Protecting group directed ring-closing metathesis (RCM): the first total synthesis of an anti-malarial nonenolide

Article abstract of DOI:10.1016/j.tetlet.2007.02.040

The first synthesis of a newly found naturally occurring anti-malarial nonenolide is described. A pivotal step in the synthesis is the ring-closing metathesis of a dienoic ester prepared by coupling an acid and alcohol that were stereoselectively synthesi

Full text of DOI:10.1016/j.tetlet.2007.02.040

Tetrahedron Letters 48 (2007) 2621–2625
Protecting group directed ring-closing metathesis (RCM):
the first total synthesis of an anti-malarial nonenolide
Debendra K. Mohapatra,a,* Dhondi K. Ramesh,a Michael A. Giardello,b
Mukund S. Chorghade,a Mukund K. Gurjara and Robert H. Grubbsc,*
aDivision of Organic Chemistry: Technology, National Chemical Laboratory, Pune 411 008, India
bMateria Inc., 60, N. Gabriel Boulevard, Pasadena, 91107, USA
cArnold and Mable Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering,
California Institute of Technology, Pasadena, 91125, USA
Received 21 December 2006; revised 23 January 2007; accepted 1 February 2007
Available online 15 February 2007
Abstract—The first synthesis of a newly found naturally occurring anti-malarial nonenolide is described. A pivotal step in the syn-
thesis is the ring-closing metathesis of a dienoic ester prepared by coupling an acid and alcohol that were stereoselectively synthe-
sized from (S)-a-hydroxy-c-butyrolactone and 1,2-O-isopropylidene D-glyceraldehyde, respectively.
Ó 2007 Elsevier Ltd. All rights reserved.
Natural resources such as plants, animals, and micro-
organisms constitute a rich and versatile source of
bioactive chemicals; many of them and their synthetic
derivatives have been proven to be beneficial to human
health. Much current interest has been focused on the
utility of naturally occurring bifidus factors and growth
inhibitors against harmful bacteria such as Clostridium
and E. coli. The ascomycetous genus Cordyceps is an
entomopathogenic fungus that has found extensive use
in food and herbal medicines in Asia.1 The approxi-
mately 400 species of Cordyceps known so far are distin-
guished from each other and classified according to the
color and shape of their fruiting bodies, possession of
spores, ascus shape, host insect species, and by other
morphological characteristics.2
attention due to their unique structures and specific bio-
logical activities. Cordycepins (30-deoxyadenosine), pos-
sessing antifungal, antivirus, and antitumor activities, is
one of a number of selected secondary metabolites that
have been previously isolated from C. militaris.4a,5 Com-
pound 1 was recently isolated as a white solid from
C. militaris BCC 2816; the structure was elucidated
and the stereochemistry confirmed by spectral data
and X-ray crystallographic analysis.6
As part of our ongoing program on the synthesis of nat-
ural lactones7 with ring-closing metathesis (RCM)8,9 as
a key step, we have devised a stereoselective synthesis
of nonenolide 1. Despite its effectiveness in the synthesis
of rings of all sizes, two factors still limit the scope of the
RCM reaction: (a) in ring sizes P8, control over E/Z
stereochemistry of the double bond generated is difficult
and not demonstrated. Stereochemical control is proba-
bly of thermodynamic origin;10 (b) reports that describe
the application of RCM to medium sized, particularly
10-membered rings, are still rare, especially when dense
functionality close to the reaction center is involved.11 A
dearth of reports on RCM reactions on substrates
wherein chiral centers with protecting groups are present
adjacent to both the reacting sites (Fig. 2), prompted us
to investigate the outcome of such RCM reactions with
promise in the synthesis of nonenolides with chiral cen-
ters on both sides of the double bond (Fig. 1).
Cordyceps militaris, an enthomopathogenic fungus
belonging to the class Ascomycetes, adheres to the sur-
face of insects during the winter, followed by penetra-
tion of its body by a fruiting body and sporangium.3
Reports on the isolation of biologically active secondary
metabolites from C. militaris have been sparse.4 In
recent years, these secondary metabolites have received
Keywords: Nonenolide; Anti-malarial agent; 1,2-O-Isopropylidene
D-glyceraldehyde; Yamaguchi esterification; Ring-closing metathesis.
*
Corresponding authors. Tel.: +91 20 25902627; fax: +91 20 25902629
(D.K.M.); e-mail: dk.mohapatra@ncl.res.in
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.02.040

2622
D. K. Mohapatra et al. / Tetrahedron Letters 48 (2007) 2621–2625
Our retrosynthetic analysis is depicted in Scheme 1. The
O
O
O
macrolactonization step relies on the RCM reaction on
a diolefinic ester. Strategic bond disconnection in ester 8
leads to chiral, nonracemic fragments 9 and 10 that
could be derived from (S)-a-hydroxy-c-butyrolactone
(11) and 1,2-O-isopropylidene D-glyceraldehyde (12),
respectively.
O
O
O
OH
OH
OH
OH
OH
OH
1 New 10-membered
macrolide
2 Z-isomer of 1
3 Decarestrictine C1
Synthesis of acid component 10 began with 13 prepared
O
O
O
by a literature procedure.12 The spectroscopic and ana-
25
25
lytical data {½aꢀD ꢁ54.8 (c 1.0, CHCl3); lit.12 ½aꢀD ꢁ56.0
(c 1.42, CHCl3)} of 13 were in excellent agreement with
that reported. The primary hydroxyl group was then
oxidized with Dess–Martin periodinane (DMP)13 to
afford the corresponding aldehyde; further treatment
O
OH
HO
OH
O
OH
4 Decarestrictine D
5 Decarestrictine A
14
with NaClO2 in the presence of NaH2PO4 and 2-
O
O
methyl-2-butene as a scavenger gave the required acid
O
O
10 in 82% overall yield15 (Scheme 2).
Compound 916 was obtained in twelve steps and 26%
overall yield, from 1,2-O-isopropylidene D-glyceralde-
hyde (12), following a standard protocol17 (Scheme 3).
O
O
OH
HO
OH
OH
OH
6 Aspinolide B
7 Achaetolide
Our next task was to couple the two fragments and con-
duct the critical RCM reaction. Carboxylic acid 10 was
coupled with alcohol 9 under Yamaguchi’s protocol18
(2,4,6-trichlorobenzoyl chloride, Et3N, DMAP) to
afford diene ester 819 in 89% yield (Scheme 4). This set
the stage for the crucial ring-closing metathesis, which
was successfully achieved with Grubbs’ second-genera-
tion catalyst 17. The extent of bias, if any, conferred
by the protecting groups on the stereochemistry of the
newly formed double bond is not readily obvious and
cannot be predicted with certainty. We envisaged that
PMB-protecting groups around the reacting centers
might act as temporary constraints to adequately shape
this particular diene and simultaneously confer selectiv-
ity upon the stereochemistry of the newly formed double
bond: we were pleased to observe this. A 0.001 M solu-
tion of 8 and 10 mol % of Grubbs’ second-generation
catalyst 17 was heated at reflux for 8 h in dry, degassed
CH2Cl2. This provided the desired 10-membered macro-
lactone (E)-15 as the major product in 78% yield. We
were unable to ascertain the precise E/Z ratio. Depro-
tection of the PMB groups yielded natural product 1
in 92% yield together with a small amount of (Z)-isomer
(2) (E/Z = 90:10). The geometry of the newly formed
double bond in the major product was unequivocally
assigned by detection of the olefinic Jtrans coupling
constant (15.9 Hz between the protons at d 5.61 and
5.73 ppm, respectively). The specific rotation does,
Figure 1. Some natural nonenolides with chiral centers on both sides
of the double bond.
OPG
OPG
OPG
RCM
or
OPG
OPG
-isomer
OPG
Z
-isomer
E
Figure 2. Outcome of the RCM reaction.
O
O
O
O
OPMB
OPMB
OH
OH
1
8
O
OH
HO
+
indeed, deviate a little, but, more importantly, it is close
OPMB
25
to the reported value and of the same sign {½aꢀD ꢁ49.8
25
OPMB
(c 0.30, MeOH); lit.6 ½aꢀD ꢁ55.0 (c 0.036, MeOH)}.
10
9
The constitution and configuration of the assigned com-
pounds are unambiguous as the NMR and elemental
analysis were in excellent accord with the proposed
structure and perfectly matched those reported in the
literature.6,20
OH
O
O
O
CHO
O
To verify the effect of the PMB group upon the stereo-
chemistry of the newly formed double bond, we carried
out the RCM reaction with diol 16 after deprotection of
12
11
Scheme 1. Retrosynthetic analysis.

D. K. Mohapatra et al. / Tetrahedron Letters 48 (2007) 2621–2625
2623
O
1. DMP, CH2Cl2,
OH
HO
HO
Ref. 12
0 oC-rt, 84%
2. NaClO2, NaH2PO4
OPMB
OPMB
O
O
2-methyl-2-butene
0 oC-rt, 82%
11
10
13
Scheme 2. Synthesis of acid 10.
OTs
to find that the newly formed double bond had the Z
conformation as evidenced by the coupling constant of
11.2 Hz. No chromatographic or spectroscopic evidence
for the formation of the E isomer was discernible.
Ref. 17
O
LAH, THF
OH
OH
O
OPMB
0 oC-rt, 88%
OPMB
CHO
Next, we wanted to verify the consistency of the results
obtained from the RCM reactions depicted above. Thus,
acid 10 was coupled with (R)-4-p-methoxybenzylhex-5-
en-1-ol (18) to afford ester 19 under Yamaguchi condi-
tions in 92% yield. The RCM reaction, as before, and
deprotection of the PMB groups furnished the 10-mem-
bered lactone 21 with the E isomer as the only product
(Scheme 5). Similarly, RCM reaction on diol 22 afforded
exclusively Z isomer 23 in 77% yield, the structure and
12
14
9
Scheme 3. Synthesis of alcohol 9.
the PMB groups of 8. The RCM reaction on 16
(0.001 M in CH2Cl2) with Grubbs’ second-generation
catalyst (10 mol %) afforded the 10-membered lactone
221 as the sole product in 76% yield. We were surprised
O
O
O
O
2,4,6-trichloro
benzoyl chloride
17, CH2Cl2
DDQ, CH2Cl2
H2O, rt, 92%
O
O
O
OH
HO
+
OPMB
reflux, 78%
OPMB
CH2Cl2, Et3N,
DMAP, rt, 89%
OPMB
OH
OPMB
OPMB
OPMB
15
OH
DDQ, CH2Cl2
H2O, rt, 92%
O
1
9
8
10
E:Z = 90:10
O
N
N
Mes
Ph
17, CH2Cl2
Mes
O
OH
O
Cl
Ru
reflux, 76%
OH
Cl
PCy3
17
OH
OH
2
16
Z-isomer exclusively
Scheme 4. Synthesis of an anti-malarial nonenolide 1 and its Z-isomer 2.
O
O
O
O
O
2,4,6-trichloro
benzoyl chloride
DDQ, CH2Cl2
H2O, rt, 90%
17, CH2Cl2
OH
O
O
HO
OPMB
OPMB
+
CH2Cl2, Et3N,
DMAP, rt, 92%
reflux, 74%
OPMB
OH
OPMB
OPMB
OPMB
OH
10
18
19
21
20
DDQ, CH2Cl2
H2O, rt, 91%
E-isomer exclusively
O
O
17, CH2Cl2
O
O
OH
reflux, 77%
OH
OH
OH
23
Z-isomer exclusively
22
Scheme 5. Synthesis of desmethyl nonenolide 21 and its Z-isomer 23.

2624
D. K. Mohapatra et al. / Tetrahedron Letters 48 (2007) 2621–2625
1
10. (a) Bourgeios, D.; Mahuteau, J.; Pancrazi, A.; Nolan, S.
stereochemistry, were unambiguously established by H
and 13C NMR analysis.
P.; Prunet, J. Synthesis 2000, 869–882; (b) Furstner, A.;
Langemann, K. Synthesis 1997, 792–803.
¨
11. (a) Sharma, G. V. M.; Cherukupallu, G. R. Tetrahedron:
Asymmetry 2006, 17, 1081–1088, and references cited
In summary, a concise first total synthesis of the E and
Z isomers of the potent anti-malarial 1 and related
congeners is presented. Our success was based on
synthesizing two coupling partners from inexpensive,
commercially available starting materials and exploiting
a diastereoselective ring-closing metathesis for the
formation of the 10-membered lactone ring. Extension
of this protocol to other members of this series and to
different ring sized derivatives is underway and will be
disclosed in due course.
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Acknowledgments
D.K.R. thanks the CSIR, New Delhi, for financial sup-
port in the form of a research fellowship. We also thank
Dr. P. R. Rajmohanan for the NMR data, respectively.
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25
15. Analytical and spectral data of 10. ½aꢀD ꢁ32.4 (c 1.1,
CHCl3); 1H NMR (200 MHz, CDCl3): d 2.48 (dd, 1H,
J = 5.2, 16.0 Hz), 2.63 (dd, 1H, J = 8.0, 16.0 Hz), 3.79 (s,
3H), 4.22 (m, 1H), 4.30 (d, 1H, J = 11.2 Hz), 4.52 (d, 1H,
J = 11.2 Hz), 5.27–5.37 (m, 2H), 5.78 (m, 1H), 6.82–6.87
(m, 2H), 7.20–7.26 (m, 2H); 13C NMR (125 MHz, CDCl3):
d 40.9, 55.1, 70.2, 76.2, 113.8, 118.3, 129.4, 130.0, 137.0,
159.2, 176.5. Anal. Calcd for C13H16O4: C, 66.09; H, 6.83.
Found: C, 65.91; H, 6.75.
25
16. Analytical and spectral data of 9. ½aꢀD +19.6 (c 1.1,
5. Kredich, N. M.; Guarino, A. J. Biochim. Biophys. Acta
1960, 41, 363–371.
CHCl3); 1H NMR (200 MHz, CDCl3): d 1.15 (d, 3H,
J = 6.0 Hz), 1.44–1.58 (m, 2H), 1.62–1.69 (m, 2H), 2.10
(br s, 1H) 3.70–3.77 (m, 2H), 3.80 (m, 3H), 4.24 (d, 1H,
J = 11.3 Hz), 4.50 (d, 1H, J = 11.3 Hz), 5.16–5.26 (m,
2H), 5.76 (m, 1H), 6.84–6.88 (m, 2H), 7.21–7.27 (m, 2H);
13C NMR (50 MHz, CDCl3): d 23.3, 31.7, 34.9, 55.1, 67.5,
69.7, 80.2, 113.7, 117.0, 129.3, 130.4, 138.8, 159.0. Anal.
Calcd for C15H22O3: C, 71.97; H, 8.86. Found C, 71.94; H,
8.72.
6. Rukachaisirikul, V.; Pramjit, S.; Pakawatchai, C.; Isaka,
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19. Analytical and spectral data of 8. ½aꢀ2D5 +8.5 (c 1.3, CHCl3);
1H NMR (200 MHz, CDCl3): d 1.16 (d, 3H, J = 6.3 Hz),
1.55–1.61 (m, 4H), 2.38 (dd, 1H, J = 5.8, 15.0 Hz), 2.56
(dd, 1H, J = 8.0, 15.0 Hz), 3.64 (m, 1H), 3.78 (s, 3H), 3.79
(s, 3H), 4.20 (m, 1H), 4.28 (d, 2H, J = 11.3 Hz), 4.46 (d,
2H, J = 11.3 Hz), 4.89 (m, 1H), 5.13–5.33 (m, 4H), 5.59–
5.85 (m, 2H), 6.81–6.87 (m, 4H), 7.19–7.27 (m, 4H); 13C
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Trans. 1 1998, 371–388.

D. K. Mohapatra et al. / Tetrahedron Letters 48 (2007) 2621–2625
2625
NMR (50 MHz, CDCl3): d 20.0, 31.3, 31.7, 41.3, 55.0,
69.7, 70.1, 71.0, 76.7, 79.8, 113.6, 117.1, 117.7, 129.2,
130.3, 130.6, 137.4, 138.9, 159.0, 170.2. Anal. Calcd for
C28H36O6: C, 71.77; H, 7.74. Found: C, 71.68; H, 7.64.
J = 3.0, 15.9 Hz); 13C NMR (100 MHz, CD3OD): d 20.6,
29.4 (cis), 31.3, 31.7 (cis), 44.0, 66.8, 70.7 (cis), 72.9, 74.3,
130.4, 133.0, 170.3. Anal. Calcd for C10H16O4: C, 59.98;
H, 8.05; Found: C, 59.87; H, 8.02.
25
25
20. Analytical and spectral data of 1. ½aꢀD Found: ꢁ49.8 (c
21. Analytical and spectral data of 2. ½aꢀD ꢁ28.0 (c 0.5,
0.3, MeOH); lit: ꢁ55.0 (c 0.036, MeOH); 1H NMR
(400 MHz, CD3OD): d 1.14 (d, 2.7H, J = 6.5 Hz), 1.18 (d,
0.3H, J = 6.8 Hz, cis), 1.57–1.64 (m, 2H), 1.77 (m, 1H),
1.95 (m, 1H), 2.04 (dd, 0.1H, J = 6.4, 13.8 Hz, cis), 2.30
(dd, 0.1H, J = 6.4, 13.8 Hz, cis), 2.46 (dd, 0.9H, J = 3.5,
11.9 Hz), 2.51 (dd, 0.9H, J = 3.7, 11.9 Hz), 2.93 (dd, 0.1H,
J = 7.6, 13.8 Hz, cis), 4.11 (m, 1H), 4.63 (m, 1H), 4.78 (m,
1H), 5.61 (ddd, 1H, J = 1.3, 8.4, 15.9 Hz), 5.73 (dd, 1H,
MeOH); 1H NMR (500 MHz, CD3OD): d 1.22 (d, 3H,
J = 3.1 Hz), 1.35–1.43 (m, 2H), 1.71 (m, 1H), 2.00 (m,
1H), 2.08 (dd, 1H, J = 11.0, 14.2 Hz), 2.83 (dd, 1H,
J = 5.9, 14.2 Hz), 4.67 (m, 1H), 4.84 (m, 1H), 4.98 (m,
1H), 5.24 (m, 1H), 5.35 (dd, 1H, J = 9.3, 11.2 Hz);
13C NMR (125 MHz, CD3OD): d 17.1, 30.7, 43.8, 65.2,
67.0, 71.8, 133.4, 134.3, 170.9. Anal. Calcd for C10H16H4:
C, 59.98; H, 8.05. Found: C, 59.79; H, 8.12.