The first total synthesis of emericellamide A

Article abstract of DOI:10.1016/j.tetlet.2008.03.107

A highly convergent asymmetric total synthesis of emericellamide A, a 19-membered antibacterial depsipeptide isolated from the co-culture of an Emericella sp. (strain CNL-878) and a Salinispora arenicola (strain CNH-665) is described.

Full text of DOI:10.1016/j.tetlet.2008.03.107

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3697–3700
The first total synthesis of emericellamide A
Subhash Ghosh *, Tapan Kumar Pradhan
Indian Institute of Chemical Technology, Hyderabad 500 607, India
Received 15 February 2008; revised 20 March 2008; accepted 24 March 2008
Available online 28 March 2008
Abstract
A highly convergent asymmetric total synthesis of emericellamide A, a 19-membered antibacterial depsipeptide isolated from the
co-culture of an Emericella sp. (strain CNL-878) and a Salinispora arenicola (strain CNH-665) is described.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Emericellamide A; Antibacterial; Macrolactamization
Emericellamides A (1) and B (2) (Scheme 1) are two
novel cyclic depsipeptides that were isolated by Fenical
et al. from Emericella sp., during co-culture with the
marine actinomycete Salinispora arenicola.1 With the help
of chemical and spectral methods, the planar structures
of these compounds were established and the absolute
stereochemistries of 1 and 2 were established with the help
of Marfey’s method,2 and the modified Mosher method.3
Emericellamide A (1) showed antimicrobial activity
against methicillin-resistant Staphylococcus aureus (MIC:
3.8 lM). It also displayed cytotoxicity against HCT-116
human colon carcinoma cell line (IC50: 23 lM). Due to
these interesting biological activities, we decided to develop
a protocol, which would allow us to synthesize not only the
natural product, but also its analogues, so that the struc-
ture–activity relationship could be established. Here, we
report the first asymmetric total synthesis of emericella-
mide A.
Retrosynthetically, emericellamide A (1) (Scheme 1) can
be obtained by the macrolactonization of the acyclic
hydroxy acid 3 which, in turn, could be prepared by the
amide coupling of the two key intermediates, the long
chain fatty acid moiety 4 and pentapeptide component 5.
For the synthesis of top half acid fragment 4, we started
from the known compound 6, which was converted to com-
pound 7 according to the reported procedure.4 The reduc-
tive removal of the chiral auxiliary5 afforded a primary
alcohol, which on oxidation followed by Wittig reaction
with Ph3P@CHCOOEt in toluene at 90 °C using a catalytic
amount of benzoic acid afforded olefinic compound 8
(E:Z = 95:5) in 70% yield.6 The reduction of the ester func-
tionality with DIBAL-H afforded allylic alcohol 9, which
on epoxidation under Sharpless conditions7 gave epoxide
1014a with de P 95% (72% yield over two steps). The regio-
selective opening of the epoxy alcohol 10 using lithium
dimethylcuprate (Me2CuLi)8 in ether at ꢀ20 °C afforded
the required 1,3-diol 1114b as the major product along
with a 1,2-diol as a minor product. To remove the minor
Me Me
Me Me Me
H
N
H
N
H
N
H
N
O
O
O
O
O
O
O
O
O
O
NH
O
O
NH
H
N
H
N
N
H
Me
N
H
Me
Me
Me
O
O
Emericellamide B (2)
Emericellamide A (1)
Me Me
HO
Me Me
H
N
H
N
4
OH
O
O
O
OH
O
O
NH
O
O
Me
COOH
Me
H
N
H
N
H
N
BocHN
COOMe
Me
N
H
N
H
N
H
Me
O
O
O
3
5
Scheme 1. Retrosynthetic analysis of 1.
*
Corresponding author. Tel.: +91 40 27193154; fax: +91 40 27193108.
E-mail address: subhash@iict.res.in (S. Ghosh).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.107

3698
S. Ghosh, T. K. Pradhan / Tetrahedron Letters 49 (2008) 3697–3700
1,2-diol, the crude reaction mixture was subjected to NaIO4
cleavage to convert the 1,2-diol to an aldehyde, which was
then easily removed from the 1,3-diol by a standard silica
gel chromatography to give pure compound 11 (70% over
two steps). The protection of the 1,3-diol compound 11
as p-methoxy-benzylidene acetal 12 in 97% yield using the
dimethyl acetal of p-methoxybenzaldehyde and a catalytic
O
H
N
H
N
COOMe
i
ii, iii
BocHN
ii, iv
COOH
O
BocHN
O
BocHN
N
H
COOMe
14
15
16
O
O
Me Me
H
N
H
N
Me
H
N
H
N
R
COOMe
vi
Me
N
H
N
H
O
O
OR'
O
O
NH
COOR''
O
O
H
N
5: R = NHBoc
N
H
Me
v
Me
O
ii
5a: R = NH2.CF3COOH
9
amount of CSA in CH2Cl2 was followed by the reductive
17: R' = PMB, R'' = Me
18: R' = H, R'' = Me
vii
opening of the PMP acetal with DIBAL-H10 in CH2Cl2 to
afford the primary alcohol 13, which was converted to acid
4 using a two-step oxidation protocol (Scheme 2).
viii
19: R' = R'' = H
1
The pentapeptide segment 5 was synthesized from the
commercially available protected (L)-amino acids (Scheme
3). The condensation of Boc-Gly-OH and valine methyl
ester using EDCI and HOBt as coupling reagents gave
dipeptide 15. The saponification of the ester functionality
with LiOH gave the free acid, which was coupled with
leucine methyl ester under similar conditions as above to
give tripeptide 16 with de P 98%. The hydrolysis of the
ester functionality of 16 followed by coupling with H-
Ala-Ala-OMe afforded pentapeptide 5 with de P 97%
(85% over two steps). Boc-deprotection of 5 with TFA in
CH2Cl2 followed by coupling with 4 under standard
peptide coupling conditions as mentioned above gave com-
pound 17. PMB deprotection using DDQ11 under biphasic
conditions gave secondary alcohol 18, which on saponifica-
tion with LiOH in THF–MeOH–H2O (3:1:1) at 0 °C affor-
ded hydroxy acid 19. The stage was now set to carry out
the crucial macrolactonization reaction. Unfortunately,
using the Yamaguchi protocol12 and varying different con-
ditions we failed to prepare the desired cyclized product.
The failure of this macrolactonization forced us to devise
Scheme 3. Reagents and conditions: (i) EDCI, HOBt, CH2Cl2, then
HClꢁNH2Val-OMe, DIPEA, 0 °C, 6 h, 90%; (ii) LiOH, THF–MeOH–H2O
(3:1:1), 0 °C, 1 h; (iii) EDCI, HOBt, CH2Cl2, then HClꢁNH2Leu-OMe,
DIPEA, 0 °C, 6 h, 80%; (iv) EDCI, HOBt, CH2Cl2, then H-Ala-Ala-OMe,
DIPEA, 0 °C, 0.5 h, 85% over two steps; (v) CF3COOH, CH2Cl2, 0 °C,
0.5 h; (vi) 4, EDCI, HOBt, DIPEA, CH2Cl2, 0 °C, 0.5 h, 80% with respect
to 4; (vii) DDQ, CHCl3–H2O (18:1), 0 °C, 3 h, 90%; (viii) 2,4,6-
trichlorobenzoyl chloride, Et3N, THF, rt, 3 h, the mixed anhydride was
then added to DMAP solution in toluene, 10ꢀ3 M, 80 °C, 5 h.
Me Me
HOOC
NH2
H
N
Me Me
OH
O
O
O
TBSO
O
O
NH
O
H
N
1
N
H
Me
21
Me
20
Scheme 4. Revised retrosynthetic analysis.
an alternative strategy by forming the ester bond first
and then carrying out a macrolactamization reaction
(Scheme 4). To avoid steric hindrance and to minimize
racemization, we decided to perform the macrolactamiza-
tion via the formation of a Gly-Val amide bond.
For this revised strategy, we started from compound 11.
The chemoselective protection of the primary hydroxy
group as a TBS ether with TBSCl and imidazole in THF
gave 21. However, the acylation of the secondary alcohol
with the acid (Boc-Val-Leu-Ala-Ala-OH) was not
successful.
Thus, we thought of first acylating alcohol 21 with Cbz-
Ala-OH, which could be followed by the extension of the
peptide chain. Thus the acylation of alcohol 21 with Cbz-
Ala-OH in the presence of DCC and a catalytic amount
of DMAP in CH2Cl2 at 0 °C gave compound 22 in 75%
yield with de P 90% (Scheme 5). Acid catalyzed TBS
Bn
Bn
Me
ii, iii, iv
i
O
N
O
N
O
O
O
O
6
7
Me
Me
v
vi
HO
EtOOC
9
8
Me
Me Me
O
vii
ix
viii
HO
HO
10
Me Me
11
OH
Me Me
x, xi
OH OPMB
13
O
O
12
PMP
Me Me
HO
deprotection, followed by a two-step oxidation of the
14c
O
OPMB
corresponding primary alcohol (23)
afforded acid 24
4
in 90% yield over three steps. Coupling of acid 24 with
H-Gly-OtBu, using EDCI and HOBt as coupling agents
gave compound 2514d in 90% yield. Cbz-deprotection of
25 under catalytic hydrogenolysis conditions gave primary
amine 26, which was coupled with Boc-Val-Leu-Ala-OH
under the same coupling condition as above to give com-
pound 2715 in 75% yield. The deprotection of the Boc
group and the hydrolysis of the tertiary butyl ester using
TFA in CH2Cl2 afforded amino acid 20, which was sub-
Scheme 2. Reagents and conditions: (i) Ref. 4 (ii) LiBH4, Et2O–H2O
(50:1), 0 °C, 10 min, 92%; (iii) (COCl)2, DMSO, Et3N, CH2Cl2, ꢀ78 °C,
2 h; (iv) Ph3P@CHCOOEt, benzoic acid (cat), toluene, 90 °C, 0.5 h, 70%
over two steps; (v) DIBAL-H, ꢀ78 °C, CH2Cl2, 0.5 h, 80%; (vi) Ti(iOPr)4,
˚
D(ꢀ)DIPT, TBHP, CH2Cl2, 4 A MS, ꢀ20 °C, 3 h, 91%; (vii) (a) Me2CuLi,
Et2O, ꢀ20 °C to 0 °C, 3 h; (b) NaIO4, THF–H2O (2:1), 0 °C, 1 h, 70% over
two steps; (viii) PMP-acetal, CSA (cat), CH2Cl2, 0 °C to rt, 0.5 h, 97%; (ix)
DIBAL-H, CH2Cl2, 0 °C, 1 h, 80%; (x) DMP, CH2Cl2, 0 °C, 1 h; (xi)
t
NaClO2, NaH2PO4ꢁ2H2O, 2-methyl-2-butene, BuOH, rt, 70% over two
steps.

S. Ghosh, T. K. Pradhan / Tetrahedron Letters 49 (2008) 3697–3700
3699
Me Me
9. Kloosterman, M.; Slaghek, T.; Hermans, J. P. G.; Van Boom, J. H.;
Recl, J. R. Neth. Chem. Soc. 1984, 103, 335–341.
10. Takano, S.; Akiyama, M.; Sato, S.; Ogasawara, K. Chem. Lett. 1983,
1593–1596.
11. Wipf, P.; Graham, T. H. J. Am. Chem. Soc. 2004, 126, 15346–
15347.
i
ii
TBSO
11
21
OH
iii
Me Me
Me Me
v
iv
TBSO
HO
22
23
O
O
O
O
12. Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989–1993.
CbzHN
Me
CbzHN
Me
13. Chen, S.; Xu, J. Tetrahedron Lett. 1991, 32, 6711–6714.
Me Me
H
N
30
Me Me
14. (a) Analytical and spectral data of compound 10: ½aꢂD +34.90 (c = 1.86,
O
HO
O
vi
CHCl3); IR (neat): mmax 3427, 2926, 2857, 1630 and 1384 cmꢀ1 1H
;
O
O
O
O
25: R = Cbz
O
O
NMR (200 MHz, CDCl3): d 3.87 (dd, J = 12.4, 2.9 Hz, 1H), 3.58 (dd,
J = 12.4, 3.6 Hz, 1H), 2.90 (m, 1H), 2.67 (m, 1H), 1.63 (m, 1H), 1.38–
1.22 (m, 10H), 1.01 (d, J = 5.9 Hz, 3H), and 0.89 (t, J = 6.6 Hz, 3H);
13C NMR (100 MHz, CDCl3): d 61.8, 60.6, 58.5, 35.4, 33.5, 31.7, 29.4,
27.0, 22.5, 17.0 and 13.9; ESIMS: [MꢀH2O+H]+ = 169; (b) Analyt-
24
vii
RHN
Me
26 : R = H
CbzHN
Me
Me Me
H
N
O
O
viii
ix
O
O
O
x
30
1
O
O
20
ical and spectral data of compound 11: ½aꢂD +14.64 (c = 4.91, CHCl3);
H
N
BocHN
N
N
H
Me
IR (neat): mmax 3355, 2959, 2925, 2855 and 1460 cmꢀ1 1H NMR
;
H
O
Me
(300 MHz, CDCl3): d 3.70 (dd, J = 10.5, 3.7 Hz, 1H), 3.59 (dd,
J = 10.5, 8.3 Hz, 1H), 3.44 (dd, J = 9.1, 3.0 Hz, 1H), 2.96–2.52 (br s,
2H), 1.82 (m, 1H), 1.59 (m, 1H), 1.40–1.21 (m, 10H), 0.93–0.84 (m,
6H) and 0.80 (d, J = 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3): d
79.6, 68.2, 37.1, 34.9, 33.9, 31.8, 29.4, 27.3, 22.5, 13.9, 13.4 and 12.1;
ESIMS: [MꢀH2O+Na]+: 207. (c) Analytical and spectral data of
27
Scheme 5. Reagents and conditions: (i) TBSCl, imidazole, THF, 8 h, 96%;
(ii) Boc-Val-Leu-Ala-Ala-OH, DCC, DMAP (cat), 0 °C to rt, 5 h, 0%; (iii)
Cbz-Ala-OH, DCC, DMAP (cat), 0 °C to rt, 5 h, 75%; (iv) CSA (cat),
MeOH–CH2Cl2 (1:1), 0 °C, 0.5 h, 85% over two steps; (v) (a) SO3–Py,
DMSO–CH2Cl2 (2:1.8), 0 °C, 1 h; (b) NaH2PO4ꢁ2H2O, NaClO2, 2-methyl-
30
compound 23: ½aꢂD ꢀ17.94 (c = 0.39, CHCl3); IR (neat): mmax 3512,
t
2-butene, BuOH, 90% over two steps; (vi) EDCI, HOBt, H-Gly-OtBu,
2957, 2928, 2857, 1465, 1386, 1254, 1077, 837 and 776 cmꢀ1; 1H NMR
(300 MHz, CDCl3): d 7.35–7.27 (m, 5H), 5.25 (d, J = 6.8 Hz, 1H),
5.08 (s, 2H), 4.88 (dd, J = 10.5, 2.2 Hz, 1H), 4.33 (dq, J = 6.8, 7.5 Hz,
1H), 3.51 (dd, J = 12.0, 3.0 Hz, 1H), 3.41 (dd, J = 12.0, 2.2 Hz, 1H),
1.88–1.67 (m, 2H), 1.44 (d, J = 6.8 Hz, 3H), 1.35–1.81 (m, 10H), 0.99
(d, J = 6.8 Hz, 3H) and 0.93–0.85 (m, 6H): 13C NMR (75 MHz,
CDCl3): d 174.1, 155.7, 136.1, 128.5, 128.1, 128.0, 78.6, 66.9, 64.1,
49.9, 36.8, 34.1, 33.6, 31.7, 29.3, 27.0, 22.5, 18.4, 14.0, 13.9 and 12.9;
HRMS (ESI): calcd for C23H37 NO5Na [M+Na]+: 430.2569, found:
0 °C to rt, 1 h, 90%; (vii) H2, Pd/C, EtOAc, 1 N, HCl, 10 min; (viii) Boc-
Val-Leu-Ala-OH, EDCI, HOBt, CH2Cl2, then 26, DIPEA, 0 °C, 1 h, 75%
over two steps; (ix) TFA–CH2Cl2 (1:1), 0 °C, 1 h; (x) FDPP, DIPEA,
CH3CN, 10ꢀ3 M, 0 °C to rt, 72 h, 75%.
jected to macrolactamization by the treatment with penta-
fluorophenyl diphenylphosphinate (FDPP)13 and DIPEA
under high dilution, 10ꢀ3 M, in CH3CN at room tempera-
ture, to afford emericellamide A in 75% yield. Spectral and
analytical data of synthetic emericellamide A were in good
agreement with those of the literature data.1,16
In conclusion, we have achieved the first total synthesis
of emericellamide A in a convergent fashion using FDPP
mediated macrolactamization as a key step. The prepara-
tion of analogues and their biological study are under
progress and will be reported in due course.
30
430.2574. (d) Analytical and spectral data of compound 25: ½aꢂD ꢀ3.99
(c = 2.75, CHCl3); IR (neat): mmax 3317, 2925, 2657, 2361, 1726, 1630,
1384, 1217 and 1156 cmꢀ1; 1H NMR (300 MHz, CDCl3): d 7.42–7.30
(m, 5H), 6.11 (t, J = 5.2 Hz, 1H), 5.70 (d, J = 7.5 Hz, 1H), 5.15–5.05
(m, 3H), 4.42 (dq, J = 6.8, 7.5 Hz, 1H), 3.94 (dd, J = 18.1, 5.1 Hz,
1H), 3.78 (dd, J = 18.1, 4.5 Hz, 1H), 2.58 (dq, J = 6.8, 15.1 Hz, 1H),
1.83–1.61 (m, 3H), 1.52–1.39 (m, 12H), 1.35–1.19 (m, 8H), 1.31 (d,
J = 7.5 Hz, 3H) and 0.91–0.81 (m, 6H): 13C NMR (75 MHz, CDCl3):
d 173.8, 171.9, 169.3, 155.7, 136.4, 128.3, 127.9, 127.8, 82.1, 78.4, 66.5,
49.7, 43.3, 41.8, 34.1, 33.4, 31.6, 29.2, 27.8, 26.8, 22.5, 18.2, 14.5, 13.9
and 13.2; HRMS (ESI): calcd for C29H46 N2O7Na [M+Na]+
=
557.3202, found: 557.3197.
30
15. Analytical and spectral data of compound 27: ½aꢂD ꢀ33.40 (c = 3.05,
CHCl3); IR (neat): mmax 3280, 3079, 2961, 2928, 2857, 1742, 1638,
1544, 1456, 1369, 1216 and 1162 cmꢀ1; 1H NMR (300 MHz, CDCl3):
Acknowledgements
One of us is thankful to UGC, New Delhi, for the
research fellowship (T.K.P.). We are also thankful to Dr.
T. K. Chakraborty, Dr. A. C. Kunwar and the Director,
IICT for their support and encouragement.
d
7.08 (d, J = 8.3 Hz, 1H), 7.04 (d, J = 8.3 Hz, 1H), 6.71 (t,
J = 5.3 Hz, 1H), 6.44 (d, J = 6.8 Hz, 1H), 5.09 (dd, J = 8.3, 3.0 Hz,
1H), 4.98 (d, J = 5.3 Hz, 1H), 4.60 (dq, J = 6.8, 7.5 Hz, 1H), 4.47 (dq,
J = 6.8, 7.5 Hz, 1H), 4.36 (m, 1H), 4.02 (dd, J = 17.3, 6.8 Hz, 1H),
3.87 (dd, J = 17.3, 4.5 Hz, 1H), 3.82 (m, 1H), 2.74 (dq, J = 7.5,
8.3 Hz, 1H), 2.16 (m, 1H), 1.68 (m, 1H), 1.45 (s, 9H), 1.44 (s, 9H),
1.48–1.37 (m, 6H), 1.33–1.20 (m, 10H), 1.13 (d, J = 6.8 Hz, 3H) and
1.05–0.83 (m, 21H): 13C NMR (75 MHz, CDCl3): d 173.9, 172.1,
171.7, 171.4, 171.1, 169.4, 156.2, 81.6, 80.1, 78.6, 60.6, 52.0, 48.7,
48.0, 43.1, 41.7, 40.9, 33.9, 33.5, 31.7, 30.6, 29.2, 28.2, 27.9, 27.0, 24.7,
22.9, 22.5, 21.7, 19.1, 18.2, 18.1, 17.8, 14.1, 13.9 and 12.9; HRMS
References and notes
1. Oh, D.-C.; Kauffman, C. A.; Jensen, P. R.; Fenical, W. J. Nat. Prod.
2007, 70, 515–520.
2. Marfey, P. Carlsberg Res. Commun. 1984, 49, 591–596.
(ESI): calcd for
806.5242.
C
40H73N5O10Na [M+Na]+ = 806.5255, found:
´
3. Seco, J. M.; Quinoa, E.; Riguera, R. Tetrahedron: Asymmetry 2001,
˜
12, 2915–2925.
30
16. Analytical and spectral data of emericellamide A: ½aꢂD ꢀ42.99 (c = 0.2,
MeOH), reported ꢀ43 (c = 0.23, MeOH); IR (KBr): mmax 3401, 3317,
3067, 2962, 2929, 2858, 1755, 1635, 1549, 1458, 1380, 1326, 1285,
4. Wipf, P.; Kim, Y.; Fritch, P. C. J. Org. Chem. 1993, 58, 7195–7203.
5. Penning, T. D.; Djuric, S. W.; Haack, R. A.; Kalish, V. J.; Miyashiro,
J. M.; Rowell, B. W.; Yu, S. S. Synth. Commun. 1990, 20, 307–312.
6. Harcken, C.; Martin, S. F. Org. Lett. 2001, 3, 3591–3593.
7. Katsuki, T.; Sharpless, K. B. J. Am. Chem. Soc. 1980, 102, 5974–5976.
8. Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135.
1239, 1169 and 1064 cmꢀ1 1H NMR (500 MHz, DMSO-d6): d 8.10
;
(d, J = 8.2 Hz, 1H), 8.02 (d, J = 3.3 Hz, 1H) 7.98 (d, J = 8.2 Hz, 1H),
7.47 (dd, J = 5.5, 2.7 Hz, 1H), 7.39 (d, J = 6.6 Hz, 1H), 4.92 (dd,

3700
S. Ghosh, T. K. Pradhan / Tetrahedron Letters 49 (2008) 3697–3700
J = 10.2, 2.0 Hz, 1H), 4.32 (dd, J = 17.0, 5.5 Hz, 1H), 4.12–4.00 (m,
0.84 (t, J = 7.0 Hz, 3H), 0.82 (d, J = 7.0 Hz, 3H) and 0.80 (d,
J = 6.0 Hz, 3H): 13C NMR (125 MHz, DMSO-d6): d 172.9, 171.4,
171.3, 171.2, 170.8, 168.8, 76.6, 60.1, 51.7, 48.2, 47.3, 42.5, 41.0, 40.8,
33.5, 33.2, 31.2, 30.2, 28.9, 26.6, 24.5, 23.2, 22.1, 20.7, 19.1, 18.8, 18.3,
16.3, 14.3, 14.0 and 12.9:HRMS (ESI): calcd for C31H55N5O7Na
[M+Na]+ = 632.3999, found: 632.3986.
3H), 3.98 (dd, J = 8.2, 8.2 Hz, 1H), 3.62 (dd, J = 17.0, 2.7 Hz, 1H),
2.86 (dq, J = 10.0, 7.1 Hz, 1H), 1.89 (m, 1H), 1.67 (m, 1H), 1.65–1.55
(m, 3H), 1.24 (d, J = 7.1 Hz, 3H), 1.23 (d, J = 7.1 Hz, 3H), 1.24–1.18
(m, 8H), 1.10 (m, 1H), 1.02 (m, 1H), 0.90 (d, J = 7.1 Hz, 3H), 0.89 (d,
J = 6.0 Hz, 3H), 0.88 (d, J = 7.0 Hz, 3H), 0.87 (d, J = 7.0 Hz, 3H),