Synthesis of pluraflavin A "aglycone"

Article abstract of DOI:10.1021/ja805936v

The "aglycone" of pluraflavin A (2) has been synthesized. The key features of this synthesis include a 1,3-dipolar cycloaddition between a nitrile oxide (cf. 14) and an olefin (22) to yield an isoxazoline followed by subsequent conversion into the γ-pyrone of pluraflavin A. The epoxide moiety linked to the pyrone is installed prior to Diels-Alder installation of the D ring, which allows access to a number of potentially active cytotoxic intermediates en route to the final compound. The preliminary in vitro results of two such compounds are also included with the racemic title compound exhibiting cytotoxicity in the nanomolar range.

Full text of DOI:10.1021/ja805936v

Published on Web 11/17/2008
Synthesis of Pluraflavin A “Aglycone”
Benjamin J. D. Wright,† John Hartung,† Feng Peng,† Ryan Van de Water,‡
Haibo Liu,‡ Quen-Hui Tan,§ Ting-Chao Chou,§ and Samuel J. Danishefsky*,†,‡
Department of Chemistry, Columbia UniVersity, HaVemeyer Hall, 3000 Broadway, New York,
New York 10027, Laboratory for Bioorganic Chemistry and Preclinical Pharmacology,
Sloan-Kettering Institute for Cancer Research, 1275 York AVenue, New York, New York 10065
Received July 29, 2008; E-mail: s-danishefsky@ski.mskcc.org
Abstract: The “aglycone” of pluraflavin A (2) has been synthesized. The key features of this synthesis
include a 1,3-dipolar cycloaddition between a nitrile oxide (cf. 14) and an olefin (22) to yield an isoxazoline
followed by subsequent conversion into the γ-pyrone of pluraflavin A. The epoxide moiety linked to the
pyrone is installed prior to Diels-Alder installation of the D ring, which allows access to a number of
potentially active cytotoxic intermediates en route to the final compound. The preliminary in vitro results of
two such compounds are also included with the racemic title compound exhibiting cytotoxicity in the
nanomolar range.
resources in oncology,8 we naturally followed emerging devel-
opments in the pluraflavin area.
Introduction
Since Maeda et al. first described the parent compound in
1956, the pluramycin (1) family of antitumor antibiotics has
expanded both in terms of structural diversity and biological
underpinnings.1 Studies have shown that, as a general mode of
action, the members of this family containing a C14/C15 oxirane
undergo apparent sequence-selective electrophilic attack upon
N7 in guanine in the major groove of DNA.2-6 This covalent
interaction leads to DNA damage and, in time, tumor cell death.
Information on the cytotoxicity of the pluramycin family has
been continually updated with the isolation of new entries. By
contrast, relatively few approaches to their total synthesis had
been reported.9,10 More recently, however, a number of elegant
attempts in this field have been described by the groups of
McDonald, Suzuki, and Tietze.11-18 This recent work in the
pluramycin field has culminated in the successful syntheses of
the aglycones of altromycin and kidamycin,12,18 both γ- and
δ-indomycinones,13,14,17,19 and episcufolin16 through a variety
of synthetic strategies. To date, no synthetic strategies toward
pluraflavin A have been described. In time, our own interest in
the pluraflavins matured into a research program aimed at
pluraflavin A. As is our custom, our efforts tend to start with
an undertaking in the total synthesis of the natural product itself.
A concurrent goal is the development of an implementable
synthesis plan, which also lays open the natural target molecule
to levels of molecular editing which are not necessarily
accessible from the natural product itself.8 We took note that
Isolated from cultures of Saccharothrix sp. DSM 12931, the
pluraflavins were first reported by Ve´rtesy and co-workers in
2001 to be pluramycin-like in terms of their structure.7
Furthermore, they showed that the epoxide-bearing isolate,
pluraflavin A (2, Figure 1), exhibits excellent potency against
a number of human tumor cell lines in the low to subnanomolar
range.7 Structurally, pluraflavins are unique in the family due
the substitution pattern at C5. This center bears a hydroxymethyl
function O-linked to a 3′-epi-vancosamine moiety. Given the
ongoing interest in our laboratory in the total synthesis and
diverted total synthesis of natural products as discovery
(8) Wilson, R. M.; Danishefsky, S. J. J. Org. Chem. 2006, 71, 8329–
8351.
(9) Hauser, F. M.; Rhee, R. P. J. Am. Chem. Soc. 1979, 101, 1628–1629.
(10) Uno, H.; Sakamoto, K.; Honda, E.; Ono, N. Chem. Commun. 1999,
1005–1006.
† Columbia University.
‡ Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for
Cancer Research.
(11) Fei, Z. B.; McDonald, F. E. Org. Lett. 2007, 9, 3547–3550.
(12) Fei, Z. B.; McDonald, F. E. Org. Lett. 2005, 7, 3617–3620.
(13) Hsu, D. S.; Matsumoto, T.; Suzuki, K. Chem. Lett. 2006, 35, 1016–
1017.
§ Preclinical Pharmacology, Sloan-Kettering Institute for Cancer Research.
(1) Maeda, K.; Takeuchi, T.; Nitta, K.; Yagishita, K.; Utahara, R.; Osato,
T.; Ueda, M.; Kondo, S.; Okami, Y.; Umezawa, H. J. Antibiot. 1956,
9, 75–81.
(14) Tietze, L. F.; Singidi, R. R.; Gericke, K. M. Org. Lett. 2006, 8, 5873–
5876.
(2) Sun, D.; Hansen, M.; Clement, J. J.; Hurley, L. H. Biochemistry 1993,
32, 8068–8074.
(15) Tietze, L. F.; Gericke, K. M.; Singidi, R. R. Angew. Chem., Int. Ed.
2006, 45, 6990–6993.
(3) Hansen, M.; Yun, S.; Hurley, L. Chem. Biol. 1995, 2, 229–240.
(4) Murray, V.; Moore, A. G.; Matias, C.; Wickham, G. Biochim. Biophys.
Acta 1995, 1261, 195–200.
(16) Tietze, L. F.; Gericke, K. M.; Singidi, R. R.; Schuberth, I. Org. Biomol.
Chem. 2007, 5, 1191–1200.
(5) Prakash, A. S.; Moore, A. G.; Murray, V.; Matias, C.; McFadyen,
W. D.; Wickham, G. Chem. Biol. Interact. 1995, 95, 17–28.
(6) Hansen, M.; Hurley, L. J. Am. Chem. Soc. 1995, 117, 2421–2429.
(7) Vertesy, L.; Barbone, F. P.; Cashmen, E.; Decker, H.; Ehrlich, K.;
Jordan, B.; Knauf, M.; Schummer, D.; Segeth, M. P.; Wink, J.; Seibert,
G. J. Antibiot. 2001, 54, 718–729.
(17) Tietze, L. F.; Singidi, R. R.; Gericke, K. M. Chem.sEur. J. 2007, 13,
9939–9947.
(18) Fei, Z.; McDonald, F. E. Abstracts of Papers, 232nd ACS National
Meeting, San Francisco, CA, Sept. 10-14, 2006; ORGN-717.
(19) Krohn, K.; Tran-Thien, H. T.; Vitz, J.; Vidal, A. Eur. J. Org. Chem.
2007, 1905–1911.
9
16786 J. AM. CHEM. SOC. 2008, 130, 16786–16790
10.1021/ja805936v CCC: $40.75
2008 American Chemical Society

Synthesis of Pluraflavin A “Aglycone”
A R T I C L E S
Figure 1. Structures of pluramycin (1), pluraflavin A (2), and its des-carbohydrate (3).
Scheme 1. Proposed Isoxazole-Based Approach to Pluraflavin A (2) from 410,21
Scheme 2. Synthesis of Oxime 14a
pluraflavin A contains both C and O glycosidic linkages. We
defined as our first subgoal the synthesis of 3, corresponding to
the “aglycone”20 of pluraflavin A. This compound, or congeners
thereof, would serve to inform whether useful biological function
is available in the absence of glycosidic domains. If this were
not the case, incorporation of handles to enable C-glycosylation
at C10 (see asterisk, Figure 1) could be undertaken. The
biological consequences of the O-glycosylation at C13 could also
be evaluated. At the level of chemical synthesis, there was
concern from the outset about possible functional group
management issues which would have to be addressed in the
co-installation of the C14/C15 oxirane and the labile glycosidic
bonds attaching the epi-vancosamine and oliose domains.
Our plan of synthesis envisioned starting with the known
naphthalene derivative 4 (Scheme 1). It was further anticipated
that the D ring could be fashioned from a Diels-Alder dynamic
on a suitable juglone-like version of 4. In addition, a formyl
group would be introduced at C4a (pluraflavin numbering, see
asterisk in compound 4). Since this center is ortho to an existing
phenolic function, a variety of options for the overall formylation
could be entertained (vida infra). At this level of planning, we
leave open the question of timing between D ring introduction
and C4a formylation. In any case, the aldehyde at C4a would be
converted to an oxime which, upon elaboration to the corre-
sponding nitrile oxide, would give rise to an isoxazole (see
5f6).
a Key: (a) K2CO3, EtOH; (b) TBSCl, imidazole, DMF, 90% (over two
steps); (c) LiAlH4, THF, 92%; (d) MOMCl, Pr2NEt, CH2Cl2; (e) TBAF,
THF, 76% (over two steps); (f) (CH2O)n, Et2AlCl, CH2Cl2; (g) BnBr, CsCO3,
DMF/acetone (2:3), 53% (over two steps, 59% based on recovered 12); (h)
TPAP, NMO, 4 Å MS, CH2Cl2; (i) NH2OH·HCl, NaOH, EtOH/H2O (2:1),
73% (over two steps).
i
when one of us was engaged as a postdoctoral fellow at
Columbia University.21 To service the case at hand, it would
of course be necessary to synthesize an appropriately function-
alized isoxazole. It was with this subgoal in mind that our
journey commenced.
The synthesis diverged from the known ester 9, which was
prepared in three steps from commercially available 2-chloro-
1,4-dimethoxybenzene according to the protocol of Bloomer and
co-workers (Scheme 2).22 Transformation of 9f10 was ac-
complished in three steps as shown (vida infra). Following
protection of the alcohol as its MOM ether and cleavage of the
silyl protecting group, phenol 11 was in hand. Employing the
hydroxymethylation conditions of Casiraghi et al., phenol 11
was converted into diol 12.23 Selective benzylation of the phenol,
under mediation by cesium carbonate, and subsequent oxidation
It was envisioned that opening of the isoxazole, in some as-
yet-unspecified manner, would set the stage for cyclization to
γ-pyrone with emergence of a side chain at C2 eventually
destined to serve as the required 2,3-oxiranyl sec-butyl group.
Below, we describe the realization of our first milestone in the
project, that is, the total synthesis of 3.
Results and Discussion
Chemistry. Our collective fascination with using isoxazoles
to store valuable functionality, which is exposed through a well-
directed ring cleavage reaction, dates back more than 45 years
(20) Technically, the term aglycone is used to describe natural products
lacking an O-glycoside, so in the case of pluraflavin A, one might
also call the title compound the “des-carbohydrate.”
(21) Stork, G.; Danishefsky, S.; Ohashi, M. J. Am. Chem. Soc. 1967, 89,
5459–5460.
(22) Bloomer, J. L.; Stagliano, K. W.; Gazzillo, J. A. J. Org. Chem. 1993,
58, 7906–7912.
(23) Bigi, F.; Casiraghi, G.; Casnati, G.; Sartori, G.; Gasparri Fava, G.;
Ferrari Belicchi, M. J. Org. Chem. 1985, 50, 5018–5022.
9
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A R T I C L E S
Wright et al.
Scheme 3. Synthesis of Isoxazole 17a
Scheme 4. Synthesis of Enamide 19 and Failed Conversion into
Pyrone 20a
a Key: (a) PPh3, NBS, NaSO2Ph, THF, 77%; (b) (i) 13, NH2OH·HCl,
t
NaOH, BuOH/H2O (2:1), 70 °C, 15 min; (ii) rt, NaHCO3, chloramine-T;
(iii) CuSO4 ·5H2O, Cu (wire), alkyne 16, 60% (over three steps from 13).
led to an aldehyde (13).24 As a side note, direct formylation25
of 11 was also possible, but the two-step protocol was more
amenable to bulk preparation of this particular material. The
aldehyde function was converted to the oxime, 14, the key
precursor for our projected 1,3-dipolar cycloaddition step.
Accordingly, our attention now turned to preparation of a
functionalized alkyne for the synthesis of pluraflavin A. The
most appropriate alkyne seemed to be 3-methylpent-3-en-1-yne.
Its synthesis, both as a mixture of alkene isomers26 as well as
a pure compound, had been previously described.27 Neverthe-
less, in our hands, challenges to obtaining required quantities
of the desired Z-isomer were particularly complicated by its
volatility. It was decided to add a functional group which would
allow for more straightforward synthesis and handling of the
required moiety. It goes without saying that the group selected
for more convenient material management had to be readily
removable at a later stage. With these considerations well in
mind, we elected to work with a sulfone function, in particular,
that of compound 16. Its synthesis from the readily available
1528 using methodology developed by Murakami29 is shown in
Scheme 3. Compound 16 was then to be combined with the
nitrile oxide derived from aldehyde 13, which often requires
multiple synthetic manipulations. Happily, however, the con-
venient conditions, reported by Fokin and co-workers, were
effectively employed to convert 13 into isoxazole 17.30
For ring opening of the isoxazole, our original plan contem-
plated Raney-Nickel-induced reductive cleavage of the N-O
bond. It was also expected that the system would be subject to
concurrent removal of the benzyl and the sulfone functions.
While we were able to open the isoxazole ring and deprotect
the phenol, in practice, removal of the sulfone was complicated
by competing reduction of the aryl chloride (see asterisk on
17) under the forcing conditions which were required (Schemes
3 and 4).
a Key: (a) OsO4 (cat.), NMO, acetone/H2O (3:1); (b) pTsOH (cat.),
acetone/2,2′-dimethoxypropane (1:1), 68% (over two steps); (c) Raney-Ni,
EtOH, 80°C, 18 h, 42% (X ) Cl), 16% (X ) H).
An even more persistent problem presented itself in the
attempted preparation of pyrone 20. In our hands, the hydrolysis
and subsequent dehydrative cyclization required of enamide 19
(or related enamides) could not be accomplished. Despite the
multitude of related examples which would have suggested that
the conversion to 20 should be possible, for some reason, the
precedents did not apply to our case.32-39
It seemed that we might be able to circumvent the challenge
at hand by recourse to an isoxazoline in place of the afore-
described isoxazole. The difficulty in the hydrolysis/cyclization
of the vinylogous amide (cf. 19f20) was indeed surprising,
but we hoped that matters might proceed more smoothly if the
product of the ring cleavage were an imine rather than the
vinylogous amide arising from reduction of an isoxazole. This
hypothesis was of course supported by well-known precedents
for the conversion of isoxazolines into their corresponding
ꢀ-hydroxy ketones.40,41 Obviously, the original enamide (19)
was in the oxidation state required for the synthesis of pyrone.
By contrast, a formal hydroxy imine derived from reduction of
an isoxazoline would require subsequent oxidation to reach our
subgoal. Fortunately, this modification did, in fact, lead to the
anthrapyran core of pluraflavin A (vida infra).
The known racemic alcohol 21,42 prepared in five steps from
tiglic acid, was converted into olefin 22 by PCC oxidation
followed by Wittig olefination.43 The alkene (22) then underwent
straightforward thermal cycloaddition with the presumed nitrile
oxide derived from oxime 14. There was obtained a ∼3:2
mixture of stereoisomers 23.44 According to the protocol due
Fortunately, allylic sulfone 17 could readily be converted into
acetonide 18 under a standard two-step protocol.31 As indicated,
upon heating with Raney-Nickel in ethanol, 18 did undergo the
desired multistep process to yield 19 in a single laboratory
operation; however, the sulfone function was not removed
cleanly under these conditions. Upon exposure to higher
temperatures and longer reaction times, the sulfone was partially
removed; however, the desulfonylated material suffered from
contamination with dechlorinated compound.
(32) Zen, S.; Harada, K.; Nakamura, H.; Iitaka, Y. Bull. Chem. Soc. Jpn.
1988, 61, 2881–2884.
(33) Jones, R. C. F.; Duller, K. A. M.; Vulto, S. I. E. J. Chem. Soc., Perkin
Trans. 1 1998, 411–416.
(34) Saxena, R.; Singh, V.; Batra, S. Tetrahedron 2004, 60, 10311–10320.
(35) Scott, M. S.; Luckhurst, C. A.; Dixon, D. J. Org. Lett. 2005, 7, 5813–
5816.
(24) Ley, S.; Norman, J.; Griffith, W.; Marsden, S. Synthesis 1994, 639–666.
(25) Hofslokken, N. U.; Skattebol, L. Acta Chem. Scand. 1999, 53, 258–262.
(26) Oroshnik, W.; Mebane, A. D. J. Am. Chem. Soc. 1954, 76, 5719–
5736.
(36) Sundberg, R. J.; Pearce, B. C. J. Org. Chem. 1985, 50, 425–432.
(37) Kenar, J. A. J. Am. Oil Chem. Soc. 2003, 80, 1027–1032.
(38) Kurykin, M. A.; Vol’pin, I. M.; German, L. S. J. Fluorine Chem. 1996,
80, 9–12.
(27) Biagetti, M.; Bellina, F.; Carpita, A.; Viel, S.; Mannina, L.; Rossi, R.
Eur. J. Org. Chem. 2002, 1063–1076.
(39) Iijima, I.; Taga, N.; Miyazaki, M.; Tanaka, T.; Uzawa, J. J. Chem.
Soc., Perkin Trans. 1 1979, 3190–3195.
(28) Mori, K.; Ohki, M.; Sato, A.; Matsui, M. Tetrahedron 1972, 28, 3739–
3745.
(40) Curran, D. P. J. Am. Chem. Soc. 1982, 104, 4024–4026.
(41) Kozikowski, A. P.; Adamczyk, M. Tetrahedron Lett. 1982, 23, 3123–
3126.
(29) Murakami, T.; Furusawa, K. Synthesis 2002, 479–482.
(30) Hansen, T. V.; Wu, P.; Fokin, V. V. J. Org. Chem. 2005, 70, 7761–
7764.
(42) Bowden, M. C.; Patel, P.; Pattenden, G. Tetrahedron Lett. 1985, 26,
4793–4796.
(31) VanRheenen, V.; Kelly, R. C.; Cha, D. Y. Tetrahedron Lett. 1976,
17, 1973–1976.
(43) Wittig, G. Chem. Ber. 1954, 87, 1318–1330.
(44) Hassner, A.; Rai, K. M. L. Synthesis 1989, 57–59.
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Synthesis of Pluraflavin A “Aglycone”
A R T I C L E S
Scheme 5. Synthesis of Cyclization Precursor 25 via Isoxazoline 23a
a Key: (a) PCC, Celite, 4 Å MS, CH2Cl2; (b) H2C)PPh3, THF, 0°C, 44% (over two steps); (c) 14, chloramine-T, CH2Cl2/EtOH:H2O (4:10:1), 81% (based
on 14); (d) Raney-Ni, B(OH)3, H2(g), EtOH/H2O (5:1), 68% (78% BRSM); (e) IBX, EtOAc, reflux, 84%.
Scheme 6. Cyclization of ꢀ-Diketone 25 and Subsequent
Conversion into Anthrapyran Core 29a
Scheme 7. Synthesis of Epoxide 33a
a Key: (a) HCl, THF/MeOH (2:1), 50°C; (b) pTsOH (cat.), acetone/2,2′-
dimethoxypropane (1:1); (c) AcOH/H2O (4:1), 78% (over three steps from
26); (d) BzCl, Et3N, CH2Cl2, 99%; (e) HCl, THF/MeOH (2:1), 50°C, 71%;
(f) MsCl, Pr2NEt, CH2Cl2, 0°C; (g) K2CO3, MeOH/THF (1:1), 0°C, 60%
(over two steps) plus 10% debenzoylated epoxide.
a Key: (a) NaOAc, AcOH, 80°C, 86%; (b) CAN, MeCN/H2O (4:1), 0°C;
(c) 1-trimethylsiloxy-1,3-butadiene, PhH, 85°C, 4 h; (d) (i) Jones’ reagent,
acetone, 0°C; (ii) Et3N, CH2Cl2, 60% (from 26).
i
to Curran, the isoxazoline was converted into the desired keto
alcohols, 24.40 Subsequent oxidation using IBX afforded ꢀ-di-
ketone 25 in good overall yield45 (Scheme 5).
protecting groups in our current system, which could well be
avoidable in a second generation approach. Furthermore, the
handling of our anthrapyran core for an extended series of
manipulations employing late stage epoxide installation was
complicated by poor solubility of various intermediates that
made purification particularly difficult. Eventually, it was found
that it would be easier to install the epoxide on the tricyclic
pyrone (26) rather than on the anthrapyran (29).
Though we did have some initial success with the selective
deprotection of the primary alcohol under nonaqueous condi-
tions, the high stability of our acetonide did not lend itself to
its selective deprotection with maintenance of the MOM ether.
The means by which that problem was solved are detailed in
Scheme 7. Heating the tricyclic pyrone 26 in methanolic acid
accomplished bis-deprotection, yielding 30. This triol was then
treated under standard conditions, as described, giving rise to a
mixture of 31 and the corresponding mixed ketal at C13 (31a).
Fortunately, this crude product, upon exposure to the action of
wet acetic acid for 10 min at room temperature, suffered
conversion to 31 in high yield. The alcohol was then benzoylated
under standard conditions. Following deprotection of the ac-
etonide, diol 32 was thus obtained. Finally, 32 gave, with desired
inversion at C15, epoxide 33 by the usual two-step sequence
without purification of the intermediate monomesylate. It should
be noted that the absolute stereochemistry of the epoxide of
pluraflavin A is currently unknown. In principle, either enan-
tiomeric olefin can be fashioned by well-established dihydroxy-
lation methods.51,52
Conversion of 25 into the desired pyrone under commonly
employed acidic conditions was initially complicated by the
presence of the acid labile protecting groups. Fortunately, this
problem was overcome by the use of mild conditions (buffered
acetic acid) to affect cyclization (Scheme 6).46 Following this
step, oxidative demethylation of 26 afforded chloroquinone 27.47
The viability of this pyrone-quinone as a dienophile was
established following a Diels-Alder reaction with 1-trimeth-
ylsiloxy-1,3-butadiene to yield compound 28 as a single isomer.
It was presumed, but not established, that it was derived from
endo approach of the diene. Without purification, oxidative
deprotection of the silyl ether using Jones’ reagent followed by
treatment with base gave, at last, 29 in good overall yield from
26.48,49 This sequence constituted a significant improvement
relative to a previously described oxidative aromatization, both
in terms of yield and ease of operation.50
Having achieved the synthesis of the anthrapyran core of
pluraflavin A, the conversion of the protected diol into an
epoxide would be required in order to reach 3. The challenge
of installing the epoxide group stems from the choice of
(45) More, J. D.; Finney, N. S. Org. Lett. 2002, 4, 3001–3003.
(46) Fukui, K.; Matsumoto, T.; Kinoshita, T. Bull. Chem. Soc. Jpn. 1964,
37, 662–664.
(47) Jacob, P.; Callery, P. S.; Shulgin, A. T.; Castagnoli, N. J. Org. Chem.
1976, 41, 3627–3629.
(48) Bowden, K.; Heilbron, I. M.; Jones, E. R. H.; Weedon, B. C. L.
J. Chem. Soc. 1946, 39–45.
(49) Baker, R.; Rao, V. B.; Ravenscroft, P. D.; Swain, C. J. Synthesis 1983,
572–574.
(51) Kolb, H. C.; VanNieuwenhze, M. S.; Sharpless, K. B. Chem. ReV.
1994, 94, 2483–2547.
(50) Boisvert, L.; Brassard, P. J. Org. Chem. 1988, 53, 4052–4059.
9
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A R T I C L E S
Wright et al.
Scheme 8. Synthesis of 3a
Table 1. Cell Growth Inhibition Against Human Lymphoblastic
T-Cell Leukemia Sublinesa
a Key: (a) CAN, MeCN/H2O (6:1), 0°C; (b) 1-trimethylsiloxy-1,3-
butadiene, PhH, 85°C, 4 h; (c) (i) Jones’ reagent, acetone, 0°C; (ii) Et3N,
CH2Cl2; (d) K2CO3, MeOH/CH2Cl2 (1:1), 0°C f rt, 80% (over five steps
from 33).
a Key: IC50 values are reported in micromolar. CCRF-CEM is a
human lymphoblastic leukemic cell line. CCRF-CEM/Taxol and
CCRF-CEM/VBL are CCRF-CEM cells that are resistant to Taxol
Early installation of the epoxide served two immediate functions.
It simplified the handling of the sensitive anthrapyran chromophores
and it allowed for straightforward access to a variety of epoxide-
bearing intermediates for use in a future structure-activity relation-
ship (SAR) study. Nevertheless, it still was necessary to demon-
strate the practicality of carrying such an epoxide through a range
of additional sequences (Scheme 8).
The conversion of epoxide 33 into 3 followed a sequence
analogous to that previously described for the synthesis of 29
(Scheme 6). In the event, epoxide 33 was first converted to the
corresponding dienophilic quinone 34. Happily, the straight-
forward Diels-Alder/oxidation/aromatization protocol could
then be realized without erosion of the epoxide linkage. Thus,
we were able to obtain 36 in high overall yield from 33. Finally,
removal of the benzoate under standard conditions gave the
target compound (3). Its NMR, IR, and high resolution mass
spectra served independently to establish the structure shown
below and were fully consistent with reported structures of this
general type.12,53 In reality, the total synthesis of 3 had proven
to be more complicated than we had anticipated by naive
examination of its structure.
Biology. In addition to the chemical questions described
herein, a fundamental interest in the potential pharmaceutical
application of this family of compounds was at the heart of
this research. Accordingly, preliminary in vitro screening was
conducted on a variety of intermediates as well as on 3. The
truncated results of this study are shown in Table 1.
In fact, we have conducted a rather extensive analysis of the
SAR relationships encompassing a range of synthetic congeners
produced in this program with respect to three cell lines shown
below. A full report of the very interesting findings of this study
will be provided in due course. However, suffice it to say for
the moment that 3 is actually a quite active compound against
each of the cell lines. Moreover, the bulk of the activity is
retained where the primary alcohol function at C13 appears as
its benzoate ester (36). Since that substantial potency is inherent
even in the bare tetracyclic core matrix of 1, it will be of
(1184-fold
resistance)
and
vinblastine
(84-fold
resistance),
respectively.54 *The numbers in brackets are fold of resistance based on
the IC50 values compared with the parent CCRF-CEM cell lines. The
dose-effect relationship data from six to seven concentrations of each
drug in duplicate was analyzed with the median-effect plot through the
use of a computer program.52
significant interest to append carbohydrate moieties to the
appropriate sites. It should thus be possible to probe the role of
the glycosidic sectors (both C- and O-linked) in enhancing
potency relative to 3 itself.
Conclusion
The synthesis of the des-carbohydrate of pluraflavin A has
been described. The key transformation was the use of a [3 +
2] cycloaddition to afford a functionalized isoxazoline. This
heterocycle was converted into the γ-pyrone of the natural
product in an efficient three-step method. Installation of an
epoxide followed by another cycloaddition-based sequence
established the D ring of 3. In addition, an in vitro study against
three leukemic cell lines has shown that, even in the absence
of any carbohydrate moieties, the anthrapyran construct is still
capable of eliciting a highly potent cytotoxic effect. Future plans
include the synthesis and attachment of one or more of the
glycosidically bound sugars of pluraflavin A, achievement of
an enantiomerically homogeneous version of 3 corresponding
to the natural series, and additional pharmacological studies on
this family of compounds.
Acknowledgment. This work was supported by the National
Institutes of Health (Grant HL25848). B.J.D.W. is grateful for an
NSF predoctoral fellowship. F.P. thanks Eli Lilly & Co. for
generous financial support. We thank Dr. W.F. Berkowitz for
helpful literature research and discussion, and Dr. Yasuhiro Itagaki
(Columbia University) for expert mass spectrometric analyses. We
also thank Drs. K. Sakamoto and H. Uno for instruction on an
improved synthesis of naphthalene starting material.
Supporting Information Available: Experimental procedures
and spectroscopic and analytical data for all new compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(52) See Supporting Information for details on the synthesis and charac-
terization of the (-)-diol and (-)-acetonide en route to 22.
(53) Uno, H.; Sakamoto, K.; Honda, E.; Fukuhara, K.; Ono, N.; Tanaka,
J.; Sakanaka, M. J. Chem. Soc., Perkin Trans. 1 2001, 229–238.
(54) Chou, T. C.; Dong, H. J.; Zhang, X. G.; Tong, W. P.; Danishefsky,
S. J. Cancer Res. 2005, 65, 9445–9454.
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