Synthesis of mimosamycin

Article abstract of DOI:10.1021/jo990767d

Mimosamycin (1) was synthesized in eight steps with an overall yield of 13% from 2-methoxy-3-methyl-1,4-benzoquinone by regioselective introduction of a chloromethyl group at C-6 and a methoxycarbonylmethyl group at C-5 and subsequent reaction of the intermediate methyl (o- (chloromethyl)phenyl)acetate derivative 16 with methylamine. Oxidation of the 5,7,8-trimethoxy-2,6-dimethyl-1,4-dihydroisoquinoline-3(2H)-one 17 thus obtained, using cerium(IV) ammonium nitrate as a selective oxidizing agent, gave mimosamycin (1) in good overall yield.

Full text of DOI:10.1021/jo990767d

VOLUME 65, NUMBER 3
FEBRUARY 11, 2000
© Copyright 2000 by the American Chemical Society
Articles
Syn th esis of Mim osa m ycin
Bart Kesteleyn and Norbert De Kimpe*
Department of Organic Chemistry, Faculty of Agricultural and Applied Biological Sciences,
University of Gent, Coupure Links 653, B-9000 Gent, Belgium
Received May 7, 1999
Mimosamycin (1) was synthesized in eight steps with an overall yield of 13% from 2-methoxy-3-
methyl-1,4-benzoquinone by regioselective introduction of a chloromethyl group at C-6 and a
methoxycarbonylmethyl group at C-5 and subsequent reaction of the intermediate methyl
(o-(chloromethyl)phenyl)acetate derivative 16 with methylamine. Oxidation of the 5,7,8-trimethoxy-
2,6-dimethyl-1,4-dihydroisoquinoline-3(2H)-one 17 thus obtained, using cerium(IV) ammonium
nitrate as a selective oxidizing agent, gave mimosamycin (1) in good overall yield.
In tr od u ction
Mimosamycin (1) (7-methoxy-2,6-dimethyl-3,5,8(2H)-
isoquinolinetrione) was the first member of a new class
of naturally occurring 3,5,8(2H)-isoquinolinetrione anti-
biotics, isolated as a satellite metabolite from the strep-
tothricin-producing microorganism Streptomyces laven-
dulae No. 314.1 Later, mimosamycin together with several
structurally related pigments, e.g., 4-aminomimosamycin
(2), 7-amino-7-demethoxymimosamycin (3), and the sulfur-
containing perfragillin (4), have been identified in marine
sponges of the genera Reniera,2 Petrosia,3 Xestospongia,4
and in the bryozoan Membranipora perfragilis.5
* To whom correspondence should be addressed. Phone: 32
264.59.51. Fax: 32 9 264.62.43.
(1) (a) Arai, T.; Yazawa, K.; Mikami, Y.; Kubo, A.; Takahashi, K. J
Antibiot. 1976, 29, 398-407. (b) Mikami, Y.; Yokoyama, K.; Omi, A.;
Arai, T. J . Antibiot. 1976, 29, 408-414. (c) Fukumi, H.; Maruyama,
F.; Yoshida, K.; Arai, M.; Kubo, A.; Arai, T. J . Antibiot. 1978, 31, 847-
849.
9
Mimosamycin has been found to be active against
mainly mycobacteria, including the streptomycin-resis-
tant strains of Mycobacterium tuberculosis.1 Further-
more, mimosamycin proved to be active against some
Gram-positive bacteria,1 but it was completely inactive
against Gram-negative bacteria and most fungi1 except
Cladosporium cucumerinum.4 Mimosamycin is also struc-
turally related to the saframycins, which represent a
more complex class of “dimeric” isoquinoline quinones
with antitumor activity.2 Mimosamycin has been syn-
thesized previously from 7-hydroxy-6-methylisoquinoline
by an appropriate oxidation sequence to create the 3,5,8-
(2) (a) He, H.-Y.; Faulkner, D. J . J . Org. Chem. 1989, 54, 5822-
5824. (b) Frincke, J . M.; Faulkner, D. J . J . Am. Chem. Soc. 1982, 104,
265-269.
(3) Kobayashi, M.; Rao, S. R.; Chavakula, R.; Sarma, N. S. J . Chem.
Res., Synop. 1994, 282-283.
(4) Edrada, R. A.; Proksch, P.; Wray, V.; Christ, R.; Witte, L.; Van
Soest, R. W. M. J . Nat. Prod. 1996, 59, 973-976.
(5) Schmitz, F. J .; DeGuzman, F. S.; Choi, Y.-H.; Hossain, M. B.;
Rizvi, S. K.; Van der Helm, D. Pure Appl. Chem. 1990, 62, 1393-1396.
10.1021/jo990767d CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/19/2000

636 J . Org. Chem., Vol. 65, No. 3, 2000
Kesteleyn and De Kimpe
Sch em e 2
Sch em e 1
(2H)-isoquinolinetrione ring system6 and, starting from
2-methoxy-3-methyl-1,4-benzoquinone, by construction of
the lactam moiety either by cycloaddition with 2-aza-1,3-
bis(tert-butyldimethylsilyloxy)-1,3-butadiene7 or via a
classical approach using a low-yielding multistep se-
quence.8 In view of the interesting antibiotic properties
found in this class of 3,5,8(2H)-isoquinolinetrione anti-
biotics and related heterocyclic derivatives, we report
herein a new and efficient route to mimosamycin.
and 16%, respectively, when a nonpurified sample of 6
was refluxed for 16 h in toluene in the presence of 5 equiv
of ketene dimethyl acetal. It is known from the litera-
ture11 that the formation of 1:2-adducts from the reaction
of p-benzoquinones with ketene acetals is stimulated by
either the presence of weak acids or the use of polar
aprotic solvents, both by stabilizing the intermediate
zwitterion resulting from the addition of one molecule of
ketene acetal and thereby allowing the addition of the
second ketene acetal. Apolar solvents, however, allow
only the addition of one molecule of ketene acetal, leading
selectively to benzofurans. Thus, upon reaction of a
carefully purified sample of 2-methoxy-3-methyl-1,4-
benzoquinone (6) with 5 equiv of ketene dimethyl acetal
under reflux in toluene for 24 h, compound 9 was
obtained as the only reaction product in 44% yield,
together with 52% of unreacted starting material 6,
which could be recovered by means of flash chromatog-
raphy. Upon reflux in 80% methanol, hydrolysis of the
ortho ester function of 9 led to the methoxycarbonyl-
methyl-containing hydroquinone 12 in 93% yield, which
was subsequently protected by methylation using di-
methyl sulfate and potassium carbonate in refluxing
acetone to give 13 in 79% yield.
For the introduction of the halomethyl group, using
anhydrous conditions to avoid the hydrolysis of the
methyl ester function, compound 13 was reacted with
excess paraformaldehyde in the presence of a solution of
33% HBr in glacial acetic acid or dry HCl in ether;
however, using these reaction conditions either no reac-
tion products or complex reaction mixtures were ob-
tained. The chloromethylation of 13 could be accom-
plished only when concentrated solutions of HBr or HCl
in water were used and only after long reaction times
and in the presence of a large excess of zinc(II) chloride
as catalyst. However, under these reaction conditions, the
hydrolysis of the methyl ester function could not be
prevented. The best results were obtained when 13 was
reacted with 20 equiv of paraformaldehyde in a mixture
of 12 M HCl and acetic acid and 10 equiv of zinc(II)
chloride as Lewis catalyst at room temperature for 3
days, leading to a mixture of chroromethylphenyl acetic
acid 14 and benzopyranone derivative 15 in a ratio of
6:4 (Scheme 4). Upon reflux of this crude reaction mixture
in acetone with 5 equiv of potassium carbonate, complete
conversion of the chloromethylphenyl acetic acid 14 into
Resu lts a n d Discu ssion
A retrosynthetic analysis of the structure of mimosa-
mycin (1) (Scheme 1) shows that the lactam moiety can
be constructed by reaction of methylamine with an
appropriately substituted p-benzoquinone 5 bearing a
halomethyl group at C-3 and an alkoxycarbonylmethyl
group at C-2. Therefore, because 2-methoxy-3-methyl-1,4-
benzoquinone (6) is available as a starting material from
the oxidation of the dimethyl ether of 2-methylresorcinol
with sodium dichromate,9 the feasibility of the strategy
depends on the regioselective introduction of both the
halomethyl group and the alkoxycarbonylmethyl group.
2-Methoxy-3-methyl-1,4-benzoquinone (6) is known to
react with nucleophiles selectively to the C-1 carbonyl
by Michael addition because the carbonyl group at C-4
is in extended conjugation with the methoxy group at
C-2.9 To our surprise, however, two well-established
procedures for the introduction of an alkoxycarbonyl-
methyl group in p-quinones,10 namely, reaction of 6 with
a pyridinium ylide, created in situ by reaction of the
pyridinium salt 7 with triethylamine, and also the radical
introduction of the same alkoxycarbonylmethyl group,
using the potassium salt of ethyl malonate in the pres-
ence of silver nitrate and ammonium persulfate, afforded
only complex reaction mixtures from which compound 8
could not be isolated (Scheme 2).
In a final attempt, the p-benzoquinone 6 was reacted
with 5 equiv of ketene dimethyl acetal at 110 °C for 1 h,
leading to a mixture of benzofuran 9 (15%) and 3,5,7-
trimethoxynaphthoquinone 10 (31%) as a result of 1:1-
addition and 1:2-addition of 6 with ketene dimethyl
acetal, respectively, together with the hydroquinone 11
(6%) (Scheme 3). The same mixture of reaction products
9, 10, and 11 was also obtained in yields of 10%, 10%,
(6) (a) Fukumi, H.; Kurihara, H.; Hata, T.; Tamura, C.; Mishima,
H.; Kubo, A.; Arai, T. Tetrahedron Lett. 1977, 43, 3825-3828. (b)
Mishima, H.; Fukumi, H.; Kurihara, H. Heterocycles 1977, 6, 1652-
1657. (c) Fukumi, H.; Kurihara, H.; Mishima, H. Chem. Pharm. Bull.
1978, 26, 2175-2180.
(7) McKillop, A.; Brown, S. P. Synth. Commun. 1987, 17, 657-667.
(8) Parker, K. A.; Casteel, D. A. J . Org. Chem. 1988, 53, 2847-2850.
(9) Mandell, L.; Roberts, E. C. J . Heterocycl. Chem. 1965, 2, 479-
480.
(10) Aldersley, M. F.; Dean, F. M.; Nayyir-Mazhir, R. J . Chem. Soc.,
Perkin Trans. 1 1983, 1753-1757.
(11) (a) Grandmaison, J .-L.; Brassard, P. Tetrahedron 1977, 33,
2047-2053. (b) Cameron, D. W.; Crossley, M. J . Aust. J . Chem. 1978,
31, 1353-1362.
(12) Luly, J . R.; Rapoport, H. J . Am. Chem. Soc. 1983, 105, 2859-
2866.

Synthesis of Mimosamycin
J . Org. Chem., Vol. 65, No. 3, 2000 637
Sch em e 3
Sch em e 4
Sch em e 5
oxidation of lactam 17 by the use of other oxidizing agents
such as fuming nitric acid or cerium(IV) ammonium
nitrate.
Using fuming nitric acid as an oxidizing agent, the
reaction of lactam 17 with 99% HNO3 at 0 °C for 30 s
led to mimosamycin in 23% after isolation by flash
chromatography and recrystallization from ethyl acetate
(Scheme 5). However, the best result was obtained by
reaction of 17 with 4 equiv of cerium(IV) ammonium
nitrate in aqueous acetonitrile at 0 °C for 30 min, which
gave rise to a reaction mixture containing only a small
amount of side products from which mimosamycin could
be isolated in 57% yield by recrystallization from ethyl
acetate. None of the side products from either oxidation
procedure could ever be isolated in pure form to allow
their characterization.
benzopyran 15 in an overall yield of 85% for both steps
was accomplished. For the conversion of lactone 15 to
the desired lactam 17, compound 15 was first treated
with a dry solution of methanol saturated with hydro-
chloric acid at room temperature for 6 h, leading to the
intermediate methyl (o-(chloromethyl)phenyl)acetate de-
rivative 16 in a crude yield of 89%. Reaction of 16 with
excess methylamine in methanol gave, after overnight
reaction at room temperature, lactam 17 in 94% yield.
Exp er im en ta l Section
1
Gen er a l Meth od s. H (270 MHz) and 13C (68 MHz) NMR
peak assignments were performed with the aid of the DEPT
technique, 2D-COSY spectra, and HETCOR spectra.
The oxidation of lactam 17 to mimosamycin using
silver(II) oxide in 6 M HNO3 and dioxane was reported
previously.8 The authors reported this oxidation to give
only 18% of a mixture rich in mimosamycin from which
the pure mimosamycin could not be isolated by means
of flash chromatography. In our hands, these conclusions
were confirmed, and we found also that the presence of
side products from this reaction might result from an
overoxidation process because prolonged reaction times
(up to 8 h) completely diminished the presence of mimo-
samycin in the reaction mixture. Oxidation of 1,2,4-
trimethoxybenzene derivatives have been reported in the
literature to give mixtures of the ortho and para quino-
nes,12 and therefore we were tempted to improve the
Keten e Dim eth yl Aceta l. Several references are available
in the literature for the synthesis of ketene dimethyl acetal.13
None of these procedures, however, proved to be of practical
use in our hands. Therefore we used a modified procedure from
the synthesis of ketene diethyl acetal,14 which allowed ketene
dimethyl acetal to be synthesized in an overall yield of 53% in
two steps from trimethyl orthoacetate and in a purity of ∼90%,
which was found to be sufficient for the cycloaddition reaction
with 2-methoxy-3-methyl-1,4-benzoquinone (6). Ketene di-
(13) (a) McElvain, S. M.; Anthes, H. I.; Shapiro, S. H. J . Am. Chem.
Soc. 1942, 64, 2525-2531. (b) Corey, E. J .; Bass, J . D.; LeMahieu, R.;
Mitra, R. B. J . Am. Chem. Soc. 1964, 86, 5570-5583.
(14) (a) Beyerstedt, F.; McElvain, S. M. J . Am. Chem. Soc. 1937,
59, 1273-1275. (b) Walters, P. M.; McElvain, S. M. J . Am. Chem. Soc.
1940, 62, 1482-1484.

638 J . Org. Chem., Vol. 65, No. 3, 2000
Kesteleyn and De Kimpe
methyl acetal was prepared as follows. To a cooled (0 °C)
solution of trimethyl orthoacetate (0.8 mol, 96 g) and pyridine
(0.8 mol, 63.2 g) was added bromine (0.8 mol, 41.2 mL)
dropwise over a period of 1 h. The reaction mixture was kept
at room temperature for 2 h and poured into dry ether (500
mL), which caused pyridinium bromide to precipitate. Filtra-
tion and evaporation of the filtrate in vacuo afforded crude
trimethyl bromoorthoacetate. Distillation (bp 68-69 °C/14
mmHg) gave pure trimethyl bromoorthoacetate (99.4 g, 62%).
A vigorously stirred mixture of sodium (1 g atom, 23 g) in
p-xylene (200 mL) was heated in an oil bath at 140-150 °C in
a three-necked flask provided with a dropping funnel and two
reflux condensers. When p-xylene started to reflux, trimethyl
bromoorthoacetate (0.5 mol, 99.4 g) was added over a period
of 20 min, causing a vigorous reaction. Heating was continued
at the same temperature for 30 min after which the suspension
was allowed to cool to room temperature, allowing a deep blue
solid to precipitate. The supernatant was collected by decanta-
tion and distilled using a Vigreux column (∼80 cm) (bp 89-
92 °C/760 mmHg) to give ketene dimethyl acetal (37.5 g, 85%),
purity ∼90% (by NMR). The 10% of impurities was recognized
as to be merely trimethyl orthoacetate.
trimethoxy-7-methyl-2,3-dihydrobenzo[b]furan-5-ol (9) (0.58 g,
44%) as a pale yellow oil.
Met h yl (2,5-Dih yd r oxy-4-m et h oxy-3-m et h ylp h en yl)-
a ceta te (12). 2,2,6-Trimethoxy-7-methyl-2,3-dihydrobenzo[b]-
furan-5-ol (9) (1 mmol, 240 mg) was heated under reflux in
80% methanol (10 mL) for 3 h. The reaction was concentrated
in vacuo to 2 mL, water was added, and the suspension was
extracted with dichloromethane and dried (MgSO4). Flash
chromatography on silica gel with ethyl acetate/light petroleum
ether (1:1) as eluent gave pure 12 (210 mg, 93%) as a colorless
oil. 1H NMR (CDCl3): δ 2.22 (3H, s, Me), 3.58 (2H, s, CH2),
3.74 (3H, s, MeO), 3.76 (3H, s, MeO), 5.32 (1H, s, OH), 6.57
(1H, s, dCH), 7.14 (1H, s, OH). 13C NMR (CDCl3): δ 9.7 (Me),
37.7 (CH2), 52.7 (MeO), 60.8 (MeO), 114.0 (dCH), 116.7 (d
C
quat), 120.4 (dCquat), 142.6 (dC-O), 145.5 (dC-O), 147.0 (d
C-O), 174.6 (CdO). IR (NaCl): νmax 3410 (OH), 1722 (CdO)
cm-1. MS m/z (%): 226 (M+, 66), 194 (68), 167 (60), 166 (68).
Anal. Calcd for C11H14O5: C 58.40, H 6.24. Found: C 58.00,
6.08.
Meth yl (2,4,5-Tr im eth oxy-3-m eth ylph en yl)acetate (13).
A mixture of methyl (2,5-dihydroxy-4-methoxy-3-methyl-
phenyl)acetate (12) (1 mmol, 230 mg), dimethyl sulfate (2
mmol, 250 mg), and potassium carbonate (5 mmol, 0.63 g) in
acetone (20 mL) was heated under reflux for 2 h, cooled to
room temperature, filtered, and evaporated in vacuo. Flash
chromatography on silica gel using ethyl acetate/light petro-
leum ether (1:4) as eluent gave 13 (200 mg, 79%) as an oil. 1H
NMR (CDCl3): δ 2.22 (3H, s, Me), 3.63 (2H, s, CH2), 3.68 (3H,
s, MeO), 3.71 (3H, s, MeO), 3.78 (3H, s, MeO), 3.82 (3H, s,
MeO), 6.65 (1H, s, dCH). 13C NMR (CDCl3): δ 9.7 (Me), 35.4
(CH2), 51.0 (MeO), 56.0 (MeO), 60.2 (MeO), 60.8 (MeO), 111.4
(dCH), 122.2 (dCquat), 125.5 (dCquat), 147.3 (dC-O), 149.2 (d
C-O), 150.8 (dC-O), 172.4 (CdO). IR (NaCl): νmax 1736 (Cd
O) cm-1. MS m/z (%): 254 (M+, 100), 239 (28), 207 (37), 195
(76), 165 (21). Anal. Calcd for C13H18O5: C 61.41, H 7.13. Found:
C 61.82, H 7.38.
5,7,8-Tr im e t h oxy-6-m e t h yl-1H -b e n z[c]p yr a n -3(4H )-
on e (15). To a mixture of methyl (2,4,5-trimethoxy-3-meth-
ylphenyl)acetate (13) (2 mmol, 0.5 g), paraformaldehyde (40
mmol, 1.2 g), and zinc(II) chloride (20 mmol, 2.7 g) in acetic
acid (5 mL) was added dropwise concentrated hydrochloric acid
(15 mL), and the reaction was stirred at room temperature
for 3 days in a stoppered flask. The reaction mixture was
poured into water, extracted with dichloromethane, dried
(MgSO4), and evaporated in vacuo. The residue was dissolved
in acetone (20 mL) and heated under reflux for 1 h in the
presence of potassium carbonate (10 mmol, 1.38 g). The
reaction mixture was allowed to cool to room temperature,
filtered, and evaporated in vacuo. Flash chromatography on
silica gel using ethyl acetate/light petroleum ether (1:4) as
eluent gave pure 15 (0.43 g, 85%) as white needles (from
ethanol), mp 92.0-92.4 °C. 1H NMR (CDCl3): δ 2.21 (3H, s,
Me), 3.66 (2H, s, CH2CdO), 3.69 (3H, s, MeO), 3.82 (3H, s,
MeO), 3.85 (3H, s, MeO), 5.35 (CH2O). 13C NMR (CDCl3): δ
9.5 (Me), 30.0 (CH2CdO), 60.3 (MeO), 60.9 (MeO), 61.0 (MeO),
65.4 (CH2O), 119.3 (dCquat), 122.8 (dCquat), 126.1 (dCquat), 145.2
(dC-O), 150.6 (dC-O), 151.3 (dC-O), 170.7 (CdO). IR
(NaCl): νmax 1744 (CdO) cm-1. MS m/z (%): 252 (M+, 34), 237
(10), 193 (25), 91 (100). Anal. Calcd for C13H16O5: C 61.90, H
6.39. Found: C 61.84, H 6.12.
Syn th esis of 2,2,6-Tr im eth oxy-7-m eth yl-2,3-dih ydr oben -
zo[b]fu r a n -5-ol (9), 3,5,7-Tr im eth oxy-2-m eth yl-1,4-n a p h -
th oqu in on e (10), a n d 2-Meth oxy-3-m eth ylh yd r oqu in on e
(11). 2-Methoxy-3-methyl-1,4-benzoquinone (6)9 (3 mmol, 0.46
g) and ketene dimethyl acetal (15 mmol, 1.32 g) were mixed
at room temperature and subsuquently heated at 110 °C in
an open recipient for 1 h. Flash chromatography on silica gel
using ethyl acetate/light petroleum ether (1:4) as an eluent
gave 2,2,6-trimethoxy-7-methyl-2,3-dihydrobenzo[b]furan-5-ol
1
(9) (110 mg, 15%) as a pale yellow oil as the first fraction. H
NMR (CDCl3): δ 2.19 (3H, s, Me), 3.19 (2H, d, J ) 0.7 Hz, CH2),
3.42 (6H, s, (MeO)2), 3.77 (3H, s, MeO), 5.26 (1H, s, OH), 6.64
(1H, d, J ) 0.7 Hz, dCH). 13C NMR (CDCl3): δ 9.4 (Me), 37.5
(CH2), 50.4 ((MeO)2), 61.0 (MeO), 108.6 (dCH), 113.2 (Cquat),
119.5 (Cquat), 125.0 (Cquat), 143.4 (dC-O), 144.9 (dC-O), 149.1
(dC-O). IR (NaCl): νmax 3429 (OH), 1465, 1285, 1052, 733
cm-1. MS m/z (%): 240 (M+, 64), 225 (47), 209 (55), 194 (37),
193 (100). Anal. Calcd for C12H16O5: C 59.99, H 6.71. Found:
C 59.64, H 6.55. Using the same solvent system, 2-methoxy-
3-methylhydroquinone (11) (30 mg, 6%) eluted as a second
compound from the column. Evaporation of the solvent gave
a solid compound, which was recrystallized from chloroform
to afford 11 as white needles, mp 110-111 °C. 1H NMR
(CDCl3): δ 2.19 (3H, s, Me), 3.78 (3H, s, MeO), 4.48 (1H, s,
OH), 5.24 (1H, s, OH), 6.48 (1H, d, J ) 8.6 Hz, dCH), 6.68
(1H, d, J ) 8.6 Hz, dCH). 13C NMR (CDCl3): δ 9.3 (Me), 60.9
(MeO), 111.0 (dCH), 112.4 (dCH), 117.8 (dCquat), 142.8 (dC-
O), 145.9 (dC-O), 147.7 (dC-O). IR (KBr): νmax 3254 (OH),
1492, 1253, 1076, 795 cm-1. MS m/z (%): 154 (M+, 100), 139
(70), 111 (29). Anal. Calcd for C8H10O3: C 62.33, H 6.54. Found:
C 61.79, H 6.60. Elution with ethyl acetate/light petroleum
ether (1:1) gave 3,5,7-trimethoxy-2-methyl-1,4-naphthoquinone
(10) (240 mg, 31%) as a yellow powder (from methanol), mp
1
139-140 °C (Lit.15 mp 141-142 °C). H NMR (CDCl3): δ 2.02
(3H, s, Me), 3.94 (3H, s, MeO), 3.96 (3H, s, MeO), 4.10 (3H, s,
MeO), 6.68 (1H, d, J ) 2.6 Hz, dCH-6), 7.26 (1H, d, J ) 2.6
Hz, dCH-8). 13C NMR (CDCl3): δ 8.9 (Me), 55.9 (MeO), 56.4
(MeO), 61.1 (MeO), 103.2 (dCH-8), 103.5 (dCH-6), 113.7 (d
C
quat), 128.2 (dCquat), 136.2 (dCquat), 158.9 (dC-O), 161.8 (d
Met h yl (6-Ch lor om et h yl-2,4,5-t r im et h oxy-3-m et h yl-
p h en yl)a ceta te (16). To a saturated solution of dry HCl in
dry methanol (10 mL, distilled from sodium) was added 5,7,8-
trimethoxy-6-methyl-1H-benz[c]pyran-3(4H)-one (15) (1.7 mmol,
0.42 g), and the reaction mixture was stirred for 6 h in a
stoppered flask at room temperature. The reaction was
quenched by the addition of water, extracted with dichlo-
romethane, dried (MgSO4), and evaporated in vacuo to give
crude 16 (0.46 g, 89%) as an oil, which was used without
C-O), 164.7 (dC-O), 179.1 (CdO), 185.7 (CdO). IR (KBr):
νmax 1658 (CdO), 1625 (CdO), 1591 (CdC) cm-1. MS m/z (%):
262 (M+, 47), 247 (12), 154 (100), 139 (75). Anal. Calcd for
C
14H14O5: C 64.12, H 5.38. Found: C 63.91, H 5.55.
A carefully purified sample of 6 (5.5 mmol, 0.84 g), obtained
by flash chromatography with ethyl acetate/light petroleum
ether (1:9) as an eluent, and ketene dimethyl acetal (27.5
mmol, 2.42 g) were heated under reflux in toluene (50 mL) for
24 h and evaporated in vacuo. Flash chromatography on silica
gel using ethyl acetate/light petroleum ether (1:4) as an eluent,
first gave 6 (0.44 g, 52%) as a yellow band and then 2,2,6-
1
further purification in the next step (purity > 95%). H NMR
(CDCl3): δ 2.22 (3H, s, Me), 3.69 (3H, s, MeO), 3.72 (3H, s,
MeO), 3.82 (2H, s, CH2CdO), 3.83 (3H, s, MeO), 3.91 (3H, s,
MeO), 4.70 (2H, s, CH2Cl). 13C NMR (CDCl3): δ 9.9 (Me), 31.6
(CH2CdO), 38.2 (CH2Cl), 52.2 (MeO), 60.1 (MeO), 60.8 (MeO),
61.2 (MeO), 123.0 (dCquat), 126.8 (dCquat), 128.5 (dCquat), 148.3
(15) Botha, M. E.; Giles, R. G. F.; Yorke, S. C. J . Chem. Soc., Perkin
Trans. 1, 1991, 85-88.

Synthesis of Mimosamycin
J . Org. Chem., Vol. 65, No. 3, 2000 639
(dC-O), 151.4 (dC-O), 153.7 (dC-O), 172.1 (CdO). IR
(NaCl): νmax 1733 (CdO) cm-1. MS m/z (%): 302/4 (M+, 100),
267 (10), 243/5 (27).
vacuo. Flash chromatography on silica gel with acetone/
chloroform (1:4) and recrystallization from ethyl acetate gave
pure mimosamycin (20 mg, 23%) as yellow needles.
5,7,8-Tr im eth oxy-2,6-dim eth yl-1,4-dih ydr oisoqu in olin e-
3(2H)-on e (17). To a solution of crude methyl (6-chloromethyl-
2,4,5-trimethoxy-3-methylphenyl)acetate (16) (1.4 mmol, 0.42
g) in methanol (30 mL) was added dropwise a solution of
methylamine 10% in water (10 mL), and the reaction mixture
was stirred for 24 h in a stoppered flask. The solvent was
concentrated in vacuo to 5 mL, water was added, and the
suspension was extracted with chloroform, dried (MgSO4), and
evaporated in vacuo. Flash chromatography on silica gel using
ethyl acetate as eluent gave pure 17 (0.35 g, 94%) as white
Syn th esis of Mim osa m ycin by Oxid a tion of 5,7,8-
Tr im eth oxy-2,6-d im eth yl-1,4-d ih yd r oisoqu in olin e-3(2H)-
on e (17) w ith Cer iu m (IV) Am m on iu m Nitr a te. To a cooled
(0 °C) solution of 5,7,8-trimethoxy-2,6-dimethyl-1,4-dihydroiso-
quinoline-3(2H)-one (17) (0.75 mmol, 200 mg) in acetonitrile
(5 mL) was added dropwise a solution of cerium(IV) am-
monium nitrate (3 mmol, 1.63 g) in water (10 mL), and after
stirring for an additional 30 min at the same temperature,
the reaction mixture was poured into water and extracted with
ethyl acetate. The combined organic extracts were stirred with
potassium carbonate (1 g) for 1 h, filtered, and evaporated in
vacuo. Recrystallization from ethyl acetate gave mimosamycin
as yellow needles, mp 228.5-229 °C (lit.1a mp 219-231 °C).
1H NMR (CDCl3): δ 2.06 (3H, s, Me), 3.67 (3H, s, N-Me), 4.17
(3H, s, MeO), 7.09 (1H, s, dCH-4), 8.27 (1H, s, dCH-1). 13C
NMR (CDCl3): δ 9.6 (Me), 38.4 (N-Me), 61.3 (MeO), 111.3 (d
Cquat), 116.7 (dCH-4), 133.2 (dCquat), 138.9 (dCquat), 142.1 (d
CH-1), 159.5 (dC-OMe), 162.8 (N-CdO), 177.3 (CdO), 183.5
(CdO). IR (NaCl): νmax 1636 (CdO), 1581 (CdO), 1549 (CdC)
cm-1. MS m/z (%): 233 (M+, 100), 218 (82), 205 (72), 190 (47),
162 (32), 134 (33). The analytical and spectral data are in
accordance with those of the natural mimosamycin as pub-
lished in the literature.1c
1
flakes (from ether), mp 104 °C. H NMR (CDCl3): δ 2.19 (3H,
s, Me), 3.12 (3H, s, N-Me), 3.57 (2H, s, CH2CdO), 3.68 (3H,
s, MeO), 3.81 (3H, s, MeO), 3.86 (3H, s, MeO), 4.48 (2H, s,
CH2N). 13C NMR (CDCl3): δ 9.0 (Me), 30.6 (CH2CdO), 34.1
(N-Me), 48.1 (CH2N), 59.9 (MeO), 59.9 (MeO), 60.1 (MeO),
120.7 (dCquat), 122.0 (dCquat), 124.4 (dCquat), 144.6 (dC-O),
149.7 (dC-O), 150.9 (dC-O), 168.0 (CdO). IR (NaCl): νmax
1650 (CdO) cm-1. MS m/z (%): 265 (M+, 100), 250 (12), 234
(66), 208 (16), 193 (44). Anal. Calcd for C13H19NO4: C 61.64, H
7.56. Found: C 61.88, H 7.65.
Syn th esis of Mim osa m ycin by Oxid a tion of 5,7,8-
Tr im eth oxy-2,6-d im eth yl-1,4-d ih yd r oisoqu in olin e-3(2H)-
on e (17) w ith Nitr ic Acid . To 5,7,8-trimethoxy-2,6-dimethyl-
1,4-dihydroisoquinoline-3(2H)-one (17) (0.37 mmol, 100 mg)
was added at 0 °C fuming nitric acid (1 mL), and the mixture
was stirred for 30 s. The reaction was quenched by the addition
of a saturated solution of sodium hydrogen carbonate until all
nitric acid was neutralized. The aqueous solution was ex-
tracted with ethyl acetate, dried (MgSO4), and evaporated in
Ack n ow led gm en t. The authors are indebted to the
J anssen Research Foundation (Beerse, Belgium) for
financial support.
J O990767D