Total syntheses of 28,29-diepi-arenamide A, 29-epi-arenamide A, and 28-epi-arenamide A

Article abstract of DOI:10.1016/j.tetlet.2012.09.018

This communication describes the synthesis of stereochemical analogs of arenamide A, a 19-membered cytotoxic depsipeptide isolated from the fermentation broth of a marine bacterial strain Salinispora arenicola. The key steps are diastereoselective aldol r

Full text of DOI:10.1016/j.tetlet.2012.09.018

Tetrahedron Letters 53 (2012) 6148–6150
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total syntheses of 28,29-diepi-arenamide A, 29-epi-arenamide A, and
28-epi-arenamide A
⇑
V. Jithender Reddy, Tapan Kumar Pradhan, Subhash Ghosh
CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 June 2012
Revised 5 September 2012
Accepted 6 September 2012
Available online 14 September 2012
This communication describes the synthesis of stereochemical analogs of arenamide A, a 19-membered
cytotoxic depsipeptide isolated from the fermentation broth of a marine bacterial strain Salinispora
arenicola. The key steps are diastereoselective aldol reaction, Mitsunobu reaction, and HATU mediated
macrolactamization.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Salinispora arenicola
Arenamide A
Depsipeptides
Cytotoxic
Macrolactamization
Cyclic peptides, which contain one or more ester linkages in
place of amide linkages called cyclic depsipeptides, produced by
marine organisms are an important source of biologically active
compounds. Arenamides A–C,1 a new class of cytotoxic cyclic dep-
sipeptides were isolated by Fenical et al. from the fermentation
broth of a marine bacterial strain Salinispora arenicola. Detailed
NMR and MS/MS spectroscopic studies revealed that arenamide
A is a 19-membered cyclic depsipeptide containing 3-hydroxy-4-
methyl decanoic acid and a pentapeptide unit of Phe, Ala, Val,
Gly, and Leu (Fig. 1). The absolute stereochemistry of the amino
acids and the aliphatic chain (HMDA) was established via Marfey’s
and modified Mosher method. Biological studies revealed that are-
namide A blocked TNF-induced activation of NF-
tion- and time-dependent manner with IC50 values of 3.7 and
1.7 M, respectively. These compounds also exhibited weak cyto-
jB in a concentra-
l
toxic activity against the human colon carcinoma cell line HCT-
116. The first total synthesis of (28S, 29S) arenamide A (1) and
(28R, 29R) arenamide A (2) was reported by Chandrasekhar et al.
and revealed that the analytical data of (28S, 29S) arenamide A
were very close to those of the natural product.2 Later Fenical et
al. reconfirmed the structure of arenamide A to be (28S, 29S) are-
namide A.3 Because of its interesting structure and functions, as
well as our continuous interest in this area,4 we have decided to
develop a synthetic strategy by which its analogs can be prepared
easily for structural activity relationship study. Herein, we report
the synthesis of stereochemical analogs (2, 3, 4) of arenamide A.
The retrosynthetic analysis of 2, 3, and 4 is depicted in Scheme
1. Inspection of the structures of 2, 3, and 4 revealed that the amide
bond between glycine and the HMDA is a preferred site of macro-
cyclization because of steric and stereoelectronic reason. Therefore
dissection of the glycine amide bond leading to seco-acids 5, 6, and
7 would be potential macrolactamization precursors of 2, 3, and 4.
Seco acids 5, 6, and 7 could be obtained from 8, 9, and 10 through
hydrogenation, oxidation, and Boc-deprotection. Compounds 8, 9,
and 10 could be obtained by means of peptide chain elongation
of 11, 12, and 13, which in turn could be synthesized through acyl-
ation of compounds 14 and 15.
H
N
25
Me
29
H
N
26
20
28
O
O
O
19
HN
O
O
O
1
H
11
13
N
2
10
N
H
CH3
O
1
arenamide A( ): 28S, 29S
28, 29-diepi-arenamide A(2): 28R, 29R
29-epi-arenamide A(3): 28S, 29R
4
28-epi-arenamide A( ): 28R, 29S
Figure 1. Structure of arenamide A and its analogs.
Thus our synthesis commenced from the known aldehyde 16,
which on treatment with the enolate generated from 17 under
⇑
Corresponding author.
E-mail address: subhash@iict.res.in (S. Ghosh).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.018

V. Jithender Reddy et al. / Tetrahedron Letters 53 (2012) 6148–6150
6149
28, 29-diepi-arenamide A(2)
OH
O
O
29-epi-arenamide A(3)
28-epi-arenamide A(4)
i
O
BnO
CHO
OH
BnO
iv, v
N
O
O
16
Bn
N
O
O
H
18
H
N
N
ii, iii
NH
Bn
N
H
17
Ph
O
O
O
O
O
O
OTBS
NH2
29
28
BnO
HO
BnO
OH
5: 28R, 29R
6: 28S, 29R
19
14
E:Z = 9:1
7
: 28R, 29S
vi
O
H
H
N
vii
N
NH
N
H
Ph
O
NHBoc
O
O
NHBoc
O
O
O
O
Ph
Ph
NHBoc
29
28
BnO
O
O
29
29
28
BnO
8: 28R, 29R
BnO
28
9
: 28S, 29R
10: 28R, 29S
12: 28S, 29R
NHBoc
O
11: 28R, 29R
Ph
OH
29
Scheme 2. Reagents and conditions: (i) TiCl4, (ꢀ)-sparteine, 0 °C, CH2Cl2, 2 h, 87%;
(ii) TBSOTf, 2,6 lutidine, 0 °C, CH2Cl2, 10 min, 81%,; (iii) LiBH4, Et2O, 0 °C, 1 h, 83%;
(iv) (a) (COCl)2, DMSO, CH2Cl2, ꢀ78 °C, NEt3, ꢀ78 °C to rt, 83%; (b)
[Ph3PCH2CH2CH2CH2CH3]Br, n-BuLi, THF, ꢀ78 °C, 74%; (v) CSA, MeOH, CH2Cl2,
O
28
29
28
BnO
BnO
14: 28R, 29R
15
85%; (vi) N-Boc-L-Phe, DCC, DMAP, CH2Cl2, 0 °C, 94%; (vii) N-Boc-L-Phe, DIAD,
: 28S, 29S
11: 28R, 29R
12: 28S, 29R
PPh3,THF, 0 °C, 76%.
13
: 28R, 29S
Scheme 1. Retrosynthetic analysis.
O
O
Crimmins conditions5 gave syn aldol product 18 in 87% yield. Pro-
tection of the secondary hydroxyl as TBS ether followed by the re-
moval of the chiral auxiliary using LiBH46 afforded primary alcohol
19 in 68% over two steps. Oxidation7 of the primary alcohol 19 fur-
nished an aldehyde, which on Wittig reaction8 with the ylide
Ph3P = CH–(CH2)3–CH3 gave an olefinic compound, which on TBS
deprotection with CSA (cat) furnished the secondary alcohol 14
(Z:E > 9:1) in good yield. Acylation9 of the secondary hydroxyl
group with BocPheOH (Scheme 2) under DCC/DMAP (cat) condi-
tions gave the acylated product 11 in 94% yield (de > 90%). The
same alcohol 14 on reaction with BocPheOH under Mitsunobu
reaction10 conditions gave compound 12 (C3-epimer of 11). In
the 1H NMR spectrum the diastereomeric aldol hydrogen (–
C28H) of 11 was appearing along with BocNH at d 5.02–4.86 (m),
whereas for 12 it appeared at d 5.04 (dd, J = 11.3, 6.04 Hz).
Following the similar strategy and changing the chiral auxiliary
compound 13 was synthesized in good overall yield (Scheme 3).
After synthesizing compounds 11–13,11 the tetrapeptide Boc-
Gly-Val-Leu-AlaOMe was needed for the peptide chain elongation
and was synthesized through solution phase peptide coupling
strategy, starting from BocGlyOH (Scheme 4). BocGlyOH was cou-
pled with NH2ValOMe using EDCI and HOBt as coupling agent to
give a dipeptide,12 which on hydrolysis with LiOH afforded acid
20. Similarly BocLeuOH was coupled with NH2AlaOMe to give Boc-
Leu-AlaOMe, which on Boc-deprotection furnished amine 21. Cou-
pling of acid 20 with amine 21 was performed under the same
coupling conditions, as used above to afford tetra peptide 22,13
which on hydrolysis furnished the acid 23 in good yield.
N
O
Bn
ent-17
OH
i-v
BnO
BnO
CHO
16
15
NHBoc
Ph
vi
O
O
29
28
BnO
13: 28R, 29S
Scheme 3. Reagents and conditions: (i) TiCl4, (ꢀ)-sparteine, 0 °C, CH2Cl2, 2 h, 86%;
(ii) TBSOTf, 2,6 lutidine, 0 °C, CH2Cl2, 10 min, 87%; (iii) LiBH4, Et2O, 0 °C, 1 h, 80%;
(iv) (a) (COCl)2, DMSO, CH2Cl2, NEt3, ꢀ78 °C; (b) [Ph3PCH2CH2CH2CH2CH3]Br, n-
BuLi, THF, ꢀ78 °C, 68%; (v) CSA, MeOH, CH2Cl2, 82%; (vi) N-Boc-
PPh3,THF, 0 °C, 71%.
L-PheOH, DIAD,
alcohols 24–26, which on oxidation14 followed by Boc-deprotec-
tion furnished amino acids 5–7. Finally macrolactamization15 un-
der high dilution conditions using HATU, HOAT afforded the
arenamide analogs 2–416 in good yields.
In conclusion we have reported a highly stereoselective and
flexible synthetic strategy for the preparation of arenamide analogs
2–4. The key steps include Crimmins modified Evans aldol reac-
tion, acylation via Mitsunobu inversion, and HATU mediated mac-
rolactamization. The strategy developed here is highly convergent
and very useful for making analogs. Research leading to more num-
ber of analogs is under progress and will be reported in due course.
For the peptide chain elongation of 11–13, (Scheme 5) the Boc
group was deprotected to obtain free amines, which were coupled
with the acid 23 to afford compounds 8–10. One pot benzyl depro-
tection and olefin reduction of compounds 8–10 furnished primary

6150
V. Jithender Reddy et al. / Tetrahedron Letters 53 (2012) 6148–6150
11. (a) Analytical data of compound 11: Rf = 0.5 silica gel, 10% EtOAc in Petroleum
ether). ½a D25 = +6.8 (c 3.5, CHCl3). IR (neat): max (br) 3441, 2924, 17111, 1498,
1366, 1167, 741 and 700 cmꢀ1 1H NMR (CDCl3, 300 MHz): d 7.38–7.13 (m,
ꢁ
m
OH
.
BocHN
BocHN
10H), 5.39 (dt, J = 10.5, 7.5 Hz, 1H), 5.13 (t, J = 10.5 Hz, 1H), 5.02–4.86 (m, 2H),
4.55 (ddd, J = 14.3, 6.8, 6.0 Hz, 1H), 4.47–4.37 (m, 2H), 3.42–3.26 (m, 2H), 3.10
(dd, J = 14.3, 6.0 Hz, 1H), 2.92 (dd, J = 14.3, 7.5 Hz, 1H), 2.71 (dt, J = 9.8, 6.7 Hz,
1H), 2.06–1.86 (m, 2H), 1.80–1.50 (m, 2H), 1.35–1.20 (m, 13H), 0.97–0.82 (m,
6H). 13C NMR (CDCl3, 75 MHz): d 171.7, 154.9, 138.2, 136.2, 131.1, 130.4, 129.3,
128.4, 128.3, 127.6, 127.5, 126.8, 79.6, 76.5, 72.9, 66.6, 54.4, 38.1, 35.6, 32.1,
31.7, 28.2, 27.2, 22.2, 16.8, 13.9. mass (ESIMS): m/z 524 [M+H]+, 546 [M+Na]+.
(b) Analytical data of compound 12: Rf = 0.5 (silica gel, 10% EtOAc in Petroleum
OH
BocHN
O
O
i, ii
iii, iv
H
N
COOH
TFA.H2N
H
N
ether). ½a D25
ꢁ
= ꢀ23.9 (c 2.5, CHCl3). IR (neat):
mmax (br) 3619, 2923, 1707, 1647,
COOMe
1515, 1462, 1169, and 742 cmꢀ1
.
1H NMR (CDCl3, 300 MHz): d 7.35–7.15 (m,
O
10H), 5.41 (dt, J = 11.3, 7.5 Hz, 1H), 5.17 (m, 1H), 5.04 (dd, J = 11.3, 6.0 Hz, 1H),
4.91 (d, J = 8.3 Hz, 1H), 4.53 (m, 1H), 4.49–4.39 (m, 2H), 3.50–3.33 (m, 2H), 3.14
(dd, J = 14.3, 6.0 Hz, 1H), 2.93 (dd, J = 14.3, 6.7 Hz, 1H), 2.70 (m, 1H), 2.06–1.93
(m, 2H), 1.81 (dd, J = 12.8, 6.0 Hz, 2H), 1.44–1.24 (m, 13H), 0.97–0.83 (m, 6H).
13C NMR (CDCl3, 75 MHz): d 171.8, 155.0, 138.3, 136.1, 131.5, 130.0, 129.3,
128.4, 128.2, 127.7, 127.5, 126.8, 79.7, 75.8, 73.1, 66.6, 54.5, 38.1, 35.5, 31.8,
31.7, 28.2, 27.1, 22.3, 16.8, 13.9. mass (ESIMS): m/z 524 [M+H]+, 546 [M+Na]+.
(c) Analytical data of compound 13: Rf = 0.5 (silica gel, 10% EtOAc in Petroleum
20
O
21
v
O
O
H
N
H
N
R
BocHN
N
H
O
ether). ½a D25
ꢁ
= +26.1 (c 2.2, CHCl3). IR (neat): mmax 2921, 2851, 1716, 1365,
O
O
1214, 1168, and 746 cmꢀ1
.
1H NMR (CDCl3, 300 MHz): d 7.38–7.12 (m, 10H),
5.40 (dt, J = 10.9, 6.7 Hz, 1H), 5.15 (t, J = 10.2 Hz, 1H), 5.06–4.86 (m, 2H), 4.55
(dd, J = 14.1, 6.4 Hz, 1H), 4.49–4.37 (m, 2H), 3.44–3.28 (m, 2H), 3.09 (dd,
J = 14.1, 6.4 Hz, 1H), 2.96 (dd, J = 14.1, 6.0 Hz, 1H), 2.71 (m, 1H), 2.07–1.92 (m,
2H), 1.79 (dd, J = 12.6, 6.2 Hz, 2H), 1.44–1.23 (m, 13H), 0.95–0.84 (m, 6H). 13C
NMR (CDCl3, 75 MHz): d 171.6, 154.9, 138.3, 136.2, 131.5, 129.8, 129.3, 128.4,
128.3, 127.7, 127.5, 126.8, 79.7, 76.1, 73.0, 66.7, 54.5, 38.2, 35.2, 31.8, 31.7,
28.2, 27.1, 22.3, 16.8, 13.9. Mass (ESIMS): m/z 524 [M+H]+, 546 [M+Na]+.
12. Chakraborty, T. K.; Ghosh, S.; Jayaprakash, S.; Sharma, J. A. R. P.; Ravikanth, V.;
Diwan, P. V.; Nagraj, R.; Kunwar, A. C. J. Org. Chem. 2000, 65, 6441–6457.
13. Analytical data of compound 22 Rf = 0.4 (silica gel, 80% EtOAc in Petroleum
22: R = CH3
23: R = H
ii
Scheme 4. Reagents and conditions: (i)
to rt, 95%.; (ii) LiOH, THF:MeOH:H2O (3:1:1); (iii)
L
-Val-OMe, EDCI, HOBT, DIPEA, CH2Cl2, 0 °C
-Ala-OMe, EDCI, HOBT, DIPEA,
L
CH2Cl2, 0 °C to rt, 97%.; (iv) TFA, CH2Cl2, 0 °C.; (v) EDCI, HOBT, DIPEA, CH2Cl2, 0 °C to
rt, 94%.
ether). ½a D25
ꢁ
= ꢀ49.0 (c 0.5, CHCl3). IR (neat): mmax 3285, 2962, 1641, 1544,
i
1368, 1164 and 754 cmꢀ1
.
1H NMR (CDCl3, 500 MHz): d 7.45–7.35 (m, 2H), 6.99
8 9 10
/ /
11/12/13
(d, J = 7.0 Hz, 1H), 5.53 (t, J = 5.4 Hz, 1H), 4.61–4.52 (m, 2H), 4.40 (t, J = 7.8 Hz,
1H), 4.00–3.84 (m, 2H), 3.72 (s, 3H), 2.09 (td, J = 7.0, 7.0, 20.2, 1H), 1.75–1.62
(m, 2H), 1.55 (m 1H), 1.44 (s, 9H), 1.38 (d, J = 7.8 Hz, 3H), 0.96–0.87 (m, 12H).
13C NMR (CDCl3, 75 MHz): d 173.0, 171.9, 171.3, 169.3, 156.1, 58.1, 52.2, 51.6,
47.9, 43.9, 41.1, 31.7, 28.2, 24.5, 22.5, 22.3, 19.0, 18.3, 17.5. Mass (ESIMS): m/z
473 [M+H]+, 495 [M+Na]+.
ii
O
H
N
H
N
NH
N
H
Ph
14. Vatele, J. M. Tetrahedron Lett. 2006, 47, 715–718.
iii
O
15. (a) Reddy, K. M.; Shashidhar, J.; Reddy, P. G.; Ghosh, S. Tetrahedron Lett. 2011,
52, 5987–5991; (b) Han, S. Y.; Kim, Y. A. Tetrahedron 2004, 60, 2447–2467.
16. (a) Analytical data of compound 2: Rf = 0.4 (silica gel, 6% MeOH in CHCl3).
O
5/6 7
/
O
O
O
29
NHBoc
28
HO
½
aꢁD25
= ꢀ43.7 (c 0.08, CHCl3). IR (neat):
mmax (br) 3330, 2958, 2925, 1639, 1531,
iv
and 672 cmꢀ1 1H NMR (CDCl3, 500 MHz): d 8.23 (d, J = 7.1 Hz, 1H), 8.13 (d,
.
24: 28R, 29R
25
: 28S, 29R
26: 28R, 29S
J = 7.1 Hz, 1H), 7.91–7.82 (m, 3H), 7.28–7.14 (m, 5H), 5.03 (m, 1H), 4.35 (ddd,
J = 8.7, 7.1, 5.5 Hz 1H), 4.11–4.00 (m, 3H), 3.93 (t, J = 7.9 Hz, 1H), 3.83 (d,
J = 5.5 Hz, 1H), 3.10 (dd, J = 14.2, 5.5 Hz, 1H), 3.0 (dd, J = 14.2, 8.7 Hz, 1H), 2.46
(m, 1H),2.33 (d, J = 13.4 Hz, 1H), 2.06 (m, 1H), 1.75–1.61 (m, 2H), 1.55 (m, 1H),
1.48 (m, 1H), 1.27 (d, J = 7.1 Hz, 3H), 1.24–1.10 (m, 10H), 0.88 (two d, J = 6.3 Hz,
6H), 0.85 (d, J = 6.3 Hz, 3H), 0.84–0.80 (m, 6H), 0.76 (d, J = 6.7 Hz, 3H). 13C NMR
(CDCl3, 75 MHz): d 171.8, 171.3, 170.6, 169.9, 169.8, 169.6, 138.1, 128.8, 128.1,
126.1, 75.2, 59.5, 54.2, 52.3, 48.5, 42.3, 38.1, 37.8, 35.9, 31.3, 31.1, 29.9, 29.8,
26.4, 24.2, 22.9, 22.1, 21.1, 19.1, 18.2, 17.6, 14.7, 13.9. HRMS (ESIMS): calcd for
2/3/4
Scheme 5. Reagents and conditions: (i) (a) TFA, CH2Cl2, 0 °C; (b) 23, EDCI, HOBT,
DIPEA, CH2Cl2, 0 °C to rt; 90% (24), 93% (25), 91% (26); (ii) H2, 10%Pd-C, MeOH, 98%;
(iii) (a) TEMPO, BAIB-CH3CN:H2O (2:1), 0 °C to rt, 2 h; (b) TFA, CH2Cl2, 0 °C; (iv)
HATU, HOAT, DIPEA, CH2Cl2, 0 °C to rt, 56% (2), 54% (3) and 59% (4) over three steps.
C
36H57N5O7Na [M+Na]+ = 694.4150, found: 694.4156.
(b) Analytical data of compound 3: Rf = 0.4 (silica gel, 6% MeOH in CHCl3).
= ꢀ12.5 (c 0.08, CHCl3). IR (neat): mmax 3308, 2926, 2858, 1749, 1639,
1543, 1173 and 694 cmꢀ1 1H NMR (CDCl3, 500 MHz): d 8.40 (d, J = 4.5 Hz, 1H),
Acknowledgments
½aꢁD25
.
The authors wish to thank CSIR (V.J.R.) and UGC (T.K.P.), New
Delhi for research fellowships. We are also thankful to Professor
Fenical for providing natural arenamide A.
8.01 (d, J = 9.0 Hz, 1H), 7.86 (d, J = 7.5 Hz, 1H), 7.60 (t, J = 4.5 Hz, 1H), 7.29–7.19
(m, 6H), 4.90 (dd, J = 7.5, 4.5 Hz, 1H), 4.28–4.18 (m, 2H), 4.15–4.05 (m, 2H),
3.94 (t, J = 7.5 Hz, 1H), 3.77 (dd, J = 16.5, 4.5 Hz, 1H), 2.99 (dd, J = 13.5, 6 Hz,
1H), 2.87 (dd, J = 13.5, 9.0 Hz, 1H), 2.45 (m, 1H), 2.11 (d, J = 13.5 Hz, 1H), 1.93
(m, 1H), 1.61–1.50 (m, 3H), 1.36–1.06 (m, 12H), 1.0 (m, 1H), 0.93–0.85 (m,
12H), 0.83 (d, J = 4.5 Hz, 3H), 0.43 (d, J = 6.0 Hz, 3H). 13C NMR (CDCl3, 75 MHz):
d 171.6, 171.2, 170.8, 169.4, 169.0, 168.9, 136.2, 129.1, 128.3, 126.7, 75.1, 60.3,
54.9, 51.4, 47.2, 42.2, 37.2, 36.7, 35.2, 31.7, 31.1, 29.7, 28.5, 26.2, 24.3, 23.2,
22.0, 20.6, 19.0, 18.6, 18.5, 13.9, 13.8. HRMS (ESIMS): calcd for C36H57N5O7Na
[M+Na]+ = 694.4150, found: 694.4154.
References and notes
1. Asolkar, R. N.; Freel, K. C.; Jensen, P. R.; Fenical, W.; Kondratyuk, T. P.; Park, E. J.;
Pezzuto, J. M. J. Nat. Prod. 2009, 72, 396–402.
2. Chandrasekhar, S.; Pavankumarreddy, J.; Sathish, K. Tetrahedron Lett. 2009, 50,
6851–6854.
3. Asolkar, R. N.; Freel, K. C.; Jensen, P. R.; Fenical, W.; Kondratyuk, T. P.; Park, E. J.;
Pezzuto, J. M. J. Nat. Prod. 2010, 73, 796.
(c) Analytical data of compound 4: Rf = 0.4 (silica gel, 6% MeOH in CHCl3);
½
a D25
ꢁ
= ꢀ4.0 (c 0.07, CHCl3); IR (neat): mmax 3301, 2926, 1648, 1535, 1248 and
696 cmꢀ1 1H NMR (CDCl3, 500 MHz):
.
d
8.20 (d, J = 7.1 Hz, 1H), 8.08 (d,
J = 7.1 Hz, 1H), 7.93–7.81 (m, 3H), 7.28–1.13 (m, 5H), 4.99 (m, 1H), 4.35 (dd,
J = 13.4, 7.9 Hz, 1H), 4.11–4.01 (m, 2H), 3.91 (t, J = 7.1 Hz, 1H), 3.88–3.79 (m,
2H), 3.13 (dd, J = 14.2, 5.5 Hz, 1H), 3.01 (dd, J = 14.2, 9.5 Hz, 1H), 2.44 (dd,
J = 15.0, 9.5 Hz, 1H), 2.35 (d, J = 13.4 Hz, 1H), 2.06 (m, 1H), 1.75–1.61 (m, 2H),
1.56 (m, 1H), 1.48 (m, 1H), 1.26 (d, J = 6.3 Hz, 3H), 1.24–1.10 (m, 10H), 0.91–
0.86 (m, 9H), 0.85 (d, J = 6.3 Hz, 3H), 0.82 (d, J = 6.3 Hz, 3H), 0.74 (d J = 6.3 Hz,
3H). 13C NMR (CDCl3, 75 MHz): d 172.0, 171.4, 170.4, 170.0, 169.9, 169.7, 138.1,
128.7, 128.0, 126.0, 75.5, 59.8, 54.2, 52.3, 48.1, 42.1, 36.7, 35.3, 31.7, 31.1, 29.6,
28.8, 28.6, 26.2, 24.2, 22.9, 21.9, 21.0, 19.0, 18.1, 17.7, 14.1, 13.8. HRMS
(ESIMS): calcd for C36H58N5O7 [M+H]+ = 672.4330, found: 672.4337.
4. Ghosh, S.; Pradhan, T. K. Tetrahedron Lett. 2008, 49, 5303–5310.
5. (a) Crimmins, M. T.; She, J. Synlett 2004, 1371–1374; (b) Tian, H.; Jiao, H.; Xie,
P.; Liang, H. Tetrahedron Lett. 2005, 46, 8579–8581.
6. Lorenz, M.; Kalesse, M. Tetrahedron Lett. 2007, 48, 2905–2907.
7. Mancuso, A. J.; Swern, D. Synthesis 1981, 165–185.
8. Harchken, C.; Martin, S. F. Org. Lett. 2001, 3, 3591–3595.
9. Gurjar, M. K.; Nagaprasad, R.; Ramana, C. V. Tetrahedron Lett. 2003, 44, 2873–
2875.
10. (a) Dodge, J. F.; Trujillo, J. I.; Presnell, M. J. Org. Chem. 1994, 59, 234–236; (b)
Ahn, C.; Correia, R.; Deshong, P. J. Org. Chem. 2002, 67, 1751–1753.