Total synthesis of (+)-geldanamycin and (-)-o-quinogeldanamycin with use of asymmetric anti- and syn-glycolate aldol reactions

Article abstract of DOI:10.1021/ol0267432

(matrix presented) Geldanamycin (GA), an antitumor Hsp90 inhibitor, was made for the first time by using an oxidative demethylation reaction as the final step. A biaryldioxanone auxiliary set the anti C11-12 hydroxy-methoxy functionality and a methylglycolate auxiliary based on norephedrine was used for the syn C6-7 methoxy-urethane. p-Quinone-forming oxidants, CAN and AgO, produced an unusual aza-quinone product. Nitric acid gave GA from a trimethoxy precursor in 55% yield as a 1:10 mixture with nonnatural o-quino-GA.

Full text of DOI:10.1021/ol0267432

ORGANIC
LETTERS
2002
Vol. 4, No. 20
3549-3552
Total Synthesis of (+)-Geldanamycin
and (−)-o-Quinogeldanamycin with Use
of Asymmetric Anti- and Syn-Glycolate
Aldol Reactions
Merritt B. Andrus,* Erik L. Meredith, Bryon L. Simmons,
B. B. V. Soma Sekhar, and Erik J. Hicken
Brigham Young UniVersity, Department of Chemistry and Biochemistry, C100 BNSN,
ProVo, Utah 84602-5700
mbandrus@chemdept.byu.edu
Received August 16, 2002
ABSTRACT
Geldanamycin (GA), an antitumor Hsp90 inhibitor, was made for the first time by using an oxidative demethylation reaction as the final step.
A biaryldioxanone auxiliary set the anti C11−12 hydroxy-methoxy functionality and a methylglycolate auxiliary based on norephedrine was
used for the syn C6−7 methoxy-urethane. p-Quinone-forming oxidants, CAN and AgO, produced an unusual aza-quinone product. Nitric acid
gave GA from a trimethoxy precursor in 55% yield as a 1:10 mixture with nonnatural o-quino-GA.
Unlike the related anasamycin antibiotics,1 macbecin I and
herbimycin which have received considerable synthetic
attention including total syntheses,2 the synthesis of geldana-
mycin (S. hydroscopicus)3 has not been reported.4 These
compounds possess various biological activities, including
great potential as antitumor agents. Geldanamycin (GA), the
most potent family member, shows broad activity with the
NCI 60 cell-line panel (13 nM).5 Its cellular target has only
recently been uncovered. GA binds to the ATP binding site
of the chaperone heat shock protein 90 (Hsp90), inhibiting
its folding and ATPase activity leading to cell cycle
disruption.6 Hsp90 inhibition by geldanamycin significantly
and selectively lowers cellular levels of various oncogenic
tyrosine kinases including v-Src, Bcr/Abl, and ErbB-2, while
the serine/threonine-specific kinases, PKA and PKC, remain
unaffected.7 In addition, recent X-ray structures of the Hsp90‚
(1) Rinehart, K. L. Acc. Chem. Res. 1972, 5, 57.
(2) Herbimycin A: (a) Nakata, M.; Osumi, T.; Ueno, A.; Kimura, T.;
Tamai, T.; Tatsuda, K. Tetrahedron Lett. 1991, 32, 6015. Macbecin I: (b)
Baker, R.; Castro, J. L. J. Chem. Soc., Perkin Trans. 1990, 47. (c) Evans,
D. A.; Miller, S. J.; Ennis, M. D. J. Org. Chem. 1993, 58, 471. (d) Panek,
J. S.; Xu, F.; Rondon, A. C. J. Am. Chem. Soc. 1998, 120, 4413.
(3) (a) De Boer, C.; Meulman, P. A.; Wnuk, R. J.; Peterson, D. H. J.
Antibiot. 1970, 23, 442. (b) Rinehart, K. L.; Sasaki, K.; Slomp, G.; Grostic,
M. F.; Olson, E. C. J. Am. Chem. Soc. 1970, 92, 7591.
(4) GA derivatives: (a) Rinehart, K. L.; McMillan, M. W.; Witty, T.
R.; Tipton, C. D.; Shield, L. S.; Li, L. H.; Reusser, F. Bioorg. Chem. 1977,
6, 353. (b) Schnur, R. C.; Corman, M. L J. Org. Chem. 1994, 59, 2581.
Schnur, R. C.; Corman, M. L.; Gallaschun, R. J.; Cooper, B. A.; Dee, M.
F.; Doty, J. L.; Muzzi, M. L.; Moyer, J. D.; DiOrio, C. I.; Barbacci, E. G.;
Miller, P. E.; O’Brien, A. T.; Morin, M. J.; Foster, B. A.; Pollack, V. A.;
Savage, D. M.; Sloan, D. E.; Pustilnik, L. R.; Moyer, M. P. J. Med. Chem.
1995, 38, 3806. (c) Kuduk, S. D.; Harris, C. R.; Zheng, F. F.; Sepp-
Lorenzino, L.; Ouerfelli, Q.; Rosen, N.; Danishefsky, S. J. Bioorg. Med.
Chem. Lett. 2000, 10, 1303.
(5) Whitesell, L.; Shifrin, S. D.; Schwab, G.; Neckers, L. M. Cancer
Res. 1992, 52, 1721.
(6) Whitesell, L.; Mimnaugh, E. G.; De Costa, B.; Myers, C. E.; Neckers,
L. M. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 8324.
(7) Lawrence, D. S.; Niu, J. Pharmacol. Ther. 1998, 77, 81.
10.1021/ol0267432 CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/11/2002

GA complex suggest modifications that may lead to en-
hanced activity.8 These findings together with its distinct
structural features clearly warrant the development of a total
synthesis. We now report the first total synthesis of GA along
with o-quino-GA using a nitric acid oxidation of trimethoxy-
lactam 1 (Scheme 1). This establishes the absolute stereo-
Scheme 2
Scheme 1
followed by oxidative benzyl ether removal with CAN (ceric
ammonium nitrate).10 Various options were considered and
explored for the installation of the C10 methyl group. Finally,
the established route with asymmetric hydroboration was
followed.2 The aldehyde derived from 7 was treated with
trimethylaluminum, followed by Dess-Martin periodinane
to give a ketone that was treated under Wittig conditions to
provide 9. Removal of the silyl ether and hydroboration gave
diol 10 with the C10 methyl possessing the proper S-
configuration (Scheme 3). Protection of the primary and
secondary hydroxyls as tert-butyldimethylsilyl (TBS) ethers
followed by treatment with aqueous camphor sulfonic acid
to remove the primary TBS ether gave a primary alcohol
intermediate. Oxidation to the aldehyde and Wittig homolo-
gation in refluxing toluene provided unsaturated ester 11 with
16:1 E:Z selectivity in 98% yield. Enal 12 was produced by
using DIBAL reduction followed by Swern oxidation condi-
tions. The boron enolate of the newly developed norephe-
drine-based glycolate 13 reacted with 12 to generate 14 in
greater than 20:1 selectivity and 90% isolated yield.11 In
accord with the norephedryl propionate boron aldol reactions
reported by Masamune and co-workers,12 glycolate 13 gives
syn-products with a wide range of substrates including
branched and unsaturated aldehydes. The auxiliary was
removed with LiOH followed by acidification with HCl (pH
∼2) and the crude mixture was converted to ester 15 with
chemistry and resolves the ambiguity of the C14 methyl
substituent. The structure of Pavletich indicated an R
configuration, while that of Pearl was S at C14.8 Key steps
to access seco acid 2 include a novel anti-selective glycolate
aldol reaction using the recently developed diaryldioxanone
auxiliary and a new syn-selective glycolate aldol using a
norephedrine based approach.
Unlike the herbimycins and macbecins, the methoxy-
quinone of GA requires the use of a pentasubstituted benzene
precursor 1 (Scheme 1). On the basis of previous ansamycin
syntheses, including other trimethoxybenzenes, oxidative
removal of the 1,4-disposed methoxyls at C18,21 of 1 was
anticipated to directly generate GA with high selectivity.2
The lack of a benzylic C15-hydroxyl together with the added
hindrance of an additional C17-methoxyl precludes the use
of an aldol reaction for GA C14-15 bond formation as
employed previously with herbimycin and macbecin.2b,c Also,
C11 bears a hydroxy group, not a methoxy as with herbi-
mycin and macbecin, thus requiring a distinct approach for
formation of the anti-C11-12 hydroxy-methoxy functional-
ity.
The previously reported R-aldehyde 3 was treated with
the boron enolate of S,S-bis-4-methoxyphenyldioxanone 4
to generate the anti-glycolate aldol adduct 5 in 70% yield
with 10:1 selectivity (Scheme 2).9 A four-step sequence was
used to generate the differentially protected anti-diol ester 7
in high overall efficiency, using catalytic sodium methoxide
(10) Andrus, M. B.; Mendenhall, K. G.; Meredith, E. L.; Soma Sekhar,
B. B. V. Tetrahedron Lett. 2002, 43, 1789-1792.
(11) Andrus, M. B.; Soma Sekhar, B. B. V.; Turner, T. M.; Meredith,
E. L. Tetrahedron Lett. 2001, 42, 7197.
(8) (a) Stebbins, C. E.; Russo, A. A.; Schneider, C.; Rosen, N.; Hartl, F.
U.; Pavletich, N. P. Cell 1997, 89, 239. (b) Roe, S. M.; Prodromou, R. O.;
Ladbury, J. E.; Piper, P. W.; Pearl, L. H. J. Med. Chem. 1999, 42, 260.
(9) (a) Andrus, M. B.; Soma Sekhar, B. B. V.; Meredith, E. L.; Dalley,
N. K. Org. Lett. 2000, 2, 3035. (b) Andrus, M. B.; Meredith, E. L.; Soma
Sekhar, B. B. V. Org. Lett. 2001, 3, 259.
(12) (a) Abiko, A.; Liu, J.-F.; Masamune, S. J. Am. Chem. Soc. 1997,
119, 2586. (b) Yoshimitsu, T.; Song, J. J.; Wang, G.-Q.; Masamune, S. J.
Org. Chem. 1997, 62, 8978. (c) Abiko, A.; Liu, J.-F.; Buske, D. C.;
Moriyama, S.; Masamune, S. J. Am. Chem. Soc. 1999, 121, 7168. (d) Liu,
J.-F.; Abiko, A.; Pei, Z.; Buske, D. C.; Masamune, S. Tetrahedron Lett.
1998, 39, 1873.
3550
Org. Lett., Vol. 4, No. 20, 2002

Scheme 3
Scheme 4
trimethylsilyldiazomethane in 4:1 benzene/methanol solu-
tion.13 The norephedrine auxiliary was also recovered at this
stage of the sequence. Protection as the triethylsilyl (TES)
ether, followed by half reduction with DIBAL, and treatment
of the resultant aldehyde with the Still-Gennari phosphonate
gave Z-16 with 13:1 Z:E selectivity in 81% yield.14 In
contrast, asymmetric aldol reaction with R-branched enal 12
using the well-known methylglycolate oxazolidinone boron
enolate gave a much reduced 2:1 mixture of diastereomeric
syn-aldol products.15
Z-Enal from 16 was generated by using DIBAL reduction
and Dess-Martin periodinane. Treatment with the allyl ester
phosphonate shown in the presence of DBU and lithium
chloride gave E,Z-diene 17 with 19:1 E:Z selectivity in high
yield (Scheme 4).16 Hydrolysis of the analogous methyl ester
of 17 with aqueous base led only to decomposition, not to
the desired unsaturated carboxylic acid. The nitro group was
first reduced to the aniline through the action of NaBH4 with
added sulfur, following the precedure of Lalancette,17 as
employed recently by Panek.2c The allyl ester was then
removed by using catalytic Pd(PPh3)4 in the presence of
morpholine to give the seco acid 2. Generation of 1 proved
uneventful, including cyclization with BOP-Cl (bis(2-oxo-
3-oxazolidinyl)phosphinic chloride, 0.001 M) to form the
lactam in 76% yield.2 Urethane formation, following TES
ether removal at C7, was performed by using 2 equiv of
trichloroacetylisocyanate then treatment with potassium
carbonate in methanol in 89% yield according to the
procedure of Kocovsky.18 Previous ansamycin routes have
used excess sodium cyanate for this transformation.2 These
conditions, in the case of the GA intermediate, gave urethane
product in less than 50% yield. Aqueous HF in acetonitrile
was then used to remove the C11-TBS ether generating
trimethoxy-GA 1 in high yield. Use of either HF‚pyridine
or tetrabutylammonium fluoride for this step failed to provide
1. The sensitivity of GA to aqueous acid and base has been
noted previously.3
Unlike the previous ansamycin routes involving dimethoxy
intermediates and other trimethoxy model substrates, silver
or manganese oxide impregnated with HNO3 reacted rapidly
with 1 to give aza-quinone GA 18 in 77 and 40% yields,
repectively. The desired p-quinone GA, as anticipated by
the precedent noted above, was not formed with these
reagents under all conditions investigated.2 Other reagents,
including CAN (ceric ammonium nitrate) in acetonitrile used
alone or under phase transfer catalysis conditions, led only
to decomposition. In these cases not a trace of 18 or GA
was observed. The unique aza-quinone 18 was confirmed
by treatment with sodium hydrosulfite to give phenol 19.19
Use of CAN with a TBS,TES-protected lactam derived from
1 also led to the formation of an aza-quinone product without
a trace of p-quinone. The hypervalent iodine based oxidants
(13) Hirai, Y.; Aida, T.; Inoue, S. J. Am. Chem. Soc. 1987, 111, 3062.
(14) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405.
(15) For a typical use of this approach see: Keck, G. E.; Palani, A.;
McHardy, S. F. J. Org. Chem. 1994, 59, 3113. 2-Methylcinnamaldehyde
gave a 3:2:2:1 mixture of products with the methoxyacetyloxazolidinone
boron enolate.
(16) Roush, W. R.; Sciotti, R. J. J. Am. Chem. Soc. 1998, 120, 7411.
(17) Lalancette, J. M.; Freche, A.; brindle, J. R.; Laliberte, M. Synthesis
1972, 526.
(18) Kocovsky, P. Tetrahedron Lett. 1986, 27, 5521.
(19) Compound 19 was formed by using the conditions found in ref 4b.
nOe enhancements for 19: C17-OH and C18-OMe 1.54%, C20-NH and
C21-OMe 4.1%.
Org. Lett., Vol. 4, No. 20, 2002
3551

bis(trifluoroacetoxy)- and bis(acetoxy)iodobenzene have also
been used for oxidative demethylation to form quinones from
1,4-dimethoxy aryl compounds.20 These reagents also failed
to produce geldanamycin from intermediate 1.
oxidation. Gratifyingly, simple treatment of 1 with nitric acid
in acetic acid for 1 min22 followed immediately by sodium
bicarbonate quenching produced both (+)-geldanamycin,
identical (NMR, TLC, UV) to authentic material, and the
22
Production of the aza-quinone in the case of silver and
manganese oxide is most likely due to nitrogen lone-pair
donation to the adjacent trimethoxybenzene moiety. The
bound conformation of GA indicates a loss of resonance of
the nitrogen lone pair with the amide carbonyl that is twisted
out of plane as seen in the Hsp90‚GA X-ray structure.8 It is
reasonable to assume that, in solution, 1 would also adopt
conformations with reduced amide resonance and enhanced
nitrogen-aryl interaction. Upon oxidation with this arrange-
ment, the intermediate radical cation is stabilized by the
available nitrogen lone pair and demethylation occurs at C17
to generate 18 instead of C18,21 as anticipated. Indeed, an
acyclic N-acetyl model substrate produced only o-quinone
upon treatment with CAN or silver oxide, not aza-quinone.21
In an effort to deactivate nitrogen-aryl delocalization, BOC
and Aloc N-protected lactams were formed from 1. In both
cases neither substrate produced the desired p-quinone upon
new nonnatural compound o-quino-GA 20, [R]D -26.7°,
in a 1:10 ratio. While the selectivity of the final step proved
disappointing, the biological activities of the new geldana-
mycin variants 1, 18, 19, and 20 can now be evaluated by
using cytotoxicity and Hsp90 client protein lowering assays.6
Efforts are also underway to solve the selectivity of the
p-quinone formation step by using labile protecting groups
at C18,21 together with improved, convergent routes to
designed analogues.
Acknowledgment. WearegratefultoTheNIH(GM57275),
The American Cancer Society, and BYU for funding, Bruce
Jackson for MS assistance, and Prof. Chaitan Khosla, Kosan
Biosciences, for a sample of authentic geldanamycin.
Supporting Information Available: Experimental pro-
cedures and characterization for all compounds and spectral
data for numbered compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
(20) Kita, Y.; Hashizume, M.; Harayama, Y.; Morioka, H.; Tohma, H.
Tetrahedron Lett. 2001, 42, 6899.
(21) These details will be reported in a full account.
(22) Musgrave, O. C. Chem. ReV. 1969, 69, 9.
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