Total Synthesis and Conformational Analysis of the Antifungal Agent (-)-PF1163B

Article abstract of DOI:10.1021/ol035309c

(Equation presented) (-)-PF1163B, a new macrocyclic antifungal antibiotic isolated from Streptomyces sp., has been prepared in eight steps from (S)-citronellene. The key step is a ring-closing metathesis reaction of an ester and amide derivative obtained from a substituted N-methyl-L-tyrosine.

Full text of DOI:10.1021/ol035309c

ORGANIC
LETTERS
2003
Vol. 5, No. 22
4049-4052
Total Synthesis and Conformational
Analysis of the Antifungal Agent
(−)-PF1163B
Fodil Bouazza,† Brigitte Renoux,† Christian Bachmann,‡ and
,†
Jean-Pierre Gesson*
UMR 6514 and UMR 6503, UniVersity of Poitiers and CNRS,
40 aVenue du recteur Pineau, 86022 Poitiers, France
jean-pierre.gesson@uniV-poitiers.fr
Received July 16, 2003
ABSTRACT
(−)-PF1163B, a new macrocyclic antifungal antibiotic isolated from Streptomyces sp., has been prepared in eight steps from (S)-citronellene.
The key step is a ring-closing metathesis reaction of an ester and amide derivative obtained from a substituted N-methyl-L-tyrosine.
The ergosterol biosynthesis cascade is a known target of
antifungal agents such as allylamines, azoles, and morpho-
broth of Penicillium sp.1,2 and subsequently shown to be the
first known inhibitors of ERG25p, a C-4 methyl oxidase.3
ERG25p is the enzyme that converts 4,4-dimethylzymosterol
to zymosterol by oxidation of the 4R-methyl group to a
carboxylic acid followed by decarboxylation. The antifungal
activity of 1 is 4 times higher than that of 2, the former being
a more potent inhibitor than fluconazole for ergosterol
synthesis in Candida albicans. Thus, these compounds are
interesting leads for the development of new antifungal
agents.
They are characterized by the presence of a 13-membered
macrocycle (incorporating both lactone and lactam moieties)
derived from O-2-hydroxyethyl-N-methyl-L-tyrosine. Al-
though several bioactive cyclic depsipeptides incorporating
N-methyl tyrosine such as geodiamolides have been re-
ported,4 the recently isolated cytotoxic spongidepsin is the
Figure 1.
(1) Nose, H.; Seki, A.; Yaguchi, T.; Hosoya, A.; Sasaki, T.; Hoshoko,
S.; Shomura, T. J. Antibiot. 2000, 53, 33.
(2) Sasaki, T.; Nose, H.; Hosoya, A.; Yoshida, S.; Kawaguchi, M.;
Watanabe, T.; Usui, T.; Ohtsuka, Y. Shomura, T.; Takano, S.; Tatsuta, K.
J. Antibiot. 2000, 53, 38.
(3) Nose, H.; Fushumi, H.; Seki, A.; Watabe, H.; Hoshiko, S. J. Antibiot.
2002, 55, 969.
lines. Recently, two new compounds, PF1163A 1 and
PF1163B 2 (Figure 1), were isolated from a fermentation
† UMR 6514.
‡ UMR 6503.
10.1021/ol035309c CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/27/2003

only known 13-membered macrocycle bearing similar lactone
and lactam functional groups.5 PF1163A differs from PF1163B
by the presence of an extra hydroxyl group in the side chain.
Their structures have been determined by NMR, degradation
studies,2 and total synthesis.6 The latter has been carried out
in 13 steps from (R)-citronellol and L-tyrosine. We now
report a shorter and more flexible synthesis of (-)-PF1163B
from (S)-citronellene using ring-closing metathesis (RCM)
as a key step (Scheme 1).
benzyl protecting group was used. Treatment of N-Boc-L-
tyrosine methyl ester 6 with 2-bromo-O-benzyl-ethanol and
Cs2CO3 in DMF at 70 °C6 thereafter afforded 7 (85%), but
chiral-phase HPLC analysis7 indicated that racemization (60:
40 ratio) was significant under these conditions. Indeed, when
the same reaction was done at room temperature, 7 was
isolated in 70% yield with minimal racemization (97:3 ratio).
N-Methylation of 7 cleanly afforded 8 (95%). Hydrolysis
of the methyl ester to give 9 was then carried out with LiOH
according to the procedure used by Boger.8
Alcohol 12 was prepared in three steps from (S)-citronel-
lene (Scheme 3).
Scheme 1. Retrosynthetic Analysis
Scheme 3. Preparation of 12a
a Alcohol 12 was prepared in three steps from (S)-citronellene
(Scheme 3).
The known aldehyde 11 was obtained by cleavage of the
trisubstituted epoxide of citronellene with periodic acid in
diethyl ether.9 The dried ethereal solution of 11 was directly
treated with dipentylzinc under the conditions of Kobayashi
and Knochel10 to give alcohol 12 (Scheme 4) in 50% yield
(ee 98%).11 Esterification of 9 with alcohol 12 was carried
out with DCC in DMF in the presence of DMAP to afford
13 (70%). Removal of the tert-butyloxycarbonyl protecting
group (TFA, CH2Cl2) provided amine 14, which was then
reacted with 4-pentenoyl chloride (THF, Et3N) to afford
amide 15 (94% over two steps). At this stage, chiral-phase
HPLC showed a 92:8 ratio of enantiomers, indicating that
some epimerization occurred during the deprotection, es-
terification, and/or amide formation steps.
We first attempted to prepare the known synthon 4
according to the procedure of Tatsuta6 from N-Boc-L-tyrosine
3. However, N-methylation of 4 to 5 using NaH and MeI
Scheme 2. Preparation of Synthon 9
Then, RCM was attempted using variable amounts of
(PCy3)2Cl2RudCHPh, but only limited cyclization was
(4) Chan, W. R., Tinto, W. F.; Manchand, P. S.; Todaro, L. J. J. Org.
Chem. 1987, 52, 3091. Coleman, J. E.; Van Soest, R.; Andersen, R. J. J.
Nat. Prod. 1999, 62, 1137.
(5) Grassia, A.; Bruno, I.; Debitus, C.; Marzocco, S.; Pinto, A.; Gomez-
Paloma, L.; Riccio, R. Tetrahedron 2001, 57, 6257.
(6) Tatsuta, K.; Takano, S.; Ikeda, Y.; Nakano, S.; Miyazaki, S. J.
Antibiot. 1999, 52, 1146.
(7) HPLC conditions: Chiralcel OD-H column (100 × 4.6 mm), eluent
2-propanol/hexane (10:90), 0.4 mL/min, UV detection at 276 nm, retention
times 7.2 min (D-7) and 7.9 min (L-7).
(8) Boger, D. L.; Yohannes, D. J. Org. Chem. 1988, 53, 487.
(9) Ireland, R. E.; Thaisrivongs, S.; Dussault, P. H. J. Am. Chem. Soc.
1988, 110, 5768.
(10) (a) Takahashi, H.; Kawakita, T.; Ohno, M.; Yoshioka, M.; Koba-
yashi, S. Tetrahedron, 1992, 48, 5691. (b) Knochel, P. Synlett 1995, 393.
(11) For a related example, see: Fu¨rstner, A.; Mu¨ller, T. J. Am. Chem.
Soc. 1999, 121, 7814.
was difficult to drive to completion and led to partial cleavage
of the silyl protecting group. To avoid the latter difficulty, a
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Org. Lett., Vol. 5, No. 22, 2003

Scheme 4. Synthesis of PF1163B
observed.12 As expected,13 the use of Grubbs II catalyst 16
Assuming that conformers A and B may be sufficiently
stable to be observed on the NMR time scale (Scheme 5),
led to better results. RCM of 15, in refluxing C2H4Cl2, gave
17 as a mixture of (E)- and (Z)- diastereoisomers (60%). A
minor isomer, which was assumed to result from the
aforementioned epimerization of the amino acid moiety, was
also isolated (10.5%). Reduction of the double bond and
hydrogenolysis of the benzyl group gave PF1163B 2 as a
colorless oil in 52% isolated yield ([R]D -105 (c 1.2, MeOH)
(lit.2 [R]D -111)).
Scheme 5
1
The H NMR spectrum of synthetic 2 in DMSO-d6 is
superimposable to the reported one. However, the H-3 signal
at 5.62 ppm accounts for only 0.5 H.14 Furthermore, as
reported,2 broad signals were observed, and this was assumed
by the authors to result from s-cis and s-trans configurations
of the amide bond. Indeed, careful examination of 1H NMR
spectra in CDCl3 revealed that three isomers are present in
solution: H-3 appeared as broad singlets at δ 5.74 (0.5 H)
and 3.38 ppm (0.4 H) and as a triplet (J ) 7 Hz) at δ 4.51
ppm (0.1 H) (Table 1). In DMSO-d6, only the lower-field
signal (δ 5.62 ppm) could be identified. It was thus obvious
that s-cis/s-trans isomerization of the amide bond was not
sufficient to explain such data.
energy minima were searched for the four possible structures
of 18 (bearing two methyl groups instead of the pentyl and
benzyl side chains), at the density functional B3-LYP/6-31G/
1
Table 1. Energy and H NMR Calculations
conformer Erel kcal/mol δcalcd ppm δobs ppm percentage
(12) For related examples of RCM of peptide derivatives, see: Boruah,
A.; Rao, I. N.; Nandy, J. P.; Kumar, S. K.; Kunwar, A. C.; Iqbal, J. J. Org.
Chem. 2003, 68, 5006 and references therein.
(13) (a) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413. (b) Trnka,
T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34.
Atrans
Btrans
Acis
0
5.47
2.72
4.14
3.17
5.74
3.38
4.51
50%
40%
10%
0%
3.6
4.2
6.2
Bcis
(14) This was not pointed out (see refs 2 and 6).
Org. Lett., Vol. 5, No. 22, 2003
4051

d3 level using the Gaussian 98 package.15 It was found that
the more stable conformer (Atrans) is similar to the reported
X-ray structure for 17, a degradation product of PF1163B.2
Similar values were obtained for the Btrans and Acis conform-
ers, the Bcis isomer being slightly less stable.
1H NMR chemical shift calculations16 revealed significant
variations for H-3 (Table 1). On the basis of these figures,
it is now proposed that the conformational population of
PF1163B in CDCl3 is 50% Atrans, 40% Btrans, and 10% Acis.
To estimate the possibilities of interconversion between these
conformers, transition structures were searched, and two of
interconversions are likely to be slow processes at room
temperature. Finally, variable-temperature experiments were
carried out. The NMR spectrum remained unchanged in
CDCl3 up to 50 °C, while the 5.62 ppm signal disappeared
at 70-80 °C in DMSO-d6 in favor of a new signal at ca. 4.6
ppm (in agreement with an averaged signal for the three
conformers). This result is in agreement with signal coales-
cence reported for lauryllactam at 60 °C.17
In conclusion, a short synthesis of PF1163B has been
completed using RCM as the key step. Such a strategy should
be extendable to the preparation of a large set of analogues,
including PF1163A, bearing different side chains and ring
size. Furthermore, the interesting conformational bias pointed
out for such macrocycles should be further studied to pinpoint
the geometrical requirements for the biological activity of
such compounds.
them were found. The first one corresponds to an Atrans
-
Acis isomerization and the second to a Btrans-Acis isomer-
ization. These two structures are, respectively, 20.7 and 23.3
kcal/mol above the lowest Atrans conformer. Thus, these
(15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(16) (a) To obtain reliable proton chemical shifts, we have used the GIAO
method available in Gaussian 98. These chemical shifts were calculated at
the Hartree-Fock level using the 6-311+G(2d,p) extended basis set on
the B3LYP/6-31G(d) optimized structures, as recommended by Cheeseman
et al.16b (b) Cheeseman, J. R.; Trucks, G. W.; Keith, T. A.; Frisch, M. J.,
J. Chem. Phys. 1996, 104, 5497.
Acknowledgment. We thank MRT, CNRS, and Re´gion
Poitou-Charentes for financial support and F. Seguin (ERM
324) for 500 MHz spectra.
Supporting Information Available: Copies of proton
spectra of compounds 7-9, 11-15, and synthetic PF1163B
and HPLC of compounds 6, 12, 15, and PF1163B. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL035309C
(17) Moriarty, R. M. J. Org. Chem. 1964, 29, 2748.
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