Organocatalytic route to enantioselective synthesis of ceramide trafficking inhibitor HPA-12

Article abstract of DOI:10.1016/j.tetlet.2016.04.087

A new organocatalytic approach to the synthesis of ceramide trafficking inhibitor HPA-12 has been described starting from phenacyl bromide. The strategy involves chiral CBS reduction of γ-ketoester and proline-catalyzed α-amination reaction of aldehyde fo

Full text of DOI:10.1016/j.tetlet.2016.04.087

Accepted Manuscript
Organocatalytic route to enantioselective synthesis of ceramide trafficking in-
hibitor HPA-12
Komal G. Lalwani, Arumugam Sudalai
PII:
S0040-4039(16)30461-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.04.087
TETL 47587
Reference:
Tetrahedron Letters
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Accepted Date:
5 February 2016
18 April 2016
21 April 2016
Please cite this article as: Lalwani, K.G., Sudalai, A., Organocatalytic route to enantioselective synthesis of ceramide
trafficking inhibitor HPA-12, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.04.087
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Graphical Abstract
Organocatalytic route to enantioselective synthesis of
ceramide trafficking inhibitor HPA-12
Komal G. Lalwani, Arumugam Sudalai
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1
Tetrahedron Letters
Organocatalytic route to enantioselective synthesis of ceramide trafficking inhibitor
HPA-12
Komal G. Lalwani, Arumugam Sudalai
Chemical Engineering & Process Development Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, Maharashtra, India- 411008
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A new organocatalytic approach to the synthesis of ceramide trafficking inhibitor HPA-12 has
been described starting from phenacyl bromide. The strategy involves chiral CBS reduction of γ-
ketoester and proline-catalyzed α-amination reaction of aldehyde followed by reduction as the
key chirality inducing steps.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Organocatalytic
Ceramide
CBS reduction
Proline
α-amination
Introduction:
unavailability and promising biological activity of HPA-12 have
prompted many groups to attempt its laboratory synthesis. The
Sphingolipids are key components of mammalian plasma
membarane that play pivotal role in cell homeostasis and human
disease.1 Ceramides, one of the sphingolipids, are not only the
structural elements of cell membranes but they also can
participate in a variety of cellular signaling which includes
regulating differentiation, proliferation and programmed cell
death of cells.2 Further, they are one of the cell component lipids
that make up sphingomyelin (SM) which is the major lipid in
lipid bilayer. De novo biosynthesis of sphingolipids in
mammalian cells takes place in two steps.3 In the first step,
ceramide is generated at the cytosolic surface of the endoplasmic
reticulum (ER).4 In the second step, ceramide is transported to
the luminal side of Golgi apparatus and converted to
sphingomyelin by sphingomyelin synthase.5 In this biosynthesis
of sphingomyelin, transport of ceramide from ER to the site of
SM synthesis is ATP-dependent.6 It has been shown that the
ceramide transportation protein CERT mediates the non-vesicular
transport of ceramide from the ER to the Golgi and has been
shown to be essential for SM biosynthesis.7 Till the year 2001, no
drug has been discovered that inhibits intracellular trafficking of
sphingolipids except some broad-spectrum inhibitors such as
brefeldin A.8 In 2001, Hanada et al. have reported that the
ceramide analogue HPA-12 (1) potently inhibits CERT-mediated
interorganelle ceramide transport.9 HPA-12 (1) mediated
inhibition of CERT, thus leads to blockage of sphingomyelin
biosynthesis. This antagonism of CERT protein by HPA-12 (1)
has been shown to interfere with (i) Hepatitis C infections;10 (ii)
to inhibit the growth of Chlamydia trachomatis;11 (iii) to sensitize
—can—cer—cells to chemotherapy12 or irradiation.13 Commercial
first total synthesis of HPA-12 was carried out by Kobayashi et
al. in which absolute stereochemistry of HPA-12 was determined
to be (1R, 3R)-HPA-12 (2).14 However, in 2011, Berkeš et al.
reported a synthesis of HPA-12 and also proposed the revision of
stereochemistry of HPA-12 (1) to be (1R, 3S)15 (Figure 1).
Figure 1: Ceramide trafficking inhibitor HPA-12
Since then, several novel syntheses of (1R, 3S)-HPA-12 (1)
have been reported in the literature.14-16 This is because HPA-12
is not commercially available, which significantly hampers basic
research on the role of HPA-12 in health and disease. However,
most of the reported synthetic sequences for HPA-12 suffer from
several drawbacks such as (i) they include specialized chemistry
and rely on catalysts that are not commercially available; (ii)
reaction sequences are lengthy; (iii) chiral pool approaches.
Hence, there is still a need to develop an easy, practical and
versatile approach for the synthesis of HPA-12 and its
derivatives. As part of our program directed towards asymmetric
synthesis of bioactive molecules,17 we became fascinated to
 Corresponding author. Tel.: +91-20-25902565; fax: +91-20-25902676; e-mail: a.sudalai@ncl.res.in

2
Tetrahedron
develop a simple route for the synthesis of (1R, 3S)-HPA-12
oxazaborolidine to give chiral alcohol 4 in 80% yield and 94% ee
(determined from chiral HPLC analysis). The free hydroxy group
was then protected as its benzyl ether (BnBr, Ag2O, dry CH2Cl2,
0-25 °C, 6 h, 95%) to furnish benzyloxy ester 8 followed by
reduction of ester functionality to produce alcohol 9. Next,
alcohol 9 was converted to hydrazino alcohol 3 via a two-step
reaction sequence: (i) oxidation of primary hydroxyl group in 9
using IBX to the corresponding aldehyde 9a, which without
purification was aminated with dibenzyl azodicarboxylate in the
presence of ʟ-proline (10 mol%) to give an aminoaldehyde in
situ. (ii) it was then subjected to reduction with NaBH4 that
afforded the protected amino alcohol 3 apparently as a single
diastereomer in 95% yield [α]25D = -25.9 (c 0.7, CHCl3)]. Finally,
the amino alcohol 3 was hydrogenated using Raney- Nickel as
catalyst to give the corresponding amino diol, which was
immediately subjected to N-acylation with lauroyl chloride to
furnish a colorless crystalline solid of target molecule HPA-12
(1) in 90% yield. The diastereomeric ratio of HPA-12 (1) was
found to be 98:2 (syn:anti) as determined from chiral HPLC
analysis of crude sample [(Chiralpak AD-H, n-hexane/iPrOH,
90:10, 0.5 mL/min)] retention time 12.591 min (3.926%) and
16.384 min (94.071%) for syn-isomer and retention time 14.224
min (0.084%) and 15.681 min (1.919%) for anti-isomer. Crude
sample of HPA-12 (1) was purified by column chromatography
to afford a colorless crystalline solid of target molecule with 92%
(1). The protocol employs chiral CBS reduction of the γ-
ketoester18 and proline catalyzed α-amination of aldehyde
followed by reduction19 as the key reaction steps.
Results and Discussion:
The retrosynthesis of (1R, 3S)-HPA-12 (1) is shown in
Scheme 1. We envisaged that the synthesis of 1 could be
achieved from hydrazino alcohol 3, which in turn could be
prepared from chiral γ-hydroxy ester 4 by ʟ-proline catalyzed α-
amination of aldehyde followed by reduction. The synthesis of
chiral hydroxy ester 4 could be realized by CBS reduction of γ-
keto malonate 5 which in turn could be readily obtained from
phenacyl bromide 6.
ee [α]25 = -32.7 (c 1.0, CHCl3). To improve enantioselectivity
D
further, purified sample of HPA-12 was recrystallized from Et2O:
hexane (1:1). The optical purity of recrystallized (1R, 3S) HPA-
12 (1) was found to be 93.7% ee as determined from chiral
HPLC analysis [α]25D = -33.2 (c 1.0, CHCl3). The spectral data of
(1R, 3S)-HPA-12 (1) were in complete agreement with the
reported values.16b, 21
Scheme 1. Retrosynthetic analysis of HPA-12 (1)
The synthetic sequence for the synthesis of (1R, 3S)-HPA-12
(1) is shown in Scheme 2. It commenced with commercially
available phenacyl bromide 6, which was condensed with tert-
butyl ethyl malonate in basic medium to afford 1-(tert-butyl) 3-
ethyl 2-(2-oxo-2-phenylethyl)malonate (5) in 92% yield.20 The
keto malonate 5 upon basic hydrolysis followed by
Conclusion:
In
summary,
an
organocatalytic
straightforward
enantioselective synthesis of (1R, 3S)-HPA-12 (1) (7 steps with
an overall yield of 36% with 93.7% ee) has been described. The
strategy employed herein mainly comprises of the chiral CBS
reduction of the ketone and α-amination of in situ formed
aldehyde followed by reduction as the key chiral inducing steps.
All of the reactions proceeded stereoselectively giving products
with high enantio- and diastereoselectivity. The synthetic strategy
described herein has potential relevance in the enantio- and
diastereoselective preparation of libraries of related HPA-12
antagonists.
Acknowledgments
We sincerely thank CSIR, New Delhi, India (Indus MAGIC
project CSC-0123) and DST-SERB (no. SB/S1/OC-42/2014), for
financial support. K.G.L. thanks CSIR, New Delhi, India for
senior research fellowship.
Scheme 2. Reactions and conditions: (i) tert-butyl ethyl malonate, NaH, THF,
0 °C, overnight, 92%; (ii) (a) 1M NaOH, EtOH:acetone, 25-55 °C, 3 h; (b)
DME, 80 °C, overnight, 90%; (iii) (R)-(+)-2-methyl-CBS-oxazaborolidine
(10 mol%), BH3.SMe2 (1.5 equiv), THF, -40 °C, 80% (ee 94%); (iv) BnBr,
Ag2O, dry CH2Cl2, 0-25 °C, 6 h, 93%; (v) LiAlH4, THF, 0 °C, 4 h, 85%; (vi)
(a) IBX, EtOAc, reflux, 3 h; (b) BnO2C-N=N-CO2Bn, ʟ-proline (10 mol %),
0–20 °C, 3 h then NaBH4, MeOH, 82% ; (vii) (a) Raney-Nickel, H2 (1 atm),
AcOH (1 drop), MeOH, 16 h; (b) C11H23COCl, Et3N, CH2Cl2, rt, 2 h, 83%
over two steps.
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3
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21. (+)-HPA-12 (1) (after recrystallization): Yield: 90%, colorless
solid; mp: 88-90 C (lit16b mp: 90-91 C); [α]25 = -33.2 (c 1.0,
D
CHCl3) [lit16b [α]25 = -34.8 (c 1.0, CHCl3)]; Optical purity:
D
93.7% ee by chiral HPLC analysis (Chiralpak AD-H, n-
hexane/iPrOH, 90:10, 0.5 mL/min) retention time 12.625 min
(3.137%) and 16.419 min (96.863%); IR (CHCl3, cm-1): υmax 3418,
3290, 3056, 1645; 1H NMR (200 MHz, CDCl3): δ 0.88 (t, J = 6.7
Hz, 3H), 1.26 (br s, 16H), 1.53-1.67 (m, 2H), 1.85-1.97 (m, 1H),
2.00-2.04 (dd, J = 3.4, 5.2 Hz, 1H), 2.16 (t, J = 8.0 Hz, 2H), 3.12
(br s, 1H), 3.67 (d, J = 3.8 Hz, 2H), 3.81 (br s, 1H), 4.02-4.13 (m,
1H), 4.78-4.85 (dd, J = 3.5, 8.6 Hz, 1H), 6.41 (d, J = 6.7 Hz, 1H),
7.29-7.36 (m, 5H); 13C NMR (50 MHz, CDCl3): δ 14.1, 22.7,

4
Tetrahedron
Highlights
synthesis of HPA-12 is described.
R)-(+)-2-methyl-CBS-oxazaborolidine and L-
proline are used as organocatalysts.
-amination of
ketone and aldehyde respectively.