Total Synthesis of Anti-Influenza Agents Zanamivir and Zanaphosphor via Asymmetric Aza-Henry Reaction

Article abstract of DOI:10.1021/acs.orglett.6b02131

The potent anti-influenza agents, zanamivir and its phosphonate congener, are synthesized by using a nitro group as the latent amino group at C4 for asymmetric aza-Henry reaction with a chiral sulfinylimine, which is derived from inexpensive d-glucono-?-lactone to establish the essential nitrogen-containing substituent at C5. This method provides an efficient way to construct the densely substituted dihydropyran core of zanamivir and zanaphosphor without using the hazardous azide reagent.

Full text of DOI:10.1021/acs.orglett.6b02131

Letter
pubs.acs.org/OrgLett
Total Synthesis of Anti-Influenza Agents Zanamivir and
Zanaphosphor via Asymmetric Aza-Henry Reaction
†
,†,‡
Long-Zhi Lin and Jim-Min Fang*
†Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
‡The Genomics Research Center, Academia Sinica, Taipei 115, Taiwan
*
S Supporting Information
ABSTRACT: The potent anti-influenza agents, zanamivir and its
phosphonate congener, are synthesized by using a nitro group as the
latent amino group at C4 for asymmetric aza-Henry reaction with a
chiral sulfinylimine, which is derived from inexpensive D-glucono-δ-
lactone to establish the essential nitrogen-containing substituent at C5.
This method provides an efficient way to construct the densely
substituted dihydropyran core of zanamivir and zanaphosphor without
using the hazardous azide reagent.
easonal influenza epidemics have been a serious health
problem to humans. Due to their high genetic variability,
zanamivir was simultaneously introduced by a 1,3-dipolar
cycloaddition between methyl acrylate (C1−C3 fragment) and
a chiral nitrone compound (C4−C9 fragment) that is derived
from inexpensive D-glucono-δ-lactone. This synthetic route
requires 17 steps to produce zanamivir in low yield (∼2%). In
Shibasaki’s synthesis, the C5−C6 strategic bond of zanamivir
is established by an asymmetric nitroaldol reaction (also known
as Henry reaction) between 4-nitro-1-butene and (E)-4-
methoxybenzyloxy-2-butenal, using the chiral heterobimetallic
S
influenza viruses may also mutate to unprecedented strains to
cause global and cross-species infections, thus claiming a vast
number of lives and causing huge economic losses. In addition
to using vaccines to prevent influenza infections, an effective
medical treatment for infected patients is to administer
14
1
,2
neuraminidase (NA) inhibitors, such as zanamivir (1),
3
,4
5,6
7,8
oseltamivir, peramivir, and laninamivir. Neuraminidase
is responsible for breaking the linkage between the influenza
virus and the sialo receptor of host cells, so that the newly
formed virus particles can be released to infect other cells. At
present, the orally available oseltamivir is the most popular anti-
influenza drug; however, emergence of oseltamivir-resistant
catalyst prepared from Nd O(O-i-Pr)13, NaN(SiMe ) , and a
5 3 2
chiral amide-based ligand. Although the nitroaldol reaction is
highly enantioselective, over 10 steps are required to convert
the C2−C3 and C7−C8 double bonds to oxoacid and glycol
moieties, respectively, to complete the synthesis of zanamivir.
9
15
viruses may limit its clinic use. In contrast, the influenza viruses
In Ma’s synthesis, an asymmetric Michael reaction of acetone
are rarely resistant to zanamivir because it carries a glycerol side
chain, as that in the structure of sialic acid (also known as N-
acetylneuraminic acid, Neu5Ac), for essential binding of
influenza hemagglutinin with host cells.
to (Z)-tert-butyl (2-nitrovinyl)carbamate is carried out by the
catalysis of chiral amine. The product containing the C1−C5
fragment is then subjected to asymmetric Henry reaction with
an aldehyde (C6−C9 fragment), which is prepared from
inexpensive D-araboascorbic acid. At the final stage, the C1
methyl group is oxidized to carboxylic acid to give zanamivir in
1
8% overall yield through 13 linear synthetic steps.
In our synthetic design of zanamivir (Figure 1), nitro-
methane was utilized as the pivotal C4 center to connect the
C5−C9 fragment E and the C1−C3 fragment D (W′ =
1
CO Et) via asymmetric aza-Henry reaction and alkylation
2
reaction, respectively. The aza-Henry reaction would provide
the two essential nitrogen-containing substituents at the C4 and
Although the nine-carbon monosaccharide structure of
zanamivir is not particularly complex, its densely substituted
dihydropyran core structure, with five consecutive stereogenic
centers, still demonstrates a synthetic challenge. Zanamivir was
16
C5 positions in the desired absolute configuration. The nitro
14−16
group is a latent amino group;
therefore, using a hazardous
azide reagent is avoided in this synthetic route to zanamivir.
The chiral imine E could be easily prepared from D-glucono-δ-
lactone (3), a carbohydrate chiral pool. Furthermore, this
10
first synthesized from sialic acid, and this synthetic method
11,12
has been modified for industrial manufacture.
Three
synthetic methods without using the relatively expensive sialic
13−15
acid as the starting material have also been explored.
In
Received: July 22, 2016
1
3
Yao’s synthesis, the C3−C4 linkage and C2 oxy group of
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02131
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 1. Synthesis of Zanamivir
Figure 1. Retrosynthetic analysis of zanamivir (1) and zanaphosphor
(2).
a
Reagents and conditions: (a) (MeO) CMe , p-TsOH, Me CO,
2
2
2
MeOH, rt, 9 h, 92%; (b) LiAlH , THF, rt, 4 h; (c) NaIO ,
synthetic approach could be applied to the synthesis of
4
4
NaHCO3(aq), CH Cl , rt, 5 h; (d) (R)-tert-butylsulfinamide, Ti(OEt) ,
2
2
4
zanaphosphor (2) by using the phosphonate reagent D [W′ =
2
THF, 65 °C, 9 h, 80% (three steps); (e) CH NO , TBAF (cat.), rt, 1
3
2
PO(OEt) ] instead of the carboxylate reagent D .
2
1
h, 99%; (f) BrCH C(CH )CO Et, Et N, THF, 40 °C, 24 h, 96%
2
2
2
3
Phosphonate groups are generally employed as a bioisostere
(
5
7a/7b = 1:1); (g) Et N, THF, 40 °C, 24 h; (h) 12 M HCl, MeOH,
3
1
7,18
of carboxylate in drug design.
Zanaphosphor has been
0 °C, 30 min, then Ac O, NaOEt, pH ≈ 7, rt, 10 min; (i) O , MeOH,
2
3
19
shown to exhibit higher anti-influenza activity than zanamivir
CH Cl , −78 °C, 15 min, then Me S, rt, 2 h; (j) Ac O, Et N, DMAP,
CH Cl , rt, 24 h, 55% (three steps); (k) Zn, HOAc, EtOH, 80 °C, 30
2 2
2
2
2
2
3
because the phosphonate ion has stronger electrostatic
interactions with the three arginine residues (R118, R292,
and R371) in the active site of influenza neuraminidase. So far,
only one synthetic method of zanaphosphor has been
min; (l) TMSOTf, CH CN, rt, 16 h; (m) MeS−C(NBoc)NHBoc,
3
HgCl
, Et
N, CH
Cl
2
, rt, 2 h, 67% (three steps); (n) NaOH(aq)
,
2
3
2
MeOH, rt, 30 min; (o) TFA, CH Cl , rt, 1 h, 95% (two steps).
2
2
19
accomplished by using sialic acid as the starting material.
Thus, our present method would represent the first synthesis of
zanaphosphor without using sialic acid.
synthesize 7a/7b by the aza-Henry reaction of 5 with ethyl 2-
methylene-4-nitrobutanoate (G) (eq 1, SI). However, the
reaction failed because reagent G decomposed under various
conditions (e.g., 1 M NaOH(aq), K CO in CH CN or THF,
Scheme 1 shows our synthesis of zanamivir. According to the
20
previously reported procedure, D-glucono-δ-lactone (3) was
first converted to a derivative 4 bearing acetal and ester groups.
2
3
3
NaH in THF, and TBAF in THF).
The ester group of 4 was reduced with LiAlH , and the diol
Fortunately, the (4S)-isomer 7a was readily extracted from
the diastereomeric mixture by n-hexane, while the (4R)-isomer
7b was retained as a solid residue. The X-ray diffraction analysis
of 7b confirmed its (4R,5R,6R,7S,8R)-configuration (Figure
4
product was subjected to oxidative cleavage with NaIO to give
4
an aldehyde intermediate, which reacted with (R)-tert-
2
1
butylsulfinamide in the presence of Ti(OEt) to give the
4
chiral sulfinylimine 5 in 80% overall yield after a simple
chromatographic purification to remove the residual titanium
and aluminum salts. The asymmetric aza-Henry reaction of 5
with nitromethane proceeded smoothly by the catalysis of
tetrabutylammonium fluoride (TBAF) to afford 99% yield of
the addition product 6 exclusively in the (5R)-configuration.
The structure of 6 was confirmed by the X-ray diffraction
analysis (Figure S1A, Supporting Information). The (R)-tert-
butylsulfinamide appeared to be an excellent chiral auxiliary to
render the remarkable stereoselectivity in the aza-Henry
S1B, SI). In the presence of Et N, the strong electron-
withdrawing nitro group rendered a base-catalyzed epimeriza-
tion at the C4 stereocenter (Table 1). For example (entry 3),
3
the recovered 7b was treated with Et N (14 equiv) in THF at
3
40 °C for 48 h to give a mixture of 7a and 7b (57:43) as shown
1
by H NMR analysis. The H-4 of major isomer 7a occurred at δ
5.12 (dt, J = 9.0, 4.5 Hz), whereas the corresponding proton of
minor isomer 7b exhibited at δ 5.06 (dt, J = 10.2, 3.2 Hz). The
desired (4S)-epimer 7a turned out to be the thermodynamically
favored product after treatment with Et N for a prolonged
3
1
6
reaction.
period (entries 3−5). The epimer 7a was simply separated by
extraction with n-hexane, and the recycling of 7b could be
repeated to give additional product 7a.
Another key step was to extend the carbon chain by
alkylation of sulfinamide 6 with ethyl 2-(bromomethyl)acrylate
(
D ). The reaction was promoted by excess Et N (5.5 equiv) in
The acetal and sulfinylimine groups of 7a were hydrolyzed
under strongly acidic conditions (12 M HCl(aq) in MeOH), and
the exposed amino group was trapped by acetylation. Without
further purification, the product was subjected to ozonolysis to
give an α-oxo ester, which was readily attacked by the C6
hydroxyl group to form a tetrahydropyran ring. All of the
1
3
THF solution (40 °C, 24 h) to give a high yield (96%) of the
alkylation product that contained 7a and 7b in equal
amounts. By using the bulky base (−)-sparteine (2 equiv),
instead of Et N, the alkylation reaction of 6 gave less 7a epimer
2
2
3
(7a/7b = 37:63). We also tested another possible approach to
B
DOI: 10.1021/acs.orglett.6b02131
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Epimerization of 7b and 10b
Scheme 2. Synthesis of Zanaphosphor
Et N
temp
time
(h)
ratio of a/b
a
3
entry
W′ =
(equiv)
(°C)
epimers
b
1
CO Et
7
14
14
14
14
16
16
16
25
40
40
40
40
40
50
50
48
24
48
72
96
24
24
48
14:86
40:60
57:43
63:37
60:40
41:59
50:50
50:50
2
c
2
CO Et
2
c
3
CO Et
2
c
4
CO Et
2
c
5
CO Et
2
d
6
PO(OEt)2
PO(OEt)2
PO(OEt)2
d
7
d
8
a
1
The ratio of 7a/7b was determined by H NMR analysis, whereas the
ratio of 10a/10b was determined by 31P NMR analysis. Compound
b (50 mg, 0.1 mmol) and Et N (0.1 mL, 0.7 mmol) in THF (0.9
b
7
3
c
mL). Compound 7b (50 mg, 0.1 mmol) and Et N (0.2 mL, 1.4
mmol) in THF (0.8 mL). Compound 10b (50 mg, 0.09 mmol) and
3
d
Et N (0.2 mL, 1.4 mmol) in THF (0.8 mL).
3
hydroxyl groups were then acetylated to provide the fully
protected sialoside 8 with retention of the (4S)-configuration
through the three-step reaction sequence. The nitro group in 8
was reduced with activated zinc powder in hot AcOH−EtOH,
followed by elimination of an AcOH molecule on treatment
with trimethylsilyl trifluoromethanesulfonate (TMSOTf), to
form the dihydropyran core of zanamivir. The intermediate
amino compound was further reacted with 1,3-bis(tert-
butoxycarbonyl)-2-methylthiopseudourea in the presence of
a
Reagents and conditions: (a) BrCH C(CH )PO(OEt) , Et N,
2
2
2
3
THF, 40 °C, 48 h, 72% (10a/10b = 1:2); (b) Et N, THF, 40 °C, 24 h;
3
(
c) Zn, AcOH, EtOH, 70 °C, 30 min; (d) 9-fluorenylmethyl
12
chloroformate, NaHCO3(aq), CH Cl , rt, 9 h, 77% (2 steps); (e) 12
2
2
M HCl, MeOH, 50 °C, 30 min, then Ac
min; (f) O , MeOH, CH Cl , −78 °C, 30 min, then Me
Ac O, I (cat.), 40 °C, 48 h, 55% (three steps); (h) Et N, HgCl ,
O, NaOEt (pH ≈ 7), rt, 10
2
S, rt, 2 h; (g)
3
2
2
2
HgCl to afford the guanidination product 9 in 67% overall
2
2
3
2
2
MeS−C(NBoc)NHBoc, CH Cl , rt, 24 h, 86%; (i) TMSBr,
2
2
yield. Saponification of 9, followed by removal of the tert-
butoxycarbonyl (Boc) protecting groups with trifluoroacetic
CH Cl , 0 °C, 24 h; (j) MeONa, MeOH, rt, 1 h, 76% (two steps).
2
2
23
acid (TFA), thus culminated in the synthesis of zanamivir.
Starting from D-glucono-δ-lactone (3), we accomplished the
total synthesis of zanamivir in 14 steps with 12% overall yield,
which does not include the added yield from recycling 7b to
reobtain the (4S)-epimer 7a.
After ozonolysis of the double bond, the presumed α-
oxophosphonate product proceeded to form a tetrahydropyran
H. The subsequent peracetylation was best performed by using
iodine as a mild Lewis acid catalyst, giving 55% yield of the
desired dihydropyran product 12, which was likely derived from
an in situ elimination reaction of intermediate I. In comparison,
In principle, the pivotal nitro compound 6 could be used to
synthesize zanaphosphor by a similar approach (Scheme 2).
Thus, the reaction of 6 with diethyl (3-bromoprop-1-en-2-
yl)phosphonate (D ) was carried out by the promotion of Et N
treatment of H with Ac O and TMSOTf afforded less yield of
2
12 (37%), and an appreciable amount of lactone J was observed
2
3
1
to give the alkylation product that contained two isomers 10a
and 10b in a ratio of 1:2. The stereoselectivity of the alkylation
in the H NMR spectrum, which showed the two diagnostic C3
protons at δ 3.08 (dd, J = 17.7, 6.6 Hz) and 2.55 (dd, J = 17.7,
19,24
reaction was not improved by using either bulky base i-Pr NEt
9.9 Hz). Like Neu5Ac phosphonate esters,
tetrahydropyran
2
(
Hu
̈
nig base) or 1,4-diazabicyclo[2.2.2]octane (DABCO). The
H, that bears both hydroxyl and phosphonate groups at the C2
position, was unstable in both acids and bases (e.g., 1 M
HCl(aq), cat. HClO , concentrated H SO , AcOH, TMSBr,
(
4S)-epimer 10a was isolated by extraction with n-hexane, and
the residual solid (4R)-epimer 10b could be recycled to afford
4
2
4
1
0a by epimerization (Table 1, entries 6−8). For example,
K CO , and Et N), and it readily lost the phosphonate
2 3(aq) 3
epimerization of 10b was carried out by treatment with excess
Et N in THF solution at 50 °C for 24 h to give equal amounts
of 10a and 10b. The (4R,5R,6R,7S,8R)-configuration of 10b
was confirmed by X-ray crystallography (Figure S1C, SI).
The nitro group of 10a was first reduced to an amino group
and then protected as the 9-fluorenylmethoxycarbonyl (Fmoc)
derivative 11. By a procedure similar to that for conversion of
substituent as detected by the resonance of diethyl phosphite at
7.33 ppm in the 31P NMR spectrum. Direct transformation of
the nitro compound 10a by a similar reaction sequence,
including acidic hydrolysis, ozonolysis and peracetylation (steps
e−g in Scheme 2), gave a complicated mixture that contained
phosphonate L and lactone M derived from the intermediate K.
Although elimination of an AcOH molecule from L could be
achieved by using iodine or TMSOTf as the promoter, the
dihydropyran product decomposed on reduction of the nitro
3
7
a to 8, compound 11 was subjected to acid-catalyzed
hydrolysis, followed by acetylation of the intermediate amine.
C
DOI: 10.1021/acs.orglett.6b02131
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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3
guanidination was carried out to give 13 in 86% yield.
Zanaphosphor was thus synthesized from 13 in a one-pot
operation by solvolysis of the phosphonate ester with TMSBr,
followed by the removal of the Boc groups on workup with
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D ) or diethyl (3-bromoprop-1-en-2-yl)phosphonate (D ),
1
2
followed by ozonolysis of the double bond, thus constructed
the densely substituted dihydropyran core of both zanamivir
and zanaphosphor with five consecutive stereogenic centers.
The nitro group was used as a latent amino group, so that no
hazardous azide reagent was required in this synthetic route.
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desired (4S)-compounds 7a and 10a were easily separated from
their (4R)-epimers by extraction with n-hexane. Furthermore,
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*
S
Supporting Information
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The Supporting Information is available free of charge on the
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(
Experimental procedures, NMR spectra, and X-ray
crystallographic data (PDF)
X-ray data for compound 6 (CIF)
(
6
X-ray data for compound 7b (CIF)
X-ray data for compound 10b (CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: jmfang@ntu.edu.tw.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Ministry of Science and Technology for financial
support.
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DOI: 10.1021/acs.orglett.6b02131
Org. Lett. XXXX, XXX, XXX−XXX