total synthesis of salinosporamide a - ccc/upcmldccc.chem.pitt.edu/wipf/current...

NH

O

O

O

Cl

H

OH

Me

Salinosporamide A

Total Synthesis of Salinosporamide A

Endo, A.; Danishefshy, S. J. JACS 2005, 127, 8298.

Joshua Pierce

Wipf Group

January 21, 2006

Josh Pierce @ Wipf Group 1 1/22/2006

Isolation

Fenical et. al. ACIE, 2003, 42, 355.

Omura et. al. J. Antibiot. 1991, 44, 113.

2,500 strains of bacteria belonging to a new taxon, which has been given the genus name

Salinospora, have been isolated from tropical or subtropical ocean systems. Preliminary

screening of the organic extracts from cultured Salinospora strains showed antibiotic and

anticancer activity, causing an increased interest in exploiting these bacteria for drug discovery.

Salinosporamide was isolated by Fenical and co-workers from the strain CNB-392 and showed

potent cancer cell cytotoxicity, apparently through its inhibition of the 20S proteasome.

Salinosporamide shares its core structure with Omuralide, a transformation product of the

microbial metabolite Lactacystin originally isolated by Omura and co-workers from terrestrial

Streptomyces. In early biological testing it has shown to be significantly more active than either

Lactacystin or Omuralide.

NH

O

O

O

Cl

H

OH

Me

NH

OMe

O

OOH

Salinosporamide A Omuralide

NH

Me

O

OH OH

O

S

NHAcHO2C

Lactacystin


Proteasome Function

Nobel Prize in Chemistry 2004:Aaron Ciechanover, Avram Hershko and Irwin Rose

The proteins that are to be destructed are markedwith a label - the molecule ubiquitin. Proteins solabelled are then rapidly broken down in

cellular "waste disposers”called proteasomes.

1.The E1 enzyme activates the ubiquitin molecule.This reaction requires energy in the form of ATP.

2.The ubiquitin molecule is transferred to a differentenzyme, E2.

3.The E3 enzyme can recognise the protein target which isto be destroyed. The E2-ubiquitin complex binds so near to

the protein target that the actual ubiquitin label can betransferred from E2 to the target.

4.The E3 enzyme now releases the ubiquitin-labelled protein.

5.This last step is repeated until the protein has a short chainof ubiquitin molecules attached to itself.

6.This ubiquitin chain is recognised in the opening of the

proteasome. The ubiquitin label is disconnected and the proteinis admitted and chopped into small pieces.

http://nobelprize.org/chemistry/laureates/2004/public.html


Current Proteasome Inhibitors

N

N

O

NH

O

HN B

OH

OH

NH

Me

O

OH OH

O

S

NHAcHO2C

O

NO

NH

O

HN

HO

OHN

O

O

Peptide aldehydes, such as Z-Leu-Leu-Leu-H, inhibit

proteasome activity in a potent but reversible manner.

O

O

NH

O

HN

O

NH

O

H

Epoxomicin is a cell permeable, potent, and selective

proteasome inhibitor, more potent than lactacystin.Lactacystin is a natural, irreversible, nonpeptide, cell

permeable inhibitor that is more selective than peptide

aldehydes but less selective than peptide boronates.

Bortezomib (Velcade) is a peptide boronate that is

the first proteasome inhibitor to be marketed for the

treatment of cancer (multiple myeloma). Produced

by Millennium Pharmaceuticals. IC50 ~ 0.03 nM


Proteasome Inhibition by !-lactone Inhibitors

20S Core (!5 subunit is the target for

!-lactone class of inhibitors)

Lactacystin bound to terminal

active site threonine.

26S Proteasome

Structure

Huber et. al. Nature, 1997, 386, 463.


Initial Synthesis and SAR Studies

NH

O

OHHO

O

Me

H

Replaceable withEt, i-Pr, n-Bu, Bnbut NOT H

Replaceable with Me

Essential

Essential

Essential

20S Proteasome

NH

O

OHHO

Me

H

O

O

CONH(!-subunit)

Me

H2N

SAR Studies:

Li, W.; Corey, E. J. Chem. Pharm. Bull. 1999, 47, 1.

First total synthesis of Lactacystin was completed by Corey et. al.. Through this work, as well

as subsequent improvements by both the Corey group and groups such as Omura-Smith,

Baldwin and Chida allowed for the rapid synthesis of both Lactacystin and Omuralide - while

allowing for a variety of analogue projects to study SAR for this class of compounds.

First enantioselective synthesis of Lactacystin by Corey in 1998:

MeS

Me

CO2Me

CO2Me

Procine liver esterase

MeS

Me

CO2H

CO2Me

62%, 95% ee

NH

Me

O

OH OH

O

S

NHAcHO2C

17 linear steps,

13.4% yield

Corey, E. J.; Li, W.; Nagamitsu, T. ACIE, 1998, 37, 1676.


Corey’s Approach to Salinosporamide A

Reddy, L. R.; Saravanan, P.;

Corey, E. J JACS, 2004, 126, 6230.

MeO

O

HN

HO Me

CO2MeMeO

O

N

Me

CO2Me

MeOO

N

Me

CO2Me

OBn

MeO

HN

HO Me

CO2Me

OBn

PMB

N

HO Me

CO2Me

OBn

O

PMB

N

O Me

CO2Me

OBn

ONO

PMB

CO2Me

Me

OBn

NOPMB

CO2Me

O

OBn

OH Me

N

Si

O

O

PMBCO2Me

OBn

MeMe

H Me

N

Si

O

O

PMBCO2Me

MeMe

H MeCHO

p-TsOH

80%

LDA, THF-HMPA

ClCH2OBn 69%

NaCNBH3

AcOH 90%

1. TMSCl

2. Acrylyl Chloride

i-Pr2NEt then H+

96%

Dess-Martin

96%

1. Quinuclidine 90%

2. BrCH2Si(CH3)2Cl Et3N, DMAP 95% +

(1:9)Si Br

Bu3SnH, AIBN

89%

1. H2, Pd/C 95%

2. Dess-Martin 95%


Completion of the Synthesis

NH

O

O

O

Cl

H

OH

Me

NO

CO2Me

PMB

Me

OH

H

SiO

H

MeMe

HNO

CO2Me

Me

OH

H

OH

H

HO

ZnCl

88%, 20:1 dr

1. KF, KHCO3

H2O2 92%

2. CAN 83%

N

Si

O

O

PMBCO2Me

MeMe

H MeCHO

1. 3N LiOH-THF2. BOPCl, Py

3. Ph3PCl2, MeCN 65% (three steps)

17 linear steps, 13.4% yield


2nd Generation Approach

Me

O

X

- 2 PMe3

X

Cp2TiO Me

Cp2Ti(PMe3)2

XO

Cp2Ti

Me

H

Ph2SiH2X

Me OSi(H)Ph2Cp2Ti

H

+ 2 PMe3

X

Me OSi(H)Ph2

Me

Precedent for Cyclization:

Kablaoui, N. M.; Buchwald, S. L. JACS, 1996, 118, 3182.

Reddy, L. J.; Fournier, J.-F.; Subba Reddy, B. V.;

Corey, E.J. OL, 2005, 13, 2699.

20 steps,

17.1% yield

PMB

N

O Me

CO2Me

OBn

O

1. Ti(Oi-Pr)4, c-C5H9MgClt-BuOMe, 30 min

2. I2, 4 h3. NEt3, 30 min

NOPMB

CO2Me

HO

OBn

Me

83% yield> 99:1 dr

NOPMB

CO2Me

HO

OBn

Me

1. [MeTeAlMe2], 12 h2. Ph3PCl2, Py 89%

3. NEt3•3HF 92%

NH

O

O

O

Cl

OH

Me

NOPMB

CO2Me

HO MeHO

ODMIPS


Danishefsky’s Approach

NO

O

Ph

NO

O

Ph

BnO

NO

O

Ph

BnO

OH

HN

OH

O

BnO

OCO2Et

N CO2t-BuEtO

BnO

OCO2Et

NEtO

BnO

O

OCO2t-Bu

NO

HO

O

OCO2t-Bu

PMB

NO

HO

CO2t-Bu

PMB

CO2Bn

SePh

1. Vinylmagnesium Bromide, TMSCl, CuI 75%

2. LDA 77%, 14:1 dr

I

OBn

O3, then NaBH4

86%

1. ClCO2Et, Py 96%

2. TfOH quant

1. Jones Oxidation2. Me2NCH(Ot-Bu)2

72% (over 2 steps)3. Et3OBF4, K2CO3 88%

LHMDS 82%

1. 1 M HCl 90%2. PMBCl, NaH 60%

3. Pd(OH)2-C, H2 quant

1. PhSeSePh, NaBH4

EtOH

2. BnBr, K2CO3

65% (over 2 steps)

Endo, A.; Danishefshy, S. J. JACS 2005, 127, 8298.


Key Reaction and Completion of the

Synthesis

NO

O

CO2t-Bu

PMB

CO2Bn NO

CO2t-Bu

PMB

CO2Bn

O

BnO

SePh

NO CHO

PMB

CO2t-Bu

O

BnO

Me

NO

HO

CO2t-Bu

PMB

CO2Bn

SePh

1. H2O2

2. toluene (reflux)

94% (over 2 steps)3. Dess-Martin

1. PhSeBr, AgBF4

BnOH 74%

(12:1 mixture)

1. n-Bu3SnH, AIBN 98%

2. NaBH4 85%3. Dess-Martin 95%

ZnCl

88% 20:1 dr

NO

CO2t-Bu

PMB

O

BnO

Me

OH

H

HNO

CO2t-Bu

OHMe

OH

H

HO

NH

O

O

O

Cl

H

OH

Me

1. CAN 90%

2. Na/NH3

3. NaBH4 97% (over 2 steps)

1. BCl32. BOPCl, TEA

3. Ph3PCl2, Py 51% (over 3 steps)

28 linear steps, 1.8% yield


Inherent Problems - Future Directions

While !-lactone inhibitors are selective and potent, their half-life in solution at pH 7 is estimated

at 5 - 10 min. This, along with other factors, has led to lackluster in-vivo activity. To circumvent

this problem, Corey et. al. has synthesized a !-lactam that greatly increases the aqueous stability

while maintaining considerable proteasome inhibition (although at a much slower rate).

NH

NH

O

O

Cl

OH Hogan, P. C.; Corey, E. J. JACS, 2005, 127, 15386.

Although the Nobel Prize in Chemistry in 2004 emphasized the importance of the proteasome pathway,

the development of novel treatments that target this pathway have been lagging. While Velcade has

shown promise, its toxicity has been a major drawback. Even though proteasome inhibitors of this type

are very selective for the proteasome over other cellular targets, their toxicity stems from complete

shutdown of the cells waste disposal system.

Since their discovery, E3 ubiqutin ligases have been viewed as ideal drug targets. “Because each E3

is responsible for the destruction of a small number of proteins, specific inhibitors or E3s should be

highly effective drugs with few side effects.” -- Dr. J. Wade Harper, Harvard University, 2001

The success of this strategy lies in the ability to interrupt protein-protein interactions with small molecules -

something that most companies are not aggressively pursuing. (Roche’s Nutlins are key exception)

Garber, K. J. Nat. Cancer Inst. 2005, 97, 166. Adams, J. DDT, 2003, 8, 307.


total synthesis of salinosporamide a - ccc/upcmldccc.chem.pitt.edu/wipf/current...

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