An enantioselective synthesis of the C3-C21 segment of the macrolide immunosuppressive agent FR252921

Article abstract of DOI:10.1016/j.tetlet.2016.05.067

An enantioselective synthesis of the C3-C21 segment of the novel immunosuppressant FR252921 is described. The C12 and C13 stereogenic centers were constructed by a non-aldol process utilizing optically active 4-phenylbutyrolactone. The C18 stereogenic cen

Full text of DOI:10.1016/j.tetlet.2016.05.067

Accepted Manuscript
An enantioselective synthesis of the C3–C21 segment of the macrolide immu-
nosuppressive agent FR252921
Arun K. Ghosh, Samuel Rodriguez
PII:
S0040-4039(16)30599-8
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.05.067
TETL 47686
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
25 April 2016
14 May 2016
17 May 2016
Please cite this article as: Ghosh, A.K., Rodriguez, S., An enantioselective synthesis of the C3–C21 segment of the
macrolide immunosuppressive agent FR252921, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.
2016.05.067
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An enantioselective synthesis of the C3–C21
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FR252921
Arun K. Ghosh,* and Samuel Rodriguez, Department of
Chemistry and Department of Medicinal Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, USA.
s

1
Tetrahedron Letters
An enantioselective synthesis of the C3–C21 segment of the macrolide
immunosuppressive agent FR252921
Arun K. Ghosh, and Samuel Rodriguez
Department of Chemistry and Department of Medicinal Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An enantioselective synthesis of the C3–C21 segment of the novel
immunosuppressant FR252921 is described. The C12 and C13 stereogenic centers
were constructed by
a non-aldol process utilizing optically active 4-
phenylbutyrolactone. The C18 stereogenic center was installed using Braun’s
highly diastereoselective acetate aldol reaction. Other key steps involved Curtius
rearrangement and Horner-Wadsworth-Emmons olefination reactions.
Available online
Keywords:
FR252921, Macrolide,
Synthesis, immunosuppressive,
asymmetric,
aldol reaction,
asymmetric reduction
Organ transplantation is often the last treatment option for
patients with organ failure. Immunosuppressive drugs such as
cyclosporin A and FR506 play a critical role for transplant
patients by suppressing the immune system’s ability to reject
transplanted organs.1,2 For decades, these calcineurin (CN)
inhibitors have been the mainstay post-transplantation
immunosuppressive regimens. However, long-term use of
these drugs is associated with issues ranging from chronic
allograft nephropathy and adverse side effects. Thus,
development of new immunosuppressive agents with a novel
mechanism of action has been a very active area of
research.3,4 In 2003, Fujine and co-workers isolated a novel
immunosuppressive agent, FK252921 (1, Figure 1), from the
culture both of Pseudomonas Fluorescens, no. 408813.5,6
This compound exhibits potent inhibition of both
lipopolysaccharide-stimulated and anti-CD3 mAb-stimulated
splenocyte proliferation in vitro without blocking T-cell
activation. Also, FK252921 shows a strong synergistic effect
with FK506 in vitro and in vivo.7 These results suggest that
the target and mechanism of action of FK252921 is different
from CN inhibitors.
Not surprisingly, the biology and chemistry of FK252921
attracted immense attention from the synthetic and medicinal
communities. FR252921 is a 19-membered macrolactone
with two amide linkages. Initial structural work of FR252921
was carried out by Fujine and co-workers and Pohanka and
co-workers.5,6 It appears that FR252921 is related to
pseudotrienic acid, which is an acid-catalyzed lactone ring
opened product followed by isomerization at C18-22 centers.8
A number of synthetic studies and total syntheses of
FR252921 have been reported. Falck and co-workers
Figure 1. Structure of FR252921 (1) and
pseudotrienic acid (2).
established the absolute configuration through total synthesis.9
Subsequently, a number of other total syntheses of this
compound were reported.10-13 In an effort to explore
biological properties of derivatives of pseudotrienic acid and
FR252921, we sought to develop an asymmetric synthesis of
FR252921. Herein, we report our enantioselective synthesis
of the C3-C21 fragment of FR252921 where the C12 and C13
stereogenic centers were constructed using a non-aldol
———
 Corresponding author. E-mail: akghosh@purdue.edu

2
1
protocol developed utilizing readily available optically
active 4-phenylbutyrolactone in our laboratory.14, 15 The C18
stereogenic center was constructed by highly
diastereoselective acetate aldol reaction as the key step.
and excellent diastereoselectivity by H-NMR (96:4 dr) and
HPLC (92:8 dr) analysis. Interestingly, the oxa-Michael
reaction of 11 with LiHMDS in THF at -78 °C afforded
tetrahydrofuran derivative 12 in 67% yield. Treatment of 12
with a catalytic amount of Zn(OTf)2 (5 mol%) in the presence
of acetic anhydride in toluene at 75 °C afforded the ring
opened product 13 in 95% yield.15 Oxidative cleavage of 13
using a catalytic amount of RuCl3 and NaIO4 as a
stoichiometric oxidant in a mixture of CH2Cl2, acetonitrile
and water as described by Sharpless and co-workers18
afforded carboxylic acid 6 in 82% yield.
a
Our retrosynthesis of FR252921 is show in Figure 1. In
order to incorporate various side chains, we planned to
synthesize the protected 19-membered macrocycle 4. We
envisioned a late stage intramolecular Horner-Wadsworth-
Emmons (HWE) olefination to construct this 19-membered
macrocycle of FR252921. The C3-C21 segment 5, containing
all stereogenic centers, would be synthesized from carboxylic
acid derivative 6, C3-N10 dienamine 7 and β-hydroxyacid 8.
This diene derivative would be obtained from 3-Fmoc-
aminopropionaldehyde and commercially available triethyl-4-
phosphonocrotonate.
Synthesis of C3-N10 dienamine segment 7 is shown in
Scheme 2. Fmoc-protected aminopropanol 14 was oxidized
Scheme 1. Synthesis of acid segment 6
o
with IBX in ethyl acetate at 80 C for 14 h to provide the
corresponding aldehyde in 95% yield. Horner-Wadsworth-
Emmons olefination was carried out by deprotonation of
commercially available triethyl-4-phosphonocrotonate with
o
LiHMDS in THF at 0 C followed by addition of aldehyde at
o
0 C for 1 h which resulted in α,β-unsaturated ester 15 as the
major product in 56% yield.19 DIBAL-H reduction of ester 15
o
in CH2Cl2 at 0 C for 1 h, provided alcohol 16 in 63% yield.
Alcohol 16 was protected as the TBS-ether using TBSCl and
imidazole in the presence of a catalytic amount of DMAP at
Figure 2. Retrosynthesis of FR252921.
Scheme 2. Synthesis of dieneamine segment 7.
As shown in Scheme 1, our synthesis of 12(S),13(R)-acid
segment 6 utilized optically active 4-phenylbutyrolactone 9 as
the key starting material. This was prepared in multigram
quantities by CBS reduction as described by Corey and co-
workers.16,17 Deprotonation of lactone 9 wih LiHMDS at -78
°C followed by reaction with methyl iodide provided the
methylated product 10 in 88% yield. The alkyation is highly
diastereoselctive, providing 10 in a 99:1 dr by HPLC analysis.
Alkylated lactone 10 was reduced by DIBAL-H and the
resulting
lactol
was
treated
with
carbethoxy-
methylenetriphenylphosphorane to afford α,β-unsaturated
ester 11 in 96% yield. Reaction of 11 with KHMDS in THF at
-78 °C furnished the oxa-Michael product 12 in 93% yield

3
o
obtained in only 23% yield. This result was inadequate for
23 C for 12 h to provide TBS-ether in 93% yield. Removal
our synthesis and we explored an alternate acetate aldol route.
As shown in Scheme 4, we investigated Braun’s
asymmetric acetate aldol reaction to install the C18-(R)-
alcohol configuration. Chiral acetate 20 was prepared as
described by Braun and co-workers.22 Acetate derivative 20
of the Fmoc group by exposure to diethylamine in CH3CN at
23 oC for 2 h afforded desired dienamine 7 in 96% yield.
For the synthesis of β-hydroxy acid fragment 8, we
initially explored the feasibility of a NaBH4/tartaric acid
reduction of the corresponding -keto ester. As shown in
Scheme 3, β-keto ester 18 was prepared by aldol reaction of
the corresponding enolate of t-BuOAc with known20 aldehyde
17, followed by oxidation of the resulting racemic alcohol
with IBX in ethyl acetate, providing 18 in 83% yield over 2-
steps. The NaBH4/tartaric acid complex was prepared as
described by Yatagai and Ohnuki.20,21 Reduction of keto ester
with freshly prepared NaBH4/tartaric acid complex (1:1) in
o
o
was deprotonated with LiHMDS in THF at -78 C to 0 C for
o
5 min. This lithium enolate was cooled to -78 C and then
o
treated with MgBr2 at -78 C for 30 min and the resulting
o
magnesium enolate was reacted with aldehyde 17 at -100 C
for 1.5 h.23 This provided aldol product 21 in 94% yield and a
diastereomeric ratio of 96:4 (by HPLC analysis). Removal of
o
the chiral auxiliary by LiOH hydrolysis at 23 C for 12 h
afforded acid 7 in 71% isolated yield.
The assembly of the C3-C21 macrocyclic core is shown
Scheme 3. Synthesis of acid 8 by chiral reduction.
Scheme 5. Synthesis of the C3-C20 segment of FR252921.
THF at -20 °C for 65 h provided β-hydroxy ester 19 in 48%
yield. However, the reduction product showed 57% ee. We
then prepared NaBH4/tartaric acid complex (1:1) in THF and
let the complex age for 144 h. Reduction of 18 with this
complex for 114 h, provided product 19 in 68% ee but the
yield was only 20%. Reduction of 18 with a freshly prepared
NaBH4/tartaric acid complex (1:2 mixture) at -20 °C for 39 h
did not improve enantioselectivity and β-hydroxy ester 19 was
in Scheme 5. Coupling of carboxylic acid 6 with amine 7 in
the presence of EDCI, HOBt, and N-methylmorpholine
(NMM) in a mixture (1:1) of CH2Cl2 and CH3CN at 0 C to
o
23 oC for 12 h afforded the coupling product 22 in 77% yield.
Selective hydrolysis of the ethyl ester in the presence of an
acetate group was achieved by exposure of 22 to Me3SnOH in
dichloroethane (DCE) at 80 oC for 12 h.24 This provided acid
23 selectively in 60% yield. A small amount of deacetylation
product was observed. Acid 23 was converted to the
corresponding amine by Curtius rearrangement.25,26 Thus, acid
23 was treated with diphenylphoryl azide and
Scheme 4. Braun’s aldol route to acid 8.
o
diisopropylethylamine (DIPEA) in CH2Cl2 at 40 C for 30
min. The reaction mixture was then reacted with fluorenyl
alcohol at 40 oC for 12 h to provide amine 24. Amine 24 was
coupled with acid 8 in the presence of HATU, HOBt and
o
NMM in a mixture (1:1) of CH2Cl2 and CH3CN at 23 C for
12 h to provide our desired C3-C21 segment 5 in 55% yield.
In conclusion, we have synthesized the C3-C21 segment
of FR252129 in a highly stereoselective manner. The C12
and C13 stereogenic centers were installed using optically
active 4-phenylbutyrolactone as the key starting material. The
key reactions involved stereoselective methylation, oxa-
Michael reaction of an α,β-unsaturated ester followed by
acyloxonium ion-mediated ring opening. The dienamine
segment
was
synthesized
from
Fmoc-protected
aminopropionaldehyde and Horner-Wadsworth-Emmons
olefination with ethyl phosphonocrotonate. For the C18-
stereogenic center, we explored chiral borane reduction with

4
24. Nicolaou, K. C.; Estrada, A. A.; Zak, M.; Lee, S. H.;
Safina, B. S. Angew. Chem. Int. Ed. 2005, 44, 1378 –1382.
25. Evans, D. A.; Black, C. J. Am. Chem. Soc. 1993, 115,
4497-4513.
26. Ghosh, A. K.; Bischoff, A. and Cappiello, J., Org. Lett.,
2001, 3, 2677-2680.
NaBH4 and tartaric acid. However, enantioselectivity and
product yield were moderate. We then utilized Braun’s
acetate aldol reaction to construct the C18-sterogenic center
efficiently. Further work toward the total synthesis of
FR252129 is in progress.
Acknowledgments
Financial support by the National Institutes of Health and
Purdue University is gratefully acknowledged.
Supplementary Data
Supplementary data associated with this article can be found
in the online version.
References and notes
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5
Highlights





Synthesis of the C3–C21 segment of the immunosuppressant FR252921 is achieved.
The C12 and C13 chiral centers were set by a diastereoselective non-aldol process.
Non-aldol route includes, alkylation, oxo-Michael reaction and ring opening.
The C18 stereogenic center was installed using Braun’s acetate aldol reaction.
Chiral reduction of β-keto-ester with NaBH4 and tartaric acid was investigated.