Studies toward asymmetric synthesis of leiodelide A

Article abstract of DOI:10.1016/j.tetlet.2014.10.102

An enantioselective route for oxazoline 4, a key fragment toward the asymmetric synthesis of leiodelide A, is described. We synthesized northern subunit 6 through a Julia-Lythgoe olefination and subsequent Sharpless asymmetric dihydroxylation. Moreover, a highly diastereoselective method using well-established Evans' asymmetric aldol condensation was developed for preparation of southern fragment 5. The additional feature of this synthetic route is the formation of oxazoline 4 through DAST-promoted cyclization of the amidation product from subunits 5 and 6.

Full text of DOI:10.1016/j.tetlet.2014.10.102

Tetrahedron Letters 55 (2014) 6903–6906
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Studies toward asymmetric synthesis of leiodelide A
⇑
Rong-Guo Ren, Ming Li, Chang-Mei Si, Zhuo-Ya Mao, Bang-Guo Wei
Department of Chemistry and Institutes of Biomedical Sciences, Fudan University, 220 Handan Road, Shanghai 200433, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An enantioselective route for oxazoline 4, a key fragment toward the asymmetric synthesis of leiodelide
A, is described. We synthesized northern subunit 6 through a Julia–Lythgoe olefination and subsequent
Sharpless asymmetric dihydroxylation. Moreover, a highly diastereoselective method using well-estab-
lished Evans’ asymmetric aldol condensation was developed for preparation of southern fragment 5.
The additional feature of this synthetic route is the formation of oxazoline 4 through DAST-promoted
cyclization of the amidation product from subunits 5 and 6.
Received 9 September 2014
Revised 9 October 2014
Accepted 17 October 2014
Available online 23 October 2014
Keywords:
Natural product
Macrolide
Ó 2014 Elsevier Ltd. All rights reserved.
Asymmetric synthesis
Oxazole
Leiodelide
Introduction
R or S
HO
15
13
O
17 16
OMe
Leiodelides A and B (1 and 2) were isolated from the marine
sponge Leiodermatium at a depth of 740 feet in Palau by Fenical
and co-workers in 2006 (Fig. 1).1 Based on the spectroscopic analy-
sis, chemical modification, and degradation, the structures of lei-
odelides A and B were determined to be an oxazole-containing
19-membered macrolides bearing a 10-carbon side chain with a
carboxylic acid terminus similar to that of okadaic acid.2 Most ste-
reo centers of the two metabolites were assigned based on detailed
interpretation of spectroscopic data, as well as chemical degrada-
tion and application of the modified Mosher ester method. How-
ever, the configuration of C13 remains to be unassigned. Both
compounds exhibit significant cytotoxic activity against HCT-116
O
N
9
HO
O
1
O
OH
4 5
OH
Leiodelide A (1)
R or S
O
O
OMe
O
HO
Br
N
O
OH
O
human colon carcinoma, with IC50 values of 1.4
and 3.8 g/mL (5.6 M), respectively. Moreover, leiodelide A shows
significant cytotoxic activity against HL-60 leukemia (GI50 = 0.26
M), NCI-H522 non-small cell lung cancer (GI50 = 0.26 M), and
OVCAR-3 ovarian cancer (GI50 = 0.25
M) cell lines.1
lg/mL (2.5 lM)
OH
l
l
Leiodelide B (2)
Figure 1. Structures of leiodelides A and B.
l
l
l
Due to the limited supply from nature, as well as their signifi-
cant biological activities and intriguing structures, leiodelides A
and B (1 and 2) have attracted great attention of many chemists.3
The first total synthesis of leiodelide B was accomplished by Fürst-
ner and co-workers,3a and two other groups finished the fragments
of leiodelide A.3b,c Unfortunately, the spectroscopic data of syn-
thetic leiodelide B and its three diastereomers were not identical
to the reported data of natural leiodelide B.3a As one part of our
continuous interests in pursuing some divergent synthesis of
alkaloids,4 and depsipeptides,5 herein we present an asymmetric
synthetic method for the C1–22 fragment of leiodelide A.
Our synthetic strategy for leiodelide A (1) is illustrated in
Scheme 1, in which the side chain could be introduced by Julia olef-
ination3b,6 between C23 and C24, and the macrolactonization could
be conducted by esterification between C1 and C17. Retrosynthetic
analysis led to two key fragments 3 and 4. We envisioned that the
oxazole ring in fragment 4 could be formed by amidation of
⇑
Corresponding author. Tel./fax: +86 21 54237757.
E-mail address: bgw1974@fudan.edu.cn (B.-G. Wei).
northern subunit 6 to
a,b-unsaturated ester 5 and subsequent
http://dx.doi.org/10.1016/j.tetlet.2014.10.102
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

6904
R.-G. Ren et al. / Tetrahedron Letters 55 (2014) 6903–6906
O
OBn
Julia-Lythgoe Olefination
O
NaHMDS
SO2R
Sharpless Asymmetric Dihydroxylation
H
O
O
+
10
HO
THF/HMPA
-78 oC, 62%
O
O
3
Condensation
Julia Olefination
OH
HO
+
12
11
O
TBSO
OMe
O
N
HO
O
Scheme 3. Preparation of olefin 12.
OMe
O
O
OH
N
O
O
Oxidation
Macrolactamization
OTBS
OH
O
OH
OBn
OH
Evans' Asymmetric Aldol Methodology
a
b
OBn
AllylO
O
12
O
OTBS
OH
Leiodelide A 1
O
O
OH
13 dr = 80 :20
OTBS
4
OTBS
O
NHP1
OH
dr = 86:14
14
AllylO
P O
2
+
OTBS
OP3 OMe
O
OTBS
c
d
OBn
6
O
OH
5
O
O
OMe
15
O
OMe
16
Scheme 1. Retrosynthetic analysis of leiodelides A (1).
OTBS
O
oxidation–cyclization or dehydrative cyclization–oxidation
between C8 and C9.7 In addition, the stereogenic centers at C15
and C16 of northern subunit 6 could be generated through Sharp-
less asymmetric dihydroxylation (SAD),8 and the R or S stereogenic
center of C13 could be achieved by stereo-controlled methylation
using Evans’ auxiliary.9
e
OH
O
O
OMe
6
Scheme 4. Preparation of northern subunit 6. Reagents and conditions: (a) AD-mix-
a
, K2CO3, K2OsO4Á2H2O, K3Fe(CN)6, CH3SO2NH2, t-BuOH, H2O, 0 °C to rt, 85%; (b)
TBSCl, imidazole, DMAP, DMF, rt, 71%; (c) NaH, CH3I, DMF, 0 °C, 72%; (d) H2/Pd/
C-Pd(OH)2/C, MeOH, rt, 90%; (e) (i) (COCl)2, DMSO, TEA, DCM, À78 °C; (ii) NaClO2,
NaH2PO4ÁH2O, t-BuOH, 2-methylbut-2-ene, 0 °C, 80% (two steps).
Results and discussion
As shown in Scheme 2, the ester 7, prepared by known
method,10 was reduced with diisobutyl aluminum hydride
(DIBAL-H)11 and the resulting alcohol was treated with benzyl bro-
mide (BnBr) to give the corresponding ether 8 in 74% overall yield.
Removal of the TBS group in 8 with camphor sulfonic acid (CSA) and
subsequent reduction with NaBH4 in the presence of NiCl6ÁH2O12
treated with a solution of NaHMDS19 in dry tetrahydrofuran and
hexamethylphosphoramide (THF/HMPA = 4:1) at À78 °C.
With olefin 12 in hand, we turned our attention to synthesize
the northern subunit 6 (Scheme 4). First, Sharpless asymmetric
dihydroxylation (AD-mix-a, K2OsO4-2H2O, K3Fe(CN)6, K2CO3,
À5.1 (c 1.41); lit.13
[
a]
À5.3
25
25
afforded the alcohol 9 {[
a]
D
D
CH3SO2NH2)8,20 of 12 generated chiral diol 13 with 80:20 diastere-
oselectivity in 85% combined yield. Separation of the two diastero-
mers was found to be difficult at diol stage using flash
chromatography on silica gel, fortunately, they were readily sepa-
rated after the selective protection (TBSCl, imidazole). The desired
silyl ether 14 was obtained with moderate chemical selectivity
(dr = 6:1) in 71% combined yield. Methylation21 (MeI, NaH) of 14
generated the ether 15 in 72% yield. Debenzylation of 15 using
hydrogenation (10% Pd/C) led to the corresponding alcohol 16 in
90% yield. Finally, the desired northern subunit 6 was obtained
after Swern oxidation [(COCl)2/DMSO] and subsequent Pinnick
oxidation (NaH2PO4Á2H2O, NaClO2)22 in 80% overall yield.
(c 1.0)} in 78% overall yield. Further conversion of the alcohol 9
to sulfone 10 was achieved through the Mitsunobu protocol (PT-SH,
DIAD, PPh3)14 and subsequent oxidation using 3-chloroperbenzoic
acid (m-CPBA)15 in 79% overall yield.
The formation of olefin 12 involved the coupling of sulfone 10
with aldehyde 11 using Julia–Lythgoe protocol.6,16 Initially, the sul-
fone 10 was treated with LiHMDS,17 followed by the addition of
aldehyde 11 in THF. To our disappointment, the desired olefination
product 12 was not produced at all while the sulfone substrate 10
was decomposed (Scheme 3). Interestingly, when the base was
switched to KHMDS,18 the desired coupling product could be iso-
lated, despite in low yield and poor selectivity. We screened sev-
eral conditions, and the predominant E isomer could be produced
in 62% yield when a mixture of sulfone 10 and aldehyde 11 was
The synthesis of fragment 5 was shown in Scheme 5. Reduction
(DIBAL-H) of the known ester 1723 and subsequent oxidation with
Swern oxidation generated aldehyde in 78% yield, which was sub-
jected to asymmetric aldol condensation9b to afford the desired
syn-product 18 with high diastereoselectivity (dr >99:1) in 85%
yield. Removal of the protective group of 18 in a mixture of acetic
acid and water (4:1) led to the diol 19 in 62% yield, and subsequent
protection (TBSCl, imidazole) of both hydroxyl groups in 19 affor-
ded 20 in 90% yield. The compound 20 was reduced by lithium
borohydride (LiBH4) to obtain the alcohol, which was then con-
verted to aldehyde through Dess–Martin oxidation24 in 60% overall
yield. The aldehyde was subjected to Wittig reaction, generating
OTBS
b
a
BnO
EtO
OTBS
O
8
7
O
S
O
c
N
BnO
OBn
N
OH
N
N
9
10
Ph
trans-a,b-unsaturated ester 21 in 80% isolated yield. Finally, selec-
tive desilylation of 21 with camphor sulfonic acid (CSA) afforded
the alcohol 22 in 67% isolated yield, which was treated with
TMSOTf25 and 2,6-lutidine at room temperature to give southern
fragment 5.
Scheme 2. Preparation of sulfone 10. Reagents and conditions: (a) (i) DIBAL-H,
toluene, À78 °C; (ii) NaH, BnBr, THF, 0 °C, 74% (two steps). (b) (i) CSA, MeOH, rt; (ii)
NiCl2Á6H2O, MeOH, NaBH4, 0 °C, 78% (two steps). (c) (i) DIAD, PPh3, 1-phenyl-1H-
tetrazole-5-thiol, 0 °C to rt; (ii) m-CPBA, DCM, rt, 79% (two steps).

R.-G. Ren et al. / Tetrahedron Letters 55 (2014) 6903–6906
6905
Bn
available material. An asymmetric synthetic method for northern
subunit 6 was achieved using Julia olefination and Sharpless asym-
metric dihydroxylation as key steps to form C15–C16 double bond
and the stereogenic centers at C15 and C16, respectively. More-
over, using well-established Evans’ asymmetric aldol methodology,
we developed a highly diastereoselective synthesis of southern
fragment 5. Further efforts on the total synthesis of leiodelide A
(1) is on-going in our laboratory.
O
BocN
a
b
O
O
OEt
N
O
NBoc
O
OH
O
17
18 dr > 99:1
Bn
Bn
N
NHBoc
NHBoc
c
O
N
OH
O
OTBS
O
OH
O
O
OTBS
O
Acknowledgments
20
19
We thank the National Natural Science Foundation of China
(21072034, 21272041, 21472022), and Key Laboratory of Synthetic
Chemistry of Natural Substances, SIOC of Chinese Academy of Sci-
ences for financial support. The authors also thank Dr. Han-Qing
Dong (Arvinas Inc.) for helpful suggestions.
NHBoc
OTBS
d
e
AllylO
O
OTBS
21
NHBoc
NH2
AllylO
f
AllylO
OH
Supplementary data
O
OTBS
22
OH
O
OTBS
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.10.
102.
5
Scheme 5. Preparation of southern subunit 5. Reagents and conditions: (a) (i)
DIBAL-H, toluene, À78 °C; (ii) (COCl)2, DMSO, TEA, DCM, À78 °C; (iii) (R)-4-benzyl-
3-propionyloxazolidin-2-one, Bu2BOTf, TEA, CH2Cl2, À78 °C, 66% (three steps); (b)
AcOH, H2O, 70 °C, 62%; (c) TBSCl, imidazole, DMAP, DMF, 0 °C, 90%; (d) (i) LiBH4,
MeOH, Et2O, 0 °C; (ii) DMP, DCM, rt; (iii) DCM, allyl 2-(triphenylphosphoranylid-
ene)acetate, rt, 48% (three steps); (e) CSA, MeOH, À45 °C, 67%; (f) TMSOTf,
2,6-lutidine, DCM, rt, in quantitative crude yield.
References and notes
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OH
O
OTBS
a
AllylO
+
6
5
N
H
O
66%
O
OTBS
O
OMe
O
23
TBSO
OMe
N
b
O
O
Leiodelide A
70%
O
AllylO
OTBS
4
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Scheme 6. Preparation of oxazoline 4. Reagents and conditions: (a) DEPBT, TEA,
THF, rt, 66%; (b) DAST, DCM, À78 °C, 70%.
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With both southern fragment 5 and northern subunit 6 in hand,
we started to investigate the amide coupling and subsequent for-
mation of oxazoline (Scheme 6). We tried several traditional cou-
pling reagents, such as HATU, FDPP,26 all of them gave the
amidation product 23 in quite low yields. Fortunately, 3-(diethoxy-
phosphoryloxy)-1,2,3-benzotriazin-4(3H)-one DEPBT)27 could
realize the amide coupling of 5 and 6 in a much higher yield
(66%). The subsequent formation of the oxazoline ring was
achieved in 70% yield by treatment of 23 with diethylaminosulfur-
trifluoride (DAST).28 At the stage of compound 4,29 we have suc-
cessfully established all stereo centers and oxazoline ring for the
macrolide unit of leiodelide A.
Conclusion
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In summary, we completed the enantioselective synthesis of the
20. Marshall, J. A.; Beaudoin, S. J. Org. Chem. 1994, 59, 6614.
key intermediate 4 for leiodelide A (1) from cheap commercially

6906
R.-G. Ren et al. / Tetrahedron Letters 55 (2014) 6903–6906
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29.
[
a
]
D
25 = À12.2 (c 0.21, CHCl3); IR (film): mmax 3416, 2923, 1714, 1649, 1586,
1557, 1462, 1382, 1258, 1039 cmÀ1
;
1H NMR (400 MHz, CDCl3) d 7.05 (dd,
J = 16.0, 7.2 Hz, 1H), 6.00–5.93 (m, 1H), 5.84 (dd, J = 15.8, 1.3 Hz, 1H), 5.37–5.33
(m, 2H), 5.26 (dd, J = 15.6, 1.2 Hz, 1H), 4.66–4.65 (m, 2H), 4.59–4.54 (m, 1H),
4.36 (m, 1H), 4.31–4.26 (m, 2H), 4.04–3.96 (m, 2H), 3.94–3.88 (m, 2H), 3.51–
3.50 (m 3H), 3.39–3.37 (m, 1H), 2.50–2.45 (m, 1H), 2.37–2.30 (m, 2H), 1.69–
1.63 (m, 8H), 1.59–1.57 (m, 4H), 1.44 (s, 3H), 1.36 (s, 3H), 1.17–1.06 (m, 2H),
1.08–1.06 (m, 3H), 0.97–0.96 (m, 3H), 0.91-0.88 (m, 21H), 0.11–0.02 (m, 12H)
ppm; 13C NMR (100 MHz, CDCl3) d 168.9, 166.3, 151.8, 135.5, 132.4, 128.7,
120.7, 117.9, 107. 9, 82.4, 75.6, 71.9, 70.9, 70.1, 65.7, 64.9, 60.3, 43.4, 40.2, 33.4,
29.4, 26.5, 26.0, 25.8, 25.6, 25.0, 19.7, 18.1, 13.7, 12.7, À4.2, À4.4, À4.9 ppm;
HRMS (ESI) calcd for [C40H73NO8Si2+H+]: 752.4953, found: 752.4930.
26. (a) Sheehan, J.; Cruickshank, P.; Boshart, G. J. Org. Chem. 1961, 26, 2525;
(b) Chen, S.; Xu, J. Tetrahedron Lett. 1991, 32, 6711.