Total synthesis of luzopeptins A-C [6]

Full text of DOI:10.1021/ja983925b

1098
J. Am. Chem. Soc. 1999, 121, 1098-1099
Total Synthesis of Luzopeptins A-C
Dale L. Boger,* Mark W. Ledeboer, and Masaharu Kume
Department of Chemistry and
The Skaggs Institute of Chemical Biology
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, California 92037
ReceiVed NoVember 12, 1998
The luzopeptins (1-3, Figure 1) are potent antitumor antibiotics
that were isolated from Actinomadura luzonensis1 and identified
through a single-crystal X-ray structure determination of 1.2 They
constitute the initial members of a growing class of C2-symmetric
cyclic decadepsipeptides which now include the quinoxapeptins
(4-5),3 quinaldopeptin,4 and sandramycin (6)5-7 that bind to DNA
with bisintercalation.6-11 In addition to their potent cytotoxic and
antitumor activity,1,8,12,13 they are potent inhibitors of HIV reverse
transcriptase (RT)7,14 including single and double mutants3
responsible for the emerging clinical resistance to recently
introduced RT inhibitors. Moreover, the cytotoxic potency of the
luzopeptins (A > B . C) and their antiviral potency/HIV RT
inhibition (C > B > A) are reversed, with luzopeptin C exhibiting
suppression of HIV replication in infected MT-4 cells at non-
cytotoxic concentrations,14 and we have observed similar divergent
structure activity relationships with a recent series of sandramycin
analogues.7
Despite their importance as prototypical DNA bisintercalators
with potent biological properties, they have been the subject of
only limited synthetic efforts.15-18 Herein, we report the first total
synthesis of luzopeptins A-C. The luzopeptins and quinoxapep-
tins contain the identical cyclic decadepsipeptide and differ only
in the attached chromophore and in the acyl substituents found
Figure 1.
on the unusual L-(4S)-hydroxy-2,3,4,5-tetrahydropyridazine-3-
carboxylic acid (L-Htp) subunit. Consequently, key elements of
the approach include the late-stage introduction of the chro-
mophore potentially providing access to both the luzopeptins and
quinoxapeptins, the late-stage L-Htp alcohol acylation permitting
the divergent synthesis of the luzopeptins and quinoxapeptins,
symmetrical pentadepsipeptide coupling and macrocyclization of
the 32-membered depsipeptide conducted at the single secondary
amide site, and a convergent assembly of the pentadepsipeptide
with introduction of the hindered and labile ester linkage in the
final coupling reaction under near racemization free conditions.
The convergent assemblage of the key pentadepsipeptide 19
from the tripeptide 17 and protected dipeptide 1818 is summarized
in Scheme 1. The protected N-methyl (3R)-hydroxyvalinol 12 for
incorporation into 17 was derived from 3-methyl-2-buten-1-ol by
Sharpless epoxidation with (+)-L-DIPT providing the known (2S)-
epoxide 7.19 Formation of the carbamate 8 upon reaction with
methyl isocyanate (1.5 equiv, CH2Cl2, 23 °C, 2 h, 94%) followed
by base-catalyzed intramolecular epoxide opening provided the
9 that cleanly rearranged to the more stable cyclic carbamate 10
(>25:1) under the reaction conditions (5.0 equiv of NaH, THF,
25 °C, 24-72 h, 66-85%).20 Protection of the primary alcohol
(1.6 equiv of DHP, 0.06 equiv of PPTs, CH2Cl2, 23 °C, 17 h,
99%), hydrolysis of the cyclic carbamate (4 equiv of KOH, (CH2-
OH)2-H2O, 150 °C, 25 h, 92-94%), and coupling with BOC-
Gly-Sar-OH (13,6 1.05 equiv of EDCI21 and HOAt,21 DMF, 23
°C, 83%) provided 14. Subsequent deprotection of the primary
(1) Konishi, M.; Ohkuma, H.; Sakai, F.; Tsuno, T.; Koshiyama, H.; Naito,
T.; Kawaguchi, H. J. Antibiot. 1981, 34, 148.
(2) Arnold, E.; Clardy, J. J. Am. Chem. Soc. 1981, 103, 1243. Konishi,
M.; Ohkuma, H.; Sakai, F.; Tsuno, T.; Koshiyama, H.; Naito, T.; Kawaguchi,
H. J. Am. Chem. Soc. 1981, 103, 1241.
(3) Lingham, R. B.; Hsu, A. H. M.; O’Brien, J. A.; Sigmund, J. M.;
Sanchez, M.; Gagliardi, M. M.; Heimbuch, B. K.; Genilloud, O.; Martin, I.;
Diez, M. T.; Hirsch, C. F.; Zink, D. L.; Liesch, J. M.; Koch, G. E.; Gartner,
S. E.; Garrity, G. M.; Tsou, N. N.; Salituro, G. M. J. Antibiot. 1996, 49, 253.
(4) Toda, S.; Sugawara, K.; Nishiyama, Y.; Ohbayashi, M.; Ohkusa, N.;
Yamamoto, H.; Konishi, M.; Oki, T. J. Antibiot. 1990, 43, 796.
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Colson, K. L.; Belofsky, G. N.; Bleiberg, B. B. J. Antibiot. 1993, 46, 162.
(6) Total synthesis of sandramycin: Boger, D. L.; Chen, J.-H. J. Am. Chem.
Soc. 1993, 115, 11624. Boger, D. L.; Chen, J.-H.; Saionz, K. W. J. Am. Chem.
Soc. 1996, 118, 1629.
(7) Boger, D. L.; Chen, J.-H.; Saionz, K. W.; Jin, Q. Bioorg. Med. Chem.
1998, 6, 85. Boger, D. L.; Saionz, K. W. Bioorg. Med. Chem. In press.
(8) Huang, C.-H.; Mong, S.; Crooke, S. T. Biochemistry 1980, 19, 5537.
Huang, C.-H.; Prestayko, A. W.; Crooke, S. T. Biochemistry 1982, 21, 3704.
Huang, C.-H.; Mirabelli, C. K.; Mong, S.; Crooke, S. T. Cancer Res. 1983,
43, 2718. Huang, C.-H.; Crooke, S. T. Cancer Res. 1985, 45, 3768.
(9) Fox, K. R.; Davies, H.; Adams, G. R.; Portugal, J.; Waring, M. J. Nucl.
Acids Res. 1988, 16, 2489. Fox, K. R.; Woolley, C. Biochem. Pharmacol.
1990, 39, 941.
(10) Searle, M. S.; Hall, J. G.; Denny, W. A.; Wakelin, L. P. G. Biochem.
J. 1989, 259, 433.
(11) Zhang, X.; Patel, D. J. Biochemistry 1991, 30, 4026. Leroy, J. L.;
Gao, X.; Misra, V.; Gueron, M.; Patel, D. J. Biochemistry 1992, 31, 1407.
(12) Rose, W. C.; Schurig, J. E.; Huftalen, J. B.; Bradner, W. T. Cancer
Res. 1983, 43, 1504. Huang, C.-H.; Crooke, S. T. Anti-cancer Drug Des. 1986,
1, 87.
(13) Boger, D. L.; Chen, J.-H. Bioorg. Med. Chem. Lett. 1997, 7, 919.
(14) Take, Y.; Inouye, Y.; Nakamura, S.; Allaudeen, H. S.; Kubo, A. J.
Antibiot. 1989, 42, 107.
(15) Olsen, R. K.; Apparao, S.; Bhat, K. L. J. Org. Chem. 1986, 51, 3079.
(16) Hughes, P.; Clardy, J. J. Org. Chem. 1989, 54, 3260. Greck, C.;
Bischoff, L.; Genet, J. P. Tetrahedron: Asymmetry 1995, 6, 1989.
(17) Ciufolini, M. A.; Swaminathan, S. Tetrahedron Lett. 1989, 30, 3027.
Ciufolini, M. A.; Xi, N. J. Chem. Soc., Chem. Commun. 1994, 1867. Xi, N.;
Ciufolini, M. A. Tetrahedron Lett. 1995, 36, 6595. Ciufolini, M.; Xi, N. J.
Org. Chem. 1997, 62, 2320. Xi, N.; Alemany, L. B.; Ciufolini, M. A. J. Am.
Chem. Soc. 1998, 120, 80.
(19) Gao, Y.; Hanson, R. M.; Klunder, J. M.; Ko, S. Y.; Masamune, H.;
Sharpless, K. B. J. Am. Chem. Soc. 1987, 109, 5765.
(20) Roush, W. R.; Adam, M. A. J. Org. Chem. 1985, 50, 3752. The
crystallinity of 10 provided the occasion to ensure pure material free of the
isomer 9 as well as any unnatural enantiomer (recrystallization, EtOAc-
hexane) was enlisted in the subsequent steps.
(21) EDCI ) 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochlo-
ride; HOAt ) 1-hydroxy-7-azabenzotriazole; DCC ) 1,3-dicyclohexylcar-
bodiimide.
(18) Boger, D. L.; Schu¨le, G. J. Org. Chem. 1998, 63, 6421.
10.1021/ja983925b CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/22/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 5, 1999 1099
Scheme 1
Linear decadepsipeptide 22 formation was accomplished by
coupling 20 with 21 in a reaction mediated by EDCI-HOAt (3
equiv/3 equiv, CH2Cl2, 0 °C, 2 h, 64% overall from 19). Single-
step deprotection of both the benzyl ester and FMOC by transfer
hydrogenolysis (25% aqueous HCO2NH4, 10% Pd-C, EtOH-
H2O, 23 °C, 4 h, 98%) followed by macrocyclization of the crude
amino acid 23 with ring closure at the single secondary amide
site provided the 32-membered cyclic decadepsipeptide 24
(EDCI-HOAt, 5.0 equiv/5.5 equiv, CH2Cl2, 0 °C, 16 h, 63%
overall from 22). Following macrocyclization, the decadepsipep-
1
tide exhibited a well-defined H NMR spectrum indicative of a
rigid conformation and material that was free of contaminate
diastereomers. The use of CH2Cl2 versus DMF as solvent and
the deliberate exclusion of added bases enhanced the coupling
and macrocyclization conversions presumably by minimizing
competitive â-elimination or retro aldol reactions of the substrates
and products. Treatment of 24 with TFA-CH2Cl2 in the presence
of anisole (1/1/0.4, 0 °C for 2 h, 0 to 23 °C, 1 h, 68%) smoothly
provided the fully functionalized cyclic decadepsipeptide 25
incorporating the protected L-Htp residue. Notably, the conversion
of 24 to 25 requires six steps involving BOC deprotection, acetal
cleavage, and imine cyclization within each L-Htp subunit and
proceeded cleanly without competitive OTBS deprotection.23
Completion of the synthesis required SES deprotection followed
by chromophore introduction. The former proved more challeng-
ing than related efforts,6 and treatment of 25 with Bu4NF or CsF
under a variety of conditions ((BOC2O)6 led to deprotection of
the OTBS groups without removal of the NSES or, under forcing
conditions, to substrate degradation. Gratifyingly, both the NSES
and OTBS groups were removed effectively upon treatment with
anhydrous HF (neat, anisole, 0 °C, 1.5 h) providing 26 and
defining a new, acidic set of conditions that may find widespread
applicability to peptide NSES deprotections.24 Coupling of 26 with
3-hydroxy-6-methoxyquinoline-2-carboxylic acid (5 equiv 27,25,26
5 equiv of EDCI-HOBt, 10 equiv of NaHCO3, DMF, 23 °C, 11
h, 80% overall from 25) without protection of the chromophore
phenol provided luzopeptin C identical in all respects (1H and
13C NMR, IR, MS, [R]D) with that reported for natural material.
Following procedures detailed in the structure elucidation studies,1
peracetylation of luzopeptin C followed by mild basic hydrolysis
of the phenol acetates provided luzopeptin A (Ac2O-py 1/1, 23
°C, 7 h; 0.1 M Na2CO3, THF/CH3OH/H2O 3/1/1, 23 °C, 50 min,
50%)27 and smaller amounts of luzopeptin B (20%) identical in
all respects with natural material.
Acknowledgment. We gratefully acknowledge the financial support
of the National Institutes of Health (CA41101), the Skaggs Institute of
Chemical Biology, a NIH postdoctoral fellowship (M.W.L., F32 GM1876),
and the postdoctoral visiting scientist support of Shionogi & Co. Ltd.
(M.K., 1998-1999). We also gratefully acknowledge the preparation of
a substantial quantity of 18 conducted by Dr. G. Schu¨le (ref 18) and the
initial development of the route to 15 by J.-H. Chen.
alcohol (0.05-0.1 equiv of TsOH, CH3OH, 24 h, 23 °C, 84%),
oxidation of 15 to the carboxylic acid 16 that was most effectively
accomplished with RuO2-NaIO422 (0.03 equiv/3 equiv, CCl4/CH3-
CN/H2O 2/2/3, 23 °C, 24 h, 87%), and FMOC/BOC exchange of
the amine protecting group provided 17 (4 M HCl-dioxane, 23
°C, 30 min; 1.05 equiv of FMOCCl, 10% aq NaHCO3-dioxane,
23 °C, 8 h, 79%). The key esterification reaction linking the
tripeptide 17 (2.0 equiv) with 18,18 suitably functionalized for
closure to the unusual L-Htp subunit, was accomplished in a
surprisingly effective manner upon treatment with DCC-DMAP6,21
(3.0 equiv/2.0 equiv, CH2Cl2, -20 to 0 °C, 17 h, 73%). Much
lower conversions and near complete racemization of the hindered
N-methyl L-â-hydroxyvaline were observed with a wide range
of alternative reagents or when the reaction was conducted in
the absence of DMAP, and the use of increasing amounts of
DMAP was found to suppress racemization and improve the
overall conversion (Table 1 in Scheme 1). Hydrogenolysis of the
benzyl ester deliberately conducted at 10-12 °C to minimize a
slow and competitive FMOC deprotection which was observed
at 20-25 °C (H2, 10% Pd-C, CH3OH, 3 h, 76-78%) and
complementary FMOC deprotection of 19 (Et2NH-CH3CN 1/2,
23 °C, 20 min, ca. 100%) provided 20 and 21, respectively.
Supporting Information Available: Characterization of 1-3, 7, 8,
10-12, 15-17, 19, epi-19, 22, 24, and 25 (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA983925B
(22) Carlsen, P. H. J.; Katsuki, T.; Martin, V. S.; Sharpless, K. B. J. Org.
Chem. 1981, 46, 3936. A related sequence enlisting KMnO4 as the oxidant
on a luzopeptin dipeptide has been reported17 and failed to provide 16.
(23) Treatment with 90% TFA-H2O16-18 gave the diol (12-42%), and
variable amounts of the mono OTBS product (10-28%) and an elimination
product (37-65%) derived from loss of one of the L-Htp hydroxy groups.
(24) Weinreb, S. M.; Demko, D. M.; Lessen, T. A. Tetrahedron Lett. 1986,
27, 2099.
(25) Prepared from methyl 2-benzyloxy-6-methoxyquinoline-2-carboxylate26
by sequential treatment with H2, 10% Pd-C, EtOH, 23 °C, 3 h (97%) and
LiOH, THF/CH3OH/H2O 3/1/1, 23 °C, 10 h (70%).
(26) Boger, D. L.; Chen, J.-H. J. Org. Chem. 1995, 60, 7369.
(27) Altering the length of time exposed to the hydrolysis conditions alters
the relative amounts of 1-3 obtained.