An improved synthesis of (-)-5,11-dideoxytetrodotoxin

Article abstract of DOI:10.1021/jo302773f

We describe an improved synthesis of (-)-5,11-dideoxytetrodotoxin from an enone, which was used for synthesis of tetrodotoxin and its analogues in this laboratory. One of the major modifications was to establish a two-step guanidinylation of trichloroacetamide of a highly functionalized intermediate, which allowed us to prepare ¹⁵N₂-labeled 5,11-dideoxytetrodotoxin for biosynthetic investigations.

Full text of DOI:10.1021/jo302773f

Note
pubs.acs.org/joc
An Improved Synthesis of (−)-5,11-Dideoxytetrodotoxin
Masaatsu Adachi, Takuya Imazu, Minoru Isobe,† and Toshio Nishikawa*
Laboratory of Organic Chemistry, Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan
S
* Supporting Information
ABSTRACT: We describe an improved synthesis of (−)-5,11-dideoxytetrodotoxin from an enone, which was used for synthesis
of tetrodotoxin and its analogues in this laboratory. One of the major modifications was to establish a two-step guanidinylation of
trichloroacetamide of a highly functionalized intermediate, which allowed us to prepare 15N2-labeled 5,11-dideoxytetrodotoxin for
biosynthetic investigations.
etrodotoxin1,2 (TTX, 1), a well-known marine natural
product, was originally isolated as a toxic principle of
puffer fish intoxication. The structure was determined in 1964
by three groups including Hirata-Goto, Tsuda, and Woodward
to be shown in Figure 1, which consists of a densely oxygenated
labeled tetrodotoxin analogues by total synthesis, because the
chemical modification of tetrodotoxin is extremely difficult due
to its complex structure and unique chemical properties.
The first total synthesis of racemic tetrodotoxin (1) was
achieved by Kishi and co-workers in 1972.12,13 We reported the
stereocontrolled synthesis of (−)-5,11-dideoxytetrodotoxin (5),
an unnatural analogue of tetodotoxin in 1999.14,15 This is the
first example of asymmetric synthesis of a tetrodotoxin
analogue, which has been a basis for the subsequent synthesis
of a variety of its analogues, including tetrodotoxin (1), in this
laboratory.16−20 In addition, we have also succeeded in the
synthesis of several naturally occurring and unnatural
tetrodotoxin analogues, such as 11-deoxytetrodotoxin (2)21
and 8,11-dideoxytetrodotoxin (3)22,23 from a common
synthetic intermediate.24 Quite recently, Yotsu-Yamashita
identified 5,11-dideoxytetrodotoxin (5) in nature,25 indicating
that 5 might be a biosynthetic precursor of tetrodotoxin. We
therefore planned to prepare stable isotope-15N2-labeled 5,11-
dideoxytetrodotoxin (at the guanidine group) for tracer
analysis. In our previous synthesis of 5,11-dideoxytetrodotoxin
(5), however, installation of the guanidine group required a
considerable number of steps26 because deprotection of N-
trichloroacetyl group was difficult. Since the route is not
suitable for the desired 15N2-labeled 5,11-dideoxytetrodotoxin
for this purpose, we reexamined the synthesis of 5 with
emphasis on improvement of the guanidinylation (vide infra).
On the basis of our previous synthesis of 5,11-dideoxyte-
trodotoxin (5), we planned to modify the synthetic route as
shown in Scheme 1. An enone 7, the same synthetic
intermediate of tetrodotoxin and its analogues as used in this
laboratory, would be transformed into a compound by cleavage
of the 1,2-diol protected as an acetonide of 7. Elaboration of the
enone A followed by ozonolysis of the vinyl group would
provide an aldehyde B. Stereocontrolled addition of acetylide
T
Figure 1. Structures of tetrodotoxin and its analogues.
cyclohexane possessing a cyclic guanidine with a hemiaminal,
an orthoester, and eight contiguous stereogenic centers.3−5 The
same year, Mosher reported that tarichatoxin isolated from
terrestrial California newts (Taricha torosa) was identical to
tetrodotoxin.6,7 Since then, tetrodotoxin and its analogues have
been isolated from many small animals such as newt, frog,
octopus, and crab.8,9 This diversity of TTX-bearing animals
suggests tetrodotoxin is not synthesized by the host animals but
is derived from external sources such as their diets. Since
tetrodotoxin-producing bacteria were identified from marine
sources in 1986,10,11 it is believed that tetrodotoxin in TTX-
bearing animals such as puffer fish is accumulated from bacteria
in the food chain. However, the biosynthetic origin of
tetrodotoxin has been completely unclear. We have been
interested in naturally occurring analogues of tetrodotoxin from
the standpoint of biosynthetic investigations, because those
analogues may be biosynthetic precursors or metabolites. One
experiment designed to elucidate the tetrodotoxin biosynthesis
is tracer analysis using isotope-labeled compounds of possible
precursors of tetrodotoxin. We planned to prepare isotope-
Received: December 21, 2012
Published: January 16, 2013
© 2013 American Chemical Society
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Note
aldehyde was protected as dimethyl acetal 8. When the
dimethyl acetal 8 was reduced under the Luche reduction used
in our previous synthesis of 5,14,15 allylic alcohol 9 was obtained
in a low yield and a considerable amount of starting material 8
was recovered, even when an excess amount of the reagents was
employed. Extensive examination revealed that DIBAL in
CH2Cl2 at −78 °C was the optimal condition for the
stereoselective reduction, giving the allylic alcohol 9 in a
moderate yield as a single product (dr >20:1). According to the
previous synthesis of 5, 9 was then transformed into an
aldehyde 12; epoxidation of 9 with MCPBA was followed by
protection of the secondary alcohol with benzyl ether to give
11. Subsequent ozonolysis of the vinyl group gave 12 (as
intermediate B in Scheme 1).
Scheme 1. Improved Synthetic Plan for (−)-5,11-
Dideoxytetrodotoxin (5)
With multigram quantities of 12 in hand, transformation into
lactone 15 by oxidative cleavage of the acetylenic moiety was
investigated (Scheme 3). Addition of magnesium trimethylsi-
and then oxidative cleavage of the acetylenic moiety would
afford lactone intermediate C. Guanidinylation of the
trichloroacetamide, followed by global deprotection under an
acidic condition, should provide 5,11-dideoxytetrodotoxin (5).
This new route includes two major modifications from the
previous one; (i) cleavage of 1,2-diol protected as acetonide to
acetal at an early stage (7 to A), (ii) two-step guanidinylation
from trichloroacetamide (C to D). The aim of the first
modification (i) is to set up the oxidation state of aldehyde at
the C-4 position before hydroxylation of the cyclohexane
moiety, which includes potentially cleavable 1,2-diol structure
with periodic acid oxidation. With regard to the second
modification (ii), we planned to utilize Cs2CO3-promoted
deprotection of N-trichloroacetyl group reported by this
laboratory in 2004,27 because the condition was compatible
with ester and acetal functional groups in some model
compounds. When amine could be obtained under the
condition, the resulting amine would be directly condensed
with a derivative of methylisothiourea to give guanidine-
containing intermediate D.
Scheme 3. Synthesis of Lactone 15 by Oxidative Cleavage of
the Acetylene and Lactonization
The synthesis commenced with cleavage of acetonide of
enone 7 with periodic acid in AcOEt (Scheme 2). The resulting
Scheme 2. Synthesis of Aldehyde 12 from Enone 7
lylacetylide in THF to 12 afforded the desired 13 as a single
diastereomer.28,29 The resulting adduct was unstable, and thus
was directly conducted to acetylation and then desilylation,
giving propargyl acetate 14. When 14 was treated with RuO4
under Sharpless conditions,30 which was employed in our first
synthesis of 5,11-dideoxytetrodotoxin (5),14,15 the product was
not the expected 15, but the different lactone 16. The structure
of 16 was determined by extensive NMR analysis; location of
the lactone was undoubtedly determined by HMBC correlation
between H-4 and C-10. The newly generated stereogenic
centers of C-4 and C-6 positions were determined by the
NOESY correlations. The reaction mechanism of the
production of 16 is proposed in Scheme 4. The acetylenic
moiety of 14 is first cleaved with RuO4 to give carboxylic acid
17. Under the acidic conditions, however, epoxide of the
intermediate 17 opens to generate a carbocation intermediate
18, which might be trapped by dimethyl acetal. Migration of
methoxy group of the resulting 19 affords an oxocarbenium
intermediate 20, which is captured by the carboxylic acid group
to give the observed lactone 16. To avoid production of 16,
alternative reagents with mild basic conditions for cleaving the
acetylenic moiety were explored. Finally, KMnO4 and NaIO4 in
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Note
Scheme 4. Proposed Mechanism for the Formation of
Lactone 16
Scheme 6. Synthesis of 5,11-Dideoxytetrodotoxin (5)
aqueous tert-butanol with an excess amount of NaHCO3 as a
base was found to be the best condition, giving the desired
lactone 15 in 88% yield without formation of 16.31
Prior to guanidinylation, the hydroxy group at the C-9
position was protected as an intramolecular acetal with the
aldehyde at the C-4 position in two steps (Scheme 5).
and H-7 indicates the boat conformation adopted in 24, which
was also supported by the observed coupling constant (J7,8
=
Scheme 5. Synthesis of di-Boc Guanidine 22 by a Two-step
Guanidinylation
4.5 Hz), whereas similar δ-lactone compounds such as 22
showed smaller coupling constant (J7,8 = 2.0 Hz). The
unexpected product 24 might be formed by intramolecular
transesterification of the desired 23. Unfortunately, retrans-
esterification from 24 back to 23 proved to be difficult under
several basic conditions. However, when Boc groups of 24 were
deprotected with TFA in MeOH, and the crude products were
then treated with 95% aqueous TFA, 4,9-anhydro-5,11-
dideoxytetrodotoxin (25) was obtained.14,15 Finally, according
to the previous synthesis, 25 was hydrolyzed with 2% TFA-d in
D2O to afford a mixture of 25 and 5,11-dideoxytetrodotoxin
(5), which were separated by an ion-exchange resin column
1
(Hitachi-gel #3013-c). The H and 13C NMR spectra of the
synthesized 4,9-anhydro-5,11-dideoxytetrodotoxin (25) and
5,11-dideoxytetrodotoxin (5) were identical to those reported
by this laboratory.14,15 It is worth noting that the successful
conversion of 24 to 25 indicates the γ-lactone-type
intermediate is also a possible precursor of tetrodotoxin
through an intramolecular transesterification.
Deacetylation with KCN in EtOH followed by treatment with
CSA in MeOH gave the acetal 21a and its epimer 21b in 73
and 13% yields, respectively.32 Guanidinylation of the resulting
acetal 21a was next attempted. As we anticipated, N-
trichloroacetyl group of 21a was deprotected with Cs2CO3 in
DMF at 100 °C to provide an amine, which was directly
subjected to the guanidinylation with N,N′-bis-Boc-S-methyl-
isothiourea in the presence of HgCl2 to give the desired di-Boc
guanidine 22 in a good overall yield. This is the first application
of the deprotection condition for the highly functionalized
synthetic intermediate in the synthesis of tetrodotoxin and its
analogues. The successful deprotection of trichloroacetamide of
21a demonstrates the usefulness of the deprotection condition
in the synthesis of complex natural products.
To complete the synthesis of 5,11-dideoxytetrodotoxin (5),
deprotection of the benzyl group of 22 was first attempted
(Scheme 6). However, hydrogenolysis of 22 under conven-
tional conditions with Pearlman’s catalyst (Pd(OH)2-C) gave
an unexpected γ-lactone 24, instead of the desired alcohol 23.
The γ-lactone structure of 24 was determined by extensive
NMR analysis; the NOESY correlation observed between H-4a
By utilizing the above-mentioned route, 15N2-labeled 4,9-
anhydro-5,11-dideoxytetrodotoxin (25A) was also synthesized
from acetal 21a as shown in Scheme 7; deprotection of the
trichloroacetamide of 21a with Cs2CO3 in DMF and
subsequent guanidinylation with 15N,15N′-bis-Boc-S-methyliso-
thiourea under the same conditions gave 15N2-labeled di-Boc
guanidine 22A. The successive removal of protective groups
was achieved by the same procedure to afford 15N2-labeled 4,9-
anhydro-5,11-dideoxytetrodotoxin (25A) in 50% yield.
In conclusion, we have developed an improved asymmetric
synthesis of (−)-5,11-dideoxytetrodotoxin (5) by utilizing the
two-step guanidinylation from trichloroacetamide. This new
route enables us to prepare 15N2-labeled 5,11-dideoxytetrodo-
toxin for biosynthetic studies of tetrodotoxin. Continuing
efforts toward biosynthetic investigations of tetrodotoxin by
using the synthesized 15N2-labeled compound are in progress.
EXPERIMENTAL SECTION
2,2,2-Trichloro-N-((1R)-6-(dimethoxymethyl)-4-methyl-2-
oxo-1-vinylcyclohex-3-en-1-yl)acetamide (8). To a solution of
■
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Note
Scheme 7. Synthesis of 15N2-labeled 4,9-Anhydro-5,11-
dideoxytetrodotoxin (25A)
2,2,2-Trichloro-N-((1R,2S,3R,6S)-4-(dimethoxymethyl)-2-hy-
droxy-6-methyl-3-vinyl-7-oxabicyclo[4.1.0]heptan-3-yl)-
acetamide (10). To a solution of allylic alcohol 9 (555 mg, 1.49
mmol) in dry CH2Cl2 (21 mL) were added Na2HPO4 (1.02 g, 7.17
mmol) and MCPBA (412 mg, 2.39 mmol) at rt. After stirring for 30 h,
the reaction mixture was quenched with an ice-cooled saturated
aqueous Na2SO3. The aqueous layer was extracted with CH2Cl2. The
combined organic layer was washed with saturated aqueous Na2SO3,
saturated aqueous NaHCO3 and brine, dried over anhydrous Na2SO4,
and concentrated. The residue was purified by flash column
chromatography (ether/hexane 1:5) to give 10 (614 mg, 98%) as a
white solid. mp 81−83 °C; [α]D30 +2.2 (c 0.88, CHCl3); IR (film) νmax
1
3651, 3318, 3137, 1703, 1535, 1274, 1121, 1053, 822 cm−1; H NMR
(400 MHz, CDCl3) δ 1.40 (3H, s, CH3−C), 1.59 (1H, dd, J = 15, 13
Hz, C−CHAHB−CH), 2.08 (1H, dd, J = 15, 4.5 Hz, C−CHAHB−CH),
2.19 (1H, ddd, J = 13, 8.5, 4.5 Hz, C−CHAHB−CH), 3.08 (1H, br s,
epoxidic), 3.27 (3H, s, CH−(OCH3)2), 3.41 (3H, s, CH-(OCH3)2),
4.04 (1H, br s, CH−OH), 4.38 (1H, d, J = 8.5 Hz, CH−(OCH3)2),
5.18 (1H, d, J = 17.5 Hz, CHCHAHB), 5.43 (1H, d, J = 11 Hz,
CHCHAHB), 5.80 (1H, dd, J = 17.5, 11 Hz, CHCHAHB), 5.81
(1H, br s, CH−OH), 9.85 (1H, br s, NH); 13C NMR (100 MHz,
CDCl3) δ 21.9, 29.0, 36.7, 50.6, 54.8, 57.6, 61.9, 65.0, 73.3, 93.2, 103.9,
117.2, 130.8, 163.2; Anal. Calcd for C14H20Cl3NO5: C, 43.26; H, 5.19;
N, 3.60. Found: C, 43.46; H, 5.26; N, 3.51.
N-((1R,2S,3R,6S)-2-(Benzyloxy)-4-(dimethoxymethyl)-6-
methyl-3-vinyl-7-oxabicyclo[4.1.0]heptan-3-yl)-2,2,2-trichlor-
oacetamide (11). NaH (60% purity, 235 mg, 5.88 mmol) was
washed with anhydrous hexane, and dry THF (10 mL) and DMF (5.0
mL) were added. To this suspension was added a solution of
epoxyalcohol 10 (614 mg, 1.47 mmol) in dry THF (5.0 mL) through a
cannula at rt. After stirring for 30 min, BnBr (0.4 mL, 2.94 mmol) was
added to the solution. After stirring for 10 h, the reaction mixture was
poured into an ice-cooled saturated aqueous NaHCO3. The aqueous
layer was extracted with AcOEt. The combined organic layer was
washed with water and brine, dried over anhydrous Na2SO4, and
concentrated. The residue was purified by flash column chromatog-
raphy (ether/hexane 1:8) to give 11 (639 mg, 91%) as a colorless oil.
[α]D30 −33.4 (c 1.55, CHCl3); IR (film) νmax 3679, 1720, 1504, 1068,
820 cm−1; 1H NMR (400 MHz, CDCl3) δ 1.35 (3H, s, CH3−C), 1.80
(1H, dd, J = 15, 13 Hz, C−CHAHB−CH), 2.17 (1H, dd, J = 15, 4.5 Hz,
C−CHAHB−CH), 2.90 (1H, br dt, J = 13, 4 Hz, C−CHAHB−CH),
3.03 (1H, s, epoxidic), 3.30 (3H, s, CH−(OCH3)2), 3.32 (3H, s, CH−
(OCH3)2), 4.22 (1H, d, J = 4 Hz, CH−(OCH3)2), 4.43 (1H, s, CH-
OBn), 4.63 (1H, d, J = 11.5 Hz, benzylic), 4.74 (1H, d, J = 11.5 Hz,
benzylic), 5.35 (1H, d, J = 17.5 Hz, CHCHAHB), 5.45 (1H, d, J =
11.5 Hz, CHCHAHB), 6.03 (1H, dd, J = 17.5, 11.5 Hz, CH
CHAHB), 6.96 (1H, br s, NH), 7.18−7.38 (5H, m, aromatic); 13C
NMR (100 MHz, CDCl3) δ 22.5, 27.4, 35.7, 54.3, 55.6, 58.5, 61.6,
63.1, 73.2, 76.7, 93.1, 104.5, 117.2, 127.8 × 2, 128.4, 134.3, 137.6,
160.2; Anal. Calcd for C21H26Cl3NO5: C, 52.68; H, 5.47; N, 2.93.
Found: C, 52.68; H, 5.60; N, 2.80.
enone 7 (2.68 g, 6.78 mmol) in AcOEt (167 mL) was added
HIO4•2H2O (2.01 g, 8.82 mmol) at rt. After stirring for 4 h, the
reaction mixture was quenched with an ice-cooled saturated aqueous
NaHCO3. The aqueous layer was extracted with AcOEt. The
combined organic layer was dried over anhydrous Na2SO4, and
concentrated. To a solution of the residue in MeOH (100 mL) were
added CSA (788 mg, 3.39 mmol) and CH(OMe)3 (50 mL) at rt. After
stirring for 1 h, the reaction mixture was quenched with an ice-cooled
saturated aqueous NaHCO3. The aqueous layer was extracted with
AcOEt. The combined organic layer was washed with brine, dried over
anhydrous Na2SO4, and concentrated. The residue was purified by
flash column chromatography (ether/hexane 1:2) to give 8 (2.07 g,
29
83% in 2 steps) as a colorless oil. [α]D +27.0 (c 1.05, CHCl3); IR
1
(film) νmax 2938, 1717, 1682, 1498, 1123, 1053, 852 cm−1; H NMR
(400 MHz, CDCl3) δ 1.98 (3H, s, CH3 −CCH), 2.45 (1H, dd, J =
20, 6.5 Hz, C−CHAHB-CH), 2.53 (1H, dd, J = 20, 11.5 Hz, C−
CHAHB−CH), 3.18 (1H, ddd, J = 11.5, 6.5, 4.5 Hz, C−CHAHB-CH),
3.34 (3H, s, CH-(OCH3)2), 3.40 (3H, s, CH−(OCH3)2), 4.39 (1H, d,
J = 4.5 Hz, CH−(OCH3)2), 5.27 (1H, d, J = 17.5 Hz, CHCHAHB),
5.36 (1H, d, J = 11 Hz, CHCHAHB), 5.97 (1H, s, CH3−CCH),
6.30 (1H, dd, J = 17.5, 11 Hz, CHCHAHB), 7.23 (1H, br s, NH);
13C NMR (100 MHz, CDCl3) δ 24.2, 29.3, 41.9, 54.7, 55.4, 65.3, 92.7,
104.8, 118.8, 124.5, 131.4, 160.6, 161.4, 191.5; Anal. Calcd for
C14H18Cl3NO4: C, 45.37; H, 4.89; N, 3.78. Found: C, 45.38; H, 4.88;
N, 3.66.
2,2,2-Trichloro-N-((1R,2R)-6-(dimethoxymethyl)-2-hydroxy-
4-methyl-1-vinylcyclohex-3-en-1-yl)acetamide (9). To a solution
of dimethyl acetal 8 (1.11 g, 2.78 mmol) in dry CH2Cl2 (25 mL) was
added DIBAL (1.01 M in toluene; 6.6 mL, 6.68 mmol) at −78 °C.
After stirring for 30 min, the reaction mixture was quenched with
acetone. The mixture was allowed to warm to rt, and a mixture of
saturated aqueous potassium sodium tartrate and water was added.
The mixture was vigorously stirred for 12 h, and the aqueous layer was
extracted with CH2Cl2. The combined organic layer was dried over
anhydrous Na2SO4, and concentrated. The residue was purified by
flash column chromatography (ether/hexane 1:4) to give 9 (738 mg,
N-((1R,2S,3R,6R)-2-(Benzyloxy)-4-(dimethoxymethyl)-3-
formyl-6-methyl-7-oxabicyclo[4.1.0]heptan-3-yl)-2,2,2-tri-
chloroacetamide (12). Benzyl ether 11 (498 mg, 1.04 mmol) was
dissolved in CH2Cl2 (21 mL). Ozone gas was passed into the solution
at −78 °C for 1.5 h. The remaining ozone gas was purged with
nitrogen, and Me2S (0.76 mL, 10.4 mmol) was added. The reaction
mixture was allowed to warm to rt and quenched with an ice-cooled
saturated aqueous NaHCO3. The aqueous layer was extracted with
CH2Cl2. The combined organic layer was dried over anhydrous
Na2SO4, and concentrated. The residue was purified by flash column
chromatography (ether/hexane 1:4) to give 12 (451 mg, 90%) as a
colorless oil. [α]D32 −67.4 (c 1.08, CHCl3); IR (film) νmax 3710, 3141,
30
66%) as a colorless oil. [α]D +32 (c 0.97, CHCl3); IR (film) νmax
1
3853, 3307, 1397, 1085, 750 cm−1; H NMR (400 MHz, CDCl3) δ
1.73 (3H, s, CH3−CCH), 1.77−1.90 (1H, m, C−CHAHB−CH),
2.05 (1H, dd, J = 18, 5 Hz, C−CHAHB−CH), 2.25 (1H, ddd, J = 12, 9,
5 Hz, C−CHAHB−CH), 3.32 (3H, s, CH-(OCH3)2), 3.45 (3H, s,
CH−(OCH3)2), 4.40 (1H, d, J = 9 Hz, CH−(OCH3)2), 4.43 (1H, br
s, CH−OH), 5.23 (1H, d, J = 18 Hz, CHCHAHB), 5.34 (1H, br s,
CH3−CCH), 5.40 (1H, d, J = 11 Hz, CHCHAHB), 5.84 (1H, dd,
J = 18, 11 Hz, CHCHAHB), 5.89 (1H, br s, CH−OH), 9.84 (1H, br
s, NH); 13C NMR (100 MHz, CDCl3) δ 22.4, 29.9, 42.0, 51.7, 55.0,
65.8, 73.7, 93.5, 104.7, 116.9, 123.3, 131.4, 133.0, 163.0; Anal. Calcd
for C14H20Cl3NO4: C, 45.12; H, 5.41; N, 3.76. Found: C, 45.12; H,
5.42; N, 3.73.
1
1714, 1594, 1498, 1066 cm−1; H NMR (400 MHz, CDCl3) δ 1.53
(3H, s, CH3−C), 2.31 (1H, dd, J = 16, 13 Hz, C−CHAHB−CH), 2.46
(1H, dd, J = 16, 5 Hz, C−CHAHB−CH), 3.22 (1H, s, epoxidic), 3.30−
3.37 (1H, m, C−CHAHB−CH), 3.34 (3H, s, CH−(OCH3)2), 3.40
(3H, s, CH-(OCH3)2), 4.17 (1H, br d, J = 2 Hz, CH−(OCH3)2), 4.59
(1H, s, CH−OBn), 4.64 (1H, d, J = 11.5 Hz, benzylic), 4.73 (1H, d, J
= 11.5 Hz, benzylic), 7.20−7.45 (5H, m, aromatic), 8.23 (1H, br s,
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Note
NH), 9.69 (1H, s, CHO); 13C NMR (100 MHz, CDCl3) δ 22.2, 25.2,
36.4, 56.7, 56.9, 58.8, 60.1, 67.0, 73.7, 74.9, 92.6, 104.4, 127.8, 128.2,
128.6, 136.6, 160.1, 194.2; Anal. Calcd for C20H24Cl3NO6: C, 49.96;
H, 5.03; N, 2.91. Found: C, 49.96; H, 5.12; N, 2.81.
(Benzyloxy)-6-hydroxy-1,7-dimethoxy-7-methyl-3-oxo-4a-(2,2,2-
trichloroacetamido)octahydro-1H-isochromen-4-yl acetate (16):
29
[α]D +1.4 (c 0.64, CHCl3); IR (film) νmax 3650, 3134, 1760, 1716,
1
1558, 1214, 1081, 822 cm−1; H NMR (400 MHz, CDCl3) δ 1.42
(1R)-1-((1R,2S,3S,6S)-2-(Benzyloxy)-4-(dimethoxymethyl)-6-
methyl-3-(2,2,2-trichloroacetamido)-7-oxabicyclo[4.1.0]-
heptan-3-yl)prop-2-yn-1-yl acetate (14). To an ice-cooled
solution of trimethylsilylacetylene (2.3 mL, 16.5 mmol) in dry THF
(53 mL) was added EtMgBr (0.98 M in THF; 14 mL, 13.7 mmol)
over 15 min. The solution was allowed to warm to rt and stirred for 15
min, and cooled to −20 °C. To the solution was added a solution of
aldehyde 12 (660 mg, 1.37 mmol) in dry THF (5.0 mL) through a
cannula. After stirring for 30 min, the reaction mixture was allowed to
warm to rt and stirred for 30 min. The reaction mixture was quenched
with an ice-cooled hydrochloric acid (0.12 N). The aqueous layer was
extracted with AcOEt. The combined organic layer was dried over
anhydrous Na2SO4, and concentrated. To a solution of the residue in
pyridine (16.7 mL) was added Ac2O (16.7 mL) at rt. After stirring for
12 h, the reaction mixture was diluted with toluene and concentrated.
To a solution of the residue in THF (25.6 mL) was added TBAF (1.0
M in THF; 1.4 mL, 1.37 mmol) at rt. After stirring for 15 min, the
reaction mixture was poured into an ice-cooled mixture of saturated
aqueous NH4Cl and H2O. The aqueous layer was extracted with
CH2Cl2. The combined organic layer was dried over anhydrous
Na2SO4, and concentrated. The residue was purified by flash column
chromatography (ether/hexane 1:4) to give 14 (568 mg, 75% in 3
(3H, s, CH3−C−OCH3), 1.64 (3H, s, CH3), 1.75 (1H, t, J = 14 Hz,
CH−CHAHB−C), 1.98 (1H, dd, J = 14, 4 Hz, CH−CHAHB−C), 3.31
(3H, s, COO−CH−OCH3), 3.48 (3H, s, CH3−C−OCH3), 3.74 (1H,
br dd, J = 14, 4 Hz, CH−CHAHB−C), 3.94 (1H, d, J = 10 Hz, BnO−
CH−CH−OH), 4.26 (1H, d, J = 10 Hz, BnO−CH−CH−OH), 4.48
(1H, d, J = 10 Hz, benzylic), 4.83 (1H, d, J = 10 Hz, benzylic), 5.06
(1H, d, J = 1 Hz, COO−CH−OCH3), 6.08 (1H, s, CH−COO), 7.25−
7.36 (5H, m, aromatic), 7.61 (1H, br s, NH); 13C NMR (100 MHz,
CDCl3) δ 16.1, 19.8, 33.8, 34.4, 49.7, 57.3, 61.9, 64.7, 76.1, 76.3, 77.1,
79.8, 92.8, 106.8, 128.1, 128.2, 128.5, 137.7, 160.9, 165.8, 168.2;
HRMS (ESI) for C23H28Cl3NO9Na [M + Na]+, calcd 590.0727, found:
590.0744.
N-((3S,3aR,4S,5S,6S,7aS)-4-(Benzyloxy)-6-hydroxy-1-me-
thoxy-6-methyl-9-oxooctahydro-5,3-(epoxymethano)iso-
benzofuran-3a-yl)-2,2,2-trichloroacetamide (21). To a solution
of lactone 15 (276 mg, 0.49 mmol) in EtOH (9.5 mL) was added
KCN (31.6 mg, 0.49 mmol) at rt. After stirring for 1 h, the reaction
mixture was diluted with a saturated aqueous NaHCO3. The aqueous
layer was extracted with AcOEt. The combined organic layer was
washed with brine, dried over anhydrous Na2SO4, and concentrated.
To a solution of the residue in MeOH (9.5 mL) was added CSA (31.6
mg, 0.49 mmol) at rt. After stirring for 12 h, the reaction mixture was
quenched with an ice-cooled saturated aqueous NaHCO3. The
aqueous layer was extracted with AcOEt. The combined organic
layer was dried over anhydrous Na2SO4, and concentrated. The
residue was purified by flash column chromatography (ether/hexane
1:2→1:1) to give 21a (175 mg, 73% in 2 steps) as a colorless oil and
29
steps) as a colorless oil. [α]D −48.7 (c 1.11, CHCl3); IR (film) νmax
1
3545, 2929, 1721, 1523, 1372, 1223, 1053, 820 cm−1; H NMR (400
MHz, CDCl3) δ 1.36 (3H, s, CH3−C), 1.98 (1H, dd, J = 16, 13 Hz,
C−CHAHB−CH), 2.07 (3H, s, CH3), 2.21 (1H, dd, J = 16, 5 Hz, C−
CHAHB−CH), 2.73 (1H, d, J = 2 Hz, CH−CCH), 3.02 (1H, dt, J =
13, 5 Hz, C−CHAHB−CH), 3.08 (1H, s, epoxidic), 3.30 (3H, s, CH−
(OCH3)2), 3.31 (3H, s, CH−(OCH3)2), 4.53 (1H, d, J = 5 Hz, CH−
(OCH3)2), 4.56 (1H, s, CH−OBn), 4.68 (1H, d, J = 11.5 Hz,
benzylic), 4.74 (1H, d, J = 11.5 Hz, benzylic), 6.34 (1H, d, J = 2 Hz,
CH−CCH), 7.25−7.40 (5H, m, aromatic), 7.74 (1H, br s, NH);
13C NMR (100 MHz, CDCl3) δ 21.1, 22.2, 27.5, 35.2, 52.9, 55.5, 58.8,
30
21b (31.0 mg, 13% in 2 steps) as a colorless oil. 21a: [α]D −1.4 (c
0.97, CHCl3); IR (film) νmax 3490, 2932, 1747, 1714, 1525, 1189,
1
1025, 822 cm−1; H NMR (400 MHz, CDCl3) δ 1.32 (1H, dd, J =
15.5, 11 Hz, CH−CHAHB−C), 1.38 (3H, s, HO−C−CH3), 2.02 (1H,
br dd, J = 15.5, 8 Hz, CH−CHAHB−C), 3.20 (1H, dd, J = 11, 8 Hz,
CH−CHAHB−C), 3.34 (3H, s, O−CH−OCH3), 4.43 (1H, br s,
COO−CH), 4.57 (1H, d, J = 11 Hz, benzylic), 4.62 (1H, s, CH−
COO), 4.66 (1H, d, J = 11 Hz, benzylic), 4.83 (1H, s, O−CH−
OCH3), 5.34 (1H, d, J = 2.5 Hz, BnO−CH), 7.03 (1H, br s, NH),
7.23−7.33 (5H, m, aromatic); 13C NMR (100 MHz, CDCl3) δ 27.5,
33.6, 44.3, 55.8, 64.5, 69.3, 71.7, 72.3, 80.6 × 2, 92.6, 111.4, 127.9,
128.2, 128.5, 137.0, 160.6, 167.5; Anal. Calcd for C20H22Cl3NO7: C,
62.3, 62.9, 64.1, 73.7, 76.8, 77.7, 78.7, 93.2, 103.9, 127.8, 128.4 × 2,
137.5, 160.2, 169.1; Anal. Calcd for C24H28Cl3NO7: C, 52.52; H, 5.14;
N, 2.55. Found: C, 52.54; H,5.17; N, 2.35.
(1S,4S,5R,6S,8S,9S)-9-(Benzyloxy)-6-(dimethoxymethyl)-8-
hydroxy-8-methyl-3-oxo-5-(2,2,2-trichloroacetamido)-2-oxa-
bicyclo[3.3.1]nonan-4-yl acetate (15). To a solution of acetate 14
(301 mg, 0.55 mmol) in t-BuOH (16.7 mL) and H2O (8.0 mL) were
added NaHCO3 (692 mg, 8.24 mmol), NaIO4 (1.41 g, 6.59 mmol)
and KMnO4 (86.7 mg, 0.55 mmol) at rt. After stirring for 30 min,
NaHCO3 (692 mg, 8.24 mmol) was added, and the mixture was
quenched with a saturated aqueous Na2SO3. The mixture was stirred
for 2 h, and the aqueous layer was extracted with CH2Cl2. To the
aqueous layer was added a saturated aqueous potassium sodium
tartrate at rt. After stirring for 12 h, the mixture was extracted with
CH2Cl2. The combined organic layer was dried over anhydrous
Na2SO4, and concentrated. The residue was purified by flash column
chromatography (ether/hexane 3:1) to give 15 (276 mg, 88%) as a
28
48.55; H, 4.48; N, 2.83. Found: C, 48.54; H, 4.66; N, 2.76. 21b: [α]D
−29 (c 0.20, CHCl3); IR (film) νmax 3330, 2926, 1716, 1519, 1456,
1262, 1215, 1178, 1074, 1019, 822 cm−1; 1H NMR (400 MHz,
CDCl3) δ 1.43 (3H, s, HO−C−CH3), 1.75−1.83 (2H, m, CH−CH2−
C), 3.38 (1H, ddd, J = 10, 9, 4 Hz, CH−CH2−C), 3.50 (3H, s, O−
CH−OCH3), 4.36 (1H, s, CH−COO), 4.42 (1H, br d, J = 2.5 Hz,
COO−CH), 4.55 (1H, d, J = 11.5 Hz, benzylic), 4.68 (1H, d, J = 11.5
Hz, benzylic), 5.12 (1H, d, J = 2.5 Hz, BnO−CH), 5.26 (1H, d, J = 4
Hz, O−CH−OCH3), 6.72 (1H, br s, NH), 7.26−7.37 (5H, m,
aromatic); 13C NMR (100 MHz, CDCl3) δ 28.0, 29.3, 40.6, 57.9, 65.4,
69.6, 71.6, 72.1, 77.6, 80.3, 108.6, 128.0, 128.3, 128.6, 136.7, 161.2,
167.7 (one peak was missing); HRMS (ESI) for C20H22Cl3NO7Na [M
+ Na]+, calcd 516.0360, found: 516.0354.
Carbamic Acid, N,N′-((3S,3aR,4S,5S,6S,7aS)-4-(Benzyloxy)-6-
hydroxy-1-methoxy-6-methyl-9-oxooctahydro-5,3-(epoxy-
methano)isobenzofuran-3a-yl)bis-, C,C′-Bis(1,1-dimethylethyl)
Ester (22). A mixture of acetal 21a (61.6 mg, 0.125 mmol) and
Cs2CO3 (81.0 mg, 0.249 mmol) in dry DMF (15.6 mL) was stirred at
100 °C for 2 h. After cooling to rt, the reaction mixture was diluted
with a saturated aqueous NaHCO3. The aqueous layer was extracted
with AcOEt. The combined organic layer was washed with brine, dried
over anhydrous Na2SO4, and concentrated. To a solution of the
residue in DMF (15.6 mL) were added Et3N (0.11 mL) followed by
N,N′-bis-Boc-S-methylisothiourea (89.2 mg, 0.249 mmol) and HgCl2
(67.6 mg, 0.249 mmol) at rt. After stirring for 1 h, the reaction
mixuture was diluted with AcOEt and filtered through a pad of Super-
Cel. The filtrate was washed with H2O and brine, dried over
28
white solid. mp 123−124 °C; [α]D −35 (c 0.94, CHCl3); IR (film)
νmax 3481, 2934, 1744, 1718, 1535, 1374, 1219, 1177, 1080, 821 cm−1;
1H NMR (400 MHz, CDCl3) δ 1.38 (3H, s, HO−C−CH3), 1.47 (1H,
dd, J = 16, 14 Hz, CH−CHAHB−C), 1.89 (1H, br dd, J = 16, 4 Hz,
CH−CHAHB−C), 2.29 (3H, s, CH3), 3.24 (3H, s, CH−(OCH3)2),
3.26 (3H, s, CH−(OCH3)2), 3.47 (1H, ddd, J = 14, 8, 4.5 Hz, CH−
CHAHB−C), 4.24 (1H, br s, COO−CH), 4.64 (1H, d, J = 8 Hz, CH−
(OCH3)2), 4.64 (1H, d, J = 11 Hz, benzylic), 4.73 (1H, d, J = 11 Hz,
benzylic), 5.36 (1H, d, J = 2 Hz, BnO−CH), 5.93 (1H, s, CH-COO),
7.28−7.37 (5H, m, aromatic), 7.66 (1H, br s, NH); 13C NMR (100
MHz, CDCl3) δ 20.8, 27.0, 33.6, 36.2, 50.2, 54.1, 63.0, 70.1, 71.0, 72.4,
73.2, 82.5, 92.6, 103.9, 128.0, 128.1, 128.5, 137.1, 160.6, 165.8, 171.3;
Anal. Calcd for C23H28Cl3NO9: C, 48.56; H, 4.96; N, 2.46. Found: C,
48.55; H, 4.96; N, 2.26.
Spectroscopic data of lactone 16 were obtained from the purified
compound in the separate experiment. (1S,4S,4aR,5S,6R,7R,8aS)-5-
1703
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The Journal of Organic Chemistry
Note
anhydrous Na2SO4, and concentrated. The residue was purified by
flash column chromatography (ether/hexane 3:1) to give 22 (84.2 mg,
5.25 (1H, s, H-4); 13C NMR (100 MHz, 4% CD3COOD/D2O) δ
27.7, 32.1, 42.4, 62.7, 65.2, 73.9, 84.0, 86.5, 86.8, 156.5, 172.1; HRMS
(ESI) for C11H16N3O5 [M + H]+, calcd 270.1090, found: 270.1086.
Carbamic Acid, 15N,15N′-((3S,3aR,4S,5S,6S,7aS)-4-(Benzyl-
oxy)-6-hydroxy-1-methoxy-6-methyl-9-oxooctahydro-5,3-
(epoxymethano)isobenzofuran-3a-yl)bis-, C,C′-Bis(1,1-dime-
thylethyl) Ester (22A). According to the procedure for synthesis of
22, 21a (43.8 mg, 88.5 μmol) was converted to 15N2-labeled di-Boc
guanidine 22A (40.7 mg, 78% in 2 steps) as a colerless oil. [α]D30 −5.0
(c 0.96, CHCl3); IR (film) νmax 3689, 2979, 2934, 1755, 1723, 1646,
30
quant. in 2 steps) as a colorless oil. [α]D −6.7 (c 0.99, CHCl3); IR
(film) νmax 3282, 2979, 1791, 1720, 1644, 1528, 1482, 1369, 1253,
1
1139, 1059, 1027 cm−1; H NMR (400 MHz, CDCl3) δ 1.35 (3H, s,
HO−C−CH3), 1.43 (9H, s, (CH3)3−C), 1.45 (1H, dd, J = 15, 12 Hz,
CH−CHAHB−C), 1.52 (9H, s, (CH3)3−C), 1.98 (1H, dd, J = 15, 8.5
Hz, CH−CHAHB−C), 3.29 (3H, s, O−CH−OCH3), 3.42 (1H, dd, J =
12, 8.5 Hz, CH−CHAHB−C), 4.41 (1H, br d, J = 2.5 Hz, COO−CH),
4.45 (1H, d, J = 12 Hz, benzylic), 4.69 (1H, s, CH−COO), 4.71 (1H,
d, J = 12 Hz, benzylic), 4.81 (1H, s, O−CH−OCH3), 5.29 (1H, d, J =
2.5 Hz, BnO−CH), 7.23−7.30 (5H, m, aromatic), 8.66 (1H, br s,
NH), 11.05 (1H, br s, NH); 13C NMR (100 MHz, CDCl3) δ 25.7,
28.0, 28.1, 32.8, 44.8, 55.3, 62.7, 69.5, 71.5, 71.8, 79.4, 80.7, 82.1, 83.6,
111.5, 128.1, 128.2, 128.3, 137.1, 152.6, 154.1, 161.4, 168.5; HRMS
(ESI) for C29H42N3O10 [M + H]+, calcd 592.2870, found: 592.2887.
5,11-DideoxyTTX (5). To a solution of di-Boc guanidine 22 (84.2
mg, 142 μmol) in MeOH (3.5 mL) was added Pd(OH)2-C (17.5 mg),
and the reaction vessel was charged with H2 gas. After stirring
vigorously at rt for 3 h, the reaction mixture was filtered through a pad
of Super-Cel, and concentrated. The residue was dissolved in Et2O and
passed through a column packed with anhydrous Na2SO4 and silica
gel, and concentrated to give γ-lactone 24, which was used without
further purification. To a solution of γ-lactone 24 in MeOH (13 mL)
was added TFA (13 mL) at rt. After stirring for 30 h, the reaction
mixture was concentrated. The residue was dissolved in 95% aqueous
TFA (26 mL) at rt. After stirring for 30 h, the reaction mixture was
concentrated to give 4,9-anhydro-5,11-dideoxyTTX (25), which was
used without further purification. 4,9-Anhydro-5,11-dideoxyTTX (25)
was dissolved in TFA-d (2% in D2O, 0.7 mL). After 7 days, the
solution was concentrated. The residue was purified by HPLC on a
Hitachi-gel #3013-c column (H+ form, 0.4 × 15 cm, 0.1N AcOH) to
give 5,11-dideoxyTTX (5) (6.9 mg, 23%), 4-epi-5,11-dideoxyTTX
(26) (3.7 mg, 12%), and 4,9-anhydro-5,11-dideoxyTTX (25) (7.3 mg,
24%), respectively. Spectroscopic data of γ-lactone 24 were obtained
from the purified compound in the separate experiment. Carbamic
acid, N,N′-((2aS,2a1R,4S,4aS,6S,7S,7aS)-6,7-dihydroxy-4-methoxy-
6-methyl-2-oxooctahydro-2H-1,3-dioxacyclopenta[cd]inden-2a1-
yl)bis-, C,C′-bis(1,1-dimethylethyl) ester (24): IR (film) νmax 3249,
2980, 1792, 1721, 1646, 1617, 1527, 1487, 1369, 1252, 1138, 1060,
751 cm−1; 1H NMR (400 MHz, CDCl3) δ 1.35 (3H, s, HO−C−CH3),
1.45 (9H, s, (CH3)3−C), 1.50 (9H, s, (CH3)3−C), 1.71 (1H, dd, J =
15.0, 10.5 Hz, CH−CHAHB−C), 1.98 (1H, dd, J = 15.0, 8 Hz, CH−
CHAHB−C), 2.36 (1H, br s, CH−OH), 2.74 (1H, dd, J = 10.5, 8 Hz,
CH−CHAHB−C), 3.37 (3H, s, O−CH−OCH3), 4.09 (1H, br s, CH−
OH), 4.92 (1H, s, CH−COO), 5.04 (1H, s, O−CH−OCH3), 5.10
(1H, d, J = 4 Hz, COO−CH), 8.97 (1H, br s, NH), 11.34 (1H, br s,
NH); 13C NMR (100 MHz, CDCl3) δ 26.8, 28.0 × 2, 34.9, 47.8, 55.5,
65.6, 71.1, 73.4, 80.1, 81.6, 83.3, 84.0, 113.6, 152.9, 154.5, 162.0, 171.8;
HRMS (ESI) for C22H36N3O10 [M + H]+, calcd 502.2401, found:
502.2397. 5: [α]D27 −8.3 (c 0.23, 0.05N AcOH); 1H NMR (400 MHz,
4% CD3COOD/D2O) δ 1.31 (1H, dd, J = 15.6, 13.2 Hz, H-5β), 1.38
(3H, s, H-11), 2.06 (1H, ddd, J = 15.6, 4.0, 1.2 Hz, H-5α), 2.26 (1H,
ddd, J = 13.2, 9.6, 4.0 Hz, H-4a), 4.37 (1H, dd, J = 2.4, 1.2 Hz, H-7),
4.43 (1H, d, J = 2.4 Hz, H-8), 4.64 (1H, s, H-9), 5.18 (1H, d, J = 9.6
Hz, H-4); 13C NMR (100 MHz, 4% CD3COOD/D2O) δ 27.5, 33.3,
42.6, 61.2, 71.2, 72.2, 73.9, 77.2, 87.4, 155.9, 176.6; HRMS (ESI) for
1
1602, 1395, 1369, 1139, 1105 cm−1; H NMR (400 MHz, CDCl3) δ
1.35 (3H, s, HO−C−CH3), 1.43 (9H, s, (CH3)3−C), 1.45 (1H, dd, J =
15, 11.5 Hz, CH−CHAHB−C), 1.52 (9H, s, (CH3)3−C), 1.98 (1H, dd,
J = 15, 9 Hz, CH−CHAHB−C), 3.29 (3H, s, O−CH−OCH3), 3.42
(1H, dd, J = 11.5, 9 Hz, CH−CHAHB−C), 4.41 (1H, br d, J = 2.4 Hz,
COO−CH), 4.45 (1H, d, J = 12.5 Hz, benzylic), 4.69 (1H, s, CH−
COO), 4.71 (1H, d, J = 12.5 Hz, benzylic), 4.81 (1H, s, O−CH-
OCH3), 5.28 (1H, d, J = 2.4 Hz, BnO−CH), 7.22−7.31 (5H, m,
aromatic), 8.66 (1H, br s, NH), 11.05 (1H, br d, 15NH); 13C NMR
(100 MHz, CDCl3) δ 25.8, 28.0, 28.1, 32.8, 44.8 (d, J = 2 Hz), 55.3,
62.7, 69.4 (d, J = 3 Hz), 71.5, 71.8, 79.3 (d, J = 2 Hz), 80.7, 82.1, 83.5,
111.5, 128.1, 128.2, 128.3, 137.1, 152.7 (d, J = 24 Hz), 154.1 (dd, J =
14, 7 Hz), 161.5 (d, J = 12 Hz), 168.5; HRMS (ESI) for
C29H42N15N2O10 [M + H]+, calcd 594.2811, found: 594.2808.
15N2-labeled 4,9-Anhydro-5,11-dideoxyTTX (25A). According
to the procedure for synthesis of 25, 22A (40.7 mg, 68.6 μmol) was
converted to 15N2-labeled 4,9-anhydro-5,11-dideoxyTTX (25A) (9.3
29
1
mg, 50%). [α]D −20 (c 0.37, 0.05N AcOH); H NMR (400 MHz,
4% CD3COOD/D2O) δ 1.24 (1H, dd, J = 16.0, 11.6 Hz, H-5β), 1.36
(3H, s, H-11), 2.11 (1H, ddd, J = 16.0, 7.2, 1.2 Hz, H-5α), 2.81 (1H,
dd, J = 11.6, 7.2 Hz, H-4a), 4.53 (1H, dd, J = 2.4, 1.2 Hz, H-7), 4.86
(1H, d, J = 2.4 Hz, H-8), 5.05 (1H, s, H-9), 5.27 (1H, d, J = 2.0 Hz, H-
4); 13C NMR (100 MHz, 4% CD3COOD/D2O) δ 28.3, 32.7, 43.0,
63.3, 65.8, 74.6, 84.6, 87.1 (d, J = 9 Hz), 87.4, 172.7 (one peak was
missing); HRMS (ESI) for C11H16N15N2O5 [M + H]+, calcd 272.1031,
found: 272.1019.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H and 13C NMR spectra for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nisikawa@agr.nagoya-u.ac.jp.
■
Present Address
†Department of Chemistry, National Tsing Hua University,
Hsinchu 30013, Taiwan.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to Prof. Mari Yotsu-Yamashita (Tohoku
University) for fruitful discussions. This work was financially
supported by Innovative Areas from the Japan Society for the
Promotion of Science (JSPS), Grants-in-Aid for Scientific
Research, the Sumitomo Foundation, the Nagase Science and
Technology Foundation, and a SUNBOR GRANT from the
Suntory Institute for Bioorganic Research.
25
C11H18N3O6 [M + H]+, calcd 288.1196, found: 288.1172. 26: [α]D
1
−46 (c 0.21, 0.05N AcOH); H NMR (400 MHz, 4% CD3COOD/
D2O) δ 1.39 (3H, s, H-11), 1.47 (1H, dd, J = 15.9, 13.8 Hz, H-5β),
1.81 (1H, ddd, J = 15.9, 4.0, 1.5 Hz, H-5α), 2.70 (1H, dt, J = 13.8, 4.0
Hz, H-4a), 4.37 (1H, dd, J = 2.2, 1.5 Hz, H-7), 4.49 (1H, d, J = 2.2 Hz,
H-8), 4.71 (1H, s, H-9), 4.95 (1H, d, J = 4.0 Hz, H-4); 13C NMR (100
MHz, 4% CD3COOD/D2O) δ 27.3, 33.1, 38.9, 60.1, 70.6, 71.9, 73.7,
74.4, 87.2, 155.6, 176.1; HRMS (ESI) for C11H18N3O6 [M + H]+,
28
REFERENCES
calcd 288.1196, found: 288.1197. 25: [α]D −22 (c 0.35, 0.05N
■
AcOH); 1H NMR (400 MHz, 4% CD3COOD/D2O) δ 1.22 (1H, dd, J
= 16.0, 11.6 Hz, H-5β), 1.34 (3H, s, H-11), 2.09 (1H, ddd, J = 16.0,
7.2, 1.2 Hz, H-5α), 2.79 (1H, dd, J = 11.6, 7.2 Hz, H-4a), 4.51 (1H, dd,
J = 2.4, 1.2 Hz, H-7), 4.84 (1H, d, J = 2.4 Hz, H-8), 5.03 (1H, s, H-9),
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1704
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The Journal of Organic Chemistry
Note
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Supporting Information.
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(28) The stereochemistry of the newly generated asymmetric center
at the C-9 position was confirmed to be the desired R from NOESY
correlation between H-5 and H-9 of bicyclic ether, which was
synthesized from the adduct 13.
(29) The high stereoselectivity of the reaction can be explained
through a chelated intermediate, in which the nucleophile attacked
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