Total synthesis of rubrolide M and some of its unnatural congeners

Article abstract of DOI:10.1016/S0040-4039(02)00202-2

Two protocols have been developed for the Pd-catalyzed regioselective synthesis of 4-aryl-3-chloro-2(5H)-furanones starting from 3,4-dichloro-2(5H)-furanone. These monochloro derivatives have then been used as precursors to (Z)-4-aryl-5-[1-(aryl)methylidene]-3-chloro-2(5H)-furanones including naturally-occurring rubrolide M.

Full text of DOI:10.1016/S0040-4039(02)00202-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 2023–2027
Total synthesis of rubrolide M and some of its unnatural
congeners
Fabio Bellina,* Chiara Anselmi and Renzo Rossi*
Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, I-56126 Pisa, Italy
Received 10 December 2001; revised 23 January 2002; accepted 25 January 2002
Abstract—Two protocols have been developed for the Pd-catalyzed regioselective synthesis of 4-aryl-3-chloro-2(5H)-furanones
starting from 3,4-dichloro-2(5H)-furanone. These monochloro derivatives have then been used as precursors to (Z)-4-aryl-5-[1-
(aryl)methylidene]-3-chloro-2(5H)-furanones including naturally-occurring rubrolide M. © 2002 Elsevier Science Ltd. All rights
reserved.
Recently, four new (Z)-4-aryl-5-[1-(aryl)methylidene]-3-
chloro-2(5H)-furanones, i.e. rubrolides I (1a), K (1b), L
(1c) and M (1d), have been isolated from the red
colonial tunicate Synoicum blochmanni.1
All these substances have been found to display signifi-
cant cytotoxicities against four cancer cell lines, i.e.
P-388 suspension culture of mouse limphoid carcinoma,
the monolayer cultures of human lung carcinoma (A-
RO
1a : R = H; X = Y = Z = Br
1b : R = Z = H; X = Y = Br
1c : R = X = H; Y = Z = Br
1d : R = X = Z = H; Y = Br
1e : R = Y = Z = H; X = Cl
1f : R = Me; X = Z = H; Y = Br
1g : R = Me; X = Cl; Y = Z = H
1h : R = Me; X = Y = Z = H
1i : R = Me; X = Cl; Y = H; Z = Br
1j : R = Me; X = Cl; Y = Z = Br
Cl
X
O
O
Y
Z
OR
R
R
X
Br
Br
X
Y
Y
O
O
O
O
O
O
2 : X = Cl
3 : X = Br
4
5
Z
X
OR1
MeO
MeO
O
O
O
O
6
7
Br
Br
OMe
OMe
* Corresponding authors. Tel.: +39 50 918214; fax: +39 50 918260; e-mail: rossi@dcci.unipi.it
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)00202-2

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F. Bellina et al. / Tetrahedron Letters 43 (2002) 2023–2027
549), human colon carcinoma (HT-29) and human
melanoma (MEL-28), but rubrolide M (1d) proved to
be the most active compound.1 To the best of our
knowledge, no synthetic route towards compounds 1a–
d has been reported to date.
reaction was very low (ca. 10%) and that the major
reaction product was biphenyl (10a) derived from
homocoupling of 9a. Interestingly, 10a was the major
reaction product also when 2 was reacted with 1.1
equiv. of 9a either in methanol at 40–60°C in the
presence of 1.5 equiv. of AcONa and 3 mol%
PdCl2(PPh3)2 or in toluene at 80°C in the presence of
3.0 equiv. of Ag2O and a catalyst system consisting of
2.5 mol% Pd2(dba)3 and 5 mol% tert-Bu3P.7 Neverthe-
less, we found that the formation of 10a could be
minimized by treatment of 2 with 1.05 equiv. of 9a in
toluene at 80°C in the presence of 3.0 equiv. of KF, 2.5
mol% Pd2(dba)3 and 5 mol% tert-Bu3P. Under these
conditions the conversion of the reaction was almost
quantitative, but 8a was obtained in a comparable
amount with 3,4-diphenyl-2(5H)-furanone (11a) (52
and 41% yield, respectively). On the other hand, the use
either of electron-rich phosphine ligands such as Cy3P
or (o-biphenyl)P(tert-Bu)2 in place of tert-Bu3P or of a
base such as Cs2CO3 or CsF in place of KF did not
allow to increase either the regioselectivity of the Pd-
catalyzed reaction between 2 and 9a or the yield of 8a.
As part of our program aimed at developing concise
and efficient protocols for the selective synthesis of
natural and unnatural substances, which are potentially
cytotoxic against human tumor cell lines, we decided to
develop a general procedure for the synthesis of
rubolide M (1d) and its congeners, such as compounds
1e–j, and to evaluate the cytotoxic activities of these
substances. It occurred to us that access to these com-
pounds might be achieved starting from readily avail-
able 3,4-dichloro-2(5H)-furanone (2)2 by a protocol
similar to that, recently used successfully to prepare a
variety of (Z)-4-aryl-5-[1-(aryl)methylidene]-3-bromo-
2(5H)-furanones 43 including the compound with the
structure corresponding to that reported for naturally-
occurring rubrolide N (4a) (R=OH; X=Br; Y=Cl;
R1=Z=H)1,4 starting from 3,4-dibromo-2(5H)-fura-
none (3).5 This protocol involved the regioselective
arylation of 3 at the bromine-bearing carbon atom 4 of
this dibromo derivative followed by the appendage of a
1-(aryl)methylidene unit at C-5 of the resulting 4-aryl-3-
bromo-2(5H)-furanone 5 by a furanolate chemistry
similar to that previously used to synthesize the
dimethyl ethers 6 and 7 corresponding to rubrolides C
and E.6
At last we found that either the 8a:11a molar ratio or
the yield of 8a could be increased when the reaction
between 2 and 1.05 equiv. of 9a was performed in
toluene at 85°C for 24 h in the presence of 2.5 mol%
Pd2(dba)3, 10 mol% (o-Tolyl)3P and 3.0 equiv. of KF
(Scheme 1). In fact, the crude mixture derived from this
reaction contained 8a contaminated by less than 15 and
7% of 10a and 11a, respectively. On the other hand,
purification of this crude mixture by MPLC on silica
gel allowed to obtain pure 8a in 63% yield. These
reaction conditions were then used to prepare com-
pounds 8b, 8c and 8d in 57, 42 and 61% yields, respec-
tively (Scheme 1).
However, the selective synthesis of 4-aryl-3-chloro-
2(5H)-furanones 8 starting from 2 proved to be prob-
lematic. In fact, in a preliminary attempt to prepare
regioselectively 3-chloro-4-phenyl-2(5H)-furanone (5a)
by reaction of 2 with 1.1 equiv. of phenylboronic acid
(9a) under the conditions commonly employed for
monoarylation of 3,3 i.e. in THF at 65°C in the pres-
ence of 5 mol% PdCl2(MeCN)2, 20 mol% AsPh3 and
3.0 equiv. of Ag2O, we found that the conversion of the
It should be mentioned that similarly to what observed
in the preparation of 8a, the crude reaction mixtures
corresponding to the preparations of compounds 8b, 8c
and 8d proved to be contaminated by less than 7% of
the corresponding 3,4-diaryl-2(5H)-furanones 11b, 11c
and 11d, respectively, and by less than 15% of the
biaryls 10b–d derived from homocoupling of the aryl-
boronic acids 9b–d.
Even though these results were satisfactory, we
searched for a new and more efficient route to com-
pounds 8 starting from dichloride 2. Thus, we found
that compounds 8 could be alternatively synthesized by
treatment of 2 with 1.1 equiv. of aryl(tributyl)stannanes
12 in NMP at 85°C for 21–22 h in the presence of the
catalyst system consisting of 2.5 mol% Pd2(dba)3 and 10
mol% (o-Tolyl)3P. This protocol allowed the prepara-
tion of compounds 8a, 8c, 8d and 8e in 53, 61, 78 and
55% yields, respectively (Scheme 2). Notably, the crude
mixtures which derived from these cross-coupling reac-
Scheme 1.

F. Bellina et al. / Tetrahedron Letters 43 (2002) 2023–2027
2025
butyldimethylsilyl trifluoromethanesulfonate (TBDM-
SOTf), 3.0 equiv. of i-Pr2NEt and 1.0 equiv. of an
arylaldehyde 14. After stirring for 23 h at room temper-
ature the reaction mixture was treated with 2.0 equiv.
of DBU at 20°C for 6 h and then with 3 M HCl to give
stereoselectively the required compound 1 (Scheme 3).
Cl
Cl
+
Ar–SnBu3
O
O
2
12a,c-e
Pd2(dba)3 (2.5 mol %)
(o-Tolyl)3P (10 mol %)
NMP, 85 °C, 21-22 h
This procedure was employed to prepare compounds 1f
and 1i in 46 and 43% yields starting from aldehyde 14a
and the 4-aryl-3-chloro-2(5H)-furanones 8d and 8b,
respectively, and compounds 1h and 1g in 65 and 51%
yields starting from aldehyde 14b and the 4-aryl-3-
chloro-2(5H)-furanones 8d and 8b, respectively.8 How-
ever, an attempt to prepare 1j starting from 14c and 8b
was fruitless, the desired product being obtained in a
negligible yield. On the other hand, another attempt to
synthesize 1j using a procedure very similar to that
previously employed to prepare (Z)-21-benzylidene-
3b-(tert-butyldimethylsilyl-oxy)-14-trimethylsilyloxy-
(55 - 78 %)
Cl
Ar
Ar
Ar
+
O
O
O
11a,c-e
O
8a,c-e
a
c
d
e
C6H5
4-ClC6H4
4-MeOC6H4
2-thienyl
Ar
Scheme 2.
5b,14b-card-20(22)-enolide
from
3b-(tert-butyldi-
methylsilyloxy) - 14 - trimethylsilyloxy - 5b,14b - card - 20-
(22)-enolide,9 i.e. by treatment of 8b with less than 1.0
equiv. of NaH in NMP at 20°C for 0.5 h followed by
addition of 14c, was also unsuccessful. In fact, no
desired compound 1j was obtained under these reaction
conditions and only the loss of 8b was observed.
tions proved to be contaminated by less than 5% of
compounds 11.
Having set up two different protocols for the regioselec-
tive synthesis of compounds 8 we then established the
feasibility of using these 2(5H)-furanone derivatives in
the preparation of a precursor to rubrolide M, i.e.
compound 1f, and for its structure of general formula
1, which are characterized by methoxyaryl moieties, by
application of a procedure similar to that recently used
to prepare dimethyl ethers 6 and 7 corresponding to
rubrolides C and E, respectively, starting from 4-(4-
methoxyphenyl)-2(5H)-furanone (13).6
We then completed the first total synthesis of rubrolide
M (1d) by reaction of 1f with 9.0 equiv. of BBr3 in
CH2Cl2 at room temperature for 23 h followed by
hydrolysis. This demethylation provided in 97% yield,
compound 1d having spectral properties in good agree-
ment with those reported for the natural product1
(Scheme 4). Under similar reaction conditions 1g gave
compound 1e in 98% yield (Scheme 4)10.
MeO
Finally, compounds 1d–i were evaluated in the NCI
3-cell line, one dose primary anticancer assay at a
1.00×10−4 M concentration. This 3-cell line panel con-
sisted of the MCF-7 (breast), NCI-H460 (lung), and
SF-268 (CNS). The results for each test agent are
reported in Table 1 as the percent of growth of the
treated cells when compared to the untreated control
cells.
O
O
13
Thus, in this one-pot protocol a CH2Cl2 solution of a
compound 8 was treated with 1.2 equiv. of tert-
MeO
Cl
1) TBDMSOTf (1.2 equiv),
X
MeO
OHC
Y
CH2Cl2, i-Pr2NEt (3.0 equiv)
O
Cl
rt, 23 h
O
+
X
OMe
2) DBU (2.0 equiv), rt, 6 h
3) 3 M HCl
O
Z
O
Y
Z
OMe
8d : X = H
14a : Y = Br; Z = H
14b : Y = Z = H
14b : Y = Z = H
14a : Y = H; Z = Br
14c : Y = Z = Br
1f : X = Z = H; Y = Br
1g : X = Cl; Y = Z = H
1h : X = Y = Z = H
1i : X = Cl; Y = H; Z = Br
1j : X = Cl; Y = Z = Br
8b : X = Cl
8d : X = H
8b : X = Cl
8b : X = Cl
Scheme 3.

2026
F. Bellina et al. / Tetrahedron Letters 43 (2002) 2023–2027
MeO
X
HO
Cl
Cl
X
1) BBr3 (7 - 9 equiv), CH2Cl2, rt, 21 - 23 h
O
O
2) H2O
O
O
(97 - 98 %)
Y
Y
Z
Z
OMe
OH
1f : X = Z = H; Y = Br
1g : X = Cl; Y = Z = H
1d : X = Z = H; Y = Br
1e : X = Cl; Y = Z = H
Scheme 4.
Table 1. Results of the cytotoxicity tests for compounds 1d–i in the NCI 3-cell line, one close primary anticancer assaya
Entry
Compound
Growth percentage
MCF-7 (breast)
Activity
NCI-H460 (lung)
SF-268 (CNS)
1
2
3
4
5
6
1d
1e
1f
1g
1h
1i
1
0
67
10
15
65
0
0
88
20
89
62
0
1
97
20
32
3
Active
Active
Inactive
Active
Active
Active
a In the protocol used, each line was inoculated and preincubated for 48 h on a microtiter plate. Test agents were then added at a 1.00×10−4
M
concentration in DMSO and the culture incubated for 48 h. End-point determinations were made with alamar blue.
As shown in Table 1, compounds 1d, 1e and 1g proved
to be significantly cytotoxic (entries 1, 2 and 4), even
though the cytotoxicity values for 1d and 1e were
higher than those for 1g. On the other hand, compound
1f was found to be inactive and compounds 1h and 1i
exhibited limited cytotoxicity. In fact, 1h reduced the
growth only of the SF-268 and the MCF-7 cell lines to
32% or less (entry 5) and compound 1i reduced signifi-
cantly (to 3%) the growth only of the SF-268 cell line
(entry 6). Thus, these data seem to suggest that, as
Acknowledgements
This work was supported by the Ministero del-
l’Istruzione, dell’Universita` e della Ricerca (MIUR)
and the University of Pisa.
References
1. Ortega, M. J.; Zub´ıa, E.; Ocan˜a, J. M.; Naranjo, S.;
Salva´, J. Tetrahedron 2000, 56, 3963–3967.
2. Lalonde, R. T.; Perakyla, H.; Cook, G. P.; Dence, C. W.
Environ. Toxicol. Chem. 1990, 9, 687–691.
3. Bellina, F.; Anselmi, C.; Viel, S.; Mannina, L.; Rossi, R.
Tetrahedron 2001, 57, 9997–10007.
4. We found that several NMR parameters of synthetic 4a
were significantly different from those reported in the
literature (Ref. 1) for rubrolide N.
5. For reviews on the synthesis of stereodefined 5-ylidene-
2(5H)-furanones, see: (a) Negishi, E.-I.; Kotora, M. Tet-
rahedron 1997, 53, 6707–6738; (b) Bru¨ckner, R. Curr.
Org. Chem. 2001, 5, 679–718. For the b-elimination route
to stereodefined 5-ylidene-2(5H)-furanones, see: Bru¨ck-
ner, R. Chem. Commun. 2001, 141–152.
previously
observed
for
(Z)-4-aryl-5-[1-(aryl)-
methylidene]-3-bromo-2(5H)-furanones,3 the presence
of phenol subunits is necessary in order for (Z)-4-aryl-
5-[1-(aryl)methylidene]-3-chloro-2(5H)-furanones
to
exhibit high cytotoxicities. These data also show that
(Z)-4-aryl-5-[1-(aryl)methylidene]-3-chloro-2(5H)-furan-
ones containing methoxyaryl subunits are less potent
than the corresponding substances characterized by
phenol subunits.
In conclusion, we have developed two protocols for the
Pd-catalyzed regioselective synthesis of 4-aryl-3-chloro-
2(5H)-furanones. We have also shown that these
monochloro derivatives represent useful precursors to
(Z)-4-aryl-5-[1-(aryl)methylidene]-3-chloro-2(5H)-furan-
ones including naturally-occurring rubrolide M. Inter-
estingly, some of these last compounds including
rubrolide M have been found to be active in the NCI
3-cell line, one dose primary anticancer assay.
6. Boukouvalas, J.; Lachance, N.; Ouellet, M.; Trudeau, M.
Tetrahedron Lett. 1998, 39, 7665–7668.
7. For the use of Pd-catalysts containing sterically hindered
alkylphosphines in cross-coupling reactions, see: (a) Dai,
C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719–2724;

F. Bellina et al. / Tetrahedron Letters 43 (2002) 2023–2027
2027
(b) Littke, A. F.; Dai, C.; Fu, G. C. J. Am. Chem. Soc.
2000, 122, 4020–4028; (c) Littke, A. F.; Fu, G. C. Angew.
Chem., Int. Ed. Engl. 1999, 38, 2411–2413; (d) Wolfe, J.
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1606, 1499, 1271, 818 cm−1. 1H NMR (200 MHz, CDCl3):
l 7.92 (1H, d, J=1.8 Hz), 7.77 (1H, dd, J=8.6 and 1.8
Hz), 7.56 (1H, d, J=8.8 Hz), 7.42 (1H, dd, J=8.7 and
2.2 Hz), 7.10 (1H, d, J=8.7 Hz), 6.91 (1H, d, J=8.8 Hz),
6.01 (1H, s), 4.00 (3H, s), 3.94 ppm (3H, s). 13C NMR
(50.3 MHz, CDCl3): l 164.09, 156.70, 156.61, 148.11,
145.23, 135.48, 131.36, 130.66, 128.85, 126.46, 123.31,
120.74, 118.06, 112.82, 112.13 (2C), 111.83, 56.33 ppm
(2C).
8. All new compounds were obtained in analytically pure
form. Selected physical and spectral properties of com-
pounds 1f–i are as follows. Compound 1f: mp 195–196°C.
1
IR (KBr): w 1769, 1607, 1498, 1267, 859 cm−1. H NMR
9. Staroske, T.; Hennig, L.; Welzel, P.; Hofmann, H.-J.;
Mu¨ller, D.; Ha¨usler, T.; Sheldrick, W. S.; Zillikens, S.;
Gretzer, B.; Pusch, H.; Glitsch, H. G. Tetrahedron 1996,
52, 12723–12744.
(200 MHz, CDCl3): l 7.91 (1H, d, J=2.2 Hz), 7.77 (1H,
dd, J=8.8 and 2.2 Hz), 7.49 (2H, m), 7.07 (2H, m), 6.90
(1H, d, J=8.8 Hz), 6.04 (1H, s), 3.94 (3H, s), 3.90 ppm
(3H, s). 13C NMR (50.3 MHz, CDCl3): l 164.46, 161.29,
156.54, 149.43, 145.57, 135.39, 131.26, 130.57 (2C),
126.65, 120.00, 117.24, 114.41 (2C), 112.75, 112.00,
111.79, 56.32, 55.43 ppm. Compound 1g: mp 148–150°C.
10. Selected physical and spectral properties of compounds
1d and 1e are as follows. Compound 1d: mp 227–230°C.
IR (KBr): w 3473, 1750, 1611, 1418, 1288, 1163, 837 cm−1
.
1H NMR (200 MHz, DMSO-d6): l 9.29 (2H, br s), 8.06
(1H, d, J=2.2 Hz), 7.74 (1H, dd, J=8.8 and 2.2 Hz),
7.53 (2H, m), 7.06 (3H, m), 6.29 ppm (1H, s). 13C NMR
(50.3 MHz, DMSO-d6): l 164.62, 160.32, 155.88, 150.84,
146.27, 136.21, 132.35, 131.83 (2C), 127.33, 119.77,
117.47, 116.88, 116.65 (2C), 113.42, 110.74 ppm. Com-
pound 1e: mp 266–270°C. IR (KBr): w 3418, 1728, 1603,
1444, 1282, 1163, 829 cm−1. 1H NMR (200 MHz, DMSO-
d6): l 10.98 (1H, br s), 10.15 (1H, br s), 7.68 (2H, m),
7.58 (1H, d, J=1.8 Hz), 7.39 (1H, dd, J=8.4 and 1.8
Hz), 7.18 (1H, d, J=8.4 Hz), 6.89 (2H, m), 6.24 ppm (1H
s). 13C NMR (50.3 MHz, DMSO-d6): l 163.77, 158.94,
154.93, 148.58, 143.35, 132.71 (2C), 130.42, 129.23,
123.54, 120.07, 119.01, 116.78, 115.82 (2C), 115.10,
114.72 ppm.
1
IR (KBr): w 1764, 1605, 1498, 1273, 824 cm−1. H NMR
(200 MHz, CDCl3): l 7.73 (2H, m), 7.56 (1H, d, J=2.2
Hz), 7.43 (1H, dd, J=8.4 and 2.2 Hz), 7.10 (1H, d,
J=8.4 Hz), 6.91 (2H, m), 6.08 (1H, s), 4.00 (3H, s), 3.85
ppm (3H, s). 13C NMR (50.3 MHz, CDCl3): l 164.44,
160.70, 156.50, 148.26, 144.46, 132.63 (2C), 130.69,
128.89, 125.23, 123.20, 121.01, 117.28, 114.59, 114.37
(2C), 112.11, 56.32, 55.36 ppm. Compound 1h: mp 169–
1
170°C. IR (KBr): w 1772, 1605, 1504, 1181, 826 cm−1. H
NMR (200 MHz, CDCl3): l 7.73 (2H, m), 7.49 (2H, m),
7.07 (2H, m), 6.90 (2H, m), 6.12 (1H, s), 3.90 (3H, s), 3.84
ppm (3H, s). 13C NMR (50.3 MHz, CDCl3): l 164.77,
161.11, 160.53, 149.57, 144.80, 132.53 (2C), 130.60 (2C),
125.40, 120.26, 116.53, 114.48, 114.33 (4C), 55.41, 55.32
ppm. Compound 1i: mp 215–217°C. IR (KBr): w 1784,