Synthetic study of kedarcidin chromophore: Revised structure

Full text of DOI:10.1021/ja973231n

12012
J. Am. Chem. Soc. 1997, 119, 12012-12013
Synthetic Study of Kedarcidin Chromophore:
Revised Structure
Shinji Kawata, Shukuko Ashizawa, and Masahiro Hirama*
Department of Chemistry, Graduate School of Science
Tohoku UniVersity, Sendai 980-77, Japan
ReceiVed September 15, 1997
Kedarcidin is a new chromoprotein antitumor antibiotic family
consisting of a carrier apoprotein and a cytotoxic nine-membered
enediyne chromophore.1 Although the naked chromophore is
highly unstable, its structure including the absolute configuration
was described as 1 in 1993.2 Because of its unusual structure,
extremely potent antitumor activity, and high degree of sequence
specificity in DNA cleavage,3 we attempted to synthesize this
chromophore. The core moiety of ent-1 has recently been
stereoselectively synthesized.4,5 In this communication, we
describe the synthesis of the chloroazatyrosyl naphthoamide
fragment 2 of ent-1. We demonstrate that the fragment degraded
from the chromophore is not an R-amino acid derivative but
â-amino ester 3 and that the structure of kedarcidin chromophore
should be revised to be 27.
Scheme 1a
An efficient route to the polysubstituted naphthoic acid 11
was first established (Scheme 1). Methyl gallate (4) was
regioselectively O-alkylated using isopropyl iodide after ac-
etalization of the vicinal dihydroxy group in 72% overall yield.6
Regioselective bromination7 of phenol 5 followed by methy-
lation of the two phenolic hydroxy groups yielded appropriately
functionalized benzoate 6. Homologation of ester 6 via Wolff
rearrangement gave phenylacetate 7. The Heck reaction of 7
with tert-butyl acrylate afforded the R,â-unsaturated ester 8.8
After methyl ester 8 was converted to acid chloride 9, cyclization
to form a naphthalene ring was examined by the intramolecular
Friedel-Crafts reaction of 9 with AlCl3. However, the yield
of 12 from 8 was low (0-36%) after esterification with MeOH,
EDC‚HCl, and DMAP. Another route via an electrocyclic
reaction of the ketene intermediate proceeded smoothly.9,10
a Reagents and conditions: (a) Ph2CCl2,, 175 °C. (b) i-PrI, K2CO3,
acetone, 50 °C. (c) AcOH, H2O, reflux, 72% (three steps). (d) NBS,
THF, room temperature (rt). (e) MeI, K2CO3, acetone, 50 °C, 86% (two
steps). (f) NaOH, MeOH, H2O, 60 °C. (g) (COCl)2, PhCH3. (h) CH2N2,
Et3N, Et2O; then PhCO2Ag, Et3N, MeOH, 75% (three steps). (i)
CH2dCHCO2t-Bu, Pd(OAc)2, (p-tol)3P, Et3N, 100 °C, 90%. (j)
Ba(OH)2‚8H2O, t-BuOH. (k) (COCl)2, CH2Cl2. (l) i-Pr2NEt, PhCH3,
rt, 62% (three steps). (m) CF3CO2H, 99%.
(1) Lam, K. S.; Hesler, G. A.; Gustavson, D. R.; Crosswell, A. R.; Veitch,
J. M.; Forenza, S.; Tomita, K. J. Antibiot. 1991, 44, 472. Hofstead, S. J.;
Matson, J. A.; Malacko, A. R.; Marquardt, H. J. Antibiot. 1992, 45, 1250.
Constantine, K. L.; Colson, K. L.; Wittekind, M.; Friedrichs, M. S.; Zein,
N.; Tuttle, J.; Langley, D. R.; Leet, J. E.; Schroeder, D. R.; Lam, K. S.;
Farmer, B. T., II; Metzler, W. J.; Bruccoleri, R. E.; Mueller, L. Biochemistry
1994, 33, 11438.
(2) (a) Leet, J. E.; Schroeder, D. R.; Hofstead, S. J.; Golik, J.; Colson,
K. L.; Huang, S.; Klohr, S. E.; Doyle, T. W.; Matson, J. A. J. Am. Chem.
Soc. 1992, 114, 7946. (b) Leet, J. E.; Schroeder, D. R.; Langley, D. R.;
Colson, K. L.; Huang, S.; Klohr, S. E.; Lee, M. S.; Golik, J.; Hofstead, S.
J.; Doyle, T. W.; Matson, J. A. J. Am. Chem. Soc. 1993, 115, 8432; 1994,
116, 2233.
Treating the acid chloride 9 with i-Pr2NEt (1 equiv) in toluene
at room temperature gave tert-butyl naphthoate 10 in 62% yield,
which was readily converted to naphthoic acid 11.
(3) Zein, N.; Colson, K. L.; Leet, J. E.; Schroeder, D. R.; Solomon, W.;
Doyle, T. W.; Casazza, A. M. Proc. Natl. Acad. Sci. U.S.A. 1993, 90, 2822.
(4) Kawata, S.; Yoshimura, F.; Irie, J.; Ehara, H.; Hirama, M. SYNLETT
1997, 250. The three-dimensional structure of neocarzinostatin [Tanaka,
T.; Hirama, M.; Fujita, K.; Imajo, S.; Ishiguro, M. J. Chem. Soc., Chem.
Commun. 1993, 1205. Imajo, S.; Ishiguro, M.; Tanaka, T.; Hirama, M.;
Teplyakov, A. Bioorg. Med. Chem. 1995, 3, 429] and a computer modeling
study of the chromophore-apoprotein binding structure for C1027 and
kedarcidin (unpublished results) as well as the absolute stereochemistry of
C1027 [Iida, K.; Fukuda, S.; Tanaka, T.; Hirama, M.; Imajo, S.; Ishiguro,
M.; Yoshida, K.; Otani, T. Tetrahedron Lett. 1996, 37, 4997] suggested a
possibility that the assigned structure of the aglycone of 1 might be antipodal
to the natural form. We therefore planned to synthesize ent-1.
(5) For related synthetic studies, see: Iida, K.; Hirama, M. J. Am. Chem.
Soc. 1994, 116, 10310. Iida, K.; Hirama, M. J. Am. Chem. Soc. 1995, 117,
8875. Sato, I.; Akahori, Y.; Iida, K.; Hirama, M. Tetrahedron Lett. 1996,
37, 5135. Hirama, M. Recent Progress in the Chemical Synthesis of
Antibiotics and Related Microbial Products; Lukacs, G., Ed.; Springer-
Verlag: Berlin, 1993; pp 293. Lhermitte, H.; Grierson, D. S. Contemp.
Org. Synth. 1996, 3, 93. Magnus, P.; Carter, R.; Davies, M.; Elliott, J.;
Pitterna, T. Tetrahedron 1996, 52, 6283.
The chloroazatyrosine derivative11 16 was efficiently syn-
thesized from commercially available 2-chloro-3-hydroxypyri-
dine (13) (Scheme 2). The palladium-catalyzed cross-coupling
reaction of iodopyridine 14 and the alkylzinc compound 15
prepared from L-serine12,13 followed Burke’s procedure.14 The
TBS group was removed to form 16 using a weakly acidic
aqueous mixture of HF-NaF. Deprotection of the Boc group
of 16 and subsequent coupling with 11 gave R-chloroazatyrosyl
(9) The same finding and a very similar synthesis of 11 were recently
reported: Myers, A. G.; Horiguchi, Y. Tetrahedron Lett. 1997, 38, 4363.
(10) For related electrocyclic reactions, see: Choshi, T.; Yamada, S.;
Sugino, E.; Kuwada, T.; Hibino, S. J. Org. Chem. 1995, 60, 5899. Saito,
T.; Morimoto, M.; Akiyama, C.; Matsumoto, T.; Suzuki, K. J. Am. Chem.
Soc. 1995, 117, 10757.
(11) Myers, A. G.; Gleason, J. L.; Yoon, T.; Kung, D. W. J. Am. Chem.
Soc. 1997, 119, 656.
(6) Jurd, L. J. Am. Chem. Soc. 1959, 81, 4606.
(12) Electrolytic zinc prepared by hydrometallurgy should be used, see:
Takai, K.; Kakiuchi, T.; Utimoto, K. J. Org. Chem. 1994, 59, 2671.
(13) Jackson, R. F. W.; Wishart, N.; Wood, A.; James, K.; Wythes, M.
J. J. Org. Chem. 1992, 57, 3397.
(7) Chow, Y. L.; Zhao, D.-C.; Johansson, C. I. Can. J. Chem. 1988, 66,
2556.
(8) Heck, R. F. Org. React. 1982, 27, 345. de Meijere, A.; Meyer, F. E.
Angew. Chem., Int. Ed. Engl. 1994, 33, 2379.
(14) Ye, B.; Burke, Jr, T. R. J. Org. Chem. 1995, 60, 2640.
S0002-7863(97)03231-9 CCC: $14.00 © 1997 American Chemical Society

Communications to the Editor
J. Am. Chem. Soc., Vol. 119, No. 49, 1997 12013
Scheme 2a
synthesis of 3 from 23,16 the R configuration of which was
defined using a modified Mosher method.17
Therefore, we reexamined the entire structure of the chro-
mophore. The structures of the sugar moieties were unequivo-
cally determined by comparison with an authentic sample2b and
by X-ray crystallographic analysis.18 Close inspection of the
reported NMR data for stable sodium borohydride reduction
product assigned as 242b indicated that ambiguity remained only
in the stereostructure of the core. There are two possible
diastereomeric structures, 25 and 26, with a (R)-â-chloroaza-
tyrosyl naphthoamide bridge, in which free rotation of the
azatyrosine unit is restricted.2b Only 26 is fully consistent with
a Reagents and conditions: (a) t-BuOCl, NaI, MeCN, H2O. (b)
TBSCl, imidazole, DMF, 71% (two steps). (c) PdCl2(PPh)2, N,N-
dimethylacetamide, THF, 60 °C. (d) 0.1 M HF, 0.1 M NaF, pH 5, THF,
73% (two steps). (e) 4 N HCl in 1,4-dioxane; then 11, EDC‚HCl, HOBt,
Et3N, DMF, 82%.
1
the reported NOEs and H-1H coupling constants:2b four key
NOEs between H4′ and H8, H5′ and H7′, H3 and H13, and H5
and H14b as well as two large coupling constants, J ) 9.9 and
9.5 Hz for H7′-H8a′ and for H13-H14b, respectively. The
NOE between H5′ and H7′ ruled out the possibility of 25, which
has the same core configuration as 24. Thus, the complete
structure of the kedarcidin chromophore should be revised to
that of 27.
Scheme 3a
a Reagents and conditions: (a) t-BuOCl, NaI, MeCN, H2O. (b)
MOMCl, K2CO3, acetone, 50 °C, 75% (two steps). (c) CH2dCHCO2-
t-Bu, Pd(OAc)2, Et3N, DMF, 60 °C, 90%. (d) (S)-(-)-benzyl-R-
methylbenzylamine, BuLi, THF, -80 °C, 1 h; then saturated NH4Cl
(aq), 83%. (e) Pd(OH)2/C, H2, MeOH, H2O, AcOH. (f) CF3CO2Et, Et3N,
MeOH, 88% (two steps). (g) TsOH, t-BuOH, 55 °C, 75%. (h) NCS,
DMF, 70 °C, 82%. (i) MOMCl, K2CO3, acetone, (j) K2CO3, MeOH,
H2O. (k) 11, EDC‚HCl, HOBt, CH2Cl2, 57% (three steps). (l) CF3CO2H;
then MeOH, EDC‚HCl, DMAP, CH2Cl2, 54%.
1
naphthoamide 2. The H and 13C NMR spectra as well as the
CD spectrum of synthetic 2, however, were not identical with
the reported data2b (see the Supporting Information).
We assumed that the chloroazatyrosine moiety of kedarcidin
would be the corresponding â-amino acid because of marked
differences in the NMR chemical shifts at C5′, 6′, 7′, and 8′ of
2. The optically active â-chloroazatyrosine derivative 3 was
synthesized via an enantio-face-selective conjugate addition of
Davies' chiral lithium amide to the R,â-unsaturated ester 18
(Scheme 3). We predicted the R configuration of the major
â-amino ester 19 (93:7) on the basis of Davies’ experimental
results.15 Subsequent selective hydrogenolysis of the benzyl
and phenethyl groups on the amine 19 proceeded under various
reduction conditions without destroying â-amino ester func-
tionality,15 but the chloride was always partially or completely
removed. Therefore, after the amino group was protected as a
trifluoroacetoamide, phenol 20 was regioselectively chlorinated
with NCS. Protection of the phenolic hydroxy group of 21 was
indispensable prior to the alkaline hydrolysis of trifluoroaceta-
mide, and the resulting amine was coupled with 11 to give
naphthoamide 22. Acidic hydrolysis of both the MOM ether
and tert-butyl ester groups followed by esterification yielded
(R)-â-chloroazatyrosyl naphthoamide 3. The spectral data (1H
and 13C NMR, UV, and CD spectra as well as optical rotation)
of 3 were identical with those reported for the fragment2b
degraded from kedarcidin chromophore (see the Supporting
Information). The R stereochemistry was confirmed by the
The present results should accelerate studies not only of
kedarcidin chromophore synthesis but also of interactions with
biopolymers at a molecular level.3
Acknowledgment. Financial support from the Ministry of Educa-
tion, Science, Sports, and Culture of Japan (Grants-in-Aid for Scientific
Research on Priority Areas No. 06240104 and 08245103) and a
fellowship to S.K. from the Japanese Society for the Promotion of
Science for Young Japanese Scientists are gratefully acknowledged.
Supporting Information Available: Spectral data of compounds
2, 3, 6-8, 10-12, 14, and 16-23 and synthetic scheme for 23 (21
pages). See any current masthead page for ordering and Internet access
instructions.
JA973231N
(16) Both enantiomers were synthesized by a conjugate addition of
hydroxylamine to the R,â-unsaturated carboxylic acid corresponding to 18
followed by the protection of the resulting amino acid (Boc2O, NaOH, 1,4-
dioxane, H2O; MeI, KHCO3, DMF) and subsequent optical resolution
(HPLC with CHIRALCEL OD).
(17) Kusumi, T.; Fukushima, T.; Ohtani, I.; Kakisawa, H. Tetrahedron
Lett. 1991, 32, 2939. Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H.
J. Am. Chem. Soc. 1991, 113, 4092.
(18) Leet, J. E.; Golik, J.; Hofstead, S. J.; Matson, J. A.; Lee, A. Y.;
Clardy, J. Tetrahedron Lett. 1992, 33, 6107.
(15) Davies, S. G.; Ichihara, O. Tetrahedron: Asymmetry 1991, 2, 183.
Costello, J. F.; Davies, S. G.; Ichihara, O. Tetrahedron: Asymmetry 1994,
5, 1999.