Stereoselective total synthesis of mucocin, an antitumor agent

Article abstract of DOI:10.1002/anie.200290038

The THP and THF rings which are regarded as the characteristic structural features of mucocin (1) were constructed by a SmI₂-induced reductive cyclization of β-alkoxy acrylate and oxidative cyclization of a homoallyl alcohol, respectively. The

Full text of DOI:10.1002/anie.200290038

COMMUNICATIONS
O
Stereoselective Total Synthesis of Mucocin, an
Antitumor Agent**
HO
O
HO
1
Me
12
4
20
O
O
Shunya Takahashi,* Akemi Kubota, and
Tadashi Nakata*
HO
OH
Me
Mucocin (1)
Mucocin (1) is a representative annonaceous acetogenin[1]
having a tetrahydropyran (THP) and a tetrahydrofuran
(THF) ring.[2] This novel type of acetogenin is known to show
remarkable inhibitory activities against A-549 (lung cancer)
and PACA-2 (pancreatic cancer) solid tumor lines with a
potency of more than 10000 times that of adriamycin. The
powerful antitumor activity and the unique structure of 1 have
consequently stimulated synthetic efforts,[3] and quite recently
three research groups (including ours) have succeeded in its
total synthesis.[4] Recently, we developed a highly efficient
method for the synthesis of tetrahydropyrans and oxepanes
based on a SmI2-induced[5] reductive cyclization of b-alkoxy
acrylate containing a formyl group.[6] We thus planned the
total synthesis of 1 to demonstrate the utility of the method
and now describe here an efficient total synthesis of 1 based
on the SmI2-induced reductive cyclization as a key step.
Our synthetic strategy directed toward 1 was based on a
convergent process which involves a Pd-catalyzed cross-
coupling reaction of the THP/THF segment 2 and a vinyl
iodide 3 as illustrated in Scheme 1. The THP ring in the
central core 2 could be constructed by the SmI2-induced
reductive cyclization, whereas the trans-THF ring should be
synthesized by oxidative cyclization[7] of a homoallyl alcohol.
This retrosynthetic approach can revert 2 back to the b-alkoxy
acrylate 4 having two formyl groups, and then to a tetraol 5.
Therefore, dialdehyde 6 with the requisite carbon backbone
was selected as the starting material. In this scenario, whether
the C-12 formyl group in the reductive cyclization of 4 could
be retained or not and the development of an efficient method
for desymmetrization of C2-symmetric 5 were problems to be
solved. On the other hand, the g-lactone 3 should be
synthesized by aldol condensation of chiral ester 7 and
aldehyde 8.[8]
O
MOMO
C10H21
O
TBDPSO
Me
+
O
I
O
MOMO
OMOM
2
3
THPO
OHC
Me
CHO
OHC
8
EtO2C
O
O
TBDPSO
CO2Me
OBn
O
MPMO
CHO
4
7
(MeO)2HC
HO
CH(OMe)2
OH
6
CHO
OH
OH
5
Scheme 1. Structure of mucocin (1) and its retrosynthetic analysis.
CHO
CH(OMe)2
a
b
CHO
CH(OMe)2
6
9
CH(OMe)2
CH(OMe)2
c, d
O
MeO
OMe
O
OH
RO
OH
HO
OH
e, f
OH
10: R = H
11: R = Bn
5
Synthesis began with acetalization of the dialdehyde 6[4a] to
afford the bis(dimethyl acetal) 9 in 99% yield (Scheme 2).
The Sharpless asymmetric dihydroxylation[9] of 9 gave the C2-
symmetric tetraol 5[10] in almost quantitative yield. To achieve
efficient desymmetrization 5 was transformed into a bisTHF
derivative 10 by treatment with CSA in methanol at ꢀ58C.
Formation of THP derivatives was observed when the
acetalization was conducted at RT. Benzylidenation of 10
followed by reduction with DIBAH gave a monobenzyl ether
S
S
g, h
S
S
O
HO
BnO
O
12
CHO
O
CHO
HO
OHC
i
20
24
EtO2C
O
O
O
H
H
OBn
O
EtO2C
OBn
O
4
[*]Dr. S. Takahashi, Prof. Dr. T. Nakata
RIKEN (The Institute of Physical and Chemical Research)
Wako-shi, Saitama 351-0198 (Japan)
Fax : (þ 81)48-462-4666
13
Scheme 2. Synthesis of the THP derivative 13: a) HC(OMe)3 (6.0 equiv),
CSA (0.02 equiv), MeOH, RT, 1 h, 99%; b) AD-mix a, MeSO2NH2, H2O/
tBuOH (1/1), 08C, 24 h; c) CSA (0.83 equiv), MeOH, ꢀ58C, 3 h, 89%
(2 steps); d) BnBr (1.1 equiv), NaH (1.1 equiv), THF, ꢀ158C!RT, 16 h,
93%; e) 1,3-propanedithiol (6.0 equiv), Zn(OTf)2 (1.5 equiv), (CH2Cl)2,
508C, 2 h; f) Me2C(OMe)2 (3.0 equiv), CSA (0.03 equiv), CH2Cl2, RT, 1 h,
86% (2 steps); g) ethyl propiolate (10 equiv), N-methylmorpholine
(5.0 equiv), CH2Cl2, RT, 5 h; h) CH3I (8.0 equiv), NaHCO3 (12 equiv),
MeCN/H2O (10/1), RT, 35 h, 73% (2 steps); i) SmI2 (3.0 equiv), MeOH
(4.4 equiv), THF, ꢀ58C, 15 min, 87%. CSA ¼ 10-camphorsulfonic acid,
Bn ¼ benzyl, Tf ¼ trifluoromethanesulfonyl.
E-mail: shunyat@riken.go.jp
nakata@riken.go.jp
A. Kubota
Graduate School of Science and Engineering
Saitama University, Saitama, Saitama 338-8570 (Japan)
[**]We are grateful to Dr. J. L. Mclaughlin (Purdue University) for
providing us with copies of the NMR spectra of mucocin. We also
thank Ms. K. Harata (RIKEN) for mass spectral measurements.
Angew. Chem. Int. Ed. 2002, 41, No. 24
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11 in 61% yield. After several attempts, a more efficient
method for preparing 11 was found. Direct benzylation of 10
with BnBr (1.1 equiv) and NaH (1.1 equiv) in THF at ꢀ158C
afforded 11 in 93% yield. At this stage, exchange of the
dimethylacetal into the corresponding dithioacetal was nec-
essary for the latter step. Transacetalization[11] of 11 was
RO
a, b
c – e
13
OMe
O
O
EtO2C
BnO
OR
14: R = H
15: R = MOM
MOM
MOM
O
achieved by using 6.0 equivalents of ethanedithiol in the
[12]
O
presence of 1.5 equivalents of Zn(OTf)2
in 1,2-dichloro-
f
ethane at 508C to give the desired bis(thioacetal) 12 in 86%
yield after isopropylidenation. Hetero-Michael addition of 12
to ethyl propiolate followed by dethioacetalization (CH3I,
NaHCO3, aq CH3CN) afforded the key intermediate 4 in 73%
yield. SmI2-induced reductive cyclization of 4 was effected by
treatment with 3.0 equivalents of SmI2 in the presence of
methanol (4.4 equiv) in THF at ꢀ58C to give a 20,24-syn-
23,24-trans-THP derivative 13[13,14] in 87% yield. The stereo-
chemistry of the THP ring system was established by NMR
analysis: J23,24 ¼ 9.9 Hz and an NOE effect was observed
between H20 and H24. The high stereoselectivity could be
explained by the transition state involving a cyclic chelate as
shown in Scheme 3. Very interestingly, the formyl group was
retained during this reaction. However, a prolonged reaction
time or the use of a large amount of SmI2 caused destruction
of this functional group and afforded higher polar substances
such as reduction or pinacol-type coupling products.
O
O
O
O
OMe
OH
C9H19
MOMO
OMOM
C9H19
MOMO
OMOM
16
17
MOM
O
g
h
OH
O
H
H
C9H19
OMOM OMOM
18
MOM
O
MOM
O
i, j
12
15
O
O
O
O
OH
C9H19
MOMO MOMO
C9H19
MOMO
OMOM
2
19
Scheme 4. Synthesis of the THP/THF segment 2: a) HC(OMe)3
(2.0 equiv), CSA (0.2 equiv), MeOH, 08C, 30 h, 93%; b) MOMBr
(4.0 equiv), iPr2NEt (8.0 equiv), (CH2Cl)2, 08C, 0.5 h; !408C, 7.5 h, 91%;
c) DIBAH (1.0 equiv), CH2Cl2, ꢀ788C, 1 h and then C8H17PPh3Br
(2.7 equiv), nBuLi (2.5 equiv), THF, ꢀ158C, 1 h, 91%; d) 10% Pd/C, H2
(1 bar), MeOH, RT, 24 h; e) MOMBr (4.0 equiv), iPr2NEt (8.0 equiv),
(CH2Cl)2, 08C, 0.5 h; !608C, 1 h, 95% (2 steps); f) AcOH/H2O (4/1), RT,
2 h; !408C, 1 h, 77%; g) CH3PPh3I (6.0 equiv), NaHMDS (5.0 equiv),
H
H
H
H
SmI2
O
SmI2
O
O
O
H
H
13
4
O
O
OEt
OEt
H
H
H
H
THF, ꢀ158C, 1 h; !RT, 1 h, 85%; h) [Co(modp)2](0.04 equiv), O
,
2
Scheme 3. SmI2-induced reductive cyclization of 4.
tBuO2H (2.0 equiv), 4ä-MS, iPrOH, 508C, 5 h, 78%; i) Tf2O (1.1 equiv),
ꢁ ꢀ
2,6-lutidine (2.3 equiv), CH2Cl2, ꢀ788C, 30 min; j) TMSC C Li
(8.0 equiv), HMPA (15 equiv), THF, ꢀ788C, 40 min, and then K2CO3
(1.1 equiv) MeOH, RT, 17 h, 90% (2 steps). MOM ¼ methoxymethyl,
DIBAH ¼ diisobutylaluminium hydride, NaHMDS ¼ sodium hexamethyl-
As several attempts for the construction of the THF ring
prior to completion of the C10 side chain gave unsatisfactory
results, the formyl group was protected as an internal acetal
with trimethyl orthoformate/CSA to give 14 in 93% yield
(Scheme 4). To enable subsequent chain elongation, the two
hydroxy groups were protected as methoxymethyl (MOM)
ethers to afford 15 (91% yield). Installation of the C10 unit on
15 was conducted in three steps: 1) DIBAH reduction of the
ester carbonyl group, 2) Wittig reaction (Ph3PþC8H17Brꢀ,
nBuLi), and 3) hydrogenation. After methoxymethylation,
exposure of the thus obtained 16 to mild acidic conditions led
to selective hydrolysis of the methyl acetal to give 17. The
resulting hemiacetal 17 was subjected to olefination with
disilazide,
[Co(modp)2] ¼ bis(1-morpholinocarbamoyl-4,4-dimethyl-1,3-
pentanedionato)cobalt(ii), Tf2O ¼ trifluoromethanesulfonic anhydride,
HMPA ¼ hexamethylphosphoric triamide, TMS ¼ trimthylsilyl.
Synthesis of terminal butenolide unit 3 began with asym-
metric allylation[16] of 20 to give 21 (> 98% ee) in 76% yield
(Scheme 5). After silylation of 21, hydroboration followed by
Jones oxidation and methylation gave methyl ester 7 in 58%
yield. Aldol reaction of the lithium enolate prepared from 7
with the chiral aldehyde 8[8a] gave a mixture of coupled
products, which was, without purification, submitted sequen-
tially to lactonization, mesylation, and b-elimination reaction,
giving butenolide 22 in 67% overall yield. After deprotection
of the MPM group, the resulting alcohol was oxidized to an
aldehyde, which was then transformed into the corresponding
vinyl iodide 3 according to the method of Takai et al.[17]
The complete carbon skeleton of 1 was assembled by
joining 2 and 3 under Hoye×s conditions[18] to give 23 in 84%
yield (Scheme 6). Finally, regioselective reduction of 23 using
Wilkinson©s catalyst gave 24 in which all the protecting groups
were removed to afford mucocin (1). The spectral data of 1
were indistinguishable from those of the natural product.
¼
Ph3P CH2 to furnish 18 in 85% yield. The homoallyl alcohol
18 was oxidized with tBuO2H in the presence of [Co(modp)2]
in an oxygen atmosphere to give rise to 5-exo cyclization[7a]
and afford alcohol 19 with a 12,15-trans THF ring system. The
structure of 19 was established by comparison with an
authentic sample, which had been previously synthesized by
us.[4b,f] Transformation of 19 into the ™left-half∫ segment 2[15]
proceeded in high overall yield (90%) as follows: 1) triflation
of the primary hydroxy group, 2) a coupling reaction with
lithium trimethylsilylacetylide, and 3) desilylation with potas-
sium carbonate.
4752
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0044-8249/02/4124-4752 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 24

COMMUNICATIONS
pp. 249 310; d) U. Koert, Synthesis 1995, 115 132; e) R. Hoppe,
H.-D. Scharf, Synthesis 1995, 1447 1464; f) B. Figadere, Acc. Chem.
Res. 1995, 28, 359 365; g) M. C. Zafra-Polo, M. C. Gonzalez, E.
Estornell, S. Sahpaz, D. Cortes, Phytochemistry 1996, 42, 253 271;
h) L. Zeng, Q. Ye, N. H. Oberlies, G. Shi, Z.-M. Gu, K. He, J. L.
McLaughlin, Nat. Prod. Rep. 1996, 13, 275 306; i) M. C. Zafra-Polo,
B. Figadere, T. Gallardo, J. R. Tormo, D. Cortes, D. Phytochemistry
1998, 48, 1087 1117; j) F. Q. Alali, X.-X. Liu, J. L. McLaughlin, J. Nat.
Prod. 1999, 62, 504 540, and references therein.
HO
b – d
a
MPMO
MPMO
CHO
21
20
THPO
OHC
Me
, e – h
TBDPSO
CO2Me
8
MPMO
MPMO
[2]G. Shi, D. Alfonso, M. O. Fatope, L. Zeng, Z-m. Gu, G-x. Zhao, K. He,
J. M. MacDougal, J. L. McLaughlin, J. Am. Chem. Soc. 1995, 117,
10409 10410.
7
O
O
[3]a) M. T. Crimmins, S. W. Rafferty, Tetrahedron Lett. 1996, 37, 5649
5652; b) P. A. Evans, J. D. Roseman, Tetrahedron Lett. 1997, 38, 5249
5252; c) P. A. Evans, V. S. Murthy, Tetrahedron Lett. 1998, 39, 9627
9628; d) P. A. Evans, V. S. Murthy, Tetrahedron Lett. 1999, 40, 1253
1256; e) S. Takahashi, K. Fujisawa, N. Sakairi, T. Nakata, Heterocycles
2000, 53, 1361 1370; f) S. Hoppen, U. Emde, T. Friedrich, L. Grubert,
U. Koert, Angew. Chem. 2000, 112, 2181 2184; Angew. Chem. Int. Ed.
2000, 39, 2099 2102.
[4]a) P. Neogi, T. Doundoulakis, A. Yazbak, S. C. Sinha, S. C. Sinha, E.
Keinan, J. Am. Chem. Soc. 1998, 120, 11279 11284; b) S. Takahashi,
T. Nakata, Tetrahedron Lett. 1999, 40, 723 726; c) S. Takahashi, T.
Nakata, Tetrahedron Lett. 1999, 40, 727 730; d) S. B‰urle, S. Hoppen,
U. Koert, Angew. Chem. 1999, 111, 1341 1344; Angew. Chem. Int. Ed.
1999, 38, 1263 1266; e) S. Hoppen, S. Baurle, U. Koert, U. Chem. Eur.
J. 2000, 6, 2382 2396; f) S. Takahashi, T. Nakata, J. Org. Chem. 2002,
67, 5739 5752.
[5]For reviews, see a) H. B. Kagan, New J. Chem. 1990, 14, 453 460;
b) G. A. Molander, Chem. Rev. 1992, 92, 29 68; c) G. A. Molander,
C. R. Harris, Chem. Rev. 1996, 96, 307 338.
[6]a) N. Hori, H. Matsukura, G. Matsuo, T. Nakata, Tetrahedron Lett.
1999, 40, 2811 2814; b) G. Matsuo, N. Hori, T. Nakata, Tetrahedron
Lett. 1999, 40, 8859 8863; c) N. Hori, H. Matsukura, T. Nakata, Org.
Lett. 1999, 1, 1099 1101; d) N. Hori, H. Matsukura, G. Matsuo, T.
Nakata, Tetrahedron 2002, 58, 1853 1864; e) G. Matsuo, H. Kadoha-
ma, T. Nakata, Chem. Lett. 2002, 148 149.
O
TBDPSO
O
TBDPSO
i – k
Me
Me
I
22
3
Scheme 5. Synthesis of the lactone 3: a) (S)-binol (0.2 equiv), Ti(OiPr)4,
(0.2 equiv), allyltributyltin (1.1 equiv), ꢀ788C, 1 h; !ꢀ258C, 5 d, 76%;
b) TBDPSCl (1.1 equiv), imidazole (2.5 equiv), DMF, RT, 6 h; c) BH3¥THF
(1.5 equiv), THF, 08C!RT, 24 h and then NaOH, 30% H2O2, 08C!RT,
1 h; d) Jones reagent, acetone, 08C, 30 min and then CH2N2, diethyl ether,
08C, 30 min, 58% (3 steps); e) LDA (1.6 equiv), 8 (2.0 equiv), THF, ꢀ788C,
1 h; f) CSA (1.1 equiv), MeOH/H2O (9/1), RT, 4 h; g) MsCl (3.0 equiv),
Et3N (5.0 equiv), CH2Cl2, ꢀ108C, 1.5 h; h) DBU (2.0 equiv), CH2Cl2,
ꢀ108C, 30 min, 67% (4 steps); i) DDQ (2.0 equiv), CH2Cl2/phosphate
buffer (pH 7.4, 10/1), 08C, 2 h; j) (COCl)2 (3.0 equiv), DMSO (6.0 equiv),
CH2Cl2, ꢀ788C, 30 min, and then Et3N (12 equiv), ꢀ788C!RT, 2 h;
k) CrCl2 (6.8 equiv), CHI3 (2.3 equiv), THF, 08C!RT, 4 h, 83% (3 steps).
binol ¼ bi-2-naphthol, TBDPS ¼ tert-butyldiphenylsilyl, LDA ¼ lithium di-
isopropylamide, DBU ¼ 1,8-diazabicyclo[5.4.0]undec-7-ene, DDQ ¼ 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone, Ms ¼ methanesulfonyl, DMSO ¼
dimethylsulfoxide.
MOM
O
[7]a) S. Inoki, T. Mukaiyama, Chem. Lett. 1990, 67 70; b) R. M.
Kennedy, S. Tang, Tetrahedron Lett. 1992, 33, 3729 3732; c) S. Tang,
R. M. Kennedy, Tetrahedron Lett. 1992, 33, 5299 5302; d) S. Tang,
R. M. Kennedy, Tetrahedron Lett. 1992, 33, 5303 5306; e) R. S.
Boyce, R. M. Kennedy, Tetrahedron Lett. 1994, 35, 5133 5136.
[8]a) H. Naito, E. Kawahara, K. Maruta, M. Maeda, S. Sasaki, J. Org.
Chem. 1995, 60, 4419 4427; b) T.-L. Wang, X. E. Hu, J. M. Cassady,
Tetrahedron Lett. 1995, 36, 9301 9304.
a
b, c
O
2
O
O
+ 3
C9H19
MOMO
OMOM
O
O
TBDPSO
23
Me
Me
O
[9]H. C. Kolb, M. S. VanNieuwenhze, K. B. Sharpless, Chem. Rev. 1994,
94, 2483 2547.
R1O
R2O
[10]The enantiopurity ( > 97% ee) was determined by the 1H NMR
analyses of the corresponding MTPA esters of 12. Furthermore, the
optical purity could be improved by recrystallization of 5 from
hexane/ethyl acetate to give an optically pure 5, m.p. 82 82.58C
(n-hexane/ethyl acetate), [a]2D6 ¼ ꢀ27.98 (c ¼ 0.84, CHCl3).
O
O
R1O
OR1
24: R1 = MOM, R2 = TBDPS
1: R1 = R2 = H (mucocin)
Me
[11]The thioacetalization of 11 with ethanedithiol and other Lewis acids
such as BF3¥Et2O and TiCl4 furnished the corresponding bis(thioace-
tal) in a low yield because of the unstability of the benzyl ether moiety
under the conditions employed.
Scheme 6. Total synthesis of mucocin (1): a) [PdCl2(Ph3P)2](0.1 equiv),
CuI (0.3 equiv), Et3N, 0 8C!RT, 1 h, 84%; b) (Ph3P)3RhCl, H2, C6H6/EtOH
(1/1), RT, 48 h, 85%; c) 10% HCl/MeOH, CH2Cl2, 08C!RT, 95 %.
[12]E. J. Corey, K. Shimoji, Tetrahedron Lett. 1983, 24, 1289 1292.
[13]Data for 13: [a]2D6 ¼ ꢀ41.18 (c ¼ 1.09 in CHCl3); 1H NMR (400 MHz,
C6D6): d ¼ 9.33 (t, J ¼ 1.5 Hz, 1H), 7.40 7.07 (m, 5H), 4.55 (dd, J ¼ 11.7,
11.7 Hz, 2H), 4.00 (m, 2H), 3.70 (ddd, J ¼ 8.8, 8.8, 4.3 Hz, 1H), 3.49,
(m, 2H), 3.44 (ddd, J ¼ 8.3, 5.4, 2.5 Hz, 1H), 3.36 (m, 1H), 3.10 (ddd,
J ¼ 9.8, 8.8, 4.3 Hz, 1H), 2.94 (dd, J ¼ 15.1, 3.4 Hz, 1H), 2.53 (dd, J ¼
15.1, 8.8 Hz, 1H), 2.20 2.02 (m, 2H), 1.85 1.37 (m, 2H), 1.70 1.59 (m,
8H), 1.35 (s, 3H), 1.34 (s, 3H), 0.95 ppm (t, J ¼ 7.3 Hz, 3H); 13C NMR
(100 MHz, C6D6): d ¼ 200.4, 171.9, 139.8, 128.5, 128.2, 128.0,
127.9, 127.8, 108.2, 80.9, 80.7, 80.3, 79.7, 73.1, 72.8, 70.0, 60.4,
40.5, 38.6, 33.1, 29.0, 27.6, 27.4, 27.2, 26.4, 25.3, 14.2 ppm; IR
(neat): n˜ ¼ 3447, 2984, 2936, 2868, 1732, 1456, 1379, 1371, 1348, 1302,
In summary, we have succeeded in an efficient synthesis of 1
through desymmetrization of 5, an SmI2-induced radical
cyclization reaction of 4, and oxidative cyclization of 18 as
the key steps. This procedure would also be useful for the
preparation of a variety of analogues of 1.
Received: September 11, 2002 [Z50140]
[1]For reviews, see a) J. K. Rupprecht, Y.-H. Hui, J. L. McLaughlin, J.
Nat. Prod. 1990, 53, 237 278; b) X.-P. Fang, M. J. Rieser, Z.-M. Gu,
G.-X. Zhao, J. L. McLaughlin, Phytochem. Anal. 1993, 4, 27 67; c) Z.-
M. Gu, G.-X. Zhao, N. H. Oberlies, L. Zeng, J. L. McLaughlin, Recent
Advances in Phytochemistry, Vol. 29, Plenum, New York, 1995,
1248, 1221, 1180, 1159, 1092, 1029, 950, 921, 887, 748, 700, 683 cmꢀ1
;
HRMS calcd for C27H40O8Na [MþNa]þ: m/z 515.2621, found:
515.2621.
[14]The numbering of mucocin was adopted.
Angew. Chem. Int. Ed. 2002, 41, No. 24
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COMMUNICATIONS
[15]Data for 2: [a]2D6 ¼ ꢀ61.48 (c ¼ 1.04 in CHCl3); 1H NMR (400 MHz,
CDCl3): d ¼ 4.81 4.59 (m, 6H), 4.10, (m, 2H), 3.47 (m, 2H), 3.39 (s,
3H), 3.38 (s, 3H), 3.36 (s, 3H), 3.33 (m, 1H), 3.21 (m, 1H), 3.10 (m,
1H), 2.47 (ddd, J ¼ 4.9, 2.9, 1.7 Hz, 1H), 2.35 (ddd, J ¼ 7.3, 2.4, 1.7 Hz,
1H), 2.24 1.22 (m, 12H), 1.25 (br s, 18H), 0.88 ppm (t, J ¼ 6.8 Hz,
3H); 13C NMR (75 MHz, CDCl3): d ¼ 96.9, 96.8, 95.3, 81.6, 81.0, 80.9,
79.7, 79.5, 79.1, 77.1, 75.8, 69.6, 55.7, 55.5, 32.1, 31.9, 31.3, 30.1, 29.7, 29.6,
29.3, 28.2, 26.9, 26.6, 26.3, 25.5, 25.3, 22.7, 14.1 ppm; IR (neat): n˜ ¼
3312, 3273, 2926, 2855, 2823, 1465, 1457, 1396, 1375, 1363, 1340, 1293,
1214, 1150, 1103, 1075, 1036, 918 cmꢀ1; HRMS calcd for C32H58O8Na
[MþNa]þ: m/z 593.4029, found: m/z 593.4037.
bulges, arising from slippage of DNA strands, particularly in
homopolymeric tracts of DNA, can result in mutation
™hotspots∫ due to DNA strand misalignment during replica-
tion.[1] Addition or deletion mutations at these sites can arise
according to whether the extra base stacks within the DNA
helix or is displaced into solution. Unpaired pyrimidines have
been shown to adopt either an intrahelical or extrahelical
conformation depending on flanking sequence, while purines
preferentially stack within the helix.[2] The interaction of drug
molecules at bulge sites has been of considerable interest
since binding close to these sites has the potential to stabilize
and increase the lifetime of the bulge state and its suscept-
ibility to frameshift mutations. It has already been demon-
strated that ethidium bromide and 9-aminoacridine have
increased affinity for sites on duplex DNA carrying an
extrahelical cytosine,[3] while neocarzinostatin induces highly
efficient site-specific strand cleavage at bulge sites in folded
single-stranded DNA.[4] In other cases, chemical modification
(base alkylation) of DNA and RNA by drugs appears to occur
primarily near bulge sites because of better binding site
access,[5] while selective drug binding to RNA bulges has been
shown to inhibit protein RNA recognition.[6] In general,
detailed structural information is lacking, although a number
of low-resolution NMR studies of bulge recognition by simple
DNA intercalators have been described.[3]
[16]G. E. Keck, D. Krishnamurthy, M. C. Grier, J. Org. Chem. 1993, 58,
6543 6544.
[17]K. Takai, K. Nitta, K. Utimoto, J. Am. Chem. Soc. 1986, 108, 7408
7410.
[18]T. R. Hoye, Z. Ye, J. Am. Chem. Soc. 1996, 118, 1801 1802.
Structure of a Drug-Induced DNA T-Bulge:
Implications for DNA Frameshift Mutations**
Michelle L. Colgrave, Huw E. L. Williams, and
Mark S. Searle*
The biological function of nucleic acids is recognized to
embrace a wide variety of structural topologies whose
stability, conformation, and dynamics may be strongly influ-
enced by the interaction of drug molecules. Single-base
The anthracycline antibiotic nogalamycin (Figure 1) has a
high sequence specificity for 5’-CG or 5’-TG sites with its
bound orientation dictated by the requirement for a guanine
(G) base on the 3’-side of the intercalation site.[7] Nogalamycin
Figure 1. a) Structure of nogalamycin, and oligonucleotide sequences H1 (b) and H2 (c) in their proposed folded conformation with frayed ends.
Conformations and base-pair alignment adopted in the bound stated with nogalamycin intercalated at the 5’-TpG step ((d) and (e)).
threads its aglycon chromophore through the DNA helix and
[*]Dr. M. S. Searle, Dr. M. L. Colgrave, Dr. H. E. L. Williams
positions bulky sugar residues in both grooves simultaneously.
Interactions in the major groove involving the two hydroxy
School of Chemistry
University Park, Nottingham NG7 2RD (UK)
groups on the bicycloaminoglucose sugar appear to account
Fax : (þ 44)115-951-3564
for the drugs orientational specificity through hydrogen
bonding to guanine. Thus, binding to the 5’-TG site results
in a unique binding orientation dictated by the G base. With
E-mail: mark.searle@nottingham.ac.uk.
[**]We thank the EPSRC of the UK, the Australian Research Council,
and Boehringer Ingelheim Fonds for supporting this work.
4754
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4124-4754 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 24