy • High-Resolution Mass Spectroscopy • X-Ray Crystallography • Before World War II • Degradation • Derivatization What’ s the Role of Total Synthesis Today? • Scale up for biological testing • Structural assignment K. C. Nicolaou, S. A. Snyder, Angew. Chem. Int. Ed., 2005, 44, 1012-1044

4. Molecules of Mistaken Identity & Total Synthesis A CLASSICAL MISASSIGNMENT: Wieland and Windaus, two researchers in Germany, won the Nobel Prize in Chemistry in 1927-1928 because of the proposed structure of cholesterol K. C. Nicolaou, S. A. Snyder, Angew. Chem. Int. Ed., 2005, 44, 1012-1044

5. Molecules of Mistaken Identity & Total Synthesis Number of inaccuracies in the core structure was revealed in 1932 by Bernal, who obtained on X-Ray cristal structure of ergosterol. Structure of Ergosterol (1932) Verified by X-Ray Crystal Analysis Correct Structure of Cholesterol(1932) K. C. Nicolaou, S. A. Snyder, Angew. Chem. Int. Ed., 2005, 44, 1012-1044

6. Molecules of Mistaken Identity & Total Synthesis In 1962, Büchi and his colleagues at the Massachussetts Institute of Technology (MIT), obtaining synthetic material that fully corresponded to authentic patchouli alcohol. K. C. Nicolaou, S. A. Snyder, Angew. Chem. Int. Ed., 2005, 44, 1012-1044

7. Molecules of Mistaken Identity & Total Synthesis In 1963, Dunitz and his colleagues at Eidgenössische Technische Hochschule Zürich, used the X-Ray Crystal Analysis of patchouli alcohol and reassigned the structure. K. C. Nicolaou, S. A. Snyder, Angew. Chem. Int. Ed., 2005, 44, 1012-1044

8. Molecules of Mistaken Identity & Total Synthesis Surprisingly, the examples outlined in the Nicolaou’s review cover virtually every compound class : stereods, terpenes, indole alkaloids and peptides and encompass molecules of all sizes and levels of stereochemical complexity. Method Original Assignment Revised Structure Basis for Revision Verified By Total Synthesis Proposed Structure K. C. Nicolaou, S. A. Snyder, Angew. Chem. Int. Ed., 2005, 44, 1012-1044

9. Molecules of Mistaken Identity & Total Synthesis Why do so many errors occur? The answer from Nicolaou is : The number of errors simply reflects the fact that every method of assignment has its weaknesses, some of which can not be resolved even if every tool for structural elucidation is also applied. • X-Ray Crystal Analysis • Does not reveal the hydrogen atoms (discern between O and N) • Confuse the identity of atoms within certain functional groups (cyano vs diazo) • High –Resolution Mass Spectroscopy • Ionisation (Fast –Atom Bombardment (FAB) vs Electron Spray Ionisation (ESI) • IR Spectroscopy • Structural information K. C. Nicolaou, S. A. Snyder, Angew. Chem. Int. Ed., 2005, 44, 1012-1044

10. Total Synthesis of (+)-Hexacyclinol: La Clair • Introduction • The state of modern structure elucidation • Total synthesis of (+)-hexacyclinol by La Clair • Formation of endoperoxides • Stability endoperoxides • Using the endoperoxide as a synthetic intermediate • Total synthesis of (+)-hexacyclinol by Porco & Rychnovsky • Conclusion

11. Story of (+)-Hexacyclinol Biological Data • Isolated by Gräfe and co-workers , in 2002. • From basidiospores collected from PanusRudis (Mushrooms growing on dead wood). • Biosynthesis of hexacyclinol occurs via the terpenoid pathway. • Moderate antibiotic activity. • Showed an antiproliferative effect. • Inhibitory activity against (P. Falciparum). (antimalariale) PanusRudis

12. Story of (+)-Hexacyclinol • German fungal cultures provided a strain of P. rudis that was able to do the biosynthesis of 1, 2 and 3. • Retrocycloaddition of 1 and 2 released oxygen to afford 3. • [2+2+2] cycloaddition of 3 with singlet O2 returned to a mixture of 1 and 2.

13. Total Synthesis of (+)-Hexacyclinol: La Clair Retrosynthetic Analysis for Hexacyclinol J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

14. Total Synthesis of (+)-Hexacyclinol: La Clair Retrosynthetic Analysis for Hexacyclinol: Jolaine

15. Total Synthesis of (+)-Hexacyclinol: La Clair Cyclization Via SN2 Isomerisation J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

16. Total Synthesis of (+)-Hexacyclinol: La Clair Cyclization Via Mitsunobu Reaction J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

17. Total Synthesis of (+)-Hexacyclinol: La Clair Kinetic Resolution of Epoxyde Synthesis of Precursor 18 D. Picq, I. Drivas, G. Carret, D. Anker, Tetrahedron, 1985, 41,13, 2681-2690 , J. Attaghrai, D. Picq, D. Anker, Carbohydr. Res. 1987, 159,159

18. Total Synthesis of (+)-Hexacyclinol: La Clair Combination of 14 & 18 follow by Mitsunobu (1:4,5) Since 19a didn’t react, that result suggested that the C6-carbinol was too hindered to form the required alkoxyphosphonium intermediate. J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

19. Total Synthesis of (+)-Hexacyclinol: La Clair Tandem Ring Opening & Closing Via Cleavage MOM Group Cyclization J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

20. Total Synthesis of (+)-Hexacyclinol: La Clair Ring Closing via Aldol Reaction Thermolysis Ring opening via esterification J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

21. Total Synthesis of (+)-Hexacyclinol: La Clair Intermolecular Cyclization Via Julia-KocienskiOlefination a-Thiolation J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

22. Total Synthesis of (+)-Hexacyclinol: La Clair Tandem Hydrolysis & b-Elimination Ring Opening Mechanism J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

23. Total Synthesis of (+)-Hexacyclinol: La Clair Cycloadditon [2+2+2] Allylic Oxidation Julia-Kocienski Olefination 0,6% for 37 steps (8:1) J. J. La Clair, Angew. Chem. Int. Ed., 2006, 45, 2769-2773

24. Total Synthesis of (+)-Hexacyclinol: La Clair Natural [a]D= 130,5o c= 0,403 Synthetic [a]D= 131,5o c= 0,40,

25. Endoperoxide • Introduction • The state of modern structure elucidation • Total synthesis of (+)-hexacyclinol by La Clair • Formation of endoperoxides • Stability of endoperoxides • Using the endoperoxide as a synthetic intermediate • Total synthesis of (+)-hexacyclinol by Porco & Rychnovsky • Conclusion

26. Endoperoxides : Natural Products & Pharmaceuticals T.C.T. Chang, M. Roseblum, J. Org. Chem., 1981, 46, 4105-4107 P. M. O’Neill, S. L. Rawe, R. C. Storr, S. A. Ward, G. H. Posnet, Tetrahedron Lett., 2005, 46, 3029-3032

27. Endoperoxides : Formation Direct Nucleophilic Attack on O2 J. S. Yadav, R. Satheesh Babu, G. Sabitha, Tetrahedron lett.,2003, 44, 387-389

28. Endoperoxide : Formation Diels-Alder Thebaine Thebaine is known to produce exclusively the adduct resulting from cycloaddition on the less hindered face of the diene P. M. O’Neill, S. L. Rawe, R. C. Storr, S. A. Ward, G. H. Posnet, Tetrahedron Lett., 2005, 46, 3029-3032 D. Lopez, E. Quinoa, R. Riguera, J. Org. Chem., 2000 ,65, 15, 4671-4678

29. Endoperoxide : Formation Nazarov A. Rostami, Y. Wang, A. M. Arif, R. McDonald, F. G. West, Org. Lett., 2007 , 9, 4, 703-706

30. Endoperoxide : Stability MukayamaAldol A. M. Szpilman, E. Korshin, H. Rozenberg, M. D. Bachi, J. Org. Chem., 2005, 70, 9, 3618-3632

31. Endoperoxide : Stability Metathesis F. Grellepois, B. Crousse, D. Bonnet-Delpon, J.-P. Bégué, Org. Lett., 2005, 7, 23, 5219-5222

32. Endoperoxide : Stability Gold Coupling V. K.-Y. Lo, Y. Liu, M.-K. Wong, C.-M. Che, Org. Lett., 2006, 8, 8, 1529-1532

33. Endoperoxide : Stability Acetalization Reduction H.-X. Jin, H.-H. Liu, Q. Zhang, Y. Wu, J. Org. Chem., 2005, 70, 4240-4247 B. J. Kim, T. Sasaki, J. Org. Chem., 2004, 69, 3242-3244

34. Endoperoxide : Synthetic Utility KornblumDeLaMare Rearrangement Mechanism Example S. T. Staben, X. Linghu, F. D. Toste, J. Am. Chem. Soc, 2006, 128, 12658-12659

35. Endoperoxide : Synthetic Utility Endoperoxide Isomerisation: 2-Vinyl-2-Cyclopentenone Mechanism Example I. Erden, N. Öcal, J. Song, C. Gleason, C. Gärtner, Tetrahedron, 2006, 62, 10676-10682

36. Endoperoxide : Synthetic Utility Ring Contraction Epoxy-1-2-dioxine 3,6-Dihydro-1-2-dioxine B.W. Greatrex, D.K. Taylor, J. Org. Chem, 2004, 69, 2577-2579 T. V. Robinson, D. K. Taylor, E. R.T. Tiekink, J. Org. Chem, 2006, 71, 19, 7236-7244

37. Endoperoxide : Synthetic Utility Reduction of Endoperoxide C. Fattorusso, G. Campiani, B. Catalanotti, M. Persico, N. Basilico, S. Parapini, D. Taramelli, C. Campagnuolo, E. Fattorusso, A. Romano, O. Taglialatela-Scafiati, J. Med. Chem, 2006, 49, 24, 7088-7094 T. V. Robinson, D. K. Taylor, E. R.T. Tiekink, J. Org. Chem, 2006, 71, 19, 7236-7244

38. Endoperoxide : Synthetic Utility Reduction of Endoperoxide T. V. Robinson, D. K. Taylor, E. R.T. Tiekink, J. Org. Chem, 2006, 71, 19, 7236-7244

39. Total Synthesis of (+)-Hexacyclinol: Porco & Rychnovsky • Introduction • The state of modern structure elucidation • Total synthesis of (+)-hexacyclinol by La Clair • Formation of endoperoxides • Stability of endoperoxides • Using the endoperoxides as a synthetic intermediate • Total synthesis of (+)-Hexacyclinol by Porco &Rychnovsky • Conclusion

40. Total Synthesis of (+)-Hexacyclinol: Porco & Rychnovsky Gräfre’ s proposed structure La Clair’s synthesis What is the wrong with the structure of (+)-hexacyclinol ? Proposed a new structure of (+)- hexacyclinol based : On calculated 13C NMR chemical shift correlation. From an acid catalyzed rearrangement panepophenanthrin. (ubiquitin activating enzyme inhibitor) Pabepophenanthrin J. A. Porco, S. Su, X. Lei, S. Bardham, S. D. Rychnovsky, Angew. Chem. Int. Ed., 2006, 45, 5790-5792

41. Total Synthesis of (+)-Hexacyclinol: Porco & Rychnovsky Revised Retrosynthetic Analysis for Hexacyclinol J. A. Porco, S. Su, X. Lei, S. Bardham, S. D. Rychnovsky, Angew. Chem. Int. Ed., 2006, 45, 5790-5792

42. Total Synthesis of (+)-Hexacyclinol: Porco & Rychnovsky Synthesis of Precursor 7 Via Oxidation Reaction Chelation-controlled reduction was attempted using DiBAL-H or Zn(BH4)2 to induce the anti-epoxy alcohol, but it’s showed no diastereoselectivity (1:1). In contrast, the reduction with the Super-Hydride cleanly afforded syn-epoxy alcohol. A. B. Hughes, M. V. Sargent, J. Chem. Soc. Perkin Trans 1, 1989, 10, 1787-1791 X. Lei, R. P. Johnson, J. A. Porco, Jr., Angew. Chem. Int. Ed., 2003, 42, 3913-3917

43. Total Synthesis of (+)-Hexacyclinol: Porco & Rychnovsky 9 introduce via Stille Coupling Hydrostannylation of propargyl alcohol ( 9) J. A. Porco, S. Su, X. Lei, S. Bardham, S. D. Rychnovsky, Angew. Chem. Int. Ed., 2006, 45, 5790-5792 L. T. Scott, G. J. DeCicco, J. L. Hyun, G. Reinhardt, J. Am. Chem. Soc., 1985, 107, 6546-6555 H.X. Zhang, F. Guibé, G. Balavoine, J. Org. Chem., 1990, 55, 1857

44. Total Synthesis of (+)-Hexacyclinol: Porco & Rychnovsky Diels-Alder Reaction & SN2’ substitution/Cyclization 28% for 11 steps 10 partially dimerized upon standing without solvent (25oC). The Diels-Alder ‘s product was cleanly obtained in 87% yield as a single diastereoisomer. J. A. Porco, S. Su, X. Lei, S. Bardham, S. D. Rychnovsky, Angew. Chem. Int. Ed., 2006, 45, 5790-5792

45. Total Synthesis of (+)-Hexacyclinol: Porco & Rychnovsky X-Ray Crystal Structure Analysis of Synthetic 2 C-5 position SN2’ cyclization places the prenylsubstituent at C-5 in a pseudo equatorial orientation, which minimizes steric interactions on the envelope conformation of the tetreahydrofuran ring. C-5 position J. A. Porco, S. Su, X. Lei, S. Bardham, S. D. Rychnovsky, Angew. Chem. Int. Ed., 2006, 45, 5790-5792

46. Total synthesis of (+)-Hexacyclinol: Porco & Rychnovsky [a]D= 133,3o c= 0,40 [a]D= 130,5o c= 0,403

47. Total synthesis of (+)-Hexacyclinol: Porco & Rychnovsky

48. Conclusion Porco & Rychnovsky [a]D= 133,3o c= 0,40, MeOH La Clair [a]D= 131,5o c= 0,40,

49. Conclusion La Clair Porco & Rychnovsky • No 13C data or experimental protocols in the supporting information. • [a]D & 1H NMR similar to the natural product. • Proof on his structure: release of O2 . • Fugal cultures can make hexacyclinol & desoxyhexacyclinol. • [a]D & 1H NMR similar to the natural product. • 13 C NMR data matched the natural product. • X-Ray Crystal obtained synthetic material (not natural product). • Proposed that hexacyclinol is an artifact obtained during the isolation procedure. • No mention of O2 release.