Total synthesis and structural revision of engelhardione

Article abstract of DOI:10.1016/j.tetlet.2011.06.112

The total synthesis of the macrocyclic natural product engelhardione is reported. This effort led to the structural revision of the published structure of engelhardione to that of pterocarine. The revision reflects the change of the substitution pattern of one phenyl ether ring from the meta to the para position. To confirm, pterocarine (2) and its close regioisomer 3 were subsequently synthesized for comparison. Moreover, to the best of our knowledge, our synthesis of 1 represents the first example of a 14-membered macrocyclic diarylheptanoid with a meta-meta substitution pattern at the diphenyl ether moiety.

Full text of DOI:10.1016/j.tetlet.2011.06.112

Tetrahedron Letters 52 (2011) 4570–4574
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total synthesis and structural revision of engelhardione
Li Shen a, Dianqing Sun a,b,
⇑
a Department of Pharmaceutical Sciences, College of Pharmacy, University of Hawai’i at Hilo, Hilo, HI 96720, USA
b Cancer Biology Program, University of Hawai’i Cancer Center, Honolulu, HI 96813, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The total synthesis of the macrocyclic natural product engelhardione is reported. This effort led to the
structural revision of the published structure of engelhardione to that of pterocarine. The revision reflects
the change of the substitution pattern of one phenyl ether ring from the meta to the para position. To con-
firm, pterocarine (2) and its close regioisomer 3 were subsequently synthesized for comparison. More-
over, to the best of our knowledge, our synthesis of 1 represents the first example of a 14-membered
macrocyclic diarylheptanoid with a meta–meta substitution pattern at the diphenyl ether moiety.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 29 May 2011
Revised 23 June 2011
Accepted 28 June 2011
Available online 2 July 2011
Keywords:
Engelhardione
Pterocarine
Ullmann coupling
Claisen–Schmidt condensation
Macrocyclization
Sealed tube
Engelhardione was originally isolated from the roots of Engel-
hardia roxburghiana (Juglandaceae).1 It belongs to a broad family
of natural plant metabolites called diarylheptanoids, which are
characterized by two phenolic aromatic rings linked by a linear se-
ven-carbon aliphatic ketone chain to form a macrocyclic architec-
ture.2 The macrocyclic diphenyl ether moiety is widely present in
many naturally occurring molecules, including vancomycin, K-13,
aceroside IV, piperazinomycin, and engelhardione (Fig. 1), which
are important medicinal molecules with antibacterial and antican-
cer properties.3–5 Several plant species containing diarylhepta-
noids are widely used in traditional medicine.2
achievable by adapting available synthetic schemes. To this end,
engelhardione represents a valuable natural product lead for fur-
ther investigation and evaluation.
Herein, we report the first total synthesis of engelhardione. Ulti-
mately, our synthesis led to the structural revision of the proposed
(published) structure 1 of engelhardione.
There are basically three different synthetic strategies to con-
struct a macrocyclic diarylheptanoid bearing a diphenyl ether skel-
eton: (i) the dianion chemistry of methyl acetoacetate and the SNAr
reaction for the diaryl ether formation reported by the Zhu
group6,7; (ii) the modified Ullmann reaction and the Wittig reac-
tion reported by the Nógrádi group13; and, (iii) the classical Ull-
mann coupling using a Cu(II) catalyst and aldol condensation
reported by the Natarajan8 and Jahng9,10 groups. We chose to adapt
the scheme of Jahng et al. for the synthesis of 1 because this clas-
sical macrocyclization procedure employing Cu(II) oxide and
potassium carbonate has also been successfully used to synthesize
the other members of naturally occurring macrocyclic ethers,14
and also, mild reaction conditions were used in the aldol and Cla-
isen–Schmidt condensation reactions. More importantly, from a
medicinal chemistry point of view, a range of diversified engelhar-
dione analogs, such as benzylated and methylated engelhardione
derivatives, can be sequentially generated. These analogs would
be invaluable compounds for subsequent structure–activity rela-
tionships (SAR) studies.
Our interest in the synthesis of engelhardione arose from the
following: engelhardione was reported to have potent in vitro
activity against the Mycobacterium tuberculosis strain H37Rv
(MIC = 0.2 l
g/mL)1; and, despite its potent antituberculosis activ-
ity, no synthetic efforts toward engelhardione have been reported
to date. Although several total syntheses of engelhardione-related
natural products, including acerogenins, galeon, and pterocarine
were reported,6–10 none of these analogs has been systematically
evaluated for antituberculosis activity. Furthermore, engelhardi-
one has desirable and druglike Lipinski’s rule of five properties,11
including a low molecular weight of 312.36, four hydrogen bond
acceptors, two hydrogen bond donors, and calculated log P
3.18.12 Compared to the other members of diaryl ether natural
products (Fig. 1), engelhardione possesses a relatively simple
structure, and we envisioned that the synthesis should be quite
The retrosynthesis of 1 is shown in Scheme 1. Initial disconnec-
tion of the diphenyl ether bond generated the linear diarylhepta-
noid intermediate, and this macrocyclization would be achieved
via intramolecular Ullmann coupling between the corresponding
⇑
Corresponding author. Tel.: +1 808 933 2960; fax: +1 808 933 2974.
E-mail address: dianqing@hawaii.edu (D. Sun).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.112

L. Shen, D. Sun / Tetrahedron Letters 52 (2011) 4570–4574
4571
OH
NH2
OH
OH
O
O
OH
OH
O
O
H
N
Cl
Cl
NH
O
O
O
CO2H
H
O
OH
HO
O
O
O
H
N
H
N
NH
O
HO
O
NHCH3
N
H
N
H
N
H
O
O
HN
O
H2N
K-13
HOOC
O
OH
OH
O
vancomycin
HO
H
N
O
N
HO
O
H
HO
HO
HO
O
1
HO
HO
O
O
O
HO
proposed (published) structure
of engelhardione
aceroside IV
piperazinomycin
Figure 1. Selected examples of macrocyclic natural products displaying a diaryl ether moiety.
available 3-hydroxy-4-methoxybenzaldehyde 4 reacted with ben-
zyl bromide in methanol to give the benzyl protected ether deriv-
ative 5 in quantitative yield. The successive aldol condensations
with acetaldehyde and then acetone yielded the substituted cinna-
maldehyde derivative 6 and 3,5-hexadien-2-one 7, respectively.
O
O
Ullmann
PG-O
O-PG
OH
Br
coupling
HO
O
Linear diarylheptanoid
(PG = protecting group)
Next, the a,b-conjugated olefin ketone 7 underwent selective pal-
Claisen-Schmidt
condensation
HO
1
ladium/carbon (Pd/C)-catalyzed hydrogenation in methanol to af-
ford the debenzylated and saturated 2-ketone intermediate 8 in
94% yield.15 Claisen–Schmidt condensation of 8 with 3-bromo-4-
methoxybenzaldehyde was performed to generate the conjugated
diarylhepten-3-one 9. Final chemoselective reduction of the olefin
9 was achieved using 10% Pd/C in the presence of diphenylsulfide16
to yield the desired 1,7-diphenylheptan-3-one derivative 10 in 90%
yield.17
Selective
reduction
O
H
O
Aldol
PG-O
PG-O
condensation
OH
6-Aryl-3,5-hexadien-2-one
OH
With the linear building block 10 in hand, Ullmann macrocycli-
zation was subsequently performed to generate the 14-membered
macrocyclic diphenyl ether product 11. To screen the experimental
conditions, conversion of 10 to 11 was examined using a variety of
Cu reagents, bases, and solvent systems and the results are shown
in Table 1. According to the procedure reported by Jahng and co-
workers,10 when CuO was used and the reaction was conducted
in pyridine using K2CO3 as base, the macrocyclic product 11 was
first provided in 25% yield after 20 h (Table 1, entry 1). It should
be noted, by comparing entries 1 and 2, that the same reaction
without N2 yielded a similar result. Further inspired by the work
of the Natarajan group,8 a sealed pressure tube was then employed
in an attempt to accelerate the reaction and to improve the yield.
Scheme 1. Retrosynthesis of 1.
suitably protected phenol derivative and the aryl bromide. The
overall synthetic strategy of the key linear 1,7-diaryl-3-ketone
can be implemented using a series of cross aldol condensation
reactions. The resulting diarylheptanoid would be further discon-
nected to the unsaturated
a,b-conjugated olefin ketone 6-aryl-
3,5-hexadien-2-one derivative following selective reduction and
Claisen–Schmidt condensation, which can be further derived from
the suitably substituted benzaldehyde.
Synthesis of 1 is illustrated in Scheme 2. First, in the presence of
potassium carbonate, the hydroxyl group of the commercially
O
O
CHO
CHO
b
c
e
d
MeO
MeO
a
MeO
MeO
OBn
6
OR
8
OBn
7
OH
7
4
R = H
O
5 R = Bn
O
3
O
g
1
f
R'O
O
MeO
OMe
MeO
OMe
R'O
10
OH
Br
OH
9
Br
11
1
R' = Me
R' = H
h
Scheme 2. Synthesis of 1. Reagents and conditions: (a) PhCH2Br, K2CO3, MeOH, reflux, 4 h, 100%; (b) CH3CHO, 10% NaOH, EtOH, rt, 3 h, 18%; (c) acetone, 10% NaOH, rt,
overnight, 95%; (d) 10% Pd/C, H2, MeOH, rt, 4 h, 94%; (e) 3-Br-4-MeOC6H3CHO, 10% NaOH, EtOH, rt, overnight, 64%; (f) 10% Pd/C, H2, Ph2S (0.05 equiv), CHCl3, 7 h, 90%; (g) CuO,
K2CO3, pyridine, 175 °C, 4.5 h, 52%; (h) AlCl3, CH2Cl2, reflux, 20 h, 31%.

4572
L. Shen, D. Sun / Tetrahedron Letters 52 (2011) 4570–4574
Table 1
Optimization of intramolecular macrocyclic Ullmann reaction of 10 to 11a
Entry
Cu reagent (equiv)
Base (equiv)
Solvent
Reaction conditionb
T (°C)
Concn (M)
Time (h)
Yieldc,d (%)
1
2
3
4
5
6
7
8
CuO (2.5)
CuO (2.5)
CuO (2.5)
CuO (2.5)
CuO (2.5)
CuO (2.5)
CuO (2.5)
CuO (2.5)
CuO (2.5)
CuBr-SMe2 (10)
CuI (0.2)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
K2CO3 (1.0)
NaH (2.0)
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
Pyridine
1,4-Dioxane
1,4-Dioxane
Reflux under N2
Reflux
Reflux
Sealed tube
Sealed tube
130
130
150
130
150
175
175
200
175
130
100
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.004
0.004
0.05
20
25
20
20
12
4.5
4.5
1
25
25
33
NR
71
52
51
44
59
NRe
NR
Sealed tube
Sealed tube under N2
Sealed tube under N2
Sealed tube
Reflux under N2
Reflux under N2
9
50
46
16.5
10
11f
CsCO3 (2.0)
a
b
c
d
e
f
Experiments were performed using 0.08 mmol of 10 except entry 9 (0.016 mmol of 10 was used) and monitored by HPLC of the reaction mixture.
Experiments were performed under N2 atmosphere except entries 2–6 and 9.
Isolated yield based on flash column chromatography.
NR indicates that no product 11 was detected by HPLC at the end of the experiment.
79% of 10 was recovered after column chromatography.
0.3 equiv of N,N-dimethyl glycine was used as additive, which has been reported to promote the intermolecular diphenyl ether reaction.19
In addition, a systematic optimization was performed to evaluate
the effect of temperature on the reaction yield. When the experi-
ment was conducted at 130 °C in a sealed tube, no reaction oc-
curred after 20 h based on HPLC analysis of the reaction mixture
(entry 4). A more encouraging result was obtained after the tem-
perature was increased to 150 °C, when the reaction was complete
in 12 h, affording the macrocyclic product 11 in 71% yield (entry 5).
As expected, when the temperature was raised to 175 °C, a signif-
icant decrease in reaction time was observed; after 4.5 h the reac-
tion went to completion to afford 11 in 52% yield. Under the same
reaction conditions with N2 atmosphere, a comparable yield of 51%
was obtained (entries 6 and 7). Increasing the reaction tempera-
ture further to 200 °C resulted in a much shorter reaction time
(1 h), nevertheless, a lower yield (44%) was also observed due to
the formation of polymeric by-products.
After completing the synthesis of 1, we were surprised to find
that both 1H and 13C NMR data of our synthetic engelhardione
were not consistent with those of the reported engelhardione,
although the 1H and 13C NMR spectra of both samples were re-
corded in CDCl3 at 400 and 100 MHz, respectively. In particular,
we did not observe the previously reported aromatic proton peak
at 5.57 ppm (1H, d, J = 2.0 Hz)1 in the 1H NMR spectrum of our syn-
thetic sample (Fig. 2a). After further reviewing the literature, it be-
came evident that this characteristic signal appeared to indicate
that the diphenyl ether moiety of the macrocyclic diarylheptanoid
was connected at meta and para positions, respectively. The mass
spectra of 1 and reported engelhardione showed the two com-
pounds had the same molecular weight. Therefore, instead of the
meta–meta linked diphenyl ether core structure as originally pro-
posed, we hypothesized that the published structure of engelhardi-
one should possess the meta and para-linked macrocyclic
architecture, which could be compound 2 or 3, as illustrated in
Scheme 3. Compound 2 was originally reported as pterocarine21
and its total synthesis has been published.9 After comparing the
published 1H and 13C NMR data of engelhardione and pterocarine,
we found that the reported spectroscopic data of engelhardione
were, indeed, consistent with those of pterocarine. To further con-
firm our hypothesis, pterocarine (2) and its close regioisomer 3
were next synthesized following the same synthetic strategy
(Scheme 3). Synthesis of 2 was achieved starting from 3-meth-
oxy-4-hydroxybenzaldehyde 12, followed by benzylation, aldol
The effect of concentration was then evaluated. A 59% yield was
obtained in a dilute solution in pyridine (0.004 M) in notably pro-
longed time (50 h) (entry 9). Finally, attempts to conduct the
experiment in milder conditions by employing alternative Cu re-
18
agents, including CuBr-SMe2 (entry 10) or CuI and N,N-dimethyl
glycine as an additive19 (entry 11) failed to afford the cyclic prod-
uct 11 in our experiments. Overall, considering the reaction time
and yield, we chose the conditions (CuO, K2CO3, 0.02 M, sealed
tube, 175 °C, 4.5 h; entry 6) as the optimum experimental condi-
tions for the macrocyclization step. Final O-demethylation of 11
by AlCl3 in CH2Cl2 at reflux afforded 1 in 31% yield.20
(a)
(d)
O
2''
2'
H2' H2''
HO
O
1
HO
O
(b)
(c)
(e)
(f)
2'
H2'
HO
O
2
HO
pterocarine
(identical with engelhardione)
O
H2''
2''
HO
O
3
HO
Figure 2. 1H and 13C NMR spectra (recorded at 400 and 100 MHz in CDCl3, respectively) of 1, pterocarine (2), and 3.

L. Shen, D. Sun / Tetrahedron Letters 52 (2011) 4570–4574
4573
O
O
CHO
CHO
b
c
e
d
BnO
RO
a
BnO
HO
OMe
14
OMe
12
16
OMe
OMe
15
R = H
O
13
R = Bn
O
O
g
f
R'O
O
HO
OMe
HO
OMe
18
R'O
19
pterocarine (2) R' = H
OMe
Br
17
OMe
Br
R' = Me
h
O
O
O
OMe
OMe
Br
i
k
j
8
R'O
l
O
MeO
Br
MeO
R'O
21
OH
OH
20
22
R' = Me
3 R' = H
Scheme 3. Synthesis of pterocarine (2) and regioisomer 3. Reagents and conditions: (a) PhCH2Br, K2CO3, MeOH, reflux, 4 h, 100%; (b) CH3CHO, 10% NaOH, EtOH, rt, 3 h, 20%;
(c) acetone, 10% NaOH, rt, overnight, 81%; (d) 10% Pd/C, H2, MeOH, rt, 2 h, 92%; (e) 3-Br-4-MeOC6H3CHO, 10% NaOH, EtOH, rt, 9.5 h, 52%; (f) 5% Pd/C, H2, Ph2S (0.05 equiv),
CHCl3, 18 h, 78%; (g) CuO, K2CO3, pyridine, 175 °C, 5 h, 54%; (h) AlCl3, CH2Cl2, reflux, 25 h, 62%; (i) 4-Br-3-MeOC6H3CHO, 10% NaOH, EtOH, rt, 3.5 h, 41%; (j) 5% Pd/C, H2, Ph2S
(0.05 equiv), 18 h, 80%; (k) CuO, K2CO3, pyridine, 175 °C, 5 h, 58%; (l) AlCl3, CH2Cl2, reflux, 11 h, 42%.
condensation reactions, and selective hydrogenations to generate
1,7-diphenylheptan-3-one variant 18, followed by the macrocyclic
Ullmann condensation and O-demethylation to afford pterocarine
(2). Accordingly, Claisen–Schmidt condensation of 8 with 4-bro-
mo-3-methoxybenzaldehyde followed by chemoselective reduc-
tion, macrocyclization, and demethylation gave the regioisomer 3.
Macrocyclic compounds 1–3 were fully characterized by mass
spectrometry, 1H and 13C NMR, and 2D HSQC, HMBC, and COSY
spectroscopy (See Supplementary data). Their complete 1H and
13C NMR spectra are shown in Figure 2a–f. The spectroscopic data
of our synthetic pterocarine (2) were identical with those of the
originally reported engelhardione1 as well as natural and synthetic
pterocarine.9,21 Moreover, on the basis of these spectra, interest-
ingly, we noted that minor structural changes in the substitution
patterns at the two aromatic rings result in dramatic differences
in their respective NMR spectra. Most notably, for meta and para
connected pterocarine (2) and regioisomer 3, high-field shifts of
and syntheses of their structural analogs are currently ongoing
and will be reported in due course.
Acknowledgments
We thank UH Hilo College of Pharmacy for the start-up funding,
and this work is also in part supported by INBRE program P20
RR016467 awarded by NCRR. We thank Dr. Philip Williams at Uni-
versity of Hawai0i at Manoa for performing high-resolution mass
spectrometric analysis of compound 11. We also thank Professor
Ih-Sheng Chen for providing NMR spectra of engelhardione.
Supplementary data
Supplementary data (experimental procedures for the synthesis
of compounds 1–22, copies of 1H and 13C NMR spectra, HPLC chro-
matographs for compounds 1–22, and HSQC, HMBC, and COSY
spectra of macrocyclic compounds 1–3, 11, 19, and 22) associated
with this article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.06.112.
H-20 (d = 5.59 ppm, d, J = 1.7 Hz) of
2
(Fig. 2b) and H-200
(d = 5.82 ppm, d, J = 2.0 Hz) of 3 (Fig. 2c) were observed due to
the anisotropic effect of the adjacent aromatic rings, as previously
reported.6,7,10 In contrast, the resonances of these aromatic protons
H-20 and H-200 from the meta–meta diaryl ether-linked 1 (Fig. 2a)
are at 6.51 and 6.45 ppm with a coupling constant value of
1.9 Hz, respectively. Differences of the chemical shifts of the ali-
phatic protons of the heptan-3-one chain among 1–3 were also ob-
served (Fig. 2a–c). Our data suggest that these macrocyclic
molecules display a high degree of conformational flexibility in
solution. Further evidence was demonstrated from the 1H NMR
data of 19, which was recorded in both non-polar CDCl3 and polar
DMSO-d6 for comparison. We noted that the signals of the geminal
protons in the heptyl chain of 19 merged together in DMSO-d6
compared to those signals recorded in CDCl3, additionally, changes
of chemical shifts from the aromatic protons were also observed
(See Supplementary data).
In conclusion, we report the first synthesis of engelhardione and
this effort led to the structural revision of this macrocyclic natural
product.22 The correct structure of the previously reported engel-
hardione should be that of pterocarine (2). To confirm, 2 and its
close regioisomer 3 were also synthesized. The published spectro-
scopic data of engelhardione were in full agreement with those of
pterocarine. Biological studies of these macrocyclic compounds
References and notes
1. Lin, W. Y.; Peng, C. F.; Tsai, I. L.; Chen, J. J.; Cheng, M. J.; Chen, I. S. Planta Med.
2005, 71, 171.
2. Keserü, G. M.; Nógrádi, M. In Studies in Natural Products Chemistry; Atta-Ur-
Rahman, Ed.; Elsevier Science, 1995; Vol. 17, p 357.
3. Ishida, J.; Kozuka, M.; Wang, H.-K.; Konoshima, T.; Tokuda, H.; Okuda, M.; Yang
Mou, X.; Nishino, H.; Sakurai, N.; Lee, K.-H.; Nagai, M. Cancer Lett. 2000, 159,
135.
4. Lee, K.-S.; Li, G.; Kim, S. H.; Lee, C.-S.; Woo, M.-H.; Lee, S.-H.; Jhang, Y.-D.; Son,
J.-K. J. Nat. Prod. 2002, 65, 1707.
5. Nomura, M.; Tokoroyama, T.; Kubota, T. Phytochemistry 1981, 20, 1097.
6. Gonzalez, G. I.; Zhu, J. J. Org. Chem. 1997, 62, 7544.
7. Gonzalez, G. I.; Zhu, J. J. Org. Chem. 1999, 64, 914.
8. Kishore Kumar, G. D.; Natarajan, A. Tetrahedron Lett. 2008, 49, 2103.
9. Wang, Q.; Son, J.-K.; Jahng, Y. Synth. Commun. 2007, 37, 675.
10. Jeong, B.-S.; Wang, Q.; Son, J.-K.; Jahng, Y. Eur. J. Org. Chem. 2007, 2007, 1338.
11. Lipinski, C. A.; Lombardo, F.; Dominy, B. W.; Feeney, P. J. Adv. Drug Deliver. Rev.
2001, 46, 3.
12. Calculated using Advanced Chemistry Development (ACD/Labs) Software
V8.19 for Solaris (Ó 1994–2011 ACD/Labs).
13. Keserü, G. M.; Nógrádi, M.; Szöllösy, Á. Eur. J. Org. Chem. 1998, 1998, 521.
14. Evano, G.; Blanchard, N.; Toumi, M. Chem. Rev. 2008, 108, 3054.
15. When chloroform was initially used as solvent, the reaction was complete after
2.5 h. However, 19% of over-reduced secondary alcohol by-product was
produced based upon HPLC and 1H NMR monitoring. We found, when

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L. Shen, D. Sun / Tetrahedron Letters 52 (2011) 4570–4574
methanol was used as solvent, the over-reduction of the carbonyl group can be
sufficiently suppressed for this reaction.
20. The reaction proceeded smoothly during the first 20 h to give a mixture of
desired product 1 (54%), partially demethylated intermediates (32%) with one
methoxy group intact, and unreacted starting material (14%) (characterized by
MS, 1H NMR, and HPLC). However, no further reaction was detected after 20 h.
21. Liu, H.; Cui, C.; Cai, B.; Gu, Q.; Zhang, D.; Zhao, Q.; Guan, H. Chinese Chem. Lett.
2005, 16, 215.
22. When we contacted Professor Ih-Sheng Chen for advice regarding the structure
revision of engelhardione to pterocarine, we were informed that the incorrect
structural elucidation has also been found very recently in his laboratory.
16. Mori, A.; Mizusaki, T.; Miyakawa, Y.; Ohashi, E.; Haga, T.; Maegawa, T.;
Monguchi, Y.; Sajiki, H. Tetrahedron 2006, 62, 11925.
17. Using the procedure reported by Sajiki and co-workers,16 the addition of
0.05 equiv of diphenylsulfide as catalyst poison proved to be effective to
achieve chemoselective hydrogenolysis between olefin and ketone.
18. Boger, D. L.; Yohannes, D.; Zhou, J.; Patane, M. A. J. Am. Chem. Soc. 1993, 115,
3420.
19. Ma, D.; Cai, Q. Org. Lett. 2003, 5, 3799.