The Future of Total Synthesis Jason M

The Future of Total Synthesis Jason M. Stevens 01.26.2012 The Future of Total Synthesis a brief forward

■ The idea for tonights topic was from discussions with all of you over the past 1.5 years

■ The intent of presentation is to:

■ Discuss a brief history of total synthesis for the purpose of context

■ Brieﬂy review the the best current work in the ﬁeld of total synthesis

■ Present examples that underscore the transitions occurring in total synthesis for the purpose of discussion Total Synthesis of Natural Products a brief history

■ It all began with urea...

■ Wöhlers synthesis of urea demonstrated that organic matter could be produced synthetically

■ Discredited vitalism, the theory that organic matter possessed a vital force inherent to living things

H2N NH2

urea

Friedrich Wöhler (1828). "Ueber künstliche Bildung des Harnstoffs". Annalen der Physik und Chemie 88 (2): 253–256. Total Synthesis of Natural Products a brief history

■ Gustaf Komppa’s industrial synthesis of camphor in 1903 via semi-synthesis from pinene

■ Camphor was a scarce natural product with a worldwide demand

■ Important milestone in synthetic organic chemistry

Me Me

Me O camphor Total Synthesis of Natural Products a brief history

■ The modern era of total synthesis began with Woodward’s synthesis of quinine

■ The ability to utilize a predictive set of known reactions to execute a synthetic plan

■ Ushered in the modern era of total synthesis

HO N MeO

quinine

Woodward, R. B.; Doering, W. E. J. Am. Chem. Soc. 1944, 66: 849-849. Total Synthesis of Natural Products why we’ve made molecules since 1828

■ Three driving forces for undertaking the total synthesis of natural products

Potential Societal Impact Assist Structural Identiﬁcation Inspire New Methods

HO H N MeO N H H

O O N H originally proposed skeleton quinine strychnine of cholesterol Total Synthesis of Natural Products why we make molecules in 2012

■ Modern analytical methods have largely eliminated the need to verify structure through synthesis

■ We’re now entering an era where chemists can make molecules with unprecedented efﬁciency

■ Focus is largely shifting toward the synthesis of molecules that have the potential for societal impact

Potential Societal Impact Assist Structural Identiﬁcation Inspire New Methods

HO H N MeO N H H

O O N H originally proposed skeleton quinine strychnine of cholesterol What is the Future of Total Synthesis? topics for discussion

■ Brief discussion of how the ﬁeld of total synthesis has changed over the past 50 years

■ Discussion will be limited to active research groups located at U.S. institutions since 1960

■ Highlight recent literature that contrast the past and present of total synthesis

■ Use insights from these examples to look toward the future

Potential Societal Impact Assist Structural Identiﬁcation Inspire New Methods

HO H N MeO N H H

O O N H originally proposed skeleton quinine strychnine of cholesterol Key Research Programs in Total Synthesis programs initiated from 1961-1972

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

Also: Phil Magnus, James Marshall, Albert Padwa, James White

■ Equipped with the knowledge that complex molecules can be made

■ The goals of synthetic efforts from this group largely focused on accessing the desired target

Syntheses completed by 1972

Strychnine - Woodward Prostaglandin - Corey

Reserpine - Woodward Progesterone - W. S. Johnson Key Research Programs in Total Synthesis programs initiated from 1961-1972

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

Also: Phil Magnus, James Marshall, Albert Padwa, James White

■ Equipped with the knowledge that complex molecules can be made

■ Only a limited selection of reliable “synthons”

Reactions and Reagents that Didn’t Exist in 1972 Active areas of research at that time Chiral Auxiliaries Hydroboration Heck, Kumada-Corriu, Stille, and Suzuki Couplings Controlling enolate geometry Sharpless epoxidation Organic photochemistry TBSCl Cross-coupling reactions Key Research Programs in Total Synthesis programs initiated from 1961-1972

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

■ Throughout their careers they produced many total syntheses which, at the time their programs began, were seeming impossible R O Cl O O

HO OH Cl O O H H H O N N N N N N Me H H H HN O O O Me

HO2C NH2 Me

OH HO OH Vancomycin (Evans 1998)

Evans, D. A.; Wood, M. R.; Trotter, W. B.; Richardson, T. I.; Barrow, J. C.; Katz, J. L. Angew. Chem. Int. Ed. 1998, 37, 2700-2704. Key Research Programs in Total Synthesis programs initiated from 1961-1972

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

■ Devoted much of their careers to developing new methods to enable the synthesis of natural products

Me OH HO

Me H O HO O O O OR Me Me HO All stereocenters set by asymmetric OH O N O O aldol, alkylation or epoxidation Me Me Ph Me OH O OH cytovaricin (Evans 1990) Me H Me

Evans, D. A.; Kaldor, S. W.; Jones, T. K.; Clardy, J.; Stout, T. J. J. Am. Chem. Soc. 1990, 112, 7001-7031. Key Research Programs in Total Synthesis programs initiated from 1961-1972

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

■ Pioneered many fundamental advances and applications for transition metal chemistry

Me TsMeN H N O NH

quadrigemine C H H N (Overman 2002) Me N Me N Bn N N TfO

OTf N Bn N N N Me N H Me H H H

HN O N H Me NMeTs Lebsack, A. D.; Link, J. T.; Overman, L. E.; Stearns, B. A. J. Am. Chem. Soc. 2002, 124, 9008-9009. Key Research Programs in Total Synthesis programs initiated from 1961-1972

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

■ Executed syntheses of natural products with the aim of exploring its therapeutic potential

OH Me HO HO O H H H Me O O spongistatin 1 X = Cl H OMe O HO O spongistatin 2 X = H OH O H H O Me (Evans 1998, Smith 2001) OH O H O X Me AcO OAc Me OH Key Research Programs in Total Synthesis programs initiated from 1961-1972

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

■ Famous molecules as a benchmark for total synthesis and a continued source of inspiration

H N H H

O O H strychnine

Magnus, Overman, Padwa (Woodward) Key Research Programs in Total Synthesis programs initiated from 1973-1984

K.C. Nicolaou (1976) Paul Wender (1976) Dale Boger (1979) Stuart Schreiber (1981)

Also: James Cook, Mike Crimmins, Gary Keck, Tom Hoye, Stephen Martin, Viresh Rawal, Bill Roush, Bob Williams Dave Williams and Jeffrey Winkler

■ Applied some of the most vigorously studied research in organic chemistry toward natural products

■ Completed brilliant total syntheses of some of the most complicated molecules ever isolated

Ph H

endiandric acids A-D (Nicolaou 1982) H H CO H H 2 HO OH H

Nicolaou, K. C.; Petasis, N. A.; Zipkin, R. E.; Uenishi, J. J. Am. Chem. Soc. 1982, 104, 5555-5557. Key Research Programs in Total Synthesis programs initiated from 1973-1984

K.C. Nicolaou (1976) Paul Wender (1976) Dale Boger (1979) Stuart Schreiber (1981)

Also: James Cook, Mike Crimmins, Gary Keck, Tom Hoye, Stephen Martin, Viresh Rawal, Bill Roush, Bob Williams Dave Williams and Jeffrey Winkler

■ Applied some of the most vigorously studied research in organic chemistry toward natural products

■ Completed brilliant total syntheses of some of the most complicated molecules ever isolated

O O CO Et HO 2 O O HO O CO2Et O O O

Me O CMe3 Me TESO hν HO O Me O TESO Me Me Me Me ginkolide B (Crimmins 1999)

Crimmins, M. T. et al. J. Am. Chem. Soc. 1999, 121, 10249-10250. Key Research Programs in Total Synthesis programs initiated from 1973-1984

K.C. Nicolaou (1976) Paul Wender (1976) Dale Boger (1979) Stuart Schreiber (1981)

Also: James Cook, Mike Crimmins, Gary Keck, Tom Hoye, Stephen Martin, Viresh Rawal, Bill Roush, Bob Williams Dave Williams and Jeffrey Winkler

■ During their careers the “synthetic toolkit” had expanded drastically

■ New transformations provided increased access to exceptionally complicated structures

HO H O Me H Me H O H Me O H Me O H H H H O O H O O O H H O H H O H O H H H

brevetoxin A (Nicolaou 1998, Crimmins 2008) Key Research Programs in Total Synthesis programs initiated from 1973-1984

K.C. Nicolaou (1976) Paul Wender (1976) Dale Boger (1979) Stuart Schreiber (1981)

Also: James Cook, Mike Crimmins, Gary Keck, Tom Hoye, Stephen Martin, Viresh Rawal, Bill Roush, Bob Williams Dave Williams and Jeffrey Winkler

■ During their careers the “synthetic toolkit” had expanded drastically

■ The synthesis of Nature’s most complicated therapeutic leads became a worthy endeavor

Ph AcO O OH HN O Me Ph Me O HO O Me

H O HO OBz OAc

taxol (Nicolaou 1994, Wender 1997) Key Research Programs in Total Synthesis programs initiated from 1973-1984

K.C. Nicolaou (1976) Paul Wender (1976) Dale Boger (1979) Stuart Schreiber (1981)

Also: James Cook, Mike Crimmins, Gary Keck, Tom Hoye, Stephen Martin, Viresh Rawal, Bill Roush, Bob Williams Dave Williams and Jeffrey Winkler

most synthetic efforts largely focused on accessing the desired target Key Research Programs in Total Synthesis programs initiated between 1985-1996

Andrew Myers (1986) Scott Rychnovsky (1988) Peter Wipf (1990) John Wood (1993)

Also: Arun Ghosh, John Montgomery, James Panek, Tom Pettus and John Rainier

■ The goals of synthetic efforts from this group largely focused on accessing the desired target

■ Constructed molecules of incredible complexity with innovative methods

Me OTMS Me H H CO2H CO2TIPS OH O HN N O O O OMe OMe OTMS H OTMS H

OH O OH O dynemicin (Myers 1995) Myers, A. G.; Fraley, M. E.; Tom, N. J. J. Am. Chem. Soc. 1994, 116, 11556-11557. Key Research Programs in Total Synthesis programs initiated between 1985-1996

Andrew Myers (1986) Scott Rychnovsky (1988) Peter Wipf (1990) John Wood (1993)

Also: Arun Ghosh, John Montgomery, James Panek, Tom Pettus and John Rainier

■ The goals of synthetic efforts from this group largely focused on accessing the desired target

■ Continued the traditions of building molecules of incredible complexity

Me Me O

Me H

HO HO ingenol (Wood 2004) HO HO Nickel, A,; Maruyama, T.; Tang, H.; Murphy, P. D.; Green, B. Yusuff, N. Wood, J. L. J. Am. Chem. Soc. 2004, 126, 16300-16301. Key Research Programs in Total Synthesis programs initiated between 1985-1996

Andrew Myers (1986) Scott Rychnovsky (1988) Peter Wipf (1990) John Wood (1993)

Also: Arun Ghosh, John Montgomery, James Panek, Tom Pettus and John Rainier

■ The goals of synthetic efforts from this group largely focused on accessing the desired target

■ Continued the traditions of building molecules of incredible complexity

Me OH NMe2 HO H H OH

NH2 OH OH O OH O

tetracycline (Myers 2005)

Charest, M. G.; Siegel, D. R.; Myers, A. G.; J. Am. Chem. Soc. 2005, 127, 8292-8293. A Paradigm Shift in Total Synthesis “can we make everything” becomes “how well can we make everything”

■ A signiﬁcant aim of the synthetic community from 1940 to ~1995 entailed accessing the desired structure

■ Once the synthetic natural product was obtained the project was over

■ Recent years have placed additional focus on how well we access desired targets

■ The shift is evident (not ubiquitous) with total synthesis programs initiated after this period

■ This shift is being increasingly adopted by the research groups initiated before this period A Paradigm Shift in Total Synthesis why the mid-1990’s

■ In 1990 Corey wins the Nobel Prize in Chemistry for the...

“...development of the theory and methodology of organic synthesis”. A Paradigm Shift in Total Synthesis why the mid-1990’s

■ A high proﬁle introspective analysis concerning synthetic efﬁciency was published in 1991.

Atom Economy

Trost B. M. Science 1991 254, 1471-1477. A Paradigm Shift in Total Synthesis why the mid-1990’s

■ The taxol problem exempliﬁed the limitations total synthesis for assembling structures that carry the potential to have societal impact (35 groups worked on taxol)

Robert Holton (1994) K.C. Nicolaou (1994) Sam Danishefsky (1996) Paul Wender (1997)

46 longest linear steps 42 longest linear steps 49 longest linear steps 37 longest linear steps

Ph AcO O OH HN O Me Ph Me O HO O Me

H O HO OBz OAc

taxol A Paradigm Shift in Total Synthesis why the mid-1990’s

■ The taxol problem exempliﬁed the limitations total synthesis for assembling structures that carry the potential to have societal impact (35 groups worked on taxol)

Robert Holton (1994) K.C. Nicolaou (1994) Sam Danishefsky (1996) Paul Wender (1997)

46 steps 55 steps 49 steps 37 steps

Consideration of the chemical complexity of baccatin III, which in suitably protected form would be the likely synthetic intermediate en route to taxol, should have engendered considerable skepticism and even disbelief that total synthesis would supplant natural sources as a route to the drug. More plausible, though as yet unrealized in practice, is the prospect that mastery of the synthesis of baccatin III will bring with it new nuclei which, upon suitable conjugation with biologically critical side chains, might provide medically promising variants of taxol. -Samuel Danishefsky Key Research Programs in Total Synthesis programs initiated from 1997-2008

David MacMillan (1998) Erik Sorensen (2001) Phil Baran (2003) Mo Movassaghi (2003) Also: Martin Burke, Steve Castle, Jef De Brabander, Justin Du Bois, Greg Dudley, Paul Floreancig, Neil Garg, Timothy Jamison, Jeff Johnson, Jeff Johnston, Glen Micalizio, Jon Njardarson, Sarah Reisman, Richmond Sarpong, Karl Scheidt, Matthew Shair, Scott Snyder, Brian Stoltz, Regan Thomson, Chris Vanderwal, Lawrence Williams, Armen Zakarian.

■ Breakthroughs in catalysis have opened new doors for powerful synthetic methods

■ Previous efforts in total synthesis have provided a framework for new researchers to build on

■ The result is that highly complex targets are being synthesized with incredible efﬁciency Key Research Programs in Total Synthesis programs initiated from 1997-2008

■ Examples of powerful synthetic methods for total synthesis developed in the last 10 years

H H H H H HO Me O O water O O Me O 70 °C, 72 h HO O O O H H H H H 71% common ladder toxin subunit

Vilotijevic, I.; Jamison, T. J. Science 2007 317, 1189-1192 Key Research Programs in Total Synthesis programs initiated from 1997-2008

■ Examples of powerful synthetic methods for total synthesis developed in the last 10 years Me Bn

OH O O O MgBr TBSO H CO2t-Bu t-BuO2C TBS O H H CO2t-Bu OAc H CO2t-Bu THF t-BuO2C Bn CO H 2 equiv OTBS O 2 -78 to -45 °C O CO2H Me HO C 50% 2 OH

zaragozic acid C Nicewicz, D. A.; Satterﬁeld, A. D.; Schmitt, D. C. Johnson, J. S. J. Am. Chem. Soc. 2008 130, 17281-17283. Key Research Programs in Total Synthesis programs initiated from 1997-2008

■ Examples of powerful synthetic methods for total synthesis developed in the last 10 years

NBoc O NHBoc Me O O ·TBA N 1-Nap (-)-strychnine N SeMe N tBu N 82%, 97% ee PMB H PMB

20 mol%

Jones, S. B.; Simmons, B.; Mastracchio, A.; MacMillan, D. W. C. Nature 2011 850, 183-188. Key Research Programs in Total Synthesis programs initiated from 1997-2008

PhO2S O O H N H N O Me Me N S N Br Me N N O N S O Me Me N Me Me Me CoCl(PPh3)3 O N O S N N O N N S SO2Ph 46% Me N H N H Me O H O SO2Ph (+)-11,11’-dideoxyverticillin A

Kim, J.; Ashenhurst, J. A.; Movassaghi, M. Science 2011 324, 238-241. Key Research Programs in Total Synthesis programs initiated from 1997-2008

■ Advances in new methodologies and synthetic strategies have changed how we view total syntheses

■ Greater emphasis on striving for an “ideal synthesis”

■ To a growing extent, attaining the natural product is no longer the ﬁnal goal

■ Total synthesis is starting to become an auxiliary function of new research in chemistry The Future of Total Synthesis representation of what we strive to accomplish in total synthesis

Me Bn O H H N N Me S N O N S O H Me OAc O H H Me N O S N CO H H H Bn O 2 N S O CO2H O O N Me Me HO C H H 2 OH H O

zaragozic acid C strychnine (+)-11,11’-dideoxyverticillin A

■ Two syntheses outlined broadly applicable concepts (cascade catalysis, controlled oligimerization)

■ All outlined powerful methods to deliver the natural product in short order(10-15 steps)

■ Two syntheses are of molecules with promising bioactivity The Future of Total Synthesis representation of what we strive to accomplish in total synthesis

Me Bn O H H N N Me S N O N S O H Me OAc O H H Me N O S N CO H H H Bn O 2 N S O CO2H O O N Me Me HO C H H 2 OH H O

zaragozic acid C strychnine (+)-11,11’-dideoxyverticillin A The Future of Total Synthesis representation of what we strive to accomplish in total synthesis

Me Bn O H H N N Me S N O N S O H Me OAc O H H Me N O S N CO H H H Bn O 2 N S O CO2H O O N Me Me HO C H H 2 OH H O

zaragozic acid C strychnine (+)-11,11’-dideoxyverticillin A

Many - some would argue most - natural products can now be synthesized if suitable resources are provided. The challenge in synthesis is therefore increasingly not whether a molecule can be made, but whether it can be made in a practical fashion, in sufﬁcient quantities for the needs of research and/ or society, and in a way that is environmentally friendly if not ‘ideal’. -Paul Wender The Future of Total Synthesis representation of what we strive to accomplish in total synthesis

Me Bn O H H N N Me S N O N S O H Me OAc O H H Me N O S N CO H H H Bn O 2 N S O CO2H O O N Me Me HO C H H 2 OH H O

zaragozic acid C strychnine (+)-11,11’-dideoxyverticillin A

■ These represent premier total syntheses for our time

■ In general, these syntheses are atypical from most syntheses that are published in top journals

■ While they embody what we strive to accomplish as synthetic chemists, they are only a small but rapidly growing representation of current work in the ﬁeld of total synthesis The Future of Total Synthesis insights from three recent total syntheses of groups from three different era’s

■ Three molecules that highlight the perceived divisions for the modern role of total synthesis

■ Which natural products do we make?

■ All, some, any? Structurally interesting, biologically active?

■ What holds more value?

■ The structure, method employed, lessons learned, or future prospects?

Me OH Me O HO Me HO + O H2N H O H H H Me NH O O O HO H Me O OMe HN O HO HO O NH2 O OH N NH O HO O O H H O Me + OH O H NH2 O OMe O Cl Me AcO OAc OH Me OH

resiniferatoxin spongistatin 1 (+)-saxitoxin Wender's Synthesis of Daphnane Diterpene Orthoesters

O Me H O

Me O

O HO O

O OMe

resiniferatoxin OH

■ Plants containing DDOs have been used medicinally for over 2000 years

■ Many DDOs are leads for treatment of cancer, diabetes, neurodegenerative disease and pain.

■ Resiniferatoxin has advanced into Phase II clinical trials

■ Study and use of DDOs are hampered by supply and cost issues

Wender, P. A.; Buschmann, N.; Cardin, N. B.; Jones, L. R.; Kan, C.; Kee, J.-M.; Kowalski, J. A.; Longcore, K. E.; Nature Chem. 2011, 3, 615-619. Wender's Synthesis of Daphnane Diterpene Orthoesters the ﬁrst synthesis of a daphnane diterpene by Wender in 1997

O HO Me Me OAc H O OH Me Me H OBn Me O H OBn H H O HO O O Me O Ph RO O OTMS OTBS OMe O TBSO resiniferatoxin OH

OAc OAc OAc Me Me Me OBn OBn O H OBn O O O HO O OAc OTBS OTBS

■ Total synthesis featured 46 stop and go steps, tour de force ■ Key disconnections: oxidopyrilium cycloaddition. Enyne ring closure. Applied in highly complex system ■ Wender’s total synthesis is widely regarded as a “classic”

Wender, P. A.; Jesudason, C. D.; Nikahira, H.; Tamura, N.; Tebbe, A. L.; Ueno, Y. J. Am. Chem. Soc. 1997, 119, 12976-12977. Wender's Synthesis of Daphnane Diterpene Orthoesters function oriented synthesis

■ Original total synthesis not ideal from an efﬁciency or a structural diversiﬁcation perspective

Me PMP Me O Me R1 O Me R1 O O OH H O H O H Me O OR H Me O Me O HO O R2 O O O HO OR O HO O O OMe O O

O PMP OH major subset of DDOs resiniferatoxin (X = H, 73 congeners)

■ Key question: ■ Is a more structurally diverse DDO collection accessible to probe function? ■ Can analog synthesis reveal a more synthetically accessible structure that retains function

■ Original total synthesis not ideal from an efﬁciency or a structural diversiﬁcation perspective

O Me H O

Me O

O HO O

O OMe

OH resiniferatoxin

■ Key question: ■ Is a more structurally diverse DDO collection accessible to probe function? ■ Can analog synthesis reveal a more synthetically accessible structure that retains function

PMP Me Me O R1 O R OH 1 O H OBn H O H Me O Me O R2 O O O O HO OR O HO O

O PMP major subset of DDOs general precursor (X = H, 73 congeners) (22 steps from commercial)

Me Me Me HO O OAc OH O Me H OBn OBn H OBn O O vs H O H O O

Ph HO TBSO OTBS Br TBSO TBSO

10 steps from tartrate revised cycloadduct original cycloadduct

PMP Me O Me Me Me O AcO AcO AcO OH O O O Me Me Me H OBn H H O H O H O Me B O O O O Me Ph Me Ph Me Ph (19 steps) O O O O O O HO OH O HO OH O HO OH O HO HO HO O PMP general precursor (22 steps) 1 2 3

PKC afﬁnity, Ki (nM)a Cellular growth inhibition

A549 EC50 (nm)b K562 EC50 (nm)c 1 0.48 +/- 0.07 150 +/- 30 7 +/- 1 2 343 +/- 6 > 10,000 > 10,000 3 1.6 +/- 0.1 1500 +/- 60 87 +/- 5

aPKC = protein kinase C, a family of serine/threonine kinases bA549 = human lung carcinoma cK562 = human chronic myleogenous leukaemia.

■ Screen of analogs revealed the high potency of DDO’s as a ligand for PKC

■ Carries the potential for treatment of cancer, alzheimers, and AIDS. Wender's Synthesis of Daphnane Diterpene Orthoesters probing the function of the “B” ring

PMP Me O Me Me Me O AcO AcO AcO OH O O O Me Me Me H OBn H H O H O H O Me B O O O O Me Ph Me Ph Me Ph (19 steps) O O O O O O HO OH O HO OH O HO OH O HO HO HO O PMP general precursor (22 steps) 1 2 3

PKC afﬁnity, Ki (nM)a Cellular growth inhibition

A549 EC50 (nm)b K562 EC50 (nm)c 1 0.48 +/- 0.07 150 +/- 30 7 +/- 1 2 343 +/- 6 > 10,000 > 10,000 3 1.6 +/- 0.1 1500 +/- 60 87 +/- 5

aPKC = protein kinase C, a family of serine/threonine kinases bA549 = human lung carcinoma cK562 = human chronic myleogenous leukaemia.

■ An assay against both cancer cell lines reveals the importance of the epoxide stereochemistry

■ Interestingly, the simpliﬁed des-epoxy analog is active Wender's Synthesis of Daphnane Diterpene Orthoesters probing the function of the “B” ring

PMP Me O Me Me Me O AcO AcO AcO OH O O O Me Me Me H OBn H H O H O H O Me B O O O O Me Ph Me Ph Me Ph (19 steps) O O O O O O HO OH O HO OH O HO OH O HO HO HO O PMP general precursor (22 steps) 1 2 3

■ Calculations showed a preservation of the oxygen spatial arrangement between 1 and 3

■ The β-epoxide of 2, signiﬁcantly perturbs the orientation of the hydoxymethyl relative to 1 Wender's Synthesis of Daphnane Diterpene Orthoesters a model for the future?

PMP Me O Me Me Me O AcO AcO AcO OH O O O Me Me Me H OBn H H O H O H O Me B O O O O Me Ph Me Ph Me Ph (19 steps) O O O O O O HO OH O HO OH O HO OH O HO HO HO O PMP general precursor (22 steps) 1 2 3

■ Original synthesis was a tour de force, 46 steps, of an incredibly complicated molecule

■ They delivered an improved synthesis of a more complicated and functionally versatile molecule

■ Is the tour de force synthesis relevant if it delivers additional compound for testing?

■ Is the second generation route more valuable than the synthesis of another natural product?

■ Does a molecules potential for societal impact alter how we perceive its total synthesis? Smiths' Synthesis of the Spongistatins

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O X Me AcO OAc Me OH spongistatin 1 X = Cl spongistatin 2 X = H

Smith, A. B., III; Doughty, V. A.; Lin, Q.; Zhuang, L; McBriar, M. D.; Boldi, A. M.; Moser, W. H.; Murase, N.; Nakayama, K. Angew. Chem. Int. Ed. 2001, 40, 196-199. Smith, A. B., III; Lin, Q.; Doughty, V. A.; Zhuang, L; McBriar, M. D.; Kerns, J. K.; Brook, C. S.; Murase, N.; Nakayama, K. Angew. Chem. Int. Ed. 2001, 40, 197-201. Smith, A. B., III; Zhu, W.; Shirakami, S.; Sfouggatakis, C.; Doughty, V. A.; Bennett, C. S.; Sakamoto, Y. Org. Lett. 2003, 5, 761-764. Smiths' Synthesis of the Spongistatins history of the spongistatins

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O X Me AcO OAc Me OH

spongistatin 1 X = Cl spongistatin 2 X = H

■ Isolated in the early 1990's by the Pettit, Fusetani, and Kitagawa laboratories

■ Pettit’s attempted re-isolation delivered 35 mg of spongistatin 1 from 13 TONS of sponge!

■ Two spiroketals, two tetrahydropyrans, hemiketal, 42 membered macrolide Smiths' Synthesis of the Spongistatins history of the spongistatins

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O X Me AcO OAc Me OH

spongistatin 1 X = Cl spongistatin 2 X = H

■ Spongistatin 1 has been recognized as one of the most selective cytotoxic agents known

■ Average IC50 value of 0.12 nM against the NCI panel of 60 human cancer cell lines

■ Proposed to bind β-tubulin near, but distinct from, the vinca domain where vinca alkaloids bind Smiths' Synthesis of the Spongistatins history of the spongistatins

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O X Me AcO OAc Me OH

spongistatin 1 X = Cl spongistatin 2 X = H

■ Promising therapeutic potential and daunting structure drew much interest as a synthetic target

■ Total syntheses of 1 and 2: Kishi & Evans (1998), Smith, Paterson, Crimmins, Ley, Heathcock and others

■ Smith completed the total synthesis of spongistatin 2 in 2001 and 1 in 2003 (48 longest linear steps) Smiths' Synthesis of the Spongistatins Smiths’ ﬁrst generation synthesis

■ Retrosynthetic analysis

OH Me OTBS HO E Me HO PPh3I O MeO E O H H H H TESO Me O O O H H H H D OMe Me F O O O HO H D OMe O C F OH OTMS OTIPS TBSO C O H O H O OTMS O H Me H O OH O H Me A O O H Cl Me OTMS A O AcO B Me OAc Cl AcO B Me OH OAc Me OH

■ Late stage Yamaguchi macrolactonization to form the macrocycle

■ A Wittig oleﬁnation unites the eastern and western halves of the molecule Smiths' Synthesis of the Spongistatins Smiths’ ﬁrst generation synthesis

■ Retrosynthetic analysis

OH Me OTBS HO E Me HO PPh3I O MeO E O H H H H TESO Me O O O H H H H OMe Me F O O O HO H OMe O F OH OTMS OTIPS TBSO O H O H O OTMS O H Me H O OH O H Me O O H Cl Me OTMS O AcO Me OAc Cl AcO Me OH OAc Me OH

Me Me SnBu OTBS 3 S O O Me O OTES MeO E H OBn TESO Me S O O OTMS H H F E Me OBn Cl O F OPMB OTES Smiths' Synthesis of the Spongistatins Smiths’ ﬁrst generation synthesis

■ Retrosynthetic analysis

OH Me OTBS HO Me HO PPh3I O MeO O H H H H TESO Me O O O H H H H D OMe Me O O O HO H D OMe O C OH OTMS OTIPS TBSO C O H O H O OTMS O H Me H O OH O H Me A O O H Cl Me OTMS A O AcO B Me OAc Cl AcO B Me OH OAc Me OH Smiths' Synthesis of the Spongistatins Smiths’ ﬁrst generation synthesis

■ Retrosynthetic analysis BnO O H H H BPSO O OMe O D D OMe TBSO C O MeO H O OTIPS TBSO O C Me A I O O H Me B OBOM O H Me H O Me Me OH O H Me A O Me PhO S ODMB AcO 2 B OAc Me OH Smiths' Synthesis of the Spongistatins Smiths’ ﬁrst generation synthesis

■ Retrosynthetic analysis BnO O H H H BPSO O OMe O OMe TBSO O MeO H O OTIPS TBSO O Me A I O O H Me B OBOM O H Me H O Me Me OH O H Me A O Me PhO S ODMB AcO 2 B OAc Me OH

Me Me OH OH OH OTBS OH O O O Me A S S Me S S O OTs BPSO BPSO B TBS Smiths' Synthesis of the Spongistatins Smiths’ ﬁrst generation synthesis

■ Retrosynthetic analysis BnO O H H H BPSO O OMe O D D OMe TBSO C O MeO H O OTIPS TBSO O C Me I O O H Me OBOM O H Me H O Me Me OH O H Me O Me PhO S ODMB AcO 2 OAc Me OH

BnO S S H Me Me O Me D OMe TBSO TBSO OMe O S S Me TBSO O O C BnO O O H D C I DMP

Me Me O OTBS O O O BnO S S

TBS Smiths' Synthesis of the Spongistatins anion relay chemistry

■ A versatile method for polyketide synthesis - used to form AB and CD fragments

O S S S S

TBS dianion phosgene equivalent umpole

S S OTBS S S S S R3Si S S base O 1,3-Brook R TBS TBS O rearrangement R R bifunctional nucleophile “anion relay” linchpin

OH O OTBS OH OTBS S S R1 R R R O 1 readily accessible R1 Smiths' Synthesis of the Spongistatins review of their initial synthetic efforts

■ Overview of their entire synthesis and strategy

OH Me Wittig (64%) OTBS HO O H HO Me O PPh I H H H H MeO 3 Me TESO O O O O OMe H OMe H H Me OTIPS TBSO O HO O O O H OH O H O 48 steps H OTMS H O H O Me Me OTMS O H OH O H O O Me Cl Me 5 steps AcO AcO OTMS OAc OAc Cl Me OH Me OH

Yamaguchi (81%) Alkylation (92%) 24 steps, 0.3% 43 steps

O H H O OMe BPSO TBSO O O MeO H O H Me I O O Me Me 19 steps Me OTES Me

PhO2S OAc 11 steps 24 steps Smiths' Synthesis of the Spongistatins review of their initial synthetic efforts

■ Overview of their synthesis and strategy

OH Me Wittig (64%) OTBS HO O H HO Me O PPh I H H H H MeO 3 Me TESO O O O O OMe H OMe H H Me OTIPS TBSO O HO O O O H OH O H O H OTMS H O H O Me Me OTMS O H OH O H O O Me Cl Me 5 steps AcO AcO OTMS OAc OAc Cl Me OH Me OH

Yamaguchi (81%) Alkylation (92%) 24 steps, 0.3% 43 steps

■ Versatile synthetic route that accommodated necessary changes in routes and strategies

■ Methods employed in the synthesis were designed to access many structurally diverse natural products

■ Methods employed were not ideally suited for this speciﬁc molecule

■ Allowed them to complete the total synthesis, not ideal for scale up or analog synthesis Smiths' Synthesis of the Spongistatins review of their initial synthetic efforts

■ Overview of their synthesis and strategy

Yamaguchi (81%) Alkylation (92%) 24 steps, 0.3% 53 steps

■ Should we attempt the total synthesis of molecules this large and complex?

■ Are these types of tour de force syntheses worth undertaking in 2012?

■ Should versatile methods (diverse array of accessible structures) continue to be employed?

■ Should methods be more ideally suited (and scalable) for a speciﬁc molecule? Smiths' Synthesis of the Spongistatins review of their second generation synthesis

■ Vastly Improved Second Generation Synthesis

OH Me Wittig (Crimmins, 64%) OTBS HO O H HO Me O PPh I H H H H MeO 3 Me TESO O O O O OMe H OMe H H Me OTIPS TBSO O HO O O O H OH O H O H OTMS H O H O Me Me OTMS O H OH O H O O Me Cl Me 4 steps AcO AcO OTMS OAc OAc Paterson Aldol Cl Me OH Me OH Evans, Crimmins 1.009 g prepared Paterson, Heathcock 24 steps, 9.5% yield 22 steps, 6.5% yield 5.8 g prepared

■ Took cues from previous syntheses to revise their overall retrosynthetic strategy

■ Adopted changes to fragment syntheses that were more speciﬁcally tuned toward this molecule

■ Vastly improved efﬁciency and scalability

Smith, A. B., III; Tomioka, T.; Risatti, C. A.; Sperry, J. B.; Sfouggatakis, C. Org. Lett. 2008, 10, 4359-4362. Smiths' Synthesis of the Spongistatins review of their second generation synthesis

■ Vastly Improved Second Generation Synthesis

OH Me OTBS HO O H HO Me O PPh I H H H H MeO 3 Me TESO O O O O OMe H OMe H H Me OTIPS TBSO O HO O O O H OH O H O H OTMS H O H O Me Me OTMS O H OH O H O O Me Cl Me 4 steps AcO AcO OTMS OAc OAc Cl Me OH Me OH 1.009 g prepared 24 steps, 9.5% yield 22 steps, 6.5% yield

O OH O H O 20% (L)-proline Me BPSO O BPSO H H H DMF, 4 °C Me Me Me OBn 84%, 5:1 anti/syn OBn 68 g prepared Smiths' Synthesis of the Spongistatins review of their second generation synthesis

■ Vastly Improved Second Generation Synthesis

■ How important is efﬁciency in a gram scale total synthesis of a bioactive natural product of low availability?

■ Do 2nd Gen syntheses have value for identifying more robust methods (proline aldol vs dithiane)?

■ Since earlier tour de force efforts enabled a highly efﬁcient synthesis, do they hold more value? Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ What was known about structural features required for activity

OH OH Me Me C23 epimer HO HO (200 nm) HO HO O O H H H H H H Me Me O O O O H OMe H OMe O HO O HO O O OH OH O H O H H O H O Me Me OH O H OH O H O O Cl Me Cl Me AcO AcO OAc OAc Me OH Me OH

spongistatin 1 Diene section required for activity Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ What was known about structural features required for activity

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O Cl Me AcO OAc Me OH

spongistatin 1 Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ Appeared that the CD spiroketal wasn’t critical but does play some role

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O Cl Me AcO OAc Me OH

spongistatin 1 Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ Appeared that the CD spiroketal wasn’t critical but does play some role

OH OH Me Me HO HO HO HO O O H H H H H Me Me O O O H OMe H O HO O O OH OH O H O H O H Me OH O H OH O H O O Cl Me Cl ( ) AcO AcO 5 OAc Me OH Me OH

spongistatin 1 480 nm (Heathcock) Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ Appeared that the CD spiroketal wasn’t critical but does play some role

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O Cl Me AcO OAc Me OH

spongistatin 1 Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ Also appears that the AB spiroketal wasn’t critical but does play some role

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O Cl Me AcO OAc Me OH

spongistatin 1 Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ Also appears that the AB spiroketal wasn’t critical but does play some role

OH OH Me Me HO HO HO O HO H H H O Me H H O O Me H OMe O H O HO O O OH O H OH H O O Me OH O H O OH Cl Me AcO Cl OAc Me OH

spongistatin 1 460 nm (Heathcock) Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ Also appears that the AB spiroketal wasn’t critical but does play some role

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O Cl Me AcO OAc Me OH

spongistatin 1 Smiths' Synthesis of the Spongistatins analog syntheses from multiple groups provide insight regarding bioactivity

■ The “western” portion of spongistatin (diene & E, F rings) constitute the recognition domain

■ The “eastern” portion of spongistatin (A,B and C,D-spiroketals) imparts conformational restraints on the western portion

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O Cl Me AcO OAc Me OH

spongistatin 1 Smiths' Synthesis of the Spongistatins examining the conformational restraint hypothesis

■ How can the hypothesis of conformational restraint imparted by the eastern half be tested?

■ Random analog synthesis seemed cumbersome and unattractive

■ Molecular modeling may provide some insights

OH Me HO HO O H H H Me O O H OMe O HO O OH O H H O Me OH O H O Cl Me AcO OAc Me OH

spongistatin 1

Smith, A. B., III; Risatti, C. A.; Atasoylu, O.; Bennett, C. S.; Liu, J.; Cheng, H.; TenDyke, K. Xu, Q. J. Am. Chem. Soc. 2011, 133, 14042-14053. Smiths' Synthesis of the Spongistatins insights from molecular modeling

■ Molecular modeling revealed two major an two minor conformations

■ Chloroform - “Flat” maximized intramolecular hydrogen bonds

■ Water - “Twisted” oxygens oriented toward solvent Smiths' Synthesis of the Spongistatins insights from molecular modeling

■ Molecular modeling revealed two major an two minor conformations

■ DMSO & Acetonitrile - “Saddle”

■ Kitagawa original solution state structure from isolation report Smiths' Synthesis of the Spongistatins molecular dynamics simulations

■ Focused on water as the solvent

■ Used molecular dynamics simulations to identify rigid and ﬂexible regions

■ Lead to the development of “DISCON” (DIstrubution of Solution CONformations) MD software Smiths' Synthesis of the Spongistatins molecular dynamics simulations

■ Focused on water as the solvent

■ Used molecular dynamics simulations to identify rigid and ﬂexible regions

■ Lead to the development of “DISCON” (DIstrubution of Solution CONformations) MD software

■ Red = Rigid, Blue = Intermediate, Green = Flexible; Number indicates bond pair where torsions change together Smiths' Synthesis of the Spongistatins molecular dynamics simulations

■ The EFAB region is extremely rigid whereas the CD region is very ﬂexible

■ The only rigidity in the “eastern” half comes from the CD spiroketal

■ The ends of the EFAB region tend to move as a single unit

■ Red = Rigid, Blue = Intermediate, Green = Flexible; Number indicates bond pair where torsions change together Smiths' Synthesis of the Spongistatins molecular dynamics simulations

■ The EFAB region is extremely rigid whereas the CD region is very ﬂexible

■ The only rigidity in the “eastern” half comes from the CD spiroketal

■ The ends of the EFAB region tend to move as a single unit

■ Explains why the C23 epimer results in loss of activity even as its not involved in recognition

OH Me C23 epimer HO E (200 nm) HO O H H H Me O O H F D OMe O HO O C OH O H H O Me OH O H O Cl A Me AcO B OAc Me OH Smiths' Synthesis of the Spongistatins molecular dynamics simulations

■ An overlay of all the most populated solution state conformations is revealing

■ The rigid EFAB region is highly conserved Smiths' Synthesis of the Spongistatins molecular dynamics simulations

■ The tether employed by Heathcock didn’t impart enough conformational restraint on the EFAB region

■ Can an appropriate tether be designed to simplify the structure while maintaining activity?

OH OH Me Me HO HO HO HO O O H H H H H Me Me O O O H OMe H O HO O O OH OH O H O H O H Me OH O H OH O H O O Cl Me Cl ( ) AcO AcO 5 OAc Me OH Me OH

spongistatin 1 ABEF analog (Heathcock)

< 1 nm 480 nm (Heathcock) Smiths' Synthesis of the Spongistatins computationally aided analog design

■ Computationally aided analog design found ABEF analog with high structural homology to spongistatin

OH OH Me Me HO E E HO E HO HO O O H H H H H Me Me O O O H OMe H F F F O HO O O O OH OH O H O H O H O Me OH O H OH O H O O Cl Me Cl AcO AcO OAc Me OH Me OH

spongistatin 1 ABEF analog (Smith)

macrolide strain energy = 8.3 kJ/mol macrolide strain energy = 9.1 kJ/mol Smiths' Synthesis of the Spongistatins computationally aided analog design

■ Computationally aided analog design found ABEF analog with high structural homology to spongistatin

OH OH Me Me HO E E HO E HO HO O O H H H H H Me Me O O O H OMe H F F F O HO O O O OH OH O H O H O H O Me OH O H OH O H O O Cl Me Cl AcO AcO OAc Me OH Me OH

spongistatin 1 ABEF analog (Smith)

macrolide strain energy = 8.3 kJ/mol macrolide strain energy = 9.1 kJ/mol Smiths' Synthesis of the Spongistatins a highly potent spongistatin analog

■ Similar activity present after having deleted nearly 1/3 of the original structure

■ ABEF analog determined to have the same mode of action

OH OH Me Me HO HO HO HO O O H H H H H Me Me O O O H OMe H O HO O O O OH OH O H O H O H O Me OH O H OH O H O O Cl Me Cl AcO AcO OAc Me OH Me OH

spongistatin 1 ABEF analog (Smith)

MDA-MB-435 HT-29 H522-T1 U937

spongistatin 0.0225 0.058 0.16 0.059

ABEF analog 82.8 161.2 297.2 60.5 Smiths' Synthesis of the Spongistatins a review of their analog work

OTBS OH Me Me PPh I MeO 3 HO TESO HO O O H H H H Me Me O O H CHO H OTES O OTIPS O OTES O OH O O H O H O 5 steps OTBS OH O H O H O Cl O Cl AcO AcO Me OH Me OTES

29 steps (20 steps shorter!!!)

■ Are the post total synthesis opportunities in computational chemistry and analog design worth the effort?

■ Do 2nd Gen syntheses have value for identifying more robust methods (proline aldol vs dithiane)?

■ Since earlier tour de force efforts enabled a highly efﬁcient synthesis, do they hold more value?

■ Are these types of projects worth undertaking in 2012? DuBois' Total Synthesis of Saxitoxin

+ H2N NH O HO HN HO O NH2 N NH

+ NH2

(+)-saxitoxin

■ A highly oxidized and polar neurotoxic agent

■ Toxicity arises from disabling ionic conductance through voltage-gated sodium channel.

■ Exhibits nanomolar afﬁnity for binding the extracellular mouth of the ion channel.

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin

■ Why synthesize a neurotoxic agent?

+ H2N NH O HO HN HO O NH2 N NH

+ NH2

(+)-saxitoxin

■ Ion ﬂux is crucial for many important biochemical processes

■ Small molecules that modulate ion ﬂux may provide the discovery of new drugs

■ Chemically modiﬁed guanidinium toxin could be used to probe structure and function of ion channels

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin Rhodium catalyzed C-H amination

O O O O 2 mol% Rh2(OAc)2 S PhI(OAc)2, MgO S H2N O HN O

Me CH2Cl2 Me Me Me Me Me

Espino, C. G.; Wehn, P. M.; Chow, J.; Du Bois, J. J. Am. Chem. Soc. 2001, 123, 6935-6936.

+ H2N NH O HO HN HO O NH2 N NH

+ NH2 DuBois' Total Synthesis of Saxitoxin Rhodium catalyzed C-H amination

O O O O 2 mol% Rh2(OAc)2 S PhI(OAc)2, MgO S H2N O HN O

Me CH2Cl2 Me Me Me Me Me

Espino, C. G.; Wehn, P. M.; Chow, J.; Du Bois, J. J. Am. Chem. Soc. 2001, 123, 6935-6936.

O O O O S HN O S 0.3 mol% Rh2(esp)2 H2N O PhI(OAc)2, MgO O

O CH2Cl2 O Me O 76% Me Me Me

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin Rhodium catalyzed C-H amination

O O O O 2 mol% Rh2(OAc)2 S PhI(OAc)2, MgO S H2N O HN O

Me CH2Cl2 Me Me Me Me Me

Espino, C. G.; Wehn, P. M.; Chow, J.; Du Bois, J. J. Am. Chem. Soc. 2001, 123, 6935-6936.

O O O O S O O HN O S S 0.3 mol% Rh2(esp)2 H N O HN O 2 R ZnCl PhI(OAc)2, MgO O

O CH2Cl2 O BF3·OEt2 Me OH O 76% Me Me Me OTs

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin ﬁrst generation total synthesis

NMbs NMbs O O H N NH OH S 2 H N NH HN O 2 AgNO3 OH

11 steps NH2 i-PrNEt2 NH OH MbsN 65% N NMbs MeS N H OTs H

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin ﬁrst generation total synthesis

NMbs NMbs O O H N NH OH S 2 H N NH HN O 2 AgNO3 OH

11 steps NH2 i-PrNEt2 NH OH MbsN 65% N NMbs MeS N H OTs H

NMbs NMbs O H N NH NH2 O 1. Cl3CC(O)NCO 2 NH2 HO O OsCl3 HO O NH2 oxone, Na2CO3 2. K2CO3, MeOH NH N NH

62%, 12:1 + 82% N NMbs NH2 H

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin ﬁrst generation total synthesis

NMbs NMbs O O H N NH OH S 2 H N NH HN O 2 AgNO3 OH

11 steps NH2 i-PrNEt2 NH OH MbsN 65% N NMbs MeS N H OTs H

NMbs NMbs O H N NH NH2 O 1. Cl3CC(O)NCO 2 NH2 HO O OsCl3 HO O NH2 oxone, Na2CO3 2. K2CO3, MeOH NH N NH

62%, 12:1 + 82% N NMbs NH2 H

+ + H2N H2N NH O NH O DMSO, DCC HO H HN B(O2CCF3)3 HO HN pyridine·TFA HO O NH2 O NH2 70% TFA, 0 °C to rt N NH N NH

82% + + NH2 (+)-saxitoxin NH2 20 steps

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin ﬁrst generation recap

+ H2N NH O HO HN HO O NH2 N NH

+ NH2

(+)-saxitoxin

■ Utilizing their C-H amination method in a total synthesis fostered the development of a better catalyst

■ The ﬁrst enantioselective synthesis

■ Du Bois synthesis was longer than both the Kishi and Jacobi racemic syntheses (17 and 15 steps) DuBois' Total Synthesis of Saxitoxin rethinking their original route

NMbs O O S H2N NH OH HN O

11 steps NH2 OH MbsN

MeS N OTs H 14 steps

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin rethinking their original route

NMbs O O S H2N NH OH HN O

11 steps NH2 OH MbsN

MeS N OTs H 14 steps

PMB OH N PMB O O N OTBDPS MeO OH SMe NHBoc H OTBDPS NHBoc NHBoc N NMbs H N-Boc serine methyl ester

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin 14 step second generation total synthesis

PMB OH alkyne N O PMB O N i-PrMgCl OTBDPS MeO OH THF, -78 °C SMe NHBoc 3 steps H OTBDPS NHBoc 78%, 5:1 anti/syn NHBoc N NMbs H N-Boc serine methyl ester

NMbs + H2N H N NH 2 NH O OH HO H HN HO O NH NH 2 6 steps 4 steps N NH N NMbs + H NH2

10 steps vs 16 steps (+)-saxitoxin

Fleming, J. J.; McReynolds, M. D.; Du Bois, J. J. Am. Chem. Soc. 2007, 129, 9964-9975. DuBois' Total Synthesis of Saxitoxin second generation recap

+ H2N NH O HO HN HO O NH2 N NH

+ NH2

(+)-saxitoxin

■ Their second generation approach provided the most efﬁcient synthesis of the (+)-saxitoxin

■ The second generation synthesis was scalable, preparing 5 g of the 9 membered ring

■ Provided enough material to initiate ion channel studies. Saxitoxin as a Small Molecule Probe for Ion Channel Studies

■ Difﬁculty in chemically modiﬁng natural saxitoxin limits its use as a small molecular probe ■ Through de novo total synthesis an array of diverse molecular probes can be synthesized readily

7,8,9-guanidine residue proposed to bind the selectivity ﬁlter carbomyl group proposed to be H bond donor

Andersen, B. M.; Du Bois, J. J. Am. Chem. Soc. 2009, 131, 12524-12525. Saxitoxin as a Small Molecule Probe for Ion Channel Studies initial question

■ Is the carbamate, speciﬁcally as an H-bond donor, important for saxitoxin binding the ion channel?

+ + H2N H2N NH O NH O HO H HO H HN HN Me HO O NH2 HO O N N NH N NH Me

+ + NH2 NH2

(+)-saxitoxin N,N-dimethyl-(+)-saxitoxin Saxitoxin as a Small Molecule Probe for Ion Channel Studies initial question

■ Is the carbamate, speciﬁcally as an H-bond donor, important for saxitoxin binding the ion channel?

+ + H2N H2N NH O NH O HO H HO H HN HN Me HO O NH2 HO O N N NH N NH Me

+ + NH2 NH2

(+)-saxitoxin N,N-dimethyl-(+)-saxitoxin ■ Strategy: Remove the hydrogen bonds and measure the voltage across the ion channel Saxitoxin as a Small Molecule Probe for Ion Channel Studies initial question

■ Is the carbamate, speciﬁcally as an H-bond donor, important for saxitoxin binding the ion channel?

+ + H2N H2N NH O NH O HO H HO H HN HN Me HO O NH2 HO O N N NH N NH Me

+ + NH2 NH2

(+)-saxitoxin N,N-dimethyl-(+)-saxitoxin

■ Increasing conc. of both saxitoxin and N,N-dimethylsaxitoxin result in decreased peak current

carbomyl unit not likely a hydrogen bond donor Saxitoxin as a Small Molecule Probe for Ion Channel Studies initial question

■ Do further modiﬁcations to the carbomyl unit effect binding?

R IC50 (nM)

C7H15 26 +/- 3

+ H2N i-Pr 83 +/- 13 NH O H HO + HN R C6H12NH3 19 +/- 0.8 HO O N N NH H - C5H10CO2 135 +/- 7

+ NH2 O

N 87 +/- 9 H

■ Despite steric, electronic, and polar modiﬁcations, all retained activity within 1-1.5 orders of magnitude

■ Allowed for the installation of the first saxitoxin photoaffinity probe Saxitoxin as a Small Molecule Probe for Ion Channel Studies additional modifications to the carbamate

■ Use of a carbamate tethered amine will allow installation of structurally complex payloads ■ Fluorogenic groups ■ Cofactors ■ Having access to synthetic saxitoxin should provide unique insights in ion channel structure and function

+H N + 2 H2N NH O NH O p-FC6H4C(O)NHS HO H H HO H HN N HN + NH3 HO O N ( )5 HO O N ( )5 N NH H N NH H F CH3CN/H2O NH + + 2 NH2 pH = 9.5

IC50 = 46 +/- 7 nm

O O N O O F

p-FC6H4C(O)NHS DuBois' Total Synthesis of Saxitoxin overview of saxitoxin synthesis

+ H2N NH O HO HN HO O NH2 N NH

+ NH2

(+)-saxitoxin

■ Their initial synthesis enabled a very elegant and scalable synthesis of an important molecule

■ Application of their chemistry toward a total synthesis identiﬁed a better C-H amination catalyst

■ The result of their work enabled a new area of academic research on ion channels. DuBois' Total Synthesis of Saxitoxin overview of saxitoxin synthesis

+ H2N NH O HO HN HO O NH2 N NH

+ NH2

(+)-saxitoxin

■ Should total syntheses be used to apply methodology if the resulting initial synthesis isn’t the “best”?

■ Is post synthetic research becoming mainstream? The Future of Total Synthesis summary of themes from selected examples

■ Three molecules that highlight the perceived divisions for the modern role of total synthesis

■ Which natural products do we make?

■ All, some, any? Structurally interesting, biologically active?

■ What holds more value?

■ The structure, method employed, lessons learned, or future prospects?

Me OH Me O HO Me HO + O H2N H O H H H Me NH O O O HO H Me O OMe HN O HO HO O NH2 O OH N NH O HO O O H H O Me + OH O H NH2 O OMe O Cl Me AcO OAc OH Me OH

resiniferatoxin spongistatin 1 (+)-saxitoxin The Future of Total Synthesis ﬁnal thoughts

■ Wender’s synthesis of resiniferatoxin

■ Which natural products to we make?

■ A tour de force synthesis is worth undertaking if the target is important and the goal of the research is to understand the SAR of the molecule to provide new therapeutic leads

■ What holds more value?

■ The structure and future prospects are what drives the value in these types of syntheses

Me OH Me O HO Me HO + O H2N H O H H H Me NH O O O HO H Me O OMe HN O HO HO O NH2 O OH N NH O HO O O H H O Me + OH O H NH2 O OMe O Cl Me AcO OAc OH Me OH

resiniferatoxin spongistatin 1 (+)-saxitoxin The Future of Total Synthesis ﬁnal thoughts

■ Smiths synthesis of spongistatin 1

■ Which natural products to we make?

■ Focused efforts toward very complex and important molecules offer a testing ground for synthetic methods and provided multiple opportunities for post total synthesis research

■ What holds more value?

■ The structure, method employed, lessons learned, and future prospects all provided value

Me OH Me O HO Me HO + O H2N H O H H H Me NH O O O HO H Me O OMe HN O HO HO O NH2 O OH N NH O HO O O H H O Me + OH O H NH2 O OMe O Cl Me AcO OAc OH Me OH

resiniferatoxin spongistatin 1 (+)-saxitoxin The Future of Total Synthesis ﬁnal thoughts

■ Du Bois synthesis of saxitoxin

■ Which natural products to we make?

■ Focused efforts toward very complex and important molecules often leads to improvements in synthetic methods and provide opportunities for post total synthesis research

■ What holds more value?

■ Methods employed, lessons learned, and future prospects drove the value of this program

Me OH Me O HO Me HO + O H2N H O H H H Me NH O O O HO H Me O OMe HN O HO HO O NH2 O OH N NH O HO O O H H O Me + OH O H NH2 O OMe O Cl Me AcO OAc OH Me OH

resiniferatoxin spongistatin 1 (+)-saxitoxin The Future of Total Synthesis ﬁnal thoughts

■ All three examples entailed focused research programs directed toward a single natural product

■ They all provided additional supplies of valuable targets that initiated further research

■ They all encountered pitfalls in synthetic strategies that facilitated future focused efforts

■ They all generated an improved synthetic transformation or method for fragment synthesis

Me OH Me O HO Me HO + O H2N H O H H H Me NH O O O HO H Me O OMe HN O HO HO O NH2 O OH N NH O HO O O H H O Me + OH O H NH2 O OMe O Cl Me AcO OAc OH Me OH

resiniferatoxin spongistatin 1 (+)-saxitoxin The Future of Total Synthesis ﬁnal thoughts

■ Powerful new methods will continue to push toward the ideal total synthesis

Me Bn O H H N N Me S N O N S O H Me OAc O H H Me N O S N CO H H H Bn O 2 N S O CO2H O O N Me Me HO C H H 2 OH H O

zaragozic acid C strychnine (+)-11,11’-dideoxyverticillin A The Future of Total Synthesis ﬁnal thoughts

■ Focused efforts toward a single natural product will continue to be a productive area of research

Me OH Me O HO Me HO + O H2N H O H H H Me NH O O O HO H Me O OMe HN O HO HO O NH2 O OH N NH O HO O O H H O Me + OH O H NH2 O OMe O Cl Me AcO OAc OH Me OH resiniferatoxin spongistatin 1 (+)-saxitoxin

new areas of research The Future of Total Synthesis ﬁnal thoughts

■ Focused efforts toward a single natural product will continue to be a productive area of research

Last total syntheses to be published in Science O H OAc H N Me Me Me OH NMe 2 S N Me Me H H H OH H N S O Me NH Me Me OH 2 O S N OH N S OH O OH O O HO O OH N H Me H O prostratin (Wender) (-)-doxycycline (Myers) dideoxyverticillin A (Movassaghi)

new areas of research The Future of Total Synthesis ﬁnal thoughts

■ Groups that undertake impractical syntheses of many different targets will become irrelevant The Future of Total Synthesis ﬁnal thoughts

■ These sentiments are being increasingly observed across the spectrum of total synthesis

■ More focused efforts toward fewer targets is likely the future of total synthesis

Barry Trost (1965) David Evans (1967) Larry Overman (1970) Amos Smith (1972)

OH Me Me HO Me OR HO MeO2C O HO O O O H H H OAc Me O O OH Me O O H H OMe O HO OH O O Me O O AcO OH H O H Me H O Me H OH O H RO Me OH Me Me O Cl Me AcO OAc CO2Me bryostatin 1 macfarlandin Me OH spongistatin 1 The Future of Total Synthesis ﬁnal thoughts

■ These sentiments are being increasingly observed across the spectrum of total synthesis

■ More focused efforts toward fewer targets is likely the future of total synthesis

K. C. Nicolaou (1976) Paul Wender (1976) Dale Boger Stuart Schreiber R Me Me HO O Cl OR MeO2C O O OAc O O Me Me HO OH OH Me Cl O O O H H H H O N N N OH N N N Me Me O O Me OH H H H Me HN O O O Me HO OH HO C RO Me OH O 2 NH2 Me

CO2Me OH bryostatin 1 prostratin HO OH vancomycin The Future of Total Synthesis ﬁnal thoughts

■ These sentiments are being increasingly observed across the spectrum of total synthesis

■ More focused efforts toward fewer targets is likely the future of total synthesis

Andrew Myers (1986) Scott Rychnovsky (1988) Peter Wipf (1990) John Wood (1993)

Me Me Me Me OH NMe2 H H H Me OH H Me H NH2 OH H OH O OH O O HO

doxycyclin cholesterol The Future of Total Synthesis ﬁnal thoughts

■ These sentiments are being increasingly observed across the spectrum of total synthesis

■ More focused efforts toward fewer targets is likely the future of total synthesis

+ H2N Me OMe O OMe NH O OH HO N HN H H HO O NH2 OH O Me Me Me N NH H Me HO + NH2 H O OH irciniastatin A saxitoxin (Du Bois) HO Me (Floreancig, De Brabander) OH O Me O OH HO Me O OH H Me H OH OH N Me O OH H H H HO O OH OH OH OH O O N Me H Me H OH Me

OO O O Me O OH Me cephalostatin 1 (Shair) amphotericin B (Burke) HO OH Me NH2 The Future of Total Synthesis ﬁnal thoughts

■ If the overall goal is for chemistry to beneﬁt society and if natural products are to play a role...

■ Continuing to strive for new reactions will deliver increasingly complex targets in short order

■ Applying methods in complex settings will lead to better and more useful methods

■ Focused efforts toward fewer targets will lead to better targets and more active areas of research

■ Whether or not total synthesis directly beneﬁts society, and thus it’s future, depends on the targets we choose and what we choose to do with those targets... The Future of Total Synthesis ﬁnal thoughts

■ If the overall goal is for chemistry to beneﬁt society and if natural products are to play a role...

■ Continuing to strive for new reactions will deliver increasingly complex targets in short order

■ Applying methods in complex settings will lead to better and more useful methods

■ Focused efforts toward fewer targets will lead to better targets and more active areas of research

■ Whether or not total synthesis directly beneﬁts society, and thus it’s future, depends on the targets we choose and what we choose to do with those targets...

which is entirely up to us