Enantioselective synthesis of protected nitrocyclohexitols with five stereocenters. Total synthesis of (+)-pancratistatin

Article abstract of DOI:10.1021/jo3022567

2-Methoxymethylpyrrolidine best performed, among several other proline derivatives, to control the enantioselective [3+3] annulation of β-(hetero)aryl-α-nitro-α,β-enals with commercial 2,2-dimethyl-1,3-dioxan-5-one, a procedure that renders highly oxygenated nitrocyclohexanes endowed with five new stereocenters. Use of this reaction allowed the development of a total synthesis of the antitumoral natural product (+)-pancratistatin; it also converted our previous racemic route to tetrodotoxin into an enantioselective one.

Full text of DOI:10.1021/jo3022567

Note
pubs.acs.org/joc
Enantioselective Synthesis of Protected Nitrocyclohexitols with Five
Stereocenters. Total Synthesis of (+)-Pancratistatin
Fernando Cagide-Fagín,† Olaia Nieto-García,‡ Hugo Lago-Santome, and Ricardo Alonso*
́
Department of Organic Chemistry, University of Santiago de Compostela, Campus Vida sn, 15782 Santiago de Compostela, A
Coruna, Spain
̃
S
* Supporting Information
ABSTRACT: 2-Methoxymethylpyrrolidine best performed,
among several other proline derivatives, to control the
enantioselective [3+3] annulation of β-(hetero)aryl-α-nitro-α,β-
enals with commercial 2,2-dimethyl-1,3-dioxan-5-one, a proce-
dure that renders highly oxygenated nitrocyclohexanes endowed
with five new stereocenters. Use of this reaction allowed the
development of a total synthesis of the antitumoral natural
product (+)-pancratistatin; it also converted our previous racemic route to tetrodotoxin into an enantioselective one.
ew reactions that can build complex chiral structures
previous efforts, the enantioselective syntheses of their
Nfrom simple achiral fragments in an enantioselective corresponding chiral nitrogen-bearing polyoxygenated cyclo-
hexane cores usually involve long multistep routes.7−10
manner are in high demand (especially when the reaction
In this context, we herein report the enantioselective
annulation of β-(hetero)aryl-α-nitro-α,β-enals (3) with enam-
ines E to form protected nitrocyclohexitols 4, a process by
which five new stereocenters are generated (Scheme 1).
products are recurrent biologically relevant units that cannot be
easily accessed through alternative methodologies). A case in
point is that of nitrogen-bearing polyhydroxylated cyclo-
hexanes. These structural units are indeed present in numerous
pharmacologically significant compounds [in particular, more
than 20000 anti-infective, antitumor, immune, anti-inflamma-
tory, peptidomimetic, anti-ulcer, dermatological, and nervous
system agents having a cyclohexane ring with (at least) one
nitrogen and three oxygens attached have been reported in
more than 82000 references to date (≈13000 in the past four
years)].1 Such a large number of compounds endowed with a
nitrogenated and heavily oxygenated cyclohexane, along with
their relevance, call for the development of appropriate
synthetic procedures, so that sufficient quantities of them
become available for the complete study of their properties and
applications. This is particularly so for natural products present
in minute amounts in their sources, a situation that fully applies
to the antitumoral isocarbostyril constituents of the plant family
Amaryllidaceae,2 with (+)-pancratistatin as one of the main
representatives,3 and to the sodium channel blocker (−)-te-
trodotoxin and its analogues (Figure 1).4−6 However, despite
Scheme 1. Convergent Synthesis of Nitrogen-Bearing
Polyoxygenated Cyclohexanes 4 by Enantioselective
Annulation of Nitroenals 3 with Chiral Enamines E (a three-
step procedure from commercially available fragments)
Because 1 and 2, precursors of E, are commercially available
and enals 3 are prepared in two steps from commercial
aldehydes and 2-nitroethanol, the nitrogen-bearing highly
oxygenated cyclohexane derivatives of type 4 can thus be
accessed in just three synthetic operations starting form
commercial sources. The power of the reaction is demonstrated
with the completion of a short total synthesis of (+)-pan-
Figure 1. Representative examples of pharmacologically relevant
natural products endowed with a nitrogen-bearing polyoxygenated
cyclohexane core.
Received: October 13, 2012
© XXXX American Chemical Society
A
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Note
cratistatin in which the enantioselective annulation procedure is
the main key step. Moreover, the reported annulations of 3a
and 3b to deliver enantiomerically pure 4a and 4b (vide infra),
respectively, provide for the asymmetric synthesis of
tetrodotoxin and some pancratistatin analogues.
Table 1. Enantioselective Annulations of Nitroenal 3a with
Dioxanone 1 Using Pyrrolidines 2a−e
a
The results described herein build on previous work in the
area, most importantly, the development of the first general
method for the preparation of β-(hetero)aryl-α-nitro-α,β-enals
(3),11 and the demonstration of their ability to annulate with
enamines E derived from achiral secondary amines, such as
morpholine or pyrrolidine.12
To develop an enantioselective version of the annulation, we
selected enal 3a (1) because the preparation of 3a, an
important step in our synthetic efforts toward tetrodotoxin,
has been optimized so that 3a is quickly obtained from cheap
furfural with no chromatography13 and (2) because 3a has
superior stability compared to those of its aromatic analogues.11
With 3a in hand, its enantioselective annulation with enamines
E formed from dioxanone 1 and a number of chiral pyrrolidines
2 was then tested; the results are listed in Table 1.
Use of D-proline (2a) to annulate 1 with 3a (entry 2) under
the conditions previously reported using pyrrolidine (DMF, rt,
0.2 equiv of PPTS)12 led to 4a in 7% yield (considerably lower
than for the racemic case, 55%, entry 1) and in 37% ee. Just
slightly better values were obtained when a 1.5:1 instead of a
1:1 molar ratio of dioxanone to enal was employed (entry 3).
The annulation took place likewise using the more convenient
acetonitrile as the solvent (entry 4).
d
e
yield
(%)
% ee,
f
b
c
entry
2
1:3a
solvent, T
DMF, rt
product
( )-4a
g
h
1
Pyrr
2a
1:1
1:1
55
2
3
4
5
6
7
8
DMF, rt
7
37, (+)-4a
39, (+)-4a
35, (+)-4a
28, (−)-4a
35, (−)-4a
51, (−)-4a
76−81,
2a
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
1.5:1
DMF, rt
8
2a
CH3CN, rt
CH3CN, rt
CH3CN, rt
CH3CN, rt
CH3CN, rt
5
2b
2c
50
45
2d
14
(S)-2e
43−47
(−)-4a
i
>99%
Employing the proline benzyl ester 2b (entry 5), the yield
notably increased (50%) and the enantioselectivity decreased
(28% ee). This last figure increased to 35 and 51% ee for
primary amide 2c (entry 6) and amine 2d (entry 7),
respectively, but the annulation yield decreased, slightly for
the first case (45%) and considerably for the latter (14%).
Use of commercial (S)-2-(methoxymethyl)pyrrolidine [(S)-
2e] afforded 4a in 43−47% yield and 76−81% ee.
Crystallization from Et2O gave the levorotatory enantiomer
(−)-4a in essentially pure form. Its absolute configuration,
shown in the scheme at the top of Table 1, was determined by
X-ray analysis of its p-bromobenzoate derivative (for details, see
the Supporting Information).14
No major changes were observed when CSA and p-TsOH
were used as additives instead of PPTS. Lower reaction
temperatures gave lower yields with no improvements in
enantioselection (entries 9−11).
Again, DMF proved to be as good as acetonitrile (42%, 74%
ee, entry 12); also, a lower yield was observed for a 1:1 molar
ratio of dioxanone to enal (30%, 73% ee, entry 13).
Except for DMSO (49% ee, entry 17), halogenated (CHCl3,
entry 14), protic (EtOH, entry 15), and other aprotic (acetone,
entry 16) solvents performed similarly in terms of enantiose-
lectivity (70−75% ee) as compared to DMF and acetonitrile.
These last two solvents remained better for annulation yields.
With an enantioselective route to protected nitrocyclitols of
type 4 available (Scheme 1 and Table 1), we next addressed the
preparation of (+)-pancratistatin, which we planned to
accomplish from nitroenal 3b and dioxanone 1, through the
key nitrocyclitol intermediate 4b and the carbamate derivative 6
(Scheme 2). Ring A of pancratistatin would ultimately come
from cheap vanillin, the precursor of nitroenal 3b; ring C would
be assembled in the key enantioselective annulation step, and
ring B would be formed by an intramolecular aromatic
electrophilic substitution of carbamate 6.
9
(R)-2e
(R)-2e
(R)-2e
(R)-2e
(R)-2e
(R)-2e
(R)-2e
(R)-2e
(R)-2e
1.5:1
1.5:1
1.5:1
1.5:1
1:1
CH3CN, 0 °C
CH3CN, −20 °C
CH3CN, −40 °C
DMF, rt
39
35
30
42
30
22
21
19
33
77, (+)-4a
75, (+)-4a
75, (+)-4a
74, (+)-4a
73, (+)-4a
75, (+)-4a
72, (+)-4a
70, (+)-4a
49, (+)-4a
10
11
12
13
14
15
16
17
DMF, rt
1.5:1
1.5:1
1.5:1
1.5:1
CHCl3, rt
EtOH, rt
acetone, rt
DMSO, rt
a
Annulations were performed according to the general procedure
reported in the Experimental Section. The structure shown for
compound 4a is that of its levorotatory enantiomer, (−)-4a. Molar
ratio. Annulation temperature in degrees Celsius. Isolated yield.
Determined by chiral HPLC. Major enantiomer of 4a. Racemic
annulation using pyrrolidine (see ref 12). Pyrrolidine. After
crystallization.
b
c
d
e
f
g
h
i
Transformation of vanillin into 5-methoxypiperonal (three
steps, 57% yield)15 and of this last one into enal 3b (two steps,
51%)11 was achieved as previously reported. Annulation of
dioxanone 1 with nitroenal 3b using (R)-2-(methoxymethyl)-
pyrrolidine [(R)-2e] gave 4b in 38% yield and 75% ee (>99%
ee after crystallization); its five stereogenic carbons matched the
configuration required by the final target at C1, C3, C4, C4a,
and C10b.
For the conversion of nitrocyclitol 4b into 6, the required
methylcarbamate unit at C4a was first installed through
reduction of the nitro group to amine and subsequent acylation
to 5 (Scheme 3). We then sujected compound 5 to a three-step
sequence (steps 3−5, 87% overall yield) to perform the
stereoselective reduction of its C2 keto group and to change
the protection group pattern so as to make it compatible with
the following step, the aromatic electrophilic substitution of 6.
Treatment of 6 with triflic anhydride and DMAP at 0 °C,16
followed by acid hydrolysis of the corresponding iminoether
B
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Note
successive treatment with BBr3 and NaMeO, thus affording
(+)-pancratistatin.
Scheme 2. Synthetic Plan for the Preparation of
(+)-Pancratistatin
Certainly, the enantioselective annulation process that yields
protected nitrocyclitols of type 4 described herein offers a way
to obtain other compounds apart from pancratistatin in
enantiomerically pure form.
This is the case for tetrodotoxin, for which we recently
reported a racemic route through ( )-4a.13 An alternative use
of (+)-4a, now available through enantioselective annulation,
would afford (−)-tetrodotoxin.
Furthermore, diverse analogues of pancratistatin could now
be obtained in enantiomerically pure form through different
nitrocyclitols of type 4 starting from a variety of aromatic
aldehydes. In particular, 7-deoxy analogues, e.g., the naturally
occurring (+)-7-deoxy-pancratistatin itself, would be accessible
from piperonal through nitroenal 3c and its annulation product,
the protected nitrocyclitol intermediate (+)-4c (Scheme 4).
Scheme 4. Enantioselective Synthesis of Protected
Nitrocyclitol (+)-4c
Scheme 3. Conversion of Nitrocyclitol 4b into
(+)-Pancratistatin
In summary, the enantioselective annulation of 2,2-dimethyl-
1,3-dioxan-5-one (1) with β-(hetero)aryl-α-nitro-α,β-enals
(3a−c) renders highly oxygenated nitrocyclohexanes 4a−c,
respectively, endowed with five new stereocenters in
enantioenriched form (enantiopure through subsequent
crystallization). As enals 3 are available from aldehydes and
2-nitroethanol in two reactions, access to 4 involves three steps
from commercially available compounds. These protected
nitrocyclitols 4 serve well as advanced key synthetic
intermediates for the (+)-pancratistatin and (−)-tetrodotoxin
families of natural products.
EXPERIMENTAL SECTION
■
Enantioselective Annulation of Dioxanone 1 with Nitroenals
3. General Procedure. A mixture of dioxanone 1 (1 or 1.5 equiv),
the chiral pyrrolidine 2 (0.8 equiv), and anhydrous Na2SO4 in a dry
solvent was magnetically stirred under argon at rt for 3 h. A solution of
nitroenal 3 (1 equiv) and PPTS (0.2 equiv) in the same dry solvent
was added. After 2 h, the reaction mixture was diluted with a 1:4 (v:v)
EtOH/H2O mixture. Solvent removal (rotary evaporator) and
chromatography (20% EtOAc/hexane) gave the corresponding
protected nitrocyclitol 4.
Protected Nitrocyclitol 4a. Prepared according to the general
procedure from 1 (117 mg, 0.9 mmol), (R)-2-(methoxymethyl)-
pyrrolidine [(R)-2e, 59 μL, 0.48 mmol], and 3a (100 mg, 0.6 mmol) in
CHCl3: 22% yield; 78% ee; 7% (first crop, unoptimized) and >99% ee
after crystallization from Et2O; [α]D20 = +181.7 (c = 1, CHCl3); white
solid; mp 190−199 °C dec (Et2O/hexane); Rf = 0.44 (silica gel plates,
30% EtOAc/hexane); 1H NMR (CDCl3, 300 MHz) δ 7.38 (d, J = 1.8
Hz, 1H), 6.39 (d, J = 3.3 Hz, 1H), 6.35 (dd, J = 3.3, 1.8 Hz, 1H), 5.33
(dd, J = 11.6, 9.5 Hz, 1H), 4.55 (dd, J = 2.4, ≈1.6 Hz, 1H), 4.51 (dd, J
≈ 2.4, ≈1.3 Hz, 1H), 4.21 (ddd, J = 10.6, ≈9.5, 1.6 Hz, 1H), 3.52 (dd,
intermediate, gave the desired lactam 7, together with its
regioisomer 7r in a 9:1 ratio and 64% overall yield. Final
cleavage of the aromatic methyl ether and removal of the
hydroxyl protecting groups were best achieved through
C
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Note
20
J = 11.6, 1.3 Hz, 1H), 2.96 (d, J = 10.6 Hz, 1H), 1.58 (s, 3H), 1.51 (s,
3H); 13C NMR (CDCl3, 125 MHz) δ 205.2, 147.3, 143.1, 110.8,
108.9, 99.9, 89.0, 79.1, 77.0, 76.2, 44.3, 28.2, 25.4; LRMS (ESI-TOF)
m/z (%) 320.0 [(M + Na)+, 46], 298.1 [(M + H)+, 100], 245.1 (18);
HRMS [ESI-TOF, (M + H)+] m/z calcd for C13H16NO7 298.0921,
found 298.0921.
EtOAc/hexane); mp 119−123 °C (EtOAc/hexane); [α]D = +129.2
(c = 1, CHCl3); 1H NMR (CDCl3, 300 MHz) δ 6.60 (s, 1H), 6.56 (m,
1H), 5.96 (s, 2H), 4.92 (br s, 1H), 4.69 (br s, 1H), 4.69−4.60 (m,
2H), 4.34−4.26 (m, 1H), 3.88 (s, 3H), 3.81 (s, 3H), 3.55 (br s, 3H),
3.05 (br s, 1H), 1.63 (s, 3H), 1.47 (s, 3H); 13C NMR (CDCl3, 75
MHz) δ 206.4, 156.2, 154.8, 148.9, 143.4, 134.9, 130.7, 108.5, 102.8,
101.5, 99.3, 79.7, 79.0, 77.6, 56.5, 55.2, 52.2, 51.1, 49.9, 28.3, 25.3;
LRMS (CI) m/z (%) 468.0 [(M + H)+, 18], 392.0 (100), 334.0 (75),
317.0 (94), 259.0 (55); HRMS [CI, (M + H)+] m/z calcd for
C21H26NO11 468.15059, found 468.15031.
Protected Nitrocyclitol 4b. Prepared according to the general
procedure from 1 (256 mg, 1.97 mmol), (R)-2-(methoxymethyl)-
pyrrolidine [(R)-2e, 195 μL, 1.58 mmol], and 3b (500 mg, 1.97
mmol) in DMF: 300 mg; 38% yield; 75% ee; 8% (first crop,
unoptimized) and >99% ee after crystallization from Et2O; white solid;
(1S,2S,3R,4R,5R,6S)-4-(7-Methoxybenzo[d][1,3]dioxol-5-yl)-
5-(methoxycarbonyl)amino-6-(methoxycarbonyloxy)cyclo-
hexane-1,2,3-triyl Triacetate (6). A mixture of 5 (150 mg, 0.32
mmol) and Dowex 50WX (687 mg) in MeOH (9.7 mL) was stirred
for 2 days at 60 °C. After filtration, the solvent was evaporated in
vacuo and the residue dissolved in a dry DCE/THF mixture (1:1, 9.7
mL) under argon. NaBH(AcO)3 (342 mg, 1.62 mmol) was added and
the mixture stirred at rt for 2 h. After the reaction had been quenched
with 30% aqueous hydrogen peroxide (30%, 600 μL) and the solvent
had been evaporated, the crude was dissolved in dry CH2Cl2 (9.7 mL)
and treated with Et3N (1.2 mL), Ac2O (0.6 mL), and DMAP (8 mg,
0.065 mmol). After being stirred for 4 h at rt, the mixture was treated
with a saturated aqueous solution of NaHCO3 (9.7 mL) and extracted
with CH2Cl2 (3 × 10 mL). Chromatography (50% EtOAc/hexane)
gave 6 (156 mg, 87%) as a white solid: Rf = 0.40 (60% EtOAc/
20
mp 167 °C (hexane/CH2Cl2); [α]D = +133.9 (c = 1, CHCl3); Rf =
1
0.51 (silica gel plates, 30% EtOAc/hexane); H NMR (CDCl3, 400
MHz) δ 6.66 (d, J = 1.5 Hz, 1H), 6.49 (d, J = 1.5 Hz, 1H), 5.97 (dd, J
= 1.5, 1.5 Hz, 1H), 5.96 (d, J = 1.5 Hz, 1H), 5.33 (dd, J = 11.7, 9.4 Hz,
1H), 4.56 (dd, J ≈ 2.4, ≈2.4 Hz, 1H), 4.39−4.34 (m, 1H), 4.22 (ddd, J
= 10.6, 9.4, 2.4 Hz, 1H), 3.88 (s, 3H), 3.11 (dd, J =11.7, 1.1 Hz, 1H),
2.97 (d, J = 10.6 Hz, 1H), 1.64 (s, 3H), 1.52 (s, 3H); 13C NMR
(CDCl3, 100 MHz) δ 205.4, 149.2, 143.6, 135.7, 128.0, 108.7, 102.4,
101.7, 99.9, 90.9, 79.2, 78.9, 76.2, 56.6, 50.2, 28.3, 25.3; LRMS (EI) m/
z 381.0 [(M)+, 94]; IR (CHCl3) 3496 (OH), 1735 (CO), 1556 (NO2)
cm−1; HRMS [EI, (M)+] m/z calcd for C17H19NO9 381.1060, found
381.1064.
Protected Nitrocyclitol 4c. Prepared according to the general
procedure from 1 (1.05 g, 8.07 mmol), (R)-2-(methoxymethyl)-
pyrrolidine [(R)-2e, 530 μL, 4.30 mmol], and 3c (1.19 g, 5.38 mmol)
in CH3CN: 39% yield; 51% ee; 13% (first crop, unoptimized) and
>99% ee after crystallization from i-PrOH; white solid; mp 203 °C
20
hexane); mp 118−122 °C (Et2O); [α]D = −124.1 (c = 1, CHCl3);
1H NMR (CDCl3, 400 MHz) δ 6.45 (s, 2H), 5.95 (d, J = 1.4 Hz, 1H),
5.94 (d, J = 1.4 Hz, 1H), 5.45 (dd, J ≈ 2.6, ≈2.6 Hz, 1H), 5.14 (dd, J ≈
2.9, ≈2.9 Hz, 1H), 5.09 (d, J = 9.2 Hz, 1H), 5.03 (br s, 1H), 4.64 (ddd,
J = 10.7, 9.2, 7.9 Hz, 1H), 4.48 (br s, 1H), 3.89 (s, 3H), 3.78 (s, 3H),
3.56 (br s, 3H), 3.38−3.26 (m, 1H), 2.19 (s, 3H), 2.17 (s, 3H), 2.01
(s, 3H); 13C NMR (CDCl3, 75 MHz) δ 169.3, 168.7, 168.2, 156.4,
155.3, 148.9, 143.3, 134.6, 130.1, 108.5, 102.8, 101.4, 74.8, 71.9, 68.4,
68.0, 56.7, 55.2, 52.2, 48.2, 47.0, 20.8, 20.8, 20.7; LRMS (ESI-TOF)
m/z (%) 578.1 [(M + Na)+, 76], 556.2 [(M + H)+, 100], 360.1 (38);
HRMS [ESI-TOF, (M + H)+] m/z calcd for C24H30NO14 556.1666,
found 556.1661.
(+)-1,2,3-Tri-O-acetyl-7-O-methyl-4-O-methoxycarbonylpan-
cratistatin (7). Trifluoromethanesulfonic anhydride (77 μL, 0.46
mmol) was added to a solution of 6 (50 mg, 0.090 mmol) and DMAP
(33 mg, 0.27 mmol) in dry CH2Cl2 (6 mL) at 0 °C under argon. After
being stirred for 4.5 h at rt, the solution was treated with a saturated
aqueous solution of Na2CO3 (6 mL) and extracted (EtOAc, 3 × 6
mL). The combined organic extracts were dried; the solvent was
evaporated in vacuo, and the crude residue was dissolved in 1,4-
dioxane (6 mL) and treated with 2 M HCl (0.6 mL). After being
stirred for 17 h, the mixture was neutralized with a saturated aqueous
solution of NaHCO3 (6 mL) and extracted (EtOAc, 3 × 6 mL).
Chromatography (60% EtOAc/hexane) afforded an inseparable 9:1
mixture of 7 and 7r (30 mg, 64%): Rf = 0.35 (80% EtOAc/hexane). 7:
1H NMR (CDCl3, 300 MHz) δ 6.30 (s, 1H), 6.04 (s, 1H), 5.99 (s,
1H), 5.94 (s, 1H), 5.53−5.52 (m, 2H), 5.25 (dd, J ≈ 2.5, ≈2.5 Hz,
1H), 4.97 (dd, J = 10.8, 3.2 Hz, 1H), 4.22 (dd, J = 12.8, 10.8 Hz, 1H),
4.08 (s, 3H), 3.85 (s, 3H), 3.38 (br d, J = 12.8 Hz, 1H), 2.17 (s, 3H),
2.08 (s, 3H), 2.04 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ 169.6,
169.1, 168.2, 163.1, 154.5, 152.4, 145.4, 137.5, 133.0, 115.7, 101.8,
98.9, 75.6, 67.6, 66.5, 66.4, 60.8, 55.5, 47.6, 40.2, 20.7, 20.6, 20.6;
LRMS (ESI-TOF) m/z (%) 646.1 [(M + Na)+, 40], 524.1 [(M + H)+,
100]; HRMS [ESI-TOF, (M + H)+] m/z calcd for C23H26NO13
524.1404, found 524.1399.
(+)-1,2,3-Tri-O-acetyl-4-O-methoxycarbonylpancratistatin
(8). Boron tribromide (1 M in CH2Cl2, 234 μL, 0.234 mmol) was
added to a solution of 7 and 7r (9:1 ratio, 41 mg, 0.078 mmol) in dry
CH2Cl2 (1.5 mL) under argon at −78 °C. After 45 min at 0 °C, the
mixture was treated with an aqueous solution of NH4OH (1.5 mL),
stirred for 20 min, and extracted with CH2Cl2 (3 × 2.5 mL).
Chromatography (45% EtOAc/hexane) gave 8 (20 mg, 50%) as a
white solid: Rf = 0.30 (50% EtOAc/hexane); mp 220−229 °C
(CH2Cl2); [α]D20 = +50 (c = 1, CHCl3); 1H NMR (CDCl3, 300 MHz)
δ 12.36 (s, 1H), 6.38 (s, 1H), 6.18 (d, J = 1.0 Hz, 1H), 6.04 (s, 2H),
20
(hexane/CH2Cl2); [α]D = +122.6 (c = 1, CHCl3); Rf = 0.36 (silica
1
gel plates, 25% EtOAc/hexane); H NMR (CDCl3, 400 MHz) δ 6.96
(d, J = 1.5 Hz, 1H), 6.77 (dd, J = 8.1, 1.5 Hz, 1H), 6.74 (d, J = 8.1 Hz,
1H), 5.97 (d, J = 1.4 Hz, 1H), 5.96 (d, J = 1.4 Hz, 1H), 5.33 (dd, J =
11.7, 9.4 Hz, 1H), 4.56 (dd, J = 2.4, 2.4 Hz, 1H), 4.35 (dd, J = 2.4, 1.4
Hz, 1H), 4.22 (br s, 1H), 3.13 (dd, J = 11.7, 1.4 Hz, 1H), 2.95 (br s,
1H), 1.65 (s, 3H), 1.51 (s, 3H); 13C NMR (CDCl3, 100 MHz) δ
205.4, 148.1, 148.1, 127.4, 122.4, 108.6, 108.5, 101.3, 99.9, 91.0, 79.2,
79.0, 76.2, 50.0, 28.4, 25.3; LRMS (CI) m/z (%) 351.0 [(M)+, 35]; IR
(CHCl3) 1733 (CO), 1556 (NO2) cm−1; HRMS [EI, (M)+] m/z calcd
for C16H17NO8 351.0954, found 351.0950.
p-Nitrobenzoate of (−)-4a. DMAP (2 mg, 0.017 mmol), DCC
(139 mg, 0.67 mmol), and 4-bromobenzoic acid (47 mg, 0.23 mmol)
were successively added to a solution of nitrocyclitol (−)-4a (50 mg,
0.17 mmol) in CH2Cl2 (3 mL) under Ar. After being stirred for 10
min, the mixture was washed with 0.1 M aqueous HCl (10 mL) and a
saturated aqueous solution of NaHCO3 (10 mL). The organic phase
was dried and the solvent removed. Column chromatography (silica
gel, 10% EtOAc/hexane) afforded the p-bromobenzoate of (−)-4a (63
mg, 78%) as a white solid: 1H NMR (CDCl3, 250 MHz) δ 7.90 (d, J =
8.7 Hz, 2H), 7.60 (d, J = 8.7 Hz, 2H), 7.41 (d, J = 1.8 Hz, 1H), 6.46
(d, J = 3.3 Hz, 1H), 6.38 (dd, J1 = 3.3 Hz, J2 = 1.9 Hz, 1H), 5.76 (dd, J1
= 11.6 Hz, J2 = 10.1 Hz, 1H), 5.49 (dd, J1 = 10.1 Hz, J2 = 2.1 Hz, J =
10.1, 2.1 Hz, 1H), 4.88 (dd, J1 ≈ 2.3 Hz, J2 ≈ 2.3 Hz, 1H), 4.56 (dd, J1
≈ 2.1 Hz, J2 ≈ 2.1 Hz, 1H), 3.65 (dd, J = 11.6, 1.7 Hz, 1H), 1.60 (s,
3H), 1.50 (s, 3H); 13C NMR (CDCl3, 62.5 MHz) δ 204.3, 163.9,
146.9, 143.3, 132.0 (2C), 131.5 (2C), 129.3, 127.1, 110.8, 109.3, 99.7,
85.2, 77.0, 76.9, 76.3, 44.4, 28.3, 25.3.
(1R,5S,6S,7R,8R)-8-(7-Methoxy-1,3-benzodioxol-5-yl)-7-
[(methoxycarbonyl)amino]-3,3-dimethyl-9-oxo-2,4-dioxa-
bicyclo[3.3.1]non-6-yl Methyl Carbonate (5). A suspension of 4b
(500 mg, 1.20 mmol), 10% Pd/C (639 mg), and ammonium formate
(342 mg, 5.43 mmol) in dry methanol (12 mL) was stirred under H2
at rt. After completion of the reduction (as monitored by TLC), the
catalyst was filtered off and washed with methanol and EtOAc. The
combined washings and the filtrate were evaporated in vacuo. The
resultant amine was dissolved in dry CH2Cl2 (12 mL) and treated with
DMAP (606 mg, 4.96 mmol) and methyl chloroformate (372 μL, 4.72
mmol). After being stirred for 2 h at rt, the reaction mixture was
treated with a saturated aqueous solution of NaHCO3 (12 mL) and
extracted with CH2Cl2 (3 × 12 mL). Chromatography (30% EtOAc/
hexane) gave 5 (487 mg, 90%) as a white foam: Rf = 0.42 (50%
D
dx.doi.org/10.1021/jo3022567 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
5.57−5.51 (m, 2H), 5.25 (dd, J ≈ 2.9, ≈2.9 Hz, 1H), 5.00 (dd, J =
10.8, 3.4 Hz, 1H), 4.31 (dd, J = 13.2, 10.8 Hz, 1H), 3.85 (s, 3H), 3.43
(br d, J = 13.2 Hz, 1H), 2.16 (s, 3H), 2.09 (s, 3H), 2.05 (s, 3H); 13C
NMR (CDCl3, 75 MHz) δ 169.8, 169.5, 169.1, 168.1, 154.4, 153.2,
146.7, 133.4, 131.4, 107.3, 102.4, 96.5, 75.4, 67.5, 66.6, 66.1, 55.6, 48.3,
39.1, 20.7, 20.6, 20.6; LRMS (ESI-TOF) m/z (%) 532.1 [(M + Na)+,
52], 510.1 [(M + H)+, 100], 307.0 (18); HRMS [ESI-TOF, (M +
H)+] m/z calcd for C22H24NO13 510.1248, found 510.1242.
(3) For studies of the anticancer activity and the pharmaceutical
potential of pancratistatin, see: (a) Griffin, C.; Hamm, C.; McNulty,
J.; Pandey, S. Cancer Cell Int. 2010, 10, 6. (b) Griffin, C.; Karnik, A.;
McNulty, J.; Pandey, S. Mol. Cancer Ther. 2011, 10, 57−68 and
references cited therein. See also the Introduction and Biomedical
Significance and Conclusion of ref 8a and the Introduction of ref 8j.
(4) Tetrodotoxin was instrumental in the identification, isolation,
purification, and subsequent sequencing of the main protein subunit of
voltage-gated sodium channels: Noda, M.; Shimizu, S.; Tanabe, T.;
Takai, T.; Kayano, T.; Ikeda, T.; Takahashi, H.; Nakayama, H.;
Kanaoka, Y.; Minamino, N.; Kangawa, K.; Matsuo, H.; Raftery, M. A.;
Hirose, T.; Inayama, S.; Hayashida, H.; Miyata, T.; Numa, S. Nature
1984, 312, 121−127.
(+)-Pancratistatin. A solution of 8 (18 mg, 0.035 mmol) in THF
(4 mL) with NaMeO (0.5 M in MeOH, 0.7 mL) was stirred for 20
min at rt. The mixture was neutralized with AcOH, and the volatiles
were removed in vacuo. Chromatography (20% MeOH/CHCl3) gave
(+)-pancratistatin (10 mg, 86%) as a white solid: Rf = 0.80 (25%
20
1
MeOH/CHCl3); [α]D = +46 (c = 1, DMSO); H NMR (DMSO,
300 MHz) δ 13.40 (s, 1H), 7.49 (s, 1H), 6.47 (s, 1H), 6.04 (s, 1H),
6.02 (s, 1H), 5.34 (d, J = 3.8 Hz, 1H), 5.07 (m, 2H), 4.81 (d, J = 7.5
Hz, 1H), 4.27 (s, 1H), 3.95 (s, 1H), 3.84 (s, 1H), 3.77−3.62 (m, 2H),
2.96 (d, J = 12.4 Hz, 1H); 13C NMR (DMSO, 75 MHz) δ 169.4,
152.0, 145.3, 135.6, 131.6, 107.4, 101.7, 97.6, 73.2, 70.1, 69.9, 68.4,
50.4, 28.9.
(5) The pharmacological significance of tetrodotoxin was put forward
by Mosher in 1986: Mosher, H. S. The chemistry of tetrodotoxin.
Ann. N.Y. Acad. Sci. 1986, 479, 32−43.
(6) Tetrodotoxin is currently being studied in the fields of analgesia
(for neuropathic pain and opioids-refractive cancer pain), drug-
addiction withdrawal treatment, and local anesthesia; it is in fact the
main lead compound in the drug development program of the
company WexPharmaceuticals (http://www.wexpharma.com/).
(7) Pancratistatin has been in preclinical development for more than
20 years. Its advancement to clinical trials has been prevented to date
by the lack of sufficient supply from natural resources. For an account
covering the progress in the synthesis of Amaryllidaceae constituents
and some truncated derivatives from 1996 to autumn 2004, with a
detailed treatment of the total synthesis of pancratistatin (also
narciclasine, 7-deoxy-pancratistatin, and lycoricidine), see: Rinner, U.;
Hudlicky, T. Synlett 2005, 365−387.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray data and copies of NMR spectra for the p-bromobenzoate
of (−)-4a, copies of NMR spectra and HPLC chromatograms
for 4a−c, and copies of NMR spectra for 5−8 and
(−)-pancratistatin. This material is available free of charge via
the Internet at http://pubs.acs.org.
(8) For a review of the total synthesis of pancratistatin, including data
about its isolation from natural sources and biomedical significance,
see: (a) Manpadi, M.; Kornienko, A. Org. Prep. Proced. Int. 2008, 40,
107−161. For the first total synthesis of pancratistatin in racemic
form, see: (b) Danishefsky, S.; Lee, J. Y. J. Am. Chem. Soc. 1989, 111,
4829−4837. For the first asymmetric total synthesis, see: (c) Tian, X.
AUTHOR INFORMATION
■
Corresponding Author
*r.alonso@usc.es.
Present Addresses
†Department of Chemistry and Biochemistry, Faculdade de
R.; Hudlicky, T.; Konigsberger, K. J. Am. Chem. Soc. 1995, 117, 3643−
̈
Cien
̂
cias, Universidade do Porto, 4169-007 Porto, Portugal.
3644. (d) Hudlicky, T.; Tian, X.; Konigsberger, K.; Maurya, R.;
̈
‡Freie Universitat Berlin, Institut fur Chemie and Biochemie,
̈
̈
Rouden, J.; Fan, B. J. Am. Chem. Soc. 1996, 118, 10752−10765. Six
additional asymmetric total syntheses have been reported to date:
(e) Trost, B. M.; Pulley, S. R. J. Am. Chem. Soc. 1995, 117, 10143−
10144. (f) Doyle, T. J.; Hendrix, M.; VanDerveer, D.; Javanmard, S.;
Haseltine, J. Tetrahedron 1997, 53, 11153−11170. (g) Magnus, P.;
Sebhat, I. K. J. Am. Chem. Soc. 1998, 120, 5341−5342. Magnus, P.;
Sebhat, I. K. Tetrahedron 1998, 54, 15509−15524. (h) Rigby, J. H.;
Maharoof, U. S. M.; Mateo, M. E. J. Am. Chem. Soc. 2000, 122, 6624−
6628. (i) Li, M.; Wu, A. M.; Zhou, P. J. Tetrahedron Lett. 2006, 47,
3707−3710. (j) Dam, J. H.; Madsen, R. Eur. J. Org. Chem. 2009,
Takustrasse 3, 14195 Berlin, Germany.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Ministry of Science and Innovation
(Project CTQ2008-03253) and by the Xunta de Galicia
(Projects 05BTF20901PR, 08CSA046209PR, 2007/XA084,
and CN2011/054) is gratefully acknowledged. F.C.-F., O.N.-
G., and H.L.-S. acknowledge contracts with financial support
from the Xunta de Galicia. O.N.-G. acknowledges fellowships
from the Xunta de Galicia (Lucas Labrada grant) and from the
4666−4673.
For a semisynthesis of (+)-pancratistatin from
(+)-narciclasine, see: (k) Pettit, G. R.; Melody, N.; Herald, D. L. J.
Org. Chem. 2001, 66, 2583−2587. Besides that of S. Danishefsky and
J. Y. Lee (ref 8b), two other total syntheses of pancratistatin in racemic
form have been reported: (l) Kim, S.; Ko, H.; Kim, E.; Kim, D. Org.
Lett. 2002, 4, 1343−1345. Ko, H. J.; Kim, E.; Park, J. E.; Kim, D.; Kim,
S. J. Org. Chem. 2004, 69, 112−121. (m) Jung, Y.-G.; Kang, H.-U.;
Cho, H.-K.; Cho, C.-G. Org. Lett. 2011, 13, 5890−5892.
“Diputacion de A Coruna”. We thank Dr. Krzysztof Kierus for
́
̃
the preparation of the p-bromobenzoate of (−)-4a.
REFERENCES
(9) For asymmetric total syntheses of (−)-tetrodotoxin, see:
(a) Ohyabu, N.; Nishikawa, T.; Isobe, M. J. Am. Chem. Soc. 2003,
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■
(1) The search was conducted with SciFinder. Structures were
required to have references and to correspond to single compounds.
Tautomeric and zwitterionic structures were excluded.
(2) For reviews on the isolation, biosynthesis, synthetic studies,
biological activity, and medical potential, mainly as antitumoral agents,
of the isocarbostyril constituents of the Amaryllidaceae, see:
(a) Ingrassia, L.; Lefranc, F.; Mathieu, V.; Darro, F.; Kiss, R.
Translational Oncology 2008, 1, 1−13. (b) Kornienko, A.; Evidente, A.
Chem. Rev. 2008, 108, 1982−2014. See also the Introduction of:
(c) Collins, J.; Rinner, U.; Moser, M.; Hudlicky, T.; Ghiviriga, I.;
Romero, A. E.; Kornienko, A.; Ma, D.; Griffin, C.; Pandey, S. J. Org.
Chem. 2010, 75, 3069−3084 and references cited therein.
E
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The Journal of Organic Chemistry
Note
(10) In his highlight Syntheses of Tetrodotoxin ( Koert, U. Angew.
Chem., Int. Ed. 2004, 43, 5572−5576 ), the author concludes, “All
three syntheses (those of Isobe and Du Bois, references 9a−9c above)
are long (>25 steps) and follow a linear strategy. The hexasubstituted
cyclohexane intermediate makes a convergent strategy, which may
reduce the number of steps to less than 20, very difficult but not
impossible. Because of its structural complexity and functional-group
density, tetrodotoxin will remain a prime synthetic target”.
(11) Martínez-Bescos, P.; Cagide-Fagín, F.; Roa, L. F.; Ortiz-Lara, J.
C.; Kierus, K.; Ozores-Viturro, L.; Fernan
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(13) Cagide-Fagín, F.; Alonso, R. Eur. J. Org. Chem. 2010, 6741−
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(14) Crystallographic data were deposited at The Cambridge
Crystallographic Data Centre as entry 903344 and can be obtained
free of charge via http://www.ccdc.cam.ac.uk/data_request/cif.
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F
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