Total synthesis of phomazarin

Article abstract of DOI:10.1021/ja983631q

A concise total synthesis of phomazarin (1) is detailed enlisting a heterocyclic azadiene inverse electron demand Diels-Alder reaction (1,2,4- triazine → pyridine) for preparation of the fully substituted and appropriately functionalized pyridine C-ring. Thus, [4 + 2] cycloaddition (85%) of triethyl 1,2,4-triazine3,5,6-tricarboxylate (2) with trimethoxyethylene (3) followed by conversion of the cycloadduct 11 to the cyclic anhydride 13 provided the phomazarin C-ring with the three carboxylates suitably differentiated. Linkage of the A- and C-rings through selective nucleophilic addition of the aryllithium reagent 9 to the least hindered anhydride carbonyl of 13 followed by Friedel-Crafts closure of the B-ring provided the fully functionalized phomazarin skeleton. The successful structural correlation of synthetic 1 with natural material and its derivatives confirmed the latest structural assignment for the natural product.

Full text of DOI:10.1021/ja983631q

J. Am. Chem. Soc. 1999, 121, 2471-2477
2471
Total Synthesis of Phomazarin
Dale L. Boger,* Jiyong Hong, Masataka Hikota, and Michio Ishida
Contribution from the Department of Chemistry and The Skaggs Institute for Chemical Biology,
The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
ReceiVed October 15, 1998
Abstract: A concise total synthesis of phomazarin (1) is detailed enlisting a heterocyclic azadiene inverse
electron demand Diels-Alder reaction (1,2,4-triazine f pyridine) for preparation of the fully substituted and
appropriately functionalized pyridine C-ring. Thus, [4 + 2] cycloaddition (85%) of triethyl 1,2,4-triazine-
3,5,6-tricarboxylate (2) with trimethoxyethylene (3) followed by conversion of the cycloadduct 11 to the cyclic
anhydride 13 provided the phomazarin C-ring with the three carboxylates suitably differentiated. Linkage of
the A- and C-rings through selective nucleophilic addition of the aryllithium reagent 9 to the least hindered
anhydride carbonyl of 13 followed by Friedel-Crafts closure of the B-ring provided the fully functionalized
phomazarin skeleton. The successful structural correlation of synthetic 1 with natural material and its derivatives
confirmed the latest structural assignment for the natural product.
Isolated from cultures of Phoma terrestris Hansen (Pyrenocha-
eta terrestris Hansen) in 1940,1 phomazarin (1) is the most
widely known and extensively studied aza anthraquinone (Figure
1). The original structural assignment1 has been revised twice,2,3
based largely on biosynthetic considerations advanced by Birch4
in conjunction with chemical degradation studies and reevalu-
ations with increasingly modern NMR spectroscopic techniques.
To date, confirmation of the structure by total synthesis has
not been realized and phomazarin has been the subject of only
limited synthetic effort.5
Herein, we report its total synthesis which serves to unam-
biguously establish the structure 1 for phomazarin on the basis
Figure 1.
of the implementation of a heteroaromatic azadiene Diels-Alder
reaction.6 Thus, inverse electron demand Diels-Alder reaction
of the electron-deficient 3,5,6-tris(ethoxycarbonyl)-1,2,4-triazine
(2)7,8 with 1,1,2-trimethoxyethylene (3)9 was anticipated to
provide 11 ideally functionalized for elaboration to the penta-
substituted pyridine C-ring (Scheme 1). The dienophile methoxy
substituents serve to enhance its nucleophilic character and
introduce the phomazarin C3 and C4 hydroxy groups. In a
complementary manner, the three electron-withdrawing ester
groups substituting the 1,2,4-triazine nucleus increase its [4 +
2] cycloaddition reactivity toward such electron-rich dienophiles
and provide the necessary functionality for introduction of the
natural product C2 carboxylic acid and C9/C10 quinone
carbonyls. Formation of cyclic anhydride 13 followed by a
projected selective nucleophilic addition to the sterically more
accessible anhydride carboxylate enlisting the aryllithium
reagent 9 would serve to introduce the phomazarin A-ring and
differentiate the three ester groups. Final intramolecular Friedel-
Crafts acylation and selective O-demethylation would then serve
to complete the total synthesis. Because the O-methyl derivatives
of phomazarin methyl ester have been extensively characterized
(1) Ko¨gl, F.; Sparenburg, J. Recl. TraV. Chim. Pays-Bas 1940, 59, 1180.
Ko¨gl, F.; Quackenbush, F. W. Recl. TraV. Chim. Pays-Bas 1944, 63, 251.
Ko¨gl, F.; van Wessem, G. C.; Elsbach, O. I. Recl. TraV. Chim. Pays-Bas
1945, 64, 23.
(2) Birch, A. J.; Butler, D. N.; Rickards, R. W. Tetrahedron Lett. 1964,
1853.
(3) Birch, A. J.; Effenberger, R.; Rickards, R. W.; Simpson, T. J.
Tetrahedron Lett. 1976, 27, 2371. Birch, A. J.; Butler, D. N.; Effenberger,
R.; Rickards, R. W.; Simpson, T. J. J. Chem. Soc., Perkin Trans. 1 1979,
807.
(4) Birch, A. J.; Simpson, T. J. J. Chem. Soc., Perkin Trans. 1 1979,
816.
(8) (a) Dittmar, W.; Sauer, J.; Steigel, A. Tetrahedron Lett. 1969, 5171.
Burg, B.; Dittmar, W.; Reim, H.; Steigel, A.; Sauer, J. Tetrahedron Lett.
1975, 2897. Reim, H.; Steigel, A.; Sauer, J. Tetrahedron Lett. 1975, 2901.
(b) Neunhoeffer, H.; Fruhauf, H.-W. Liebigs Ann. Chem. 1972, 758, 120.
Neunhoeffer, H.; Werner, G. Liebigs Ann. Chem. 1973, 1955. Neunhoeffer,
H.; Frey, G. Liebigs Ann. Chem. 1973, 1963. Oeser, E. Liebigs Ann. Chem.
1973, 1970. Neunhoeffer, H.; Werner, G. Liebigs Ann. Chem. 1973, 1955.
(c) Boger, D. L.; Duff, S. R.; Panek, J. S.; Yasuda, M. J. Org. Chem. 1985,
50, 5790. Boger, D. L.; Duff, S. R.; Panek, J. S.; Yasuda, M. J. Org. Chem.
1985, 50, 5782. Boger, D. L.; Panek, J. S. J. Am. Chem. Soc. 1985, 107,
5745. Boger, D. L.; Panek, J. S. Tetrahedron Lett. 1984, 25, 3175. Boger,
D. L.; Panek, J. S. J. Org. Chem. 1982, 47, 3763.
(9) Bakker, C. G.; Scheeren, J. W.; Nivard, R. J. F. Recl. TraV. Chim.
Pays-Bas 1981, 100, 13. Spinning band distillation required to separate 3
from methyl methoxyacetate and 1,1,1,2-tetramethoxyethane was unneces-
sary, and material that was as low as 40% 3 worked satisfactorily in the
thermal cycloaddition reaction. See also: Boger, D. L.; Mullican, M. D. J.
Org. Chem. 1984, 49, 4033. and Boger, D. L.; Brotherton, C. E. J. Org.
Chem. 1984, 49, 4050.
(5) Guay, V.; Brassard, P. J. Heterocycl. Chem. 1987, 24, 1649. Guay,
V.; Brassard, P. Synthesis 1987, 294.
(6) Boger, D. L. Chemtracts: Org. Chem. 1996, 9, 149. Boger, D. L.
Bull. Chim. Soc., Belg. 1990, 99, 599. Boger, D. L.; Patel, M. In Progress
in Heterocyclic Chemistry 1989; Suschitzky, H., Scriven, E. F. V., Eds.;
Pergamon: Oxford, U.K., 1989; Vol. 1; p 30. Boger, D. L.; Weinreb, S.
M. Hetero Diels-Alder Methodology in Organic Synthesis; Academic: San
Diego, CA, 1987. Boger, D. L. Chem. ReV. 1986, 86, 781. Boger, D. L.
Tetrahedron 1983, 39, 2869.
(7) Boger, D. L.; Panek, J. S.; Yasuda, M. Org. Synth. 1987, 66, 142.
10.1021/ja983631q CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/05/1999

2472 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Boger et al.
Scheme 1
Scheme 3
Scheme 2
and typically provided a mixture of the unaromatized cyclo-
adduct and 11 (Scheme 3). Thus, the complementary match of
the electron-rich character of the dienophile and the electron-
deficient nature of the 1,2,4-triazine imparted by the substituents
provides an uncatalyzed Diels-Alder cycloaddition reaction that
proceeds effectively even at room temperature. In the optimiza-
tion of the reaction, we established that the slow and final
elimination step involving the loss of CH3OH was promoted at
higher reaction temperatures (100 °C, dioxane, 30 min) and
could be driven to completion by a brief Et3N treatment (25
°C, 1 h) prior to purification isolation. Exhaustive ester
hydrolysis of 11 (4 equiv of LiOH, THF/CH3OH/H2O 3:2:1,
reflux, 36 h, 87%) cleanly provided the tricarboxylic acid 12
and set the stage for differentiation of the three carboxylates.12
Treatment of 12 with TFAA followed by CH2N2 esterification
provided the key cyclic anhydride 13 in which the C2 carbox-
ylate had been converted to a methyl ester.13 The two-step
conversion of 12 to 13 proved uniquely successful with TFAA
and was not achieved with Ac2O. Presumably, this may be
attributed to the enhanced reactivity of the trifluoroacetyl mixed
anhydrides, the entropic assistance for closure of an intermediate
half acid anhydride to the cyclic anhydride, and the subsequent
ease of hydrolysis of the remaining C2 trifluoroacetyl mixed
anhydride14 by adventitious water upon exposure to the diazo-
methane esterification conditions.15 The remaining addition of
the A-ring aryllithium reagent 9 provided 14 with selective
and are considerably easier to work with than the insoluble,
oxidatively labile, and polar (zwitterionic) phomazarin itself,
the approach was designed to pass through 3,4-O-dimethyl
phomazarin methyl ester as a more manageable comparison
derivative despite the challenge this poses for the final depro-
tection to provide the natural product.
Preparation of the A-Ring. Selective methylation of the
more acidic C2-hydroxy group of 2,3-dihydroxybenzaldehyde
(4, 1 equiv of K2CO3, 1.3 equiv CH3I, DMF, 25 °C, 96%) and
subsequent MOM protection of the remaining C3-hydroxy group
of 510 provided 6 (95%), Scheme 2. The MOM protecting group
was enlisted to promote directed metalation at C4 for linkage
of the A-ring with the pyridyl C-ring. The n-butyl side chain
was introduced by Wittig reaction of 6 with propylidenetri-
phenylphosphorane (96%, 11:1 cis:trans) followed by catalytic
hydrogenation of 7 to provide 8 (97%) in 85% overall yield
from 4. Directed ortho metalation of 8 occurred cleanly at
temperatures >-20 °C with n-BuLi or more effectively with
t-BuLi (1.1 equiv, Et2O, 0 °C 1 h, 23 °C, 1 h) to provide 9 in
situ. Preliminary studies on this metalation were conducted using
1-chloro-2-iodoethane as an electrophile providing 1011 (74%),
an alternative metalation precursor to 9.
Heterocyclic Azadiene Diels-Alder Preparation of the
C-Ring and Coupling with the A-Ring. The key inverse
electron demand Diels-Alder reaction of the 1,2,4-triazine 27
with 1,1,2-trimethoxyethylene (3)9 proceeded effectively provid-
ing the fully substituted C-ring precursor 11 (85%). The [4 +
2] cycloaddition reaction was observed even at 25 °C, albeit
proceeding more slowly (5 h, CHCl3, 22%; 12 h, 60-70%),
(11) Data for 10: 1H NMR (CDCl3, 400 MHz) δ 7.42 (d, 1H, J ) 8.2
Hz), 6.68 (d, 1H, J ) 8.2 Hz), 5.15 (s, 2H), 3.78 (s, 3H), 3.68 (s, 3H), 2.56
(t, 2H, J ) 7.3 Hz), 1.50-1.48 (m, 2H), 1.35 (sextet, 2H, J ) 7.3 Hz),
0.91 (t, 3H, J ) 7.3 Hz); IR (neat) νmax 2929, 2856, 1569, 1463, 1440,
1416, 1390, 1263, 1204, 1161, 1080, 1010, 839 cm-1; FABHRMS (NBA-
CsI) m/z 483.9439 (M+ + Cs, C13H19O3I requires 482.9433).
(12) When this reaction was conducted at room temperature (10 equiv
of LiOH, 25 °C, 16 h, quantitative), the C3 ethyl ester was not hydrolyzed.
Data for 4,5-dimethoxy-3-(ethoxycarbonyl)pyridine-2,6-dicarboxylic acid:
1H NMR (DMSO-d6, 400 MHz) δ 4.31 (q, 2H, J ) 7.2 Hz), 3.97 (s, 3H),
3.90 (s, 3H), 1.28 (t, 3H, J ) 7.2 Hz).
(13) Deliberate hydrolysis of 13 provided 4,5-dimethoxy-6-(methoxy-
carbonyl)pyridine-2,3-dicarboxylic acid: 1H NMR (D2O, 400 MHz) δ 3.88
(s, 3H), 3.82 (s, 3H), 3.80 (s, 3H).
(14) The crude product representing the corresponding cyclic anhydride/
C2 mixed trifluoroacetyl mixed anhydride exhibits the following data: 1H
1
NMR (CDCl3, 400 MHz) δ 4.57 (s, 3H), 4.07 (s, 3H); H NMR (DMSO-
d6, 400 MHz) δ 4.39 (s, 3H), 3.90 (s, 3H); 19F NMR (CDCl3, 376 MHz) δ
-7.03 (s).
(10) Kessar, S. V.; Gupta, Y. P.; Mohammad, T.; Goyal, M.; Sawal, K.
K. J. Chem. Soc., Chem. Commun. 1983, 400.
(15) Boger, D. L.; Baldino, C. M. J. Am. Chem. Soc. 1993, 115, 11418.

Total Synthesis of Phomazarin
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2473
Scheme 4
Scheme 5
respectively (Scheme 4). The single-crystal X-ray structure
determination17 of 16b unambiguously provided the structure
of the minor product and established the regioselectivity
preference of nucleophilic attack. Hydrogenolysis deprotection
of both 15a,b provided the same carboxylic acid 17, and
deprotection of both 16a,b provided the isomeric acid 18.
Although equilibration isomerism between pyridine-2,3-dicar-
boxylic acid monoesters and monoacid chlorides has been
observed,18 no isomerism between 17 and 18 was observed at
25 °C (48 h, CH3OH). However at reflux, a 62:38 mixture of
18:17 was observed after 48 h (CH3OH) starting with 18.
Activation of 17 and 18 for nucleophilic attack by conversion
to the acid chlorides followed by addition of the C-ring
aryllithium reagent 9 or its corresponding higher order cuprate
provided 21 and 22, respectively. Diazomethane esterification
of 14, derived from the route detailed in Scheme 3, also provided
21 correlating it with the sample that was established to contain
the A-ring linked to the C6 versus C5 carboxylate.
Concurrent with these efforts, we briefly examined the
potential of regioselective esterification of 13 with the A-ring
phenol 23 followed by Fries rearrangement of 24 to provide 26
as an alternative approach to linking the A and C rings (Scheme
5). Esterification of anhydride 13 with 23 followed by CH2N2
esterification of the liberated carboxylic acid proceeded surpris-
ingly effectively (72%) albeit to provide a 57:43 mixture of 24
and 25. Initial attempts to promote the Fries rearrangement of
24 (BF3‚OEt2, CH2Cl2) were not successful. Given the rather
nonselective esterification of 23, these studies were not pursued
further.
Intramolecular Friedel-Crafts Acylation and Completion
of the Phomazarin Total Synthesis. In contrast to initial
expectations, efforts to promote the Friedel-Crafts closure of
the B-ring with 14 directly were not successful (Scheme 6).
Under a range of mild reaction conditions only MOM depro-
tection19 was observed without evidence of ring closure. More
forcing conditions led to the consumption of 14 and 27 but
nucleophilic addition to the least hindered of the cyclic
anhydride carbonyls. The four-step sequence for the conversion
of triacid 12 to 21 was typically carried out without intermediate
purifications providing 21 in 30-42% yield overall.
Establishment of the Regioselectivity of the A and C Ring
Coupling. In preliminary efforts to differentiate the three
carboxylates on the pyridine 11, simple regioselective esterifi-
cation of the cyclic anhydride 13 was also examined. Although
this did not prove especially successful, an X-ray structure
determination of an intermediate and its subsequent correlation
with the coupling product 14 confirmed the structural assign-
ments. Thus, reaction of 13 with benzyl alcohol or 3,5-
dinitrobenzyl alcohol followed by CH2N2 esterification of the
liberated carboxylic acid provided a separable 63:37 mixture
of 15 and 16 (50-81%) derived from alcohol nucleophilic attack
at the less hindered and more hindered anhydride carbonyl,16
(16) In an analogous manner, treatment of 13 with t-BuOH (30%) and
(Ph)2CHOH (70%) provided 70:30 and 58:42 mixtures of the corresponding
C6:C5 esters (15c,d:16c,d). Details are provided in the Supporting Informa-
tion.
(17) Details of the X-ray structure of 16b are supplied in the Supporting
Information and have been deposited with the Cambridge Crystallographic
Data Centre.
(18) Kenyon, J.; Thaker, K. J. Chem. Soc. 1957, 2531. Chase, B. H.;
Hey, D. H. J. Chem. Soc. 1952, 553.

2474 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Boger et al.
Table 1. Cytotoxic Activity
IC50, µg/mL
Scheme 6
IC50, µg/mL
(L-1210)
compd
(L-1210)
compd
32
33
34
0.3
0.03
2.5
1
35
36
>100
1.6
25
detection of the intermediate phenol. The resulting 3,4-O-
dimethyl phomazarin methyl ester (32) proved identical in all
respects (1H and 13C NMR, IR) to authentic material3 derived
from the natural product confirming the Birch 1976 structural
assignment.3
Completion of the total synthesis required selective cleavage
of the C3 and C4 versus C7 methyl ethers and methyl ester
hydrolysis to provide 1. A variety of conditions were explored,
and no single-step deprotection was found capable of converting
32 directly to 1. However, clean C4 methyl ether deprotection
to provide 33 was accomplished with BCl3 (10 equiv, CH2Cl2,
-78 to -50 °C, 3 h, g95%), AlCl3 (14-15 equiv, CH2Cl2, 25
°C, 24 h or reflux, 7 h, 90%), or NaBr-TsOH (20 equiv each,
CH3OH, reflux, 12 h, 74%), and the latter reaction also provided
phomazarin methyl ester (34) in 10%. With enlistment of CF3-
CH2OH as solvent,21 the latter treatment with NaBr-TsOH (20
equiv each) cleanly provided only 33 at 25 °C (24 h, 100%)
and, more importantly, effectively provided phomazarin methyl
ester (34) at 100 °C (36 h, 95-100%) derived from the desired
selective C3 and C4 methyl ether deprotections. In contrast,
treatment of either 32 or 33 with BBr3 (10 equiv, -78 to -40
°C, 6 h) provided a 1:1 mixutre of 34 and 35 derived from C4
and nonselective C3 or C7 methyl ether deprotections along
with a trace of 36. Both 33 and 34 correlated with the
corresponding materials derived from phomazarin described by
Birch further confirming the structure of the natural product.
Diagnostic of the site of methyl ether deprotection,3 the adjacent
carbonyls exhibited perturbed IR carbonyl stretches derived from
H-bonding to the proximal free phenol: C4-OH/C10 quinone
(1670 f 1640 cm-1), C3-OH/ester carbonyl (1740 f 1680
cm-1). Final hydrolysis of phomazarin methyl ester (34) with
LiOH (20 equiv, THF-H2O, 30 °C, 24 h, 70%) provided
phomazarin identical in all respects with authentic material.1-3
In Vitro Cytotoxic Activity. A preliminary cytotoxicity
screen of 1 and the closely related synthetic intermediates
revealed that phomazarin was inactive but that two of its
immediate precursors, 32 and 33, exhibit useful levels of activity
(Table 1). Although the free carboxylic acids were less potent
(e.g. 1 versus 34),22 the methyl esters exhibited interesting levels
of cytotoxic activity, and capping both the C7 and C3 phenols
as methyl ethers (34 versus 36 and 33 versus 34 or 35 versus
36) increased the cytotoxic activity 10-100× while conversion
of the C4 phenol to a methyl ether (32 versus 33) lowered the
potency 10×.
without evidence of B-ring closure. The electron-withdrawing
properties of the A-ring C1 carbonyl, the competitive MOM
deprotection, coupled with reaction of the A-ring C2 hydroxy
group and, more significantly, the C1 keto carbonyl with the
intermediate acylium ion appeared to be contributing to this
failure to close the B-ring with 14. This was successfully
addressed by removal of the ketone by NaBH4 reduction of 21
to provide the lactone 28 (93%) followed by hydrogenolysis to
provide 29 (100%). Exposure of 29 to TFAA cleanly resulted
in Friedel-Crafts closure of the B-ring and concurrent MOM
deprotection providing a variable and inconsequential mixture
of 30 and 31 with no detection of the intermediate bisphenol.
Conditions were devised that provided cleanly the single mono-
(trifluoroacetate) 30 (88%; TFAA, 50 °C, 72 h) which has been
tentatively assigned as the C5 trifluoroacetate on the basis of
its air oxidation stability and the downfield C10-H chemical
shift of δ 9.05. Subsequent methanolysis of 30 or the mixture
of trifluoroacetates (30 °C, CH3OH) in the presence of air led
directly to 32 (83%) resulting from deprotection followed by
an unusually facile oxidation to the B-ring quinone without
Experimental Section
3-Hydroxy-2-methoxybenzaldehyde (5). A solution of 4 (6.0 g,
43.4 mmol) in anhydrous DMF (100 mL) was treated with K2CO3 (6.0
g, 43.4 mmol) at 25 °C, and the mixture was stirred for 30 min. Methyl
iodide (3.50 mL, 56.2 mmol) was added, and the reaction mixture was
further stirred for 20 h before being quenched by the addition of H2O
(100 mL). The aqueous layer was extracted with Et2O (2 × 100 mL).
The combined organic layer was dried (MgSO4) and concentrated under
vacuum. Recrystallization (EtOAc-hexane) afforded 510 (6.31 g, 96%)
(19) Data for 27: 1H NMR (CDCl3, 400 MHz) δ 7.37 (d, 1H, J ) 8.4
Hz), 6.68 (d, 1H, J ) 8.4 Hz), 4.16 (s, 3H), 4.02 (s, 3H), 3.97 (s, 3H), 3.89
(s, 3H), 2.65 (t, 2H, J ) 7.8 Hz), 1.56 (m, 2H), 1.37 (m, 2H), 0.92 (t, 3H,
1
J ) 7.3 Hz); H NMR (CD3OD, 400 MHz) δ 7.19 (d, 1H, J ) 8.3 Hz),
6.68 (d, 1H, J ) 8.3 Hz), 4.11 (s, 3H), 3.96 (s, 3H), 3.91 (s, 3H), 3.87 (s,
3H), 2.65 (t, 2H, J ) 7.7 Hz), 1.56 (m, 2H), 1.37 (m, 2H), 0.93 (t, 3H, J
) 7.3 Hz); 1H NMR (acetone-d6, 400 MHz) δ 7.48 (s, 1H), 7.22 (d, 1H, J
) 7.8 Hz), 6.82 (d, 1H, J ) 7.8 Hz), 5.01 (s, 1H), 4.18 (s, 3H), 4.00 (s,
3H), 3.93 (s, 3H), 3.89 (s, 3H), 2.73 (t, 2H, J ) 7.4 Hz), 1.66 (m, 2H),
1.48 (m, 2H), 1.04 (t, 3H, J ) 7.3 Hz).
1
as a pale yellow solid: mp 109 °C; H NMR (CDCl3, 400 MHz) δ
(20) Details are provided in the Supporting Information.
(21) Boger, D. L.; Cassidy, K. C.; Nakahara, S. J. Am. Chem. Soc. 1993,
115, 10733.

Total Synthesis of Phomazarin
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2475
10.27 (s, 1H), 7.38 (dd, 1H, J ) 1.7, 7.7 Hz), 7.24 (dd, 1H, J ) 1.7,
8.0 Hz), 7.15 (t, 1H, J ) 7.9 Hz), 5.86 (br s, 1H), 3.98 (s, 3H); 13C
NMR (CDCl3, 100 MHz) δ 189.5, 149.4, 149.1, 129.0, 125.1, 121.83,
121.76, 63.9; IR (neat) νmax 3193, 1666, 1605, 1582, 1475, 1444, 1372,
(CDCl3, 400 MHz) δ 4.46 (q, 2H, J ) 7.2 Hz), 4.43 (q, 2H, J ) 7.2
Hz), 4.41 (q, 2H, J ) 7.2 Hz), 4.03 (s, 3H), 3.96 (s, 3H), 1.41 (t, 3H,
J ) 7.2 Hz), 1.382 (t, 3H, J ) 7.2 Hz), 1.380 (t, 3H, J ) 7.2 Hz); 13
C
NMR (CDCl3, 100 MHz) δ 164.2, 163.9, 163.2, 157.6, 150.0, 146.4,
141.0, 129.2, 62.4, 62.2, 62.1, 62.0, 61.5, 14.0 (2C), 13.9; IR (neat)
νmax 2982, 1743, 1569, 1550, 1466, 1371, 1343, 1268, 1155, 1047,
963, 864 cm-1; FABHRMS (NBA-NaI) m/z 356.1352 (M+ + H,
C16H21NO8 requires 356.1345). Anal. Calcd for C16H21NO8: C, 54.08;
H, 5.96; N, 3.94. Found: C, 53.78; H, 5.91; N, 3.67.
1295, 1220, 997 cm-1; FABHRMS (NBA-NaI) m/z 153.0557 (M+
+
H, C8H8O3 requires 153.0552).
2-Methoxy-3-(methoxymethoxy)benzaldehyde (6). A suspension
of NaH (60% in oil, 600 mg, 15.0 mmol) in anhydrous THF (20 mL)
was treated with 5 (1.98 g, 13.0 mmol) in anhydrous THF (5 mL)
dropwise at 0 °C over 5 min. The mixture was stirred for 10 min before
(MOM)Cl (1.20 mL, 15.7 mmol) was added. The resulting mixture
was stirred at 25 °C for 2 h before it was poured into H2O (20 mL).
The aqueous phase was extracted by Et2O (3 × 20 mL), and the
combined organic layer was dried (MgSO4) and concentrated under
vacuum. Chromatography (SiO2, 15% EtOAc-hexane) afforded 6 (2.44
g, 95%) as a colorless oil: 1H NMR (CDCl3, 400 MHz) δ 10.43 (s,
1H), 7.49 (dd, 1H, J ) 1.6, 7.8 Hz), 7.41 (dd, 1H, J ) 1.6, 8.0 Hz),
7.12 (t, 1H, J ) 8.0 Hz), 5.26 (s, 2H), 4.01 (s, 3H), 3.54 (s, 3H); 13C
NMR (CDCl3, 100 MHz) δ 189.9, 153.2, 150.5, 130.0, 124.2, 122.5,
120.9, 95.2, 62.5, 56.4; IR (neat) νmax 2938, 2900, 2832, 1690, 1584,
1480, 1386, 1250, 1154, 1087, 1023, 1000, 923, 905 cm-1; FABHRMS
(NBA-NaI) m/z 197.0820 (M+ + H, C10H12O4 requires 197.0814).
Anal. Calcd for C10H12O4: C, 61.22; H, 6.16. Found: C, 60.89; H,
6.39.
1-(1-Butenyl)-2-methoxy-3-(methoxymethoxy)benzene (7). A so-
lution of n-PrPPh3Br (500 mg, 1.30 mmol) in anhydrous THF was
treated with t-BuOK (157 mg, 1.40 mmol) at 25 °C, and the resulting
orange solution was stirred for 30 min before a solution of 6 (197 mg,
1.00 mmol) in anhydrous THF (5 mL) was added. The reaction mixture
was stirred for 1 h during which time the orange color disappeared.
The reaction mixture was poured into saturated aqueous NH4Cl (10
mL), and the aqueous phase was extracted by Et2O (3 × 10 mL). The
combined organic layer was dried (MgSO4) and concentrated under
vacuum. Chromatography (SiO2, 5% EtOAc-hexane) afforded 7 (213
mg, 96%) as a mixture of cis and trans isomers (11:1) as a colorless
oil. Data for major isomer: 1H NMR (CDCl3, 400 MHz) δ 7.05 (dd,
1H, J ) 1.6, 8.0 Hz), 6.99 (t, 1H, J ) 8.0 Hz), 6.93 (dd, 1H, J ) 1.4,
7.6 Hz), 6.50 (d, 1H, J ) 11.6 Hz), 5.73 (dt, 1H, J ) 7.4, 11.6 Hz),
5.21 (s, 2H), 3.80 (s, 3H), 3.53 (s, 3H), 2.27 (m, 2H), 1.03 (t, 3H, J )
7.4 Hz); 13C NMR (CDCl3, 100 MHz) δ 150.2, 147.7, 135.3, 132.2,
123.6, 123.4, 123.3, 115.4, 95.2, 60.6, 56.1, 22.0, 14.3; IR (neat) νmax
2952, 1575, 1471, 1427, 1401, 1261, 1222, 1154, 1087, 1034, 1010,
926 cm-1; FABHRMS (NBA-NaI) m/z 222.1247 (M+, C13H18O3
requires 222.1256). Anal. Calcd for C13H18O3: C, 70.25; H, 8.16.
Found: C, 69.91; H, 8.47.
1-Butyl-2-methoxy-3-(methoxymethoxy)benzene (8). A suspension
of 7 (144 mg, 0.649 mmol) and 10% Pd-C (7 mg) in EtOH (5 mL)
was stirred for 17 h under 1 atm of H2. The reaction mixture was filtered
through a pad of Celite (EtOH, 3 × 5 mL), and the solvent was removed
under vacuum. Chromatography (SiO2, 5% EtOAc-hexane) afforded
8 (141 mg, 97%) as a colorless oil: 1H NMR (CDCl3, 400 MHz) δ
6.97 (dd, 1H, J ) 1.9, 8.2 Hz), 6.93 (t, 1H, J ) 8.1 Hz), 6.82 (dd, 1H,
J ) 1.9, 8.2 Hz), 5.20 (s, 2H), 3.82 (s, 3H), 3.50 (s, 3H), 2.60 (t, 2H,
J ) 7.8 Hz), 1.53-1.61 (m, 2H), 1.36 (sextet, 2H, J ) 7.3 Hz), 0.91
(t, 3H, J ) 7.3 Hz); 13C NMR (CDCl3, 100 MHz) δ 150.2, 147.9,
136.9, 123.7, 123.2, 114.1, 95.1, 60.7, 56.1, 32.9, 29.6, 22.6, 13.9; IR
(neat) νmax 2959, 2930, 1579, 1477, 1266, 1221, 1154, 1087, 1032,
924 cm-1; FABHRMS (NBA-NaI) m/z 224.1416 (M+, C13H20O3
requires 224.1412). Anal. Calcd for C13H20O3: C, 69.61; H, 8.99.
Found: C, 69.37; H, 9.31.
4,5-Dimethoxy-2,3,6-tris(ethoxycarbonyl)pyridine (11). A solution
of triethyl 1,2,4-triazine-3,5,6-tricarboxylate7 (2, 70 mg, 0.24 mmol)
in anhydrous 1,4-dioxane (1 mL) was treated with trimethoxyethylene
(3, purity 40%,9 400 mg, 1.34 mmol) at 25 °C, and the mixture was
warmed at 100 °C with stirring for 30 min. The reaction mixture was
cooled to 25 °C, and Et3N (0.5 mL) was added to complete elimination
of CH3OH from the Diels-Alder product. The mixture was stirred for
1 h at 25 °C, and the volatiles were removed under vacuum.
Chromatography (SiO2, 15% EtOAc-hexane) afforded 11 (71 mg,
85%) as a pale yellow oil which upon recrystallization from Et2O-
hexane (-78 °C) provided a colorless solid: mp 49-50 °C; 1H NMR
This reaction was conducted on 100 mg-2.5 g scales in yields
ranging from 73 to 86%.
4,5-Dimethoxypyridine-2,3,6-tricarboxylic Acid (12). A solution
of 11 (60 mg, 0.619 mmol) in THF/CH3OH/H2O (3/2/1, 3 mL) was
treated with LiOH‚H2O (106 mg, 2.54 mmol) at 25 °C, and the mixture
was warmed at 90 °C and was stirred for 36 h. The reaction mixture
was cooled to 25 °C, and H2O (3 mL) was added. The aqueous phase
was washed with Et2O (3 mL) before it was acidified to pH 2-3 by
the addition of 1 N aqueous HCl. The aqueous phase was saturated
with NaCl and extracted with EtOAc (5 × 5 mL). The combined organic
layer was dried (MgSO4) and concentrated under vacuum. After the
trituration with Et2O-hexane, 12 (40 mg, 87%) was obtained as a white
1
solid: mp 157-159 °C (decomp); H NMR (CD3OD, 400 MHz) δ
1
4.06 (s, 3H), 4.01 (s, 3H); H NMR (D2O, 400 MHz) δ 4.13 (s, 3H),
3.98 (s, 3H); 13C NMR (CD3OD, 100 MHz) δ 167.7, 167.0, 165.9,
159.3, 152.1, 146.8, 141.3, 131.5, 62.8, 62.3; IR (neat) νmax 3700-
2700 (br), 1722, 1462, 1380, 1283, 1216, 1038 cm-1; FABHRMS
(NBA-NaI) m/z 272.6415 (M+ + H, C10H9NO8 requires 272.0406).
Anal. Calcd for C10H9NO8: C, 44.29; H, 3.35; N, 5.17. Found: C,
44.11; H, 3.42; N, 4.96.
4,5-Dimethoxy-6-(methoxycarbonyl)-2,3-dicarboxylic Anhydride
(13). A sample of 12 (16 mg, 0.059 mmol) was stirred in trifluoroacetic
anhydride (1 mL) at 25 °C for 2 h. The volatile material was removed
under vacuum, the crude product14 was dissolved in THF (1 mL), and
ethereal CH2N2 was added at 0 °C until the color of the solution
remained yellow. The resulting solution was stirred for 5 min before
the solvent was removed under vacuum. The crude product 1313 (12
mg) was used for the next reaction without purification: 1H NMR
(CDCl3, 400 MHz) δ 4.51 (s, 3H), 4.00 (s, 3H), 3.98 (s, 3H);
FABHRMS (NBA) m/z 267.0376 (M+, C11H9NO7 requires 267.0379).
2-[(4’-Butyl-3’-methoxy-2’-(methoxymethoxy)phenyl)carbonyl]-
4,5-dimethoxy-6-(methoxycarbonyl)pyridine-3-carboxylic Acid (14)
and Methyl 2-[(4’-Butyl-3’-methoxy-2’-(methoxymethoxy)phenyl)-
carbonyl]-4,5-dimethoxy-6-(methoxycarbonyl)pyridine-3-carboxyl-
ate (21). The lithium reagent 9, generated by the treatment of 8 (28
mg, 125 µmol) with t-BuLi (1.5 M in hexane, 75 µL, 113 µmol) in
Et2O (200 µL) at 0 °C for 45 min, was added at -78 °C through a
cannula to a solution of 13 (crude product generated from the triacid
12, 28.4 mg, 105 µmol) in anhydrous THF (500 µL). After further
stirring at -78 °C for 5 min, the reaction mixture was quenched by
the addition of saturated aqueous NH4Cl (200 µL). The aqueous phase
was extracted with EtOAc (4 × 2 mL). The combined organic layer
was dried (Na2SO4) and concentrated under vacuum to give 46.4 mg
of crude material. A 3.3 mg of the sample of crude 14 in THF (0.3
mL) was treated with ethereal CH2N2 at 25 °C. Concentration and
chromatography (SiO2, 30% EtOAc-hexane) gave the single isomer
21 (1.2 mg) identical in all respects with authentic 21 prepared as shown
in Scheme 4.20
The residual 43.1 mg of the crude sample was purified by
chromatography (SiO2, 30% EtOAc, 5% CH3OH + 1% HOAc-CH2-
Cl2) afforded 14 (21.9 mg, 42%) as a pale yellow oil: 1H NMR (CD3-
OD, 400 MHz) δ 7.18 (d, 1H, J ) 8.2 Hz), 6.98 (d, 1H, J ) 8.2 Hz),
4.99 (s, 2H), 4.07 (s, 3H), 3.94 (s, 3H), 3.84 (s, 3H), 3.82 (s, 3H), 3.35
(s, 3H), 2.66 (t, 2H, J ) 7.8 Hz), 1.55-1.63 (m, 2H), 1.39 (sextet, 2H,
J ) 7.3 Hz), 0.95 (t, 3H, J ) 7.3 Hz); 13C NMR (CDCl3, 100 MHz)
δ 164.9, 151.1, 149.7, 141.9, 130.2, 128.9, 128.5, 126.0, 124.6, 99.5,
61.8, 61.5, 60.6, 57.4, 52.5, 32.6, 29.8, 22.7, 14.0; IR (neat) νmax 3435,
2946, 1739, 1667, 1596, 1448, 1397, 1346, 1265, 1214, 1163, 1041,
995 cm-1; FABHRMS (NBA-NaI) m/z 514.1661 (M+ + Na, C24H29-
NO10 requires 514.1689).
Typically, crude 14 (0.1-0.2 mmol) in THF (0.1 M) was treated
with a slight excess of CH2N2 at 25 °C. Concentration and chroma-

2476 J. Am. Chem. Soc., Vol. 121, No. 11, 1999
Boger et al.
tography (SiO2, 30% EtOAc-hexane) gave 21 quantitatively and in
30-42% overall yield from 12: 1H NMR (acetone-d6, 400 MHz) δ
7.21 (d, 1H, J ) 7.9 Hz), 7.07 (d, 1H, J ) 7.9 Hz), 4.92 (s, 2H), 4.06
(s, 3H), 3.98 (s, 3H), 3.85 (s, 3H), 3.84 (s, 3H), 3.81 (s, 3H), 3.22 (s,
3H), 2.68 (t, 2H, J ) 7.8 Hz), 1.60 (br p, 2H, J ) 7.7 Hz), 1.40 (sextet,
2H, J ) 7.6 Hz), 0.94 (t, 3H, J ) 7.3 Hz); 13C NMR (CDCl3, 100
MHz) δ 192.3, 165.0, 164.6, 157.9, 151.0, 149.8, 149.7, 149.2, 145.3,
142.4, 130.2, 126.6, 125.8, 124.8, 99.2, 62.1, 61.7, 60.6, 57.2, 53.0,
52.8, 32.6, 29.3, 22.7, 13.9; IR (neat) νmax 2946, 1739, 1673, 1596,
1448, 1402, 1346, 1265, 1153, 1041, 995 cm-1; FABHRMS (NBA-
CsI) m/z 638.1024 (M+ + Cs, C25H31NO10 requires 638.1002).
Methyl 3-(4’-Butyl-3’-methoxy-2’-(methoxymethyl)phenyl)-6,7-
dimethoxy-1-oxo-1,3-dihydrofuro[3,4-b]pyridine-5-carboxylate (28).
A solution of 21 (19.7 mg, 0.04 mmol) in CH3OH (3 mL) was treated
with NaBH4 (4.43 mg, 0.12 mmol) and stirred at -10 °C for 40 min.
The reaction mixture was quenched with the addition of saturated
aqueous NH4Cl (0.2 mL) and extracted with EtOAc (2 × 2 mL), and
the organic layer was dried (Na2SO4) and concentrated under reduced
pressure. Chromatography (SiO2, 20% EtOAc-hexane) afforded 28
(17.2 mg, 93%) as a colorless oil: 1H NMR (CDCl3, 400 MHz) δ 6.87
(d, 1H, J ) 8.0 Hz), 6.62 (d, 1H, J ) 8.0 Hz), 6.62 (s, 1H), 5.17 (d,
1H, J ) 5.0 Hz), 4.99 (d, 1H, J ) 5.0 Hz), 4.50 (s, 3H), 3.92 (s, 3H),
3.91 (s, 3H), 3.77 (s, 3H), 3.51 (s, 3H), 2.57 (t, 2H, J ) 7.8 Hz), 1.51
(br p, 2H, J ) 7.7 Hz), 1.34 (sextet, 2H, J ) 7.6 Hz), 0.90 (t, 3H, J )
7.3 Hz); 13C NMR (CDCl3, 100 MHz) δ 165.9, 165.2, 165.0, 159.9,
151.0, 150.2, 149.1, 146.1, 139.6, 126.4, 125.4, 123.8, 111.9, 99.3,
78.9, 63.6, 62.4, 60.6, 57.9, 53.0, 32.7, 29.5, 22.7, 13.9; IR (neat) νmax
2955, 1765, 1740, 1582, 1480, 1448, 1415, 1300, 1277, 1247, 1163,
1089, 1027, 996, 930, 778 cm-1; FABHRMS (NBS-CsI) m/z 608.0875
(M+ + Cs, C24H29NO9 requires 608.0897).
Methyl 7-Butyl-5,10-dihydro-9-hydroxy-3,4,8-trimethoxy-5,10-
dioxobenzo[g]quinoline-2-carboxylate (32; 3,4-O-Dimethyl Phomaz-
arin Methyl Ester). A solution of 30 (21.9 mg, 0.04 mmol) in CH3OH
(15 mL) was stirred at 30 °C open to the air for 72 h. Concentration in
vacuo and chromatography (SiO2, 20% EtOAc-hexane) afforded 32
(15.2 mg, 83%) identical in all compared respects with material derived
from the natural product:3 mp 127 °C (lit. mp 131-133 °C);3 1H NMR
(CDCl3, 400 MHz) δ 12.66 (s, 1H), 7.63 (s, 1H), 4.10 (s, 3H), 4.06 (s,
3H), 4.04 (s, 3H), 4.01 (s, 3H), 2.73 (t, 2H, J ) 7.8 Hz), 1.60 (br p,
2H, J ) 7.7 Hz), 1.39 (sextet, 2H, 7.3 Hz), 0.94 (t, 3H, J ) 7.2 Hz);
13C NMR (CDCl3, 100 MHz) δ 185.1, 180.5, 164.2, 161.2, 155.7, 152.4,
151.8, 149.3, 145.43, 145.38, 128.0, 126.2, 121.6, 115.0, 62.55, 62.48,
60.8, 53.2, 32.1, 30.5, 22.6, 13.9; IR (neat) νmax 2954, 2863, 1741,
1672, 1641, 1446, 1409, 1271 cm-1; FABHRMS (NBA-CsI) m/z
562.0494 (M+ + Cs, C22H23NO8 requires 562.0478).
Methyl 7-Butyl-5,10-dihydro-4,9-dihydroxy-3,8-dimethoxy-5,10-
dioxobenzo[g]quinoline-2-carboxylate (33). A solution of 32 (1.3 mg,
3.0 µmol) in CF3CH2OH (0.5 mL) was treated with TsOH (20 equiv,
11.5 mg) and NaBr (20 equiv, 6.2 mg), and the mixture was stirred at
25 °C for 24 h. Concentration in vacuo and chromatography (SiO2,
CHCl3/EtOAc/CH3OH/TFA ) 40/2/2/3) afforded 33 (1.3 mg, 100%)
as an orange-yellow solid: mp 178 °C (lit. mp 180-182 °C);3 1H NMR
(CDCl3, 500 MHz) δ 13.27 (s, 1H), 12.81 (s, 1H), 7.71 (s, 1H), 4.14
(s, 3H), 4.08 (s, 3H), 3.99 (s, 3H), 2.73 (t, 2H, J ) 7.5 Hz), 1.60 (br
p, 2H, J ) 7.6 Hz), 1.39 (sextet, 2H, 7.5 Hz), 0.94 (t, 3H, J ) 7.5 Hz);
13C NMR (CDCl3, 100 MHz) δ 187.9, 184.8, 164.2, 161.8, 156.6, 153.6,
147.60, 147.53, 145.1, 143.6, 125.8, 122.5, 117.3, 115.3, 62.0, 61.1,
53.2, 32.0, 30.5, 22.6, 13.9; IR (neat) νmax 2951, 2867, 1744, 1637,
1447, 1413, 1267 cm-1; FABHRMS (NBA-NaI) m/z 438.1179 (M+
+ Na, C21H21NO8 requires 438.1165).
2-(4’-Butyl-3’-methoxy-2’-(methoxymethyl)phenyl)methyl-4,5-
dimethoxy-6-(methoxycarbonyl)pyridine-3-carboxylic Acid (29). A
solution of 28 (17.5 mg, 0.04 mmol) in EtOH (4 mL) under H2 was
treated with 20% Pd(OH)2 on carbon (17.5 mg) and stirred at 25 °C
for 2 h. The reaction mixture was filtered through Celite and
concentrated to afford 29 (17.5 mg, 100%) as a colorless oil. Following
this procedure, 28 (5-20 mg, 0.01-0.04 mmol) typically provided
analytically pure 29 in over 95% yield: 1H NMR (CDCl3, 400 MHz)
δ 6.78 (d, 1H, J ) 8.0 Hz), 6.74 (d, 1H, J ) 8.0 Hz), 5.06 (s, 2H),
4.24 (s, 2H), 4.01 (s, 3H), 3.93 (s, 3H), 3.88 (s, 3H), 3.72 (s, 3H), 3.54
(s, 3H), 2.51 (t, 2H, J ) 7.8 Hz), 1.50 (br p, 2H, J ) 7.7 Hz), 1.33
(sextet, 2H, J ) 7.6 Hz), 0.89 (t, 3H, J ) 8.2 Hz); 13C NMR (CDCl3,
100 MHz) δ 167.7, 165.1, 157.7, 153.6, 150.4, 149.1, 147.7, 146.7,
135.6, 130.1, 126.7, 125.2, 125.1, 99.0, 62.0, 61.2, 60.4, 57.7, 52.9,
35.5, 32.9, 29.3, 22.7, 13.9; IR (neat) νmax 2955, 1734, 1560, 1457,
1406, 1348, 1269, 1161, 1069, 1041, 997, 758 cm-1; FABHRMS
(NBS-CsI) m/z 610.1072 (M+ + Cs, C24H31NO9 requires 610.1053).
Methyl 7-Butyl-5,10-dihydro-3,4,9-trihydroxy-8-methoxy-5,10-
dioxobenzo[g]quinoline-2-carboxylate (34, Phomazarin Methyl Es-
ter). Method A. A solution of 32 (1.5 mg, 3.5 µmol) in CF3CH2OH
(1.0 mL) in a sealed vessel was treated with TsOH (20 equiv, 13.3
mg) and NaBr (20 equiv, 7.2 mg), and the mixture was stirred in a
100 °C oil bath for 36 h. Concentration in vacuo and chromatography
(SiO2, CHCl3/EtOAc/CH3OH/TFA ) 40/2/2/3) afforded 34 (1.4 mg,
100%).
Method B. A solution of 32 (3.8 mg, 8.9 µmol) in CH2Cl2 was
treated with BBr3 (10 equiv, 1 M in CH2Cl2, 90 µL) at -78 °C and
stirred at -78 °C for 2 h. The mixture was slowly warmed to -40 °C
over 2 h and stirred at -40 °C for an additional 2 h before the reaction
was quenched with the addition of CH3OH (0.1 mL). Concentration in
vacuo and chromatography (SiO2, CHCl3/EtOAc/CH3OH/TFA ) 40/
2/2/3) afforded 34 (1.2 mg, 34%) along with 33 (1.0 mg, 27%) and 35
(1.2 mg, 34%).
Data for 34: mp 208 °C (lit. mp 213 °C);3 1H NMR (CDCl3, 500
MHz) δ 13.20 (s, 1H), 12.94 (s, 1H), 11.41 (s, 1H), 7.72 (s, 1H), 4.11
(s, 3H), 4.09 (s, 3H), 2.73 (t, 2H, J ) 7.8 Hz), 1.60 (br p, 2H, J ) 7.6
Hz), 1.39 (sextet, 2H, 7.5 Hz), 0.94 (t, 3H, J ) 7.5 Hz); IR (neat) νmax
2925, 1682, 1631, 1452, 1327, 1282, 1231 cm-1; FABHRMS (NBA-
CsI) m/z 534.0187 (M+ + Cs, C20H19NO8 requires 534.1065).
Methyl 7-Butyl-9-hydroxy-5-(trifluoroacetoxy)-3,4,8-trimethoxy-
benzo[g]quinoline-2-carboxylate (30). A solution of 29 (15.8 mg, 0.03
mmol) in trifluoroacetic anhydride (3 mL) in a sealed vessel was
warmed in a 50 °C oil bath for 72 h. Concentration in vacuo and
chromatography (SiO2, 20% EtOAc-hexane) afforded 30 (14.8 mg,
88%) as a pale yellow oil: 1H NMR (CDCl3, 400 MHz) δ 9.05 (s,
1H), 7.29 (s, 1H), 6.20 (s, 1H), 4.09 (s, 3H), 4.06 (s, 3H), 3.98 (s, 3H),
3.89 (s, 3H), 2.83 (t, 2H, J ) 7.8 Hz), 1.71 (br p, 2H, J ) 7.7 Hz),
1.43 (sextet, 2H, 7.6 Hz), 0.96 (t, 3H, J ) 7.3 Hz); 13C NMR (CDCl3,
125 MHz) δ 165.2, 156.1, 156.0 (q, J ) 39.0 Hz), 153.2, 149.9, 143.6,
142.2, 141.3, 141.1, 140.0, 137.0, 125.2, 123.0, 115.4, 115.0 (q, J )
284.5 Hz), 110.8, 62.2, 61.5 (2C’s), 53.2, 31.9, 30.7, 22.6, 13.9; IR
Data for methyl 7-butyl-5,10-dihydro-4,8,9-trihydroxy-3-methoxy-
5,10-dioxobenzo[g]quinoline-2-carboxylate (35): mp 203-208 °C; 1H
NMR (CDCl3, 400 MHz) δ 13.41 (s, 1H), 12.94 (s, 1H), 7.73 (s, 1H),
6.49 (s, 1H), 4.14 (s, 3H), 3.99 (s, 3H), 2.76 (t, 2H, J ) 7.6 Hz), 1.63
(br p, 2H, J ) 7.6 Hz), 1.40 (sextet, 2H, 7.6 Hz), 0.95 (t, 3H, J ) 7.3
Hz); IR (neat) νmax 2915, 2850, 1741, 1721, 1626, 1452, 1268 cm-1
;
FABHRMS (NBA-NaI) m/z 424.0092 (M+ + Na, C20H19NO8 requires
(neat) νmax 3821, 2956, 1804, 1740, 1522, 1458, 1225, 1133, 760 cm-1
;
424.1008).
FABHRMS (NBA-NaI) m/z 512.1547 (M+ + H, C24H24F3NO8 re-
Data for methyl 7-butyl-5,10-dihydro-3,4,8,9-tetrahydroxy-5,10-
dioxo-benzo[g]quinoline-2-carboxylate (36): mp 198-200 °C; 1H NMR
(CDCl3, 400 MHz) δ 13.35 (s, 1H), 12.77 (s, 1H), 11.41 (s, 1H), 7.75
(s, 1H), 6.50 (s, 1H), 4.11 (s, 3H), 2.76 (t, 2H, J ) 7.8 Hz), 1.62 (br
p, 2H, J ) 7.6 Hz), 1.40 (sextet, 2H, 7.6 Hz), 0.95 (t, 3H, J ) 7.2 Hz);
quires 512.1532).23
(22) The corresponding carboxylic acid of 32 was 25× less potent (IC50
) 8 µg/mL).
(23) In preliminary studies of the reaction before optimization, small
amounts of 31 were detected in the crude reaction products: 1H NMR
(CDCl3, 400 MHz) δ 8.58 (s, 1H), 7.74 (s, 1H), 4.12 (s, 3H), 4.07 (s, 3H),
3.99 (s, 3H), 3.95 (s, 3H), 2.85 (t, 2H, J ) 7.8 Hz), 1.70 (br p, 2H, J ) 7.6
Hz), 1.48 (sextet, 2H, J ) 7.4 Hz), 0.98 (t, 3H, J ) 7.3 Hz); FABMS
(NBA) m/z 608 (M+ + H, C26H23F16NO8 requires 608). Under the optimized
reaction conditions, 31 was not detected.
IR (film) νmax 2917, 2858, 1681, 1631, 1442, 1282, 1208 cm-1
;
MALDIHRMS (CHCA) m/z 388.1043 (M+ + H, C19H17NO8 requires
388.1032).
7-Butyl-5,10-dihydro-3,4,9-trihydroxy-8-methoxy-5,10-dioxoben-
zo[g]quinoline-2-carboxylic Acid (1, Phomazarin). A solution of 34

Total Synthesis of Phomazarin
J. Am. Chem. Soc., Vol. 121, No. 11, 1999 2477
(0.7 mg, 1.8 µmol) in THF/H2O (3/1, 0.1 mL) was treated with aqueous
1 N LiOH (35 µL, 20 equiv), and the mixture was stirred in a 30 °C
oil bath for 24 h. After removal of solvent, the residue was dissolved
in CH3OH and acidified with cation-exchange resin (Dowex 50WX8-
100) until the color of the solution turned yellow. Filtration through a
cotton plug and concentration in vacuo afforded 1 (0.5 mg, 70%): mp
191 °C (lit. mp 196 °C);3 1H NMR (TFA-d, 400 MHz) δ 8.00 (s, 1H),
4.17 (s, 3H), 2.86 (t, 2H, J ) 7.7 Hz), 1.68 (br p, 2H, J ) 7.6 Hz),
1.44 (sextet, 2H, J ) 7.2 Hz), 1.00 (t, 3H, J ) 7.2 Hz); IR (film) νmax
1693, 1633, 1445, 1301, 1203, 1138 cm-1; FABHRMS (NBA-NaI)
m/z 410.0841 (M+ + Na, C19H17NO8 requires 410.0852) identical in
all compared respects with the natural product.
Acknowledgment. We gratefully acknowledge the financial
support of the National Institutes of Health (Grant CA42056)
and the Skaggs Institute for Chemical Biology.
Supporting Information Available: Text of experimental
details for the preparation and characterization of 15a-d, 16a-
d, 17, 18, and 21-25 and text, tables, and figures providing
details of the X-ray structure determination of 16b (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA983631Q