molecules that changes the world

7/28/2019 Molecules That Changes the World

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Taxol

Chapter 25 1994


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he legend of Taxol reveals

a prime example of how nat-

ural products and chemical

V\QWKHVLV LQXHQFH WKLQJV

at the very heart of our lives. It is a tale

of hope for humanity which also brimsZLWK VFLHQWLF GUDPD DQG LQWULJXH

Taxol is one of the best-selling anti-

cancer drugs, saving and improving

the quality of countless lives around

the world. It is also a scarce natural

product with a beautiful and complex

molecular architecture that chemists

were able to isolate and construct in

the laboratory, thus learning about its

intricate properties and facilitating itsadvancement to the clinic as an effec-

WLYHFDQFHUJKWLQJGUXJ

Cancer is a word that instills adeep-seated fear because we imme-diately associate it with grave illnessand a high mortality rate. Almost all of

us know someone whose life has been

blighted by a cancer diagnosis, and whohas suffered the prolonged pain of theillness. Cancer patients are forced totolerate a tough treatment regime withall the accompanying side effects andsubsequent problems. Few people arefortunate enough to escape the dis-

tress of cancer over their lifetime, sincethe frightening statistics would suggestthat the vast majority of us will eitherH[SHULHQFHLWUVWKDQGRUKDYHDORYHGRQH DILFWHG+RZHYHU DVZH DGYDQFHour understanding of the mechanismsinvolved in causing and propagatingcancer, we are gradually uncovering ahost of new leads and hopes for cures.

Scientists and clinicians diligently andcontinuously harness such intelligenceand powerful resources, laboring to con-vert them into practical strides forward,giving us hope for a future where cancer

is not a death sentence, but a curabledisease. The story of the development

of Taxol provides a striking example ofKRZDGYDQFHVLQVHYHUDOHOGVRIVFLHQFHand technology can be combined to pro-vide new lifesaving therapies.

Cancer is a collection of a numberof rather disparate diseases, all charac-terized by the uncontrolled proliferationof abnormal cells which invade and dis-

rupt tissues, beginning locally and thenspreading through the body to extendthe reach of their destructive behavior.Both external causes (e.g. chemicals,

radiation, viruses) and internal factors(e.g. hormones, immune conditions,inherited genes), acting either alone orin combination, may be responsible for

the initiation and promotion of carcino-genesis. Furthermore, many years canpass between cause and detection, and,with some types of cancer, there existsthe obstinate problem of detecting malig-

Cancer cell

Taxus brevifolia

Taxol2 0 8

Taxol


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Julius Caesar

nant growths early enough

for intervention to standany chance of success.

The global impact ofcancer cannot be over-stated. It constitutes amajor public health prob-lem with an ever growing

worldwide occurrence.In 2003, approximately1.34 million people werediagnosed with cancer (notincluding basal and squamouscell skin cancers) and more than halfa million patients died from the diseasein the United States alone. The only con-dition attributed with more deaths per

annum is cardiovascular disease, which isresponsible for one in every four deathsin the industrialized world. The AmericanCancer Society estimates that men haveslightly less than a one in two lifetime

risk of developing cancer; for women therisk is slightly more than one in three.Quite apart from the personal suffering

caused by this high incidence, there isDQLPPHQVH QDQFLDO FRVW WRERWK WKHindividual and society as a whole in theform of direct medical expenses and lostproductivity. As a consequence, eachsmall step scientists make in advancingour ability to treat this heinous group of

diseases translates into a major unbur-dening for society, not to mention thelives saved.

The seriousness of the prob-

lem was recognized in theUnited States in the 1950s,prompting the rapid pas-sage of new legislation

over the ensuing twodecades as part of acampaign against thedisease. This legisla-tion included the 1971

National Cancer Act

which aimed to invig-orate cancer research

through a directed andsubstantial increase in

funding. As a result of thisact proclaiming the conquest of

cancer a national crusade, several newcancer centers were established aroundWKHFRXQWU\DQGUHVHDUFKLQWKHHOGZDV

DFFHOHUDWHGFRQVLGHUDEO\+RZHYHUZKLOHthe long and arduous journey of Taxol

WRWKHFOLQLFZRXOGFHUWDLQO\EHQHWIURPthis infusion of funds, the story beginsPXFKHDUOLHU $ VSHFLFSURMHFWGLUHFWHG

by the National Cancer Institute (NCI),already underway during the 1960s,

aimed to conduct a widespread screen-ing of substances and extracts obtainedfrom a variety of natural sources for anti-neoplastic activity. Thus, United StatesDepartment of Agriculture (USDA) bota-nist Arthur S. Barclay collected samples ofEDUNIURPWKHUHODWLYHO\UDUH3DFLF\HZTaxus brevifoliaZKLOHRQDHOGWULSWR

an Oregon forest in the summer of 1962.Barclays choice of the yew was an astute

one, since these trees had a long andproud history of use by man-

kind, and buried withinthis ancient knowledge

were the clues thatthey might contain

cytotoxic compounds.For example, JuliusCaesar recorded that,after defeat at thehands of the Roman

NeedlesofTaxus b

acca

ta

Bark

ofTaxusbrevif

olia

Tubulin polymerization andmicrotubules

Box 1

Chapter 25

2 0 9


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Monroe E. Wall and Mansukh C. Wani Susan B. Horwitz

legions, the Gallic chieftain Cativolcus

committed suicide by drinking teamade from yew bark. Barclays sampleswere eventually packed up and sent tothe Research Triangle Institute (NorthCarolina, USA) in 1964, where two chem-ists, Monroe E. Wall and Mansukh C.Wani, were charged with their investiga-tion. The crude extracts of the yew barkwere shown to exhibit an unusually broad

spectrum of cytotoxic activity against leu-kemia cells, as well as against a variety ofother cancer cells. This activity spurredon Wall and his colleagues to isolate theactive constituent of the extracts, the

FRPSOHWHVWUXFWXUHRIZKLFKZDVGHQL-tively established, in collaboration withresearchers at Duke University (North

Carolina, USA), by x-ray crystallographicanalysis in 1971. Of the more than110,000 compounds from 35,000 plantspecies tested by the NCI between 1960and 1981, this active ingredient, namedTaxol, proved to be the most promising.Despite its promise, however, Taxol was

set to languish on laboratory shelves foralmost another full decade before thenext advance was made. The reluctanceto pursue investigations into the potential

WKHUDSHXWLFEHQHWVRI7D[RO was largelyGXHWRGLIFXOWLHVDVVRFLDWHGZLWKLWVLVR -lation and low solubility, as well as thebelief that it was simply another microtu-

bule-destabilizing agent akin to the otheravailable natural products, colchicine andthe vinca alkaloids, already in use as anti-FDQFHUDJHQWV+RZHYHULQ6XVDQ%+RUZLW]


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The Nicolaou Taxol team

cer cells suffer from a

GHFLHQF\LQWKHFKHFN-points which regulate

the rate of division sothat the problem of exces-

sive expression of thesecells is grossly exacerbated.

+RUZLW] VKRZHG WKDW WXPRU FHOOVtreated in vitrowith Taxol were arrestedat the transition from metaphase to ana-

phase. Morphologically these cells exhib-ited unnatural bundles of microtubulesand no mitotic spindle. Other researchJURXSV TXLFNO\H[WHQGHG WKHVH QGLQJVproving that Taxol causes the general

and irreversible formation of bundlesof microtubules at several cell lifecycle

stages, visually reminiscent of a log jam.The net effect of this phenomenon is thesequestering of tubulin in the form ofstable structures, ultimately preventing

the formation of a mitotic spindle,halting cell division, and lead-

ing to rapid cell death byapoptosis (suicidal death).

Furthermore, the distur-bance to microtubulemobility was also shownto have dramatic effects

on other processeswithin the cell, such as

inhibiting the secretionof certain proteins, lead-

ing to the general conclu-sion that the full nature of

the attack of Taxol on cancer-ous cells is likely to be a complex con-

glomeration of cell function shutdowns.

bound GTP molecules. What

+RUZLW]V JURXS VKRZHGwas that Taxol affectsthe tubulin-microtubuleequilibrium. Taxol

decreases both theconcentration of freetubulin (to almost

zero) and the induc-tion time for polymer-ization, with the resultthat microtubules form-ing in the presence ofTaxol have a distinctly dif-ferent morphology from the con-trol variants. They have a shorter averagelength and are resistant to change under

conditions that would depolymerize nor-mal microtubules. In add ition, the Taxol

promoted polymerization of tubulin doesnot require GTP.

From the brief discussion above, we

can now understand the interactions ofTaxol with tubulin and the subsequent

effects on the gross architecture of micro-tubules, but how does this translate intothe powerful cytotoxicity of the drug?Most cells, excluding a few that cannotreplicate, spend the majority oftheir time in a nonproliferativestate called quiescence, yetthey are able to switch

rapidly into a repro-ductive cycle (mitosis)when their popula-tions are low and in

need of a boost. Themitotic cycle itself hasD QXPEHU RI GHQHGstages (Box 2). Cancer

cells, unlike their normalcounterparts, do not enterthe quiescent state when thepopulation density is high, and fur-thermore, during the mitotic cycle, can-

The Nicolaou retrosynthetic analysis of TaxolBox 3

K. C. Nicolaou

Taxusbaccata be

rries

Chapter 252 1 1


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As these groundbreaking

investigations were graduallyrevealed, scientists becameincreasingly excited about

Taxol because of its emerging portrayalDVD ORQJ VRXJKWDIWHU DOO\ LQWKH JKWagainst cancer. In the 1980s and 1990s,the media was buzzing with stories of amiraculous cure and a drug that couldchange the face of cancer treatment for-

HYHU+RZHYHUEHIRUHWKHVHGUHDPVFRXOGbe realized, passage around a number ofGLIFXOWORJLVWLFDOREVWDFOHVVWLOOKDGWREHnavigated. The most pressing of theseLVVXHV ZDV WKDW RI VXSSO\ 7KH 3DFLF

yew is a slow growing tree and has atendency to grow in dispersed microsites

hidden in ecosystems containing mostlyolder growth and larger tree species.Prior to the discovery of Taxol, thesetrees were regarded as scrub and were,for the most part, ignored in primary log-ging operations, instead being destroyedLQWKHVODVKDQGEXUQGLVSRVDOUHVWKDWfollowed. Large numbers of mature

specimens concentrated in sizeable andharvestable regions were therefore notavailable. The extraction of Taxol itselfwas also a tedious process requiring large

quantities of tree bark to yield relativelysmall amounts of the compound. Forexample, 38,000 Taxus brevifolia treesZHUHVDFULFHG WRREWDLQMXVW NJRI

Taxol, enough to treat 12,000 patients(that is more than three trees per per-son) in one of the initial clinical studies ofthis new medication. Such short supplieswere hampering the progress of these

early clinical trials, and yet the problem

was complicated even further by envi-ronmental concerns. Indeed, there wasa major controversy and a lively debatesurrounding the harvest of so many treesfrom the delicate forest environments.The concern was voiced that the successof TaxolZRXOGGULYHWKH3DFLF\HZWR

the verge of extinction and cause irrepa-rable damage to other members of thesefragile ecosystems, including the endan-gered Northern Spotted owl.

The frustrated medicinal promise thatTaxol held in these heady days createdunprecedented interest among syntheticorganic chemists, who were motivatedby the recognition that a chemical syn-

thesis could potentially provide a solu-tion to the supply problem. This urgencyspurred additional teams of researchersto follow the many groups, from a host ofcountries, who had already embarked on

the quest for a total synthesis of Taxol.The lure was not only the issue of the

limited stocks, but also the intrigue andchallenge presented by the highly com-plex and unique architecture of this mol-ecule. The structure of Taxol consists ofa diterpene core that is distinguished byits ring arrangement, a periphery denselypopulated with oxygen functionalities,and nine asymmetric centers. Appended

to this core, at a relatively hindered site,is an ester side chain bearing two addi-tional asymmetric carbon atoms. Taxol

EHFDPHWKH+RO\*UDLORIWRWDOV\QWKHVLV

and the race to reach it took on manyWZLVWVDQGWXUQVXQWLOLWQDOO\HQGHGLQsuccess in 1994.

7KHUVWWRWDOV\QWKHVLVRI7D[RO to

be published was that emanating fromthe Nicolaou laboratories at The ScrippsResearch Institute in La Jolla, California(USA), reported in Natureearly in 1994.This achievement, together with another

Highlights of the Nicolaou groups total synthesisof Taxol

Box 4

Ball and stick model ofthe molecule of Taxol

6SDFHOOLQJPRGHOof the molecule

of Taxol

Taxol2 1 2


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The Holton Taxol team

reaction, and was

then coupled to theC-ring in a subse-quent Shapiro reaction.Between each of thesekey stages adjustment ofoxidation levels and functionalgroup manipulations steered the grow-LQJPROHFXOHWRZDUGVLWVQDOGHVWLQDWLRQPleasingly, the challenge of cyclization to

furnish the highly strained and crowded8-membered B-ring was met, as pro-

jected, by the McMurry reaction. Thisremarkable process proved capable ofuniting the two aldehyde groups to close

the highly strained ring, yielding thetricyclic framework of Taxol molecule,GHVSLWH WKHVLJQLFDQW RSSRVLQJ IRUFHV

A more advanced intermediate exhib-iting all of the structural motifs of theTaxol was then reached after a numberof sensitive and frequently treacherousFKHPLFDOPRGLFDWLRQV,QFOXGHGDPRQJthese stringent tests were the generationof the highly strained oxetane ring and

the regioselective cleavage of the cycliccarbonate to form the desired

hydroxy benzoate moiety. ToFRPSOHWH WKH +HUFXOHDQ

task of Taxols totalsynthesis, the hinderedester side chain wasQDOO\LQVWDOOHGE\WKH

reaction of the com-pleted tetracyclic corewith an appropriate

electrophilic -lactamring, according to the

total synthesis which appeared at almost

the same time and was accomplished inWKHODERUDWRULHV RI 5REHUW $+ROWRQDWFlorida State University in Tallahassee(USA), ended nearly two decades ofrecalcitrance exhibited by Taxol. TheNicolaou groups success relied on theimplementation of a highly convergent

strategy devised by retrosynthetic analy-VLV%R[7KXVYHVWUDWHJLFERQGVKDGEHHQLGHQWLHGDQGUHWURV\QWKHWLFDOO\GLV-connected, disassembling the moleculeof Taxol into three key building blocks:a -lactam corresponding to the sidechain, a hydrazone encoding the A-ring,and an aldehyde containing the neces-sary carbon framework and functionality

to complete rings C and D. It was antici-pated that ring B, which perhaps posedthe thorniest problem of the synthesis,might be cast through an intermolecularShapiro reaction followed by an intramo-

lecular McMurry coupling process. Finally,the venerable DielsAlder reaction was to

be called upon twice, to assemble eachof the two requisite cyclohexene rings.

This strategy was successfully imple-mented, more or less the way it wasdesigned, after a relentless campaignthat included many twists and turns anddramatic moments, as summarized in Box4. Thus, the six-membered C-ring was

assembled through the application of aninnovative boron-tethered Diels

Alder reaction, developed byKoichi Narasaka (University

of Tokyo, Japan), whichcaused a switch ofregioselectivity fromthe unwanted natural

bias of the reactantstowards the desiredisomer. The A-ringwas prepared througha different DielsAlder

Highlights of the Holton groups total synthesisof Taxol

Box 5

RobertA. Holton

Taxusbaccata

Chapter 252 1 3

molecules that changes the world

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