A chemoenzymatic synthesis of ceramide trafficking inhibitor HPA-12

Article abstract of DOI:10.3762/bjoc.15.42

A chemoenzymatic synthesis of the title compound has been developed using an efficient and highly enantioselective lipase-catalyzed acylation in a hydrophobic ionic liquid, [bmim][PF₆], followed by a diastereoselective asymmetric dihydroxylatio

Full text of DOI:10.3762/bjoc.15.42

A chemoenzymatic synthesis of ceramide trafficking
inhibitor HPA-12
Seema V. Kanojia1, Sucheta Chatterjee1, Subrata Chattopadhyay1
and Dibakar Goswami*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2019, 15, 490–496.
1Bio-Organic Division, Bhabha Atomic Research Centre, Mumbai
400 085, India and 2Homi Bhabha National Institute, Training School
Complex, Anushakti Nagar, Mumbai 400 094, India
doi:10.3762/bjoc.15.42
Received: 03 October 2018
Accepted: 26 January 2019
Published: 18 February 2019
Email:
Dibakar Goswami* - dibakarg@barc.gov.in
Associate Editor: S. Bräse
* Corresponding author
© 2019 Kanojia et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
AD mix-β; [bmim][PF6]; DDQ; HPA-12; lipase
Abstract
A chemoenzymatic synthesis of the title compound has been developed using an efficient and highly enantioselective lipase-cata-
lyzed acylation in a hydrophobic ionic liquid, [bmim][PF6], followed by a diastereoselective asymmetric dihydroxylation as the key
steps for incorporating the stereogenic centers. The further conversion to the appropriate intermediates and subsequent acylation
with lauric acid furnished the target compound.
Introduction
Ceramides belong to the family of sphingolipids (SLs) and are ever, the initially determined (1R,3R) configuration of the most
synthesized de-novo in the endoplasmic reticulum (ER) [1]. active HPA-12 stereoisomer (compound 1, Figure 1) was later
Once formed, ceramide transport protein (CERT), a 68 kDa revised to (1R,3S) configuration (compound 2, Figure 1) by
cytosolic protein, delivers the compound to the Golgi apparatus Berkeš et al. in 2011 [12]. Since then, HPA-12 has been the
for further conversion to sphingomyelins, which play important subject of extensive biological evaluation. The HPA-12-medi-
roles in cell-signaling pathways [2]. In the last few years, the ated CERT knockdown has been associated with restoration of
downregulation of CERT-mediated ceramide transfer from the cell death in paclitaxel-resistant ovarian cancer cells [3] and
ER to Golgi has gained increased attention in antioncogenic as also with the increased rate of ceramide-induced apoptosis
well as antineurodegenerative therapeutic research [3-10]. How- following UVB irradiation, suggesting the possibility of its use
ever, chemical entities which inhibit CERT are scarce. In 2001, as an anticancer compound. Besides, inhibition of sphingolipid
Hanada et al. reported (1R,3R)-N-(3-hydroxy-1-hydroxymethyl- biosynthesis using HPA-12 has also been reported to inhibit
3-phenylpropyl)dodecanamide (HPA-12, 1, Figure 1) as the hepatitis C virus replication substantially [4]. In another study,
first inhibitor of CERT-mediated ceramide transport [11]. How- the CERT knockdown disrupted the normal oxidative stress
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Beilstein J. Org. Chem. 2019, 15, 490–496.
response in Drosophila [5]. More recently, a fluorinated ana- tion of a propargylic N-hydroxylamine [23], from β-sulfin-
logue of HPA-12 has been studied for its BBB permeability and amido ketones derived from chiral sulfinimines [24], and a
subsequent brain uptake, showcasing its possible use in Kornblum–DeLaMare/aza-Michael reaction of 3,6-dihydro-1,2-
neurodegenerative disorders [9].
dioxines followed by diastereoselective reduction [25]. Most of
these methods employ starting materials or catalysts, which are
not commercially available, and also require operationally
demanding reaction conditions. Hence a need to develop a
simple, efficient and inexpensive synthesis of HPA-12 was felt.
Our own interest in developing anticancer agents also prompted
us to develop a new and practical enantioselective synthesis of 2
[26-29].
To realize our objective, we paid particular attention to obtain 2
using reactions that are high-yielding and can be executed under
simple reaction conditions with commercially available and
inexpensive materials/reagents. In this regard biocatalytic reac-
Figure 1: Structure of most active HPA-12 isomers, originally pro-
posed (1) and revised (2).
However, a limited commercial availability and high cost of tions offer green and sustainable alternative routes to develop
HPA-12 have hindered its application in basic researches in- asymmetric syntheses of pharmaceuticals with varied stereo-
volving CERT inhibition. For this, a number of groups have chemical features [30-33]. Our group has been using lipases for
successfully accomplished the synthesis of HPA-12 [13-25]. the chemoenzymatic syntheses of several bioactive molecules
The first synthesis of HPA-12 comprised [13] a three-compo- [34-39]. The prevalence of PhCH(OH) in 2, and in many other
nent asymmetric Mannich reaction catalyzed by a chiral zirco- biochemicals, attracted our attention to formulate an enantiose-
nium catalyst. However, after the structural revision of the most lective lipase-catalyzed transacetylation strategy to obtain a
active stereoisomer, Kobayashi et al. synthesized (1R,3S)-HPA- suitable molecule bearing the designated chiral segment (com-
12 (2, Figure 1) using a Zn-catalyzed asymmetric Mannich-type pound 2, vide infra).
reaction in water, and unambiguously ascertained the revised
configuration by X-ray crystallography [14]. The other synthe- Results and Discussion
ses of (1R,3S)-HPA-12 (2) used the chiral pool approach For the synthesis, commercially available benzaldehyde (3) was
[15,16], crystallization-induced asymmetric transformation allylated using Zn/allyl bromide in moist THF following
[17], diastereoselective reduction of γ-aryl-γ-oxo-β-amino Luche’s protocol to obtain the homoallylic alcohol (±)-4
alcohol [18], cycloaddition of oxime with alkenes [19], enantio- (Scheme 1) [40]. We envisaged that the enantiomers of 4 were
selective carbonyl reduction followed by an organocatalyzed ideal substrates for the asymmetric synthesis of different stereo-
α-amination reaction [20], tandem approach from (S)-Wynberg isomers of 2. Several biocatalytic protocols for the preparation
lactone [21], chiral ruthenium-catalyzed N-demethylative rear- of (R) or (S)-4 were reported earlier. The hydrolysis of the cor-
rangement of 1,2-isoxazolidines [22], gold(I)-catalyzed cycliza- responding acetate with crude enzyme preparations from pig
Scheme 1: Lipase-catalyzed trans-acylation of (±)-4 and subsequent Mitsunobu inversion. Conditions: (i) Zn/THF/allyl bromide/aqueous sat. NH4Cl/
25 °C/3 h; (ii) vinyl acetate/lipase (Table 1); (iii) KOH/EtOH/25 °C/6 h; (iv) Ph3P/DIAD/p-nitrobenzoic acid/THF; (v) KOH/EtOH/25 °C/8 h.
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Beilstein J. Org. Chem. 2019, 15, 490–496.
liver and chicken liver esterase proceeded with modest enantio- acetylation of (±)-4 with vinyl acetate in diisopropyl ether using
selectivity [41,42]. Rhizopus arrhizus-mediated hydrolysis of Candida rugosa lipase (CRL) and Pseudomonas fluorescens
the acetate furnished the enantiomerically pure alcohol (99% lipase (PFL) were futile (Table 1, entries 2 and 3). In contrast, a
ee), however, the enantiomeric excess (ee) of the antipode better yield and enantiocontrol was achieved using Amano
acetate was very poor (5–9%) [43]. On the other hand, the lipase from P. fluorescens (Amano PFL) in conjunction with
Amano PS lipase-catalyzed trans-acylation of (±)-4 furnished vinyl acetate in diisopropyl ether, where the (R)-acetate 5 and
(S)-4 with good ees, while the ee of the (R)-acetate was poor (S)-alcohol 4 were obtained in 85% and 88% ee, respectively, at
[44]. The acylase 1-catalyzed resolution protocol gave (S)-4 in a 48% conversion after 192 h (Table 1, entry 4). The efficacy of
poor % ee [45]. A unified approach where both the enantiomers Amano PFL for the resolution of (±)-4 is in corroboration with
of 4 are obtained in good optical purities is rare. This is impor- the reported method using Amano lipase PS [44]. However,
tant as the individual enantiomers can be converted to the since the reaction was very slow, we attempted to improve the
antipodes by Mitsonobu inversion [46], thereby maximizing the reaction rate by carrying out the resolution at 50 °C, near the
yield of the desired enantiomer. In addition, the availability of optimum temperature of the enzyme (55 °C). Under these
both the enantiomers of 4 would be useful for the synthesis of conditions, a 50% conversion was achieved within 48 h to
all diastereomers of 2 (as per our synthetic plan) and also obtain both (R)-5 and (S)-4 in 90% ee (Table 1, entry 5). A
several other pharmacologically important compounds. To the second acetylation (12% conversion, 20 h) of the resolved (S)-4
best of our knowledge, only the lipase PS-catalyzed acylation of under the same conditions improved its ee to 99% (Table 1,
(±)-4 proceeded with good to excellent ees for both the acetate entry 6). The products were isolated by filtering the insoluble
and alcohol. However, details of the protocol are unavailable, enzyme from the reaction mixture, followed by concentration of
and also the resolution was quite slow [47]. Hence, we screened the filtrate. The obtained residue was subjected to silica gel
different lipases for the trans-acetylation of (±)-4 for its effec- column chromatography to isolate unreacted alcohol and acety-
tive resolution and the results are summarized in Table 1. Based lated product. We have reused the recovered lipase at least three
on our past experience in lipase-catalyzed resolution of homoal- times without any significant loss of enzyme activity.
lylic alcohols [38], initially a Novozym 435®-catalyzed acetyla-
tion of (±)-4 with vinyl acetate in diisopropyl ether (DIP) was There are similar instances in literature [48] for a substantial
attempted. However, the yield and enantioselectivity of the slow-down of the transesterification reaction rate in organic sol-
desired alcohol (S)-4 were very poor (Table 1, entry 1). Also the vents. This issue can be overcome by using room temperature
Table 1: Lipase-catalyzed resolution of (±)-4.
Entry Lipase
Acylating Solvent
agent
Time
(h)
%
% ee of
% ee of
(R)-5a
% cb Yield of
Yield of
Ed
Conversiona (S)-4a
enriched enriched
(S)-4c
76
(R)-5c
16
1
2
3
4
5
6
7
8
Novozym 435 vinyl
DIP
96
72
96
20
23
26
22
19
35
88
90
99
ND
92
71
68
89
85
90
ND
ND
91
23.7
21.8
28.2
50.9
50.0
ND
7.21
6.35
24.58
35.40
58.75
ND
acetate
CRL
vinyl
DIP
DIP
DIP
DIP
DIP
71
20
acetate
Lipase from
vinyl
67
21
P. fluorescens acetate
Amano PFL
Amano PFL
Amano PFL
Amano PFL
Amano PFL
vinyl
acetate
192e 48
45
42
vinyl
acetate
48f
20
48
6f
50
46
43
vinyl
acetate
12g
10e
48
82
10
vinyl
acetate
[bmim]
[BF4]
ND
ND
45
ND
42
ND
vinyl
[bmim]
[PF6]
50.3
68.62
acetate
aExperimentally determined from chiral HPLC analysis using AD-H column and 5% isopropanol/hexane as eluent @1.0 mL min−1, λ = 254 nm. bThe
% c values were calculated from the enantiomeric excess of the starting material (ees) and the product (eep) according to % c = ees/(ees + eep).
cIsolated yield. dThe enantiomeric factor (E) was calculated from the enantiomeric excess of the starting material (ees) and the c value according to
E = ln[(1 − c)(1 − ees)]/ln[(1 − c)(1 + ees). eReaction done at 25 °C. fReaction done at 50 °C. gReaction done on the partially resolved alcohol ob-
tained from the previous entry.
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Beilstein J. Org. Chem. 2019, 15, 490–496.
ionic liquids, which not only substitute the environment All the reactions were carried out at least 3–4 times and the best
damaging organic solvent, but also increase the reaction rate, results are presented in Table 1. To the best of our knowledge,
and provide many other technological advantages [49]. Towards this is the first attempt towards the resolution of (±)-4 in a room
this, we have chosen [bmim][BF4] and [bmim][PF6] as two temperature ionic liquid at an elevated temperature.
model ionic liquids. Of them, [bmim][BF4] is water soluble, and
[bmim][PF6] is immiscible with water. The acetylation of (±)-4 To make the synthesis enantio-convergent, and also to offset the
with vinyl acetate in [bmim][BF4] using Amano PFL was very limitations of a resolution-based protocol, (R)-5 was hydro-
slow (Table 1, entry 7), showing only 10% completion after lyzed with alcoholic KOH to furnish (R)-4 (Scheme 1). Its
48 h. Carrying out the reaction at 50 °C also did not improve the inversion under the Mitsunobu conditions (Ph3P/DIAD/p-
reaction rate. However, the acetylation in [bmim][PF6] at 50 °C nitrobenzoic acid/THF; KOH/EtOH/25 °C/8 h, 91% over two
using vinyl acetate and Amano PFL was much faster (Table 1, steps) gave (S)-4 [46]. The benzylation of the hydroxy function
entry 8), and a 48% conversion was achieved in only 6 h to in (S)-4 with benzyl bromide (BnBr) and Bu4NI in the presence
obtain (R)-5 and (S)-4 in 91% and 92% ee, respectively. This of NaH produced compound 6 (Scheme 2). This was subjected
kind of dependency of the reaction rate on the nature of the to asymmetric dihydroxylation (ADH) using AD mix-β
anion in the ionic liquid has been reported earlier [50]. Whatso- [K2OsO2(OH)4 and (DHQD)2-PHAL]. The reaction proceeded
ever, the reaction protocol in [bmim][PF6] was fruitful for the predominantly from the α-face, resulting in the formation of the
successful resolution of racemic (±)-4. The products were 1,3-anti diol 7a and 1,3-syn diol 7b in a 91:9 ratio (based on the
extracted from the [bmim][PF6] medium with diethyl ether and isolated yields of 7a and 7b, separated by column chromatogra-
the ethereal phase was concentrated. The residue was then sub- phy). Previously the ADH reaction of a homologue of 6, bear-
jected to silica gel column chromatography to obtain the unre- ing a methyl substitution and a hydroxy group (instead of
acted alcohol and the acetate. We have reused the recovered benzyloxy group) with AD mix-β also produced the correspond-
lipase in ionic liquid at least three times without any significant ing α-alcohol [53]. However, unlike in our case, the reaction
loss of enzyme activity.
proceeded with poor diastereoselectivity irrespective of the
dihydroxylating agent used. To confirm the 1,3-anti diol stereo-
The % ee of the (R)-5 and (S)-4 were determined from chiral chemistry of 7a, it was debenzylated using DDQ/CH2Cl2–H2O
HPLC analyses (150 mm × 4.6 mm, 5 μm, chiral AD-H to furnish the trihydroxy compound 7a'. The 1H and 13C NMR
column, 5% isopropanol/hexane @ 1.0 mL min−1, UV detec- spectra, and the optical rotation of 7a' were in conformity with
tion at 254 nm). The absolute configurations of (R)-5 and (S)-4 those reported [54]. In addition, the 1,3-anti stereochemistry of
were assigned by comparison of the chiroptical data with those the diol 7a was confirmed by converting it to the target com-
reported [45]. The stereochemical outcome of the reaction is pound 2, and comparing its chiroptical data with the reported
consistent with Kazlauskas’ empirical rule [51]. The conver- values, as described afterwards.
sion (% c) and the enantiomeric excess (E) values were calcu-
lated according to the method described by Sih et al. [52]. For the synthesis of 2, the primary hydroxy function of 7a was
Besides the high yield and % ee, the biocatalytic protocol is benzoylated to get 8. Compound 8 was mesylated with
operationally simple and convenient. We carried out the trans- methanesulfonyl chloride (MsCl)/Et3N and the product reacted
formation using inexpensive vinyl acetate as the acyl donor due with NaN3/DMF at 90 °C to obtain the azide 9 (Scheme 2). We
to its volatility that would assist easy isolation of the products. first attempted to convert 9 to the target compound 2 by i)
Scheme 2: Synthesis of azide 9 from (S)-4. Conditions: (i) NaH/Bu4NI/BnBr/THF/25 °C/4 h; (ii) AD-mix-β/t-BuOH–H2O 1:1/0 °C/72 h; (iii) DDQ/
CH2Cl2–H2O 4:1/3 h; (iv) Et3N/BzCN/0 °C/CH2Cl2/2 h; (v) a) MsCl/Et3N/CH2Cl2/0 °C/2.5 h, b) NaN3/DMF/90 °C/3 h.
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converting the azide group to the amine using LiAlH4 with con- reaction as the key steps. The synthesis was accomplished by
comitant debenzoylation, followed by the acylation of the employing reactions that use inexpensive reagents, are opera-
amine with lauric acid to afford 9a, and finally, ii) reductive tionally simple and proceed with good to excellent yields and
cleavage by hydrogenolysis using Pd–C/H2 leading to debenzy- excellent stereoselectivities. The target compound was obtained
lation (Scheme 3). However, during hydrogenolysis, the elimi- in 23% overall yield starting from (S)-4. The protocol can also
nation of the -OBn group led to product 9b, which was undesir- be used to access different HPA analogues and derivatives.
able. A similar elimination was earlier observed by Sharf et al.
during hydrogenolysis of dibenzyl ether [55]. To avoid this, an
Supporting Information
oxidative debenzylation of 9a using DDQ/CH2Cl2–H2O was
carried out. However, this led to a very poor yield of the target
Supporting Information File 1
compound 2 (Scheme 3). Hence, we decided to debenzylate
compound 9 prior to its reduction to the amine and subsequent
acylation. Towards this (Scheme 4), oxidative debenzylation of
9 using DDQ yielded 10 in 84% yield. Treatment of 10 with
LiAlH4 led to the reduction of the azide group to amine along
with debenzoylation to furnish an intermediate aminodiol,
which, without further purification, was acylated with lauric
acid in the presence of dicyclohexylcarbodiimde (DCC) and
4-dimethylaminopyridine (DMAP) to give the target compound
HPA-12 (1R,3S)-2. The optical and spectroscopic data of com-
pound 2 were in accordance with those reported [18].
Experimental details and analytical data.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-42-S1.pdf]
Supporting Information File 2
NMR spectra.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-15-42-S2.pdf]
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