First Total Synthesis of Calicheamicin Achieved Stu Borman, C&EN Washington

The natural product calicheamicin γ^, a Mount Everest among molecules that has been difficult to conquer syntheti­cally, has just met its Sir Edmund Hil­lary. Chemistry professor K. C. Nico-laou of Scripps Research Institute and the University of California, San Diego, and coworkers have achieved a total synthesis of the compound.

The work, reported last week in the Journal of the American Chemical Society [114,10082 (1992)], is the first synthesis of any member of the enediyne class of anticancer antibiotics, a family of com­pounds whose biological activity stems from its ability to bind and cleave DNA. Carried out at Scripps, the work was supported by the National Insti­tutes of Health.

"The Nicolaou effort represents a landmark in organic synthesis/7 says chemistry professor Stuart L. Schreiber of Harvard University. "The calicheami­cin structure presented a multitude of challenges not previously encountered in synthetic studies of natural products. Nicolaou's ability to overcome these challenges is breathtaking/7

Chemistry professor Samuel J. Dan-ishefsky of Yale University calls the

K. C. Nicolaou (front) and coworkers break out the champagne after their successful total synthesis of calicheamicin. Team members include (left to Hght) visiting scientist Katsuhiro Shibayama, graduate student Conrad W. Hummel (holding calicheamicin model), postdoctoral fellow Masahisa Nakada, graduate student Emmanuel N. Pitsinos, and postdoctoral fellow Hiroyuki Saimoto. Postdoctoral fellow AdHan L. Smith, now at Merck, Sharpe & Dohme, Terlings Park, Harlow, U.K., is not shown

work "a major accomplishment. It is a masterful effort that demonstrates Nicolaou's extraordinary courage and vision and that helps to move our whole field forward/7

Originally, many researchers in the field believed that the total synthesis of calicheamicin was not an achievable goal. However, says Danishefsky, "K.C. was absolutely committed to the

Calicheamicin synthesis couples carbohydrate, aglycone precursors

CH3 Ο

H3C-7-W 0 C H 3

O-^V-OCHs Ο ( C 2 H 5 ) 3 S i O > ^ 0 — W + J OCHo i ^ 0 0 I NOP FMOC

Si(C2H5)3

H 3 C ^ / N

OSi(C2H5)3 H 3 C d

Carbohydrate precursor

(C2H5)3SiO„,

BzO

OH

Aglycone precursor

I Multiple steps I

Bz (benzoate) = ·- y^ ^

Ο FMOC (9-f luorenyl- y methyloxycarbonyl) = ^ " \

CH3 Ο QHa H3CSSS

Q 0 H

Ρ NHC02CH3

cr ^ r OCH3 OH H3C-7-0-7' 0 C H 3

HoCO ·

s?np H3C^

OH

HO

Η _

H3CO

Calicheamicin

DECEMBER 7,1992 C&EN 29

SCIENCE/TECHNOLOGY

idea that it could be done. I can't say enough superla­tives about it."

According to bioorganic chemist George Ellestad of Lederle Laboratories—a member of the team that re­ported the isolation and structure determination of the calicheamicins in 1987— the total synthesis is "a re­markable achievement that certainly confirms the origi­nal structural conclusions published by Lederle. This opens up the way for the preparation of simpler com­pounds for biological evalu­ation." Calicheamicin γ^ is considered the most promis­ing of the calicheamicins in terms of its biological activity.

The enediynes, naturally occurring substances from bacterial sources, have been a focus of intense interest because of their strong antibiotic and antitumor activity, their unique mode of action, and their complex molecular struc­tures. The family includes the cali­cheamicins, the esperamicins, dynemi-cin, neocarzinostatin chromophore, and the recently discovered kedarcidin chromophore [/. Am. Chem. Soc, 114, 7946 (1992)].

All five groups share the characteris­tic enediyne structure—a pair of dou­ble-bonded carbons flanked by two pairs of triple-bonded carbons within a complex tricyclic ring system. This structure is often called the "warhead" because it is the part of the molecule that cleaves DNA. Either before or after

Space-filling model of calicheamicin γ/. Carbon atoms hydrogen, white; oxygen, red; sulfur, yellow; nitrogen,

iodine, purple

the enediyne compound binds to DNA, a triggering reaction leads to a cycliza-tion of the enediyne group in which a 1,4-benzenoid diradical forms. This re­active species abstracts two hydrogen atoms from DNA's sugar-phosphate backbone, causing strand cleavage.

The total synthesis of calicheamicin by Nicolaou and coworkers is the cul­mination of work by several groups, including Nicolaou's, that helped set up base camps on the way to the sum­mit. This intellectual history begins with chemistry professor Robert G. Bergman of the University of Califor­nia, Berkeley, who first discovered enediyne cyclization—now called the Bergman reaction—in 1972. This find­ing anticipated by almost two decades the recently discovered mode of action of the enediynes.

In 1989, chemistry professor Philip

D. Magnus and coworkers at the University of Texas, Austin, became the first to assemble the trisulfide moi­ety of calicheamicin on a simplified version of the aglycone (the noncarbohy-drate part of the molecule). "People had been in abso­lute dread of doing this, in­cluding ourselves," says Danishefsky. "When Mag­nus put in the trisulfide he showed us it wasn't such a terrible thing. That's a very important thing in synthe-

are black; sis—the person who shows blue; and that fears are misplaced,

that it can be done." In 1990, Danishefsky and

coworkers achieved the first synthesis of racemic calicheamicinone, the race-mic aglycone of calicheamicin. This ac­complishment was also a case of "push­ing the envelope," says Schreiber. "It b e gan to change people's views of what could be accomplished in this area." Nicolaou and coworkers later reported a conceptually different approach that yielded the naturally occurring enantio-mer of calicheamicinone.

In 1990, Nicolaou and coworkers re­ported the first total synthesis of the pentacyclic oligosaccharide sector of calicheamicin, which is fiendishly sub­stituted and complicated. Danishefsky later synthesized this sector by a differ­ent approach. Chemistry professor Daniel E. Kahne of Princeton Universi­ty has also made key contributions to synthesis of the carbohydrate sector of the molecule.

Calicheamicin uses complex mechanism to cleave DNA

HO-,) HO-Bergman

cyclization •

O-sugar

HO-,)

O-sugar

DNA cleavage HO'

O-sugar

H3CS Nucleophile

A nucleophile such as glutathione attacks the central sulfur atom of the trisulfide group, causing a thioether ring to form and converting an sp2 (double-bonded) carbon to an sp3 (single-bonded) carbon. This also makes two triple-bonded car­bons in the enediyne group move closer together (double arrow), leading to a Bergman cyclization in which a 1,4-ben­zenoid diradical is formed. The diradical abstracts two hydrogens from DNA, causing strand cleavage

Adapted with permission from K. C. Nicolaou and W.-M. Dai, Angew. Chem. Int. Ed. Engl., 30,1387 (1991)

30 DECEMBER 7,1992 C&EN

With the aglycone and carbohydrate parts of calicheamicin in hand, re­searchers began to focus on efforts to couple the two together. In 1991, Nico-laou's group developed a strategy for coupling the calicheamicin oligosaccha­ride with aglycones, using dynemicin enediyne models. However, there was no simple way to do the coupling on the final calicheamicin aglycone. Dan-ishefsky says, "The question was, how close to the final aglycone could one get, do the coupling, and then still get to the final aglycone structure?"

Early this year, Danishefsky's group reported the first coupling of the cali­cheamicin carbohydrate to an imma­ture version of the calicheamicin agly­cone. Unfortunately, this aglycone was too immature to get to the final prod­uct because it didn't hold up to subse­quent reactions. The achievement of Nicolaou and coworkers was finding a way to couple the oligosaccharide structure to a nearly mature aglycone.

"They did the coupling reaction about four steps later into the synthe­sis," says Danishefsky, "which enabled them to go to the end. Coupling every­thing together at the right strategic point and deprotecting everything is extremely difficult. They've showed what's possible at the frontiers of plan­ning and execution."

At this point, the enediynes are still a long way from being commercial drugs, the problem being that they cleave DNA only too well. "The com­pounds themselves are too toxic," says Ellestad. Lederle has been investigating the ability of monoclonal antibody con­jugates of calicheamicin to target spe­cific .tumor cell lines. The idea—still unproven—is that a drug targeted to a specific diseased tissue will not tend to damage normal body cells.

"K.C.'s synthesis will make it possible to prepare simpler analogs that may or may not have the toxicity the natural products do," says Ellestad. "The hope is that a synthetic program could come up with derivatives that are less toxic and that wouldn't have to be conjugated with monoclonal antibodies."

According to Schreiber, "The cali­cheamicin challenge has resulted in a remarkable body of innovative re­search— The ability to construct the natural product, and therefore structur­al variants, in the laboratory promises an exciting new chapter in this evolv­ing saga." •

Surface phosphating process developed Technology to modify textiles, catalysts, and chromatographic packings with alkyl phosphate ester groups has been developed by chemists at the University of Massachusetts, Amherst. Their work could lead to such products as fire-

retarded cotton and rayon, zeolite cata­lysts with altered activities and selectivi-ties, and silica gel column packings for separation of enantiomers of chiral com­pounds or simply cleaner separations among other compound types.

Metaphosphates attach phosphate groups to substrates

(C2H5)3N » (C2H5)2NH I POCI3 + C2H5OH » C2H5OPCI2 ^ C2H50—P—N(C2H5)2

CI

(1) NaOH (2) H+-lon

exchange resin

C 2H 50-P-OH Substrates c ^ 0 _ ^ = o

Substrate— Ο Ethyl metaphosphate

C2H50-P—N(C2H5)2

OH

THE COMPETITION DEADLINE IS FEBRUARY 16 ,1993

The BFGoodrich Company and the National Invention Center are seeking student applicants to compete in the 1993 BFGoodrich Collegiate Inventors Program. This highly visible national awards pro­gram recognizes student inventors, their advisors and institutions for their contributions and inventiveness.

Open to any full time student at a college or university in the U.S., this showcase competition rewards student achievements in patent development with awards of $5,000, $2,500 to their advisors, and a patent seminar for the institution. Call Rose Heintz (216) 762-4463 (or application information.

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DECEMBER 7,1992 C&EN 31