Synthesis of conformationally north-locked pyrimidine nucleosides built on an oxabicyclo[3.1.0]hexane scaffold

Article abstract of DOI:10.1021/jo201716c

Beginning with a known 3-oxabicyclo[3.1.0]hexane scaffold (I), the relocation of the fused cyclopropane ring bond and the shifting of the oxygen atom to an alternative location engendered a new 2-oxabicyclo[3.1.0]hexane template (II) that mimics more closely the tetrahydrofuran ring of conventional nucleosides. The synthesis of this new class of locked nucleosides involved a novel approach that required the isocyanate II (B = NCO) with a hydroxyl-protected scaffold as a pivotal intermediate that was obtained in 11 steps from a known dihydrofuran precursor. The completion of the nucleobases was successfully achieved by quenching the isocyanate with the lithium salts of the corresponding acrylic amides that led to the uracil and thymidine precursors in a single step. Ring closure of these intermediates led to the target, locked nucleosides. The anti-HIV activity of 29 (uridine analogue), 31 (thymidine analogue), and 34 (cytidine analogue) was explored in human osteosarcoma (HOS) cells or modified HOS cells (HOS-313) expressing the herpes simplex virus 1 thymidine kinase (HSV-1 TK). Only the cytidine analogue showed moderate activity in HOS-313 cells, which means that the compounds are not good substrates for the cellular kinases.

Full text of DOI:10.1021/jo201716c

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pubs.acs.org/joc
Synthesis of Conformationally North-Locked Pyrimidine Nucleosides
Built on an Oxabicyclo[3.1.0]hexane Scaffold
Olaf R. Ludek and Victor E. Marquez*
Chemical Biology Laboratory, Center for Cancer Research, National Cancer Institute at Frederick, National Institutes of Health,
Frederick, Maryland 21702, United States
S
* Supporting Information
ABSTRACT: Beginning with a known 3-oxabicyclo[3.1.0]-
hexane scaffold (I), the relocation of the fused cyclopropane
ring bond and the shifting of the oxygen atom to an alternative
location engendered a new 2-oxabicyclo[3.1.0]hexane template
(II) that mimics more closely the tetrahydrofuran ring of
conventional nucleosides. The synthesis of this new class of
locked nucleosides involved a novel approach that required the
isocyanate II (B = NCO) with a hydroxyl-protected scaffold as a pivotal intermediate that was obtained in 11 steps from a known
dihydrofuran precursor. The completion of the nucleobases was successfully achieved by quenching the isocyanate with the
lithium salts of the corresponding acrylic amides that led to the uracil and thymidine precursors in a single step. Ring closure of
these intermediates led to the target, locked nucleosides. The anti-HIV activity of 29 (uridine analogue), 31 (thymidine
analogue), and 34 (cytidine analogue) was explored in human osteosarcoma (HOS) cells or modified HOS cells (HOS-313)
expressing the herpes simplex virus 1 thymidine kinase (HSV-1 TK). Only the cytidine analogue showed moderate activity in
HOS-313 cells, which means that the compounds are not good substrates for the cellular kinases.
In order to introduce an oxygen atom into the original
bicyclo[3.1.0]hexane system, we previously synthesized a set of
locked North/South isomers with an oxabicyclo[3.1.0]hexane
template represented by structures 3 and 4 (Figure 2).4
INTRODUCTION
■
The conformationally rigid bicyclo[3.1.0]hexane scaffold in 1
and 2 has been employed successfully to discern the North/
South conformational preferences of several nucleoside(tide)
binding enzymes for their natural substrates (Figure 1).1
Figure 2. Structures of two oxabicyclo[3.1.0]hexane nucleosides and
their positions in the pseudorotational cycle.
Figure 1. Conformationally locked nucleosides showing the value of P
of the embedded cyclopentane ring in the pseudorotational cycle.
Nucleoside numbering (clockwise) is used beginning with 1 at the
point of attachment of the base. C5 occupies the site of the oxygen; the
carbon at the tip of the cyclopropane ring is excluded from the
numbering.
However, the position of the oxygen in compound 3 is not
equivalent to that of a typical nucleoside, and hence, it is not a
good partner for compound 4, where the oxygen is in the
corresponding position of a normal nucleoside. To overcome
this deficiency, we propose reshuffling some bonds and atoms
in compound 3as depicted in Figure 3to restore the
oxygen to a position equivalent to that corresponding to a
conventional nucleoside.
However, in some enzymes, such as adenosine and cytidine
deaminases, the degree of North/South discrimination occurs
with a significant loss in binding affinity, even for the preferred
rigid conformer.2,3 This drop in affinity has been attributed to
the missing oxygen, which could either interact with key amino
acids at the active site through hydrogen bonding or participate
in important electronic interaction with the base through the
anomeric effect.3,4
With these changes implemented, both the South 4 (1E) and
the North 5 (4E) isomers contain an embedded tetrahydrofuran
ring that corresponds to equivalent nucleoside antipodes in the
Received: August 17, 2011
Published: October 25, 2011
© 2011 American Chemical Society
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Figure 3. “Reshuffling” of the North oxabicyclo[3.1.0]hexane template 3 to generate a new North oxabicyclo[3.1.0]hexane scaffold (5) with a
conventional glycosyl bond. The carbon at the tip of the cyclopropane ring is excluded from the numbering system.
South and North hemispheres, respectively, although both
conformations appear significantly shifted toward the East
(Figure 4). In the present investigation, we describe the
Scheme 1. Retrosynthetic Analysis of the (North)-
Oxabicyclo[3.1.0]hexane Scaffold
dihydrofuran 9, a compound already reported by Jung et al.7
The enantiomerically pure starting dihydrofuran 9 was thus
prepared according to the published seven-step procedure using
commercially available (S)-glycidol as a chiral precursor.7,8
Initially, we attempted to introduce a one-carbon side chain
by regioselective lithiation of the enol ether moiety in 9
followed by reaction with an appropriate electrophile (Scheme
2). Although metalation of 9 with t-BuLi proceeded very fast at
the desired 5-position in greater than 95% yield, as judged by
1H NMR and TLC analysis of the reaction of 10 with CD3OD
showing no decomposition,9 we were unable to alkylate 10 with
one-carbon fragments such as m-formaldehyde or benzyl
chloromethyl ether. In contrast to reports from Meyers et
al.,10 no reaction was observed with m-formaldehyde, while
TLC showed a range of products resulting from decomposition
of the starting material 9 when benzyl chloromethyl ether was
used as the aklylating agent. The use of dimethyl carbonate as a
one-carbon fragment in the alkylations of 10 resulted in
formation of the methyl ester 11; however, the yield was below
20%. Interestingly, when benzaldehyde was used as electro-
phile, 10 was alkylated in 68% yield (results not shown).
Since we were unable to introduce the side chain at position
5 of the dihydrofurane system by this approach, we pursued a
strategy derived from classical carbohydrate chemistry employ-
ing an OsO4-catalyzed cis-dihydroxylation of the double bond
in 9 with subsequent acetylation of the resulting hydroxy
groups (92% yield, two steps, Scheme 3). The α- and β-
anomers of 12 were obtained in a 1:1 ratio.7 Introduction of the
one-carbon fragment at C1 was accomplished by a C-
glycosylation reaction using (CH3)3SiCN (TMSCN) as the
nucleophile.11 Because of the neighboring effect of the C2
acetate in 12, only the β-anomer 13 was obtained in 93% yield.
We found that the reported solvent for this reaction,
nitromethane, could be easily replaced with the more
convenient solvent acetonitrile. DBU-promoted elimination of
acetic acid from 13 led directly to the dihydrofuran
intermediate 8 with a double bond suitable for cyclo-
Figure 4. Pseudorotational cycle positions of both types of
conformationally locked nucleosides: the bicyclo[3.1.0]hexanes 1
(2E) and 2 (3E) and the oxabicyclo[3.1.0]hexanes 4 (1E) and 5 (4E)
relative to the perfect North (3T2) and South (3T2) conformations.
synthesis of three pyrimidine analogues with the template
exhibited by 5.
RESULTS AND DISCUSSION
■
The proposed approach was inspired on the strategy that was
successful in the synthesis of the locked carbocyclic South
series,5 where the adjacency of the cyclopropane moiety to the
anomeric/pseudoanomeric position prevented the use of a
convergent glycosylation reaction. Therefore, we decided to
pursue a linear strategy for the construction of the heterocyclic
nucleobase, starting from the bicyclic amine 6 suitably
protected with an easily removable group (Scheme 1). This
amine, in turn, could be made from the corresponding
carboxylic acid 7 via Curtius degradation, which is known to
proceed under mild conditions without isomerization. In our
previous work,5 we have shown that carboxylic acids at the
bridgehead of a bicyclo[3.1.0]hexane template could be
obtained by reaction of an α,β-unsaturated nitrile with
diazomethane and subsequent saponification of the nitrile.
On the basis of such precedent, compound 8 was expected to
react similarly. Carbon−carbon bond formation at the anomeric
position of glycals is well documented6 and could be used for
the introduction of functionality at the 5-position of the chiral
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Scheme 2
Scheme 3
propanation. However, the reaction time for the elimination
was very long (4 days); therefore, the secondary hydroxy group
in 13 was readily deacetylated with sodium methoxide in
methanol and subsequently mesylated. Elimination of the
mesylate proceeded smoothly within minutes, and 8 was
obtained in 82% over three steps.
Scheme 4
From our expertise in constructing the bicyclo[3.1.0]hexane
system locked in the southern hemisphere of the pseudorota-
tional cycle,5 we decided to introduce the cyclopropane ring to
the double bond in 8 by forming a fused pyrazoline moiety via
an 1,3-dipolar cycloaddition of diazomethane and subsequent
light-mediated extrusion of nitrogen.12 Surprisingly, the
dihydrofuran 8 proved to be stable toward reaction with
diazomethane, even under Pd(OAc)2 catalysis.13 It is likely that
the fused vinyl ether system raises the energy of the LUMO of
the dipolarophile and prevents formation of the cycloaddition
product.14 Switching to Simmons−Smith based methods
(CH2I2/diethylzinc;15 CH2I2/diethylzinc/O2;16 CH2ICl/dieth-
ylzinc;17 CH2I2/triethylaluminum;18 CH2I2/Zn(Cu);19 and
CH2I2, Zn, CuCl, acetyl chloride20), which are used for
cyclopropanating activated olefins, was also unsuccessful and in
all cases unchanged starting material 8 was reisolated. In an
attempt to influence the electronic properties of the CC
double bond, we converted the nitrile in 8 to the corresponding
methyl ester (11) by reaction with sodium methoxide and
subsequent hydrolysis of the imidate (see Scheme 4).
Unfortunately, repeating the aforementioned methods for
cyclopropanation on this α,β-unsaturated ester also failed, and
no bicyclic product was isolated. Interestingly, cyclopropana-
tion of 8 with dichlorocarbene,21 generated from chloroform
and sodium hydroxide, appeared to have been successful but
subsequent attempts to remove the halogens with lithium
aluminum hydride led to decomposition.
application of the Furukawa protocol of the Simmons−Smith
reaction (Scheme 4).15 Because the intermediate alcohol 15
was not stable at room temperature, the crude was immediately
subjected to the cyclopropanation reaction. Aided by the bulky
tert-butyldiphenylsilyl protecting group, the reaction proceeded
with high stereoselectivity from the less shielded α-side of the
molecule to form the desired isomer 16 and only a small
amount of the undesired β-regioisomer 17 was formed (ratio
15:1, 78% combined yield after two steps). The bicyclic
alcohols 16 and 17 are stable compounds and were easily
separated by regular silica gel chromatography.
The stereochemical assignment of both isomers (16/17) was
1
based on the distinct H NMR spectra of both compounds
(Figure 5). Because of the preferred boat-like conformation and
the intrinsic rigidity of the oxobicyclo[3.1.0]hexane system,
coupling constants can easily be predicted by Karplus’ equation.
The major isomer 16 shows only a dd-system for H5, with
Finally, an effective cyclopropanation was achieved after
diisobutylaluminium hydride (DIBAL-H) reduction of the ester
side chain in 11 to the primary alcohol 15 and subsequent
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1
Figure 5. Stereochemical assignment of both isomers 16 and 17 based on H NMR spectroscopy.
coupling constants of 9.3 and 4.8 Hz to the cyclopropyl protons
H6a and H6b, respectively. No coupling is observed to H4 as a
result of a dihedral angle of 90° between H4 and H5. This result
is in accordance with the predicted α-cyclopropanation. As for
the minor isomer 17, clearly the H5 proton shows a ddd-
coupling pattern, with coupling constants of 9.4 and 4.9 Hz to
both cyclopropyl protons (H6a and H6b), as well as a 4.9 Hz
coupling to H4. A coupling constant of 4.9 Hz translates into a
dihedral angle of about 40°, supporting the proposed β-side
attack for the isolated minor isomer.
After having established the relative stereochemistry of both
bicyclic alcohols 16 and 17, the hydroxymethyl moiety in 16
had to be reoxidized to the corresponding carboxylic acid 7.
Therefore, the alcohol 16 was first oxidized to the aldehyde 18
by a Swern procedure, followed by sodium chlorite oxidation
(Scheme 5). Later, this two-step procedure was changed in
favor of a more convenient, one-step oxidation with sodium
periodate and ruthenium(III) catalysis.22
Finally, introduction of the nucleobase precursor was
successfully achieved by quenching the intermediate isocyanate
20 with the lithium salts of the acrylic amides 24 and 25,
leading to the uracil and thymidine precursors 26 and 27, which
were obtained in 34 and 41% yield, respectively (Scheme 5).24
Due to the instability of the glycon in strong acid media, the
ring-closure was performed in aqueous ammonium hydroxide
solution, followed by tetrabutylammonium fluoride (TBAF)
desilylation in THF, to yield the uridine and thymidine
analogues 29 and 31 in 81 and 86% yield, respectively, after two
steps.
The cytidine derivative 34 was prepared from the uracil 28
by transformation into the triazolo compound 32 and
subsequent ammonolysis (Scheme 6). Removal of the
TBDPS group from 33 with TBAF afforded the targeted,
conformationally locked cytidine derivative 34 in 73% yield
over three steps.
The anti-HIV activity of compounds 29 (uridine analogue),
31 (thymidine analogue), and 34 (cytidine analogue) was
initially explored using an already reported method.25 Briefly,
the procedure involves the use of an HIV-1 based vector
containing wild-type HIV-1 RT that lacks a functional Env
coding region and contains a luciferase reporter gene in the nef
coding region. The vector was used to infect conventional
human osteosarcoma (HOS) cells or a modified HOS cell line
(HOS-313) that express the herpes simplex virus 1 thymidine
kinase (HSV-1 TK). After 48 h, anti-HIV activity was
determined by measuring the decrease in luciferase activity in
infected cells. Only cells, which contain integrated viral DNA,
will produce luciferase in the infectivity assay. All the
compounds were inactive with the exception of the cytidine
analogue, which had an EC50 of 23 μM in HOS-313 cells and
an EC50 of 440 μM in HOS cells. These results mean that the
compounds are not good substrates for the cellular kinases and
The acid 7 was subjected to a Curtius degradation, using
diphenylphosphoryl azide (DPPA) as azide source. Heating of
the crude acyl azide in toluene afforded the intermediate
isocyanate 20, which was quenched by addition of benzyl
alcohol to form the stable benzyl carbamate 6 (73%, three
steps). Unfortunately, all attempts to generate the amine by
hydrogenolytic benzyl cleavage failed and only products
resulting from decomposition were observed by TLC analysis.
Presumably, the strained nature of the hemiaminal in 21 leads
to an opening of the cyclopropyl moiety and loss of structural
integrity.23 As an alternative, the intermediate isocyanate 20
was reacted with ammonia, leading to the urea 22 in 72% yield.
Although acylations of ureas to form precursors of pyrimidine
nucleobases are common procedures in nucleoside chemistry,
we were unable to obtain the acyl urea upon reaction with 23
by this strategy, possibly due to the poor nucleophilicity of the
urea moiety in 22.
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Scheme 5
even when phosphorylated by HSV-TK, as in HOS-313 cells,
the anti-HIV activity was not very impressive.
achieved by quenching the isocyanate intermediate 20 with the
lithium salts of the corresponding acrylic amides.
In summary, by exploiting the rigid nature of the
oxobicyclo[3.1.0]hexane template we have been able to
construct a new template, conceptually designed from known
3-oxabicyclo[3.1.0]hexane scaffold (compound 3). The reloca-
tion of the fused cyclopropane ring bond and the shifting of the
oxygen atom to an alternative location provided a new 2-
oxabicyclo[3.1.0]hexane template target. This template posed a
significant synthetic challenge since it has the nucleobase
attached to an anomeric carbon that forms part of a fused
cyclopropane ring. The synthesis of the 2-oxabicyclo[3.1.0]-
hexane template involved a novel approach that was completed
in eleven steps from a known dihydrofuran precursor. The
completion of the pyrimidine nucleobase was successfully
EXPERIMENTAL SECTION
■
General Synthetic Methods. All experiments involving water-
sensitive compounds were conducted under dry conditions (positive
argon pressure) using standard syringe, cannula, and septa apparatus.
All solvents were purchased anhydrous and stored over activated
molecular sieves. Hexanes, ethyl acetate, methylene chloride, and
methanol employed in chromatography were HPLC grade. Cen-
trifugally accelerated, radial, thin-layer chromatography was performed
on silica gel GF coated rotors with UV detection at 254 nm. Medium-
pressure flash chromatography was performed on prepacked silica gel
columns. TLC (analytical thin-layer chromatography) was performed
on precoated plates of silica gel (250 μm) containing a fluorescence
indicator. Sugar-containing compounds were visualized with the sugar
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mL), and stirring was continued for 30 min at room temperature. The
aqueous phase was diluted with water (100 mL) and extracted with
CH2Cl2 (3 × 50 mL). The combined extracts were dried (MgSO4)
and concentrated. The crude was purified by flash chromatography on
silica gel (EtOAc in hexanes 15−35%) to yield the title compound 13
(947 mg, 93%) as a colorless syrup: [α]20D = −8.30 (c 1.0, CHCl3); IR
(neat) 3071, 2931, 2857, 2358, 1477, 1589, 1472, 1427, 1364, 1225,
Scheme 6
1
1104, 1041, 996, 936, 919, 822, 796, 740 cm−1; H NMR (400 MHz,
CDCl3) δ 7.60−7.55 (m, 4 H), 7.40−7.30 (m, 6 H), 5.26 (dd, 1 H, J =
3.4, 2.3 Hz), 4.53 (d, 1 H, J = 2.3 Hz), 4.11 (dd, 1 H, J = 9.3, 7.5 Hz),
3.83 (dd, 1 H, J = 9.3, 7.4 Hz), 3.73 (dd, 1 H, J = 9.0, 4.9 Hz), 3.70
(dd, 1 H, J = 9.0, 5.2 Hz), 2.55−2.45 (m, 1 H), 2.01 (s, 3 H), 1.48 (s,
9 H); 13C NMR (100 MHz, CDCl3) δ 170.1, 135.5, 132.8, 129.9,
127.8, 116.3, 79.2, 72.1, 70.6, 61.7, 48.0, 26.7, 20.6, 19.2; FAB-MS (m/
z, relative intensity) 424.2 (MH+, 42), 366 (100). Anal. Calcd for
C24H29NO4Si: C, 68.05; H, 6.90; N, 3.31. Found: C, 68.17; H, 6.96; N,
3.21.
(2S,3R,4R)-4-tert-Butyldiphenylsilyloxymethyl-3-hydroxyte-
trahydrofuran-2-carbonitrile (14). The acetate 13 (5.83 g, 13.8
mmol) was dissolved in dry methanol (150 mL), and the mixture was
cooled to 0 °C. Sodium hydride (660 mg of 50% oil suspension, 13.8
mmol) was added in one portion, and the mixture was stirred at 0 °C
until all starting material was consumed (15 min.). After neutralization
with 1 N HCl, the solvent was evaporated and the residue was
partitioned between water (250 mL) and CH2Cl2 (250 mL). The
aqueous phase was extracted with CH2Cl2 (2 × 150 mL), and the
combined extracts were dried (MgSO4) and concentrated. The crude
was purified by flash chromatography on silica gel (EtOAc in hexanes,
25−50%) to yield the title compound 14 (5.19 g, 99%) as a colorless
spray reagent (5 mL of 4-methoxybenzaldehyde, 90 mL of ethanol, 5
mL of concentrated sulfuric acid, and 10 mL of glacial acetic acid) by
heating with a heat gun. NMR spectra were recorded in a 400 MHz
spectrometer. The coupling constants are reported in hertz, and the
peak shifts are reported on the δ (ppm) scale: abbreviations s (singlet),
d (doublet), t (triplet), q (quartet), m (multiplet), br (broad) and app
(apparent). Mass spectra (FABMS) were obtained at an accelerating
voltage of 6 kV and at a resolution of 2000. Glycerol was used as the
sample matrix, and ionization was effected by a beam of xenon atoms.
APCI and ESI mass spectra were obtained using a multimode ion
source (LC/MSD SL) using the loop injection mode. Optical
rotations were measured at 589 nm. Infrared spectroscopy data was
obtained neat and an independent laboratory performed elemental
analyses and HR MS.
oil: [α]20 = 0.38 (c 1.0, CHCl3); IR (neat) 3470, 3071, 2931, 2857,
D
1589, 1471, 1427, 1390, 1361, 1188, 1109, 997, 938, 822, 796, 740
cm−1; 1H NMR (400 MHz, CDCl3) δ 7.65−7.60 (m, 4 H), 7.45−7.35
(m, 6 H), 4.56 (dd, 1 H, J = 7.7, 4.0 Hz), 4.44 (d, 1 H, J = 4.0 Hz),
4.10 (dd, 1 H, J = 9.1, 7.7 Hz), 3.77 (dd, 1 H, J = 9.1, 8.0 Hz), 3.74
(dd, 2 H, J = 6.8, 1.0 Hz), 2.47−2.38 (m, 1 H), 2.34 (d, 1 H, J = 4.0
Hz), 1.05 (s, 9H); 13C NMR (100 MHz, CDCl3) δ 135.5, 132.8, 132.7,
130.0, 127.9, 117.5, 78.6, 72.9, 70.3, 62.3, 50.4, 26.8, 19.2; APCI-MS
+
(m/z) 399.2 (M + NH4 ). Anal. Calcd for C22H27NO3Si: C, 69.25; H,
(R)-Methyl 4-(tert-Butyldiphenylsilyloxymethyl)-4,5-dihy-
drofuran-2-carboxylate (11). The dihydrofuran 9 (100 mg, 0.295
mmol) was dissolved in dry THF (200 μL), and the solution was
cooled to −78 °C under argon. At this temperature, tert-butyllithium
(1.7 M in pentane, 225 μL, 0.384 mmol) was added dropwise, and the
yellow solution was stirred for 15 min. The solution was warmed to 0
°C and stirred over an ice bath, whereupon the solution became
colorless. After 45 min, the solution was cooled again to −78 °C, and
dimethyl carbonate (133 mg, 1.48 mmol) was added. The reaction was
stirred for 15 min at −78 °C, warmed to 0 °C, and stirred for an
additional 1 h. Saturated ammonium chloride solution (50 mL) was
added, and the aqueous phase was extracted with CH2Cl2 (3 × 20
mL). The combined organic extracts were dried (MgSO4) and
concentrated. The crude was purified by flash chromatography on
silica gel (EtOAc in hexanes 0−25%) to yield the methyl ester 11
(21.0 mg, 18%) as a colorless oil: [α]20D = −84.16 (c 1.0, CHCl3); IR
(neat) 3071, 2952, 2857, 1739, 1631, 1471, 1428, 1294, 1219, 1109,
7.13; N, 3.67. Found: C, 69.07; H, 6.98; N, 3.70.
(R)-4-tert-Butyldiphenylsilyloxymethyl-4,5-dihydrofuran-2-
carbonitrile (8). The nitrile 14 (1.38 g, 3.62 mmol) was dissolved in
dry CH2Cl2 (40 mL) under argon. The solution was cooled to 0 °C,
and triethylamine (1.51 mL, 10.9 mmol) was added. After stirring,
mesyl chloride (420 μL, 5.43 mmol) was added dropwise, and the
reaction was allowed to warm to room temperature. After 30 min, all
starting material was consumed according to TLC (EtOAc/hexanes
1:3), and the mixture was treated dropwise with DBU (1.35 mL, 9.05
mmol). Stirring was continued until no mesylate could be detected by
TLC analysis (30 min). Aqueous 1 N HCl (100 mL) was added, and
after phase separation the aqueous phase was extracted with CH2Cl2
(2 × 50 mL). The combined organic extracts were dried (MgSO4) and
concentrated. The crude was purified by flash chromatography on
silica gel (EtOAc in hexanes, 0−25%) to yield the α,β-unsaturated
nitrile 8 (1.16 g, 88%) as a colorless oil: [α]20 = −111.07 (c 1.0,
D
CHCl3); IR (neat) 3073, 2931, 2857, 2237, 1623, 1589, 1471, 1427,
1389, 1362, 1227, 1176, 1107, 997, 959, 917, 822 cm−1; 1H NMR (400
MHz, CDCl3) δ 7.63−7.58 (m, 4 H), 7.45−7.35 (m, 6 H), 5.70 (dd, 1
H, J = 2.6, 2.6 Hz), 4.47−4.40 (m, 1 H), 4.32 (ddd, 1 H, J = 9.1, 6.5,
2.6 Hz), 3.65−3.55 (m, 2 H), 3.35−3.25 (m, 1 H), 1.03 (s, 9 H); 13C
NMR (100 MHz, CDCl3) δ 135.5, 135.5, 133.1, 133.0, 131.2, 129.9,
127.8, 117.5, 111.6, 73.5, 64.7, 46.3, 26.7, 19.2. Anal. Calcd for
C22H25NO2Si: C, 72.69; H, 6.93; N, 3.85. Found: C, 72.66; H, 6.97; N,
3.79.
1
997, 941, 823, 781, 739 cm−1; H NMR (400 MHz, CDCl3) δ 7.60−
7.55 (m, 4 H), 7.40−7.30 (m, 6 H), 5.79 (d, 1 H, J = 3.0 Hz), 4.44
(dd, 1 H, J = 10.0, 9.5 Hz), 4.31 (dd, 1 H, J = 9.5, 6.6 Hz), 3.73 (s, 3
H); 3.60 (dd, 1 H, J = 10.0, 5.9 Hz), 3.53 (dd, 1 H, J = 10.0, 7.0 Hz),
2.32−2.22 (m, 1 H), 0.98 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ
160.6, 148.9, 135.5, 135.5, 133.3, 133.2, 129.7, 127.7, 112.2, 73.5, 65.4,
52.1, 46.2, 26.7, 19.2. Anal. Calcd for C23H28O4Si: C, 69.66; H, 7.12.
Found: C, 69.55; H, 7.17.
(2S,3R,4R)-4-tert-Butyldiphenylsilanyloxymethyl-2-cyanote-
trahydrofuran-3-yl Acetate (13). The diacetate 127 (1.10 g, 2.41
mmol) was dissolved in dry MeCN (20 mL) and TMSCN (1.08 g,
10.8 mmol) was added under argon at 0 °C. After adding a catalytic
amount of BF3·Et2O (60 μL, 0.48 mmol) the mixture was allowed to
reach room temperature and was stirred for 30 min. The reaction was
quenched by the slow addition of satd aqueous NaHCO3 solution (20
(R)-Methyl 4-(tert-Butyldiphenylsilyloxymethyl)-4,5-dihy-
drofuran-2-carboxylate (11). The nitrile 8 (4.68 g, 12.8 mmol)
was dissolved in dry methanol (150 mL), and the mixture was cooled
to 0 °C. Sodium hydride (560 mg, 55% oil suspension, 12.8 mmol)
was added in portions, and the mixture was warmed to room
temperature and stirred overnight. TLC analysis showed that all
starting material had reacted and the mixture was acidified by the
820
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addition of 20% aqueous acetic acid. The solvent was evaporated, and
the residue was partitioned between water (250 mL) and CH2Cl2 (250
mL). The aqueous phase was extracted with CH2Cl2 (2 × 150 mL),
and the combined extracts were dried (MgSO4) and concentrated. The
crude was purified by flash chromatography on silica gel (EtOAc in
hexanes, 5−30%) to yield the methyl ester 11 (3.20 g, 62%) as a
colorless oil. The spectroscopic data were identical to those reported
above.
7.58−7.53 (m, 4 H), 7.40−7.29 (m, 6 H), 4.04 (dd, 1 H, J = 9.7, 2.4
Hz), 3.71 (dd, 1 H, J = 9.7, 6.8 Hz), 3.49 (d, 2 H, J = 7.3 Hz), 2.45
(dddd, 1 H, J = 7.2, 7.2, 7.2, 2.4 Hz), 2.01 (dd, 1 H, J = 9.6, 6.1 Hz),
1.51 (dd, 1 H, J = 9.6, 6.4 Hz), 1.39 (dd, 1 H, J = 6.2, 6.2 Hz), 0.98 (s,
9 H); 13C NMR (100 MHz, CDCl3) δ 197.8, 135.5, 133.2, 133.2,
129.8, 127.7, 74.3, 71.0, 64.8, 43.6, 31.0, 26.8, 19.2, 18.4.
The crude aldehyde 18 (450 mg, 1.18 mmol), dissolved in acetone
(5.0 mL), was slowly added to a stirred mixture of sodium chlorite
(374 mg, 4.13 mmol), sodium dihydrogenphosphate (142 mg, 1.18
mmol), and 2-methylbut-2-ene (543 μL, 5.13 mmol) in water/acetone
(2:1, 15 mL) at 0 °C. The cooling bath was removed, and the mixture
was stirred at room temperature for 1 h. The mixture was acidified
with 1 N HCl to pH 2−3, and the aqueous phase was extracted with
CH2Cl2 (3 × 50 mL). The combined organic extracts were dried
(MgSO4) and concentrated to yield the crude acid 7 (465 mg, quant)
as a slightly yellow syrup, which was used in the following Curtius
reaction without any further purification: 1H NMR (400 MHz,
CDCl3) δ 7.60−7.54 (m, 4 H), 7.38−7.28 (m, 6 H), 4.07 (dd, 1 H, J =
9.6, 2.2 Hz), 3.67 (dd, 1 H, J = 9.6, 7.0 Hz), 3.54 (dd, 1 H, J = 10.4, 7.3
Hz), 3.50 (dd, 1 H, J = 10.4, 8.0 Hz), 2.47−2.40 (m, 1 H), 2.00 (dd, 1
H, J = 9.5, 6.1 Hz), 1.54 (dd, 1 H, J = 9.5, 6.1 Hz), 1.31 (dd, 1 H, J =
6.1, 6.1 Hz), 0.97 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ 175.3,
134.5, 134.5, 132.3, 132.2, 128.9, 126.7, 70.2, 64.7, 63.8, 42.9, 29.1,
25.8, 18.2, 17.7; ESI-MS (m/z) 419.2 (M + Na+).
((1R,4R,5R)-4-tert-Butyldiphenylsilyloxymethyl-2-
oxabicyclo[3.1.0]hexan-1-yl)methanol (16) and ((1S,4R,5S)-4-
tert-Butyldiphenylsilyloxymethyl-2-oxabicyclo[3.1.0]hexan-1-
yl)methanol (17). The ester 11 (1.25 g, 3.15 mmol) was dissolved in
anhydrous toluene (20 mL) and DIBAL-H (12.6 mL, 1.0 M in
toluene, 12.6 mmol) was added at −78 °C under argon. The mixture
was warmed to room temperature over 30 min and methanol (5 mL)
was carefully added. An aqueous satd solution of Seignette salt (100
mL) was added, and the aqueous phase was extracted with CH2Cl2 (3
× 50 mL). The combined organic extracts were dried (MgSO4) and
concentrated. The crude alcohol was used for the subsequent
cyclopropanation without any further purification. Thus, the crude
alcohol (1.02 g) was dissolved in dry CH2Cl2 (40 mL) at 0 °C under
argon, and Et2Zn (14.4 mL, 1.1 M in toluene, 15.8 mmol) was added
dropwise. Immediately after the addition, CH2I2 (1.27 mL, 15.8
mmol) was added and the cooling bath was removed. The reaction
was stirred at room temperature for 1 h, and satd ammonium chloride
solution (100 mL) was added carefully. The aqueous phase was
acidified with 1 N HCl to pH 3−2 and extracted with CH2Cl2 (3 × 50
mL). The combined extracts were dried (MgSO4) and concentrated.
The crude was purified by flash chromatography on silica gel (EtOAc
in hexanes 30−50%) to yield the bicyclic alcohol 16 (895 mg, 74%) as
a colorless syrup. As a minor byproduct, the diastereomer 17 (55.0 mg,
5%) was obtained as a colorless syrup.
An analytical sample was prepared by quantitative conversion to the
methyl ester with an etheral solution of diazomethane. The solvent
was evaporated, and the crude was purified by flash chromatography
(EtOAc in hexanes 0−30%) to yield the methyl ester 19 as a colorless
oil: [α]20D = 61.32 (c 1.0, CHCl3); IR (neat) 3073, 2930, 2857, 1729,
1440, 1428, 1377, 1349, 1193, 1150, 1109, 1040, 822, 771, 740 cm−1
;
1H NMR (400 MHz, CDCl3) δ 7.65−7.60 (m, 4 H), 7.45−7.30 (m, 6
H), 4.10 (dd, 1 H, J = 9.6, 2.4 Hz), 3.73 (s, 3 H), 3.70 (dd, 1 H, J =
9.6, 7.0 Hz), 3.58 (dd, 1 H, J = 8.5, 5.5 Hz), 3.54 (dd, 1 H, J = 8.5, 6.0
Hz), 2.46 (dddd, 1 H, J = 7.0, 7.0, 7.0, 2.4 Hz), 1.99 (dd, 1 H, J = 9.5,
6.0 Hz), 1.48 (dd, 1 H, J = 9.5, 6.0 Hz), 1.29 (dd, 1 H, J = 6.0, 6.0 Hz),
1.01 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ 171.8, 135.5, 133.4,
133.3, 129.7, 127.7, 70.9, 66.1, 65.0, 52.3, 43.9, 29.1, 26.7, 19.2, 18.3;
Compound 16: [α]20 = 8.44 (c 1.0, CHCl3); IR (neat) 3448,
D
3070, 2929, 2857, 1589, 1471, 1427, 1389, 1361, 1231, 1185, 1107,
1029, 940, 910, 822, 791 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.60−
7.55 (m, 4 H), 7.40−7.30 (m, 6 H), 3.90 (d, 1 H, J = 9.6 Hz), 3.87 (d,
1 H, J = 12.5 Hz), 3.55−3.43 (m, 4 H), 2.39 (ddd, 1 H, J = 7.2, 7.2, 7.2
Hz), 1.70 (bs, 1 H), 1.23 (dd, 1 H, J = 9.3, 4.8 Hz), 0.99 (s, 9 H), 0.95
(dd, 1 H, J = 6.4, 4.8 Hz), 0.55 (dd, 1 H, J = 9.3, 6.4 Hz); 13C NMR
(100 MHz, CDCl3) δ 135.5, 133.6, 133.5, 129.7, 127.7, 68.7, 68.4,
65.4, 64.6, 44.3, 26.8, 21.0, 20.,11.4; ESI-MS (m/z) 383.2 (M + H+),
405.1 (M + Na+). Anal. Calcd for C23H30O3Si·0.1H2O: C, 71.83; H,
7.92. Found: C, 71.61; H, 7.90.
ESI-MS (m/z) 428.2 (M + NH4 ), 433.1 (M + Na+). Anal. Calcd for
+
C24H30O4Si: C, 70.21; H, 7.36. Found: C, 70.42; H, 7.66.
Method B. The bicyclic alcohol 16 (800 mg, 2.09 mmol) was
dissolved in a mixture of water (30 mL), acetonitrile (20 mL), and
chloroform (20 mL) to which sodium periodate (2.46 g, 11.5 mmol)
and sodium bicarbonate (1.05 g, 12.5 mmol) were added. To this
mixture was added a catalytic amount of ruthenium(III) chloride (43.4
mg, 0.21 mmol), and the reaction was stirred overnight at room
temperature. After full conversion to the carbocylic acid, the mixture
was diluted with water (100 mL) and acifified to pH 2−3 by the
addition of 1 N HCl. The aqueous phase was extracted with CH2Cl2
(3 × 50 mL), and the combined extracts were filtered over a short pad
of Celite and concentrated. The crude acid 7 (830 mg, quant) was
used as such without any further purification. The analytical data were
identical to those reported in method A.
Compound 17. [α]20 = −30.27 (c 1.0, CHCl3); IR (neat) 3430,
D
3073, 2930, 2875, 1783, 1589, 1471, 1427, 1389, 1242, 1109, 1044,
1
953, 823, 792, 739 cm−1; H NMR (400 MHz, CDCl3) δ 7.68−7.60
(m, 4 H), 7.43−7.33 (m, 6 H), 4.08 (dd, 1 H, J = 9.7, 8.5 Hz), 3.99 (d,
1 H, J = 12.4 Hz), 3.72 (dd, 1 H, J = 10.0, 6.8 Hz), 3.62 (d, 1 H, J =
12.4 Hz), 3.60 (dd, 1 H, J = 10.0, 6.9 Hz), 3.20 (dd, 1 H, J = 9.7, 9.7
Hz), 2.86−2.76 (m, 1 H), 1.85−1.75 (bs, 1 H), 1.48 (ddd, 1 H, J = 9.4,
4.9, 4.9 Hz), 1.02 (s, 9 H), 0.99 (dd, 1 H, J = 6.8, 4.9 Hz), 0.47 (dd, 1
H, J = 9.4, 6.8 Hz); 13C NMR (100 MHz, CDCl3) δ 135.5, 135.5,
133.6, 133.5, 129.7, 127.7, 127.7, 70.0, 69.2, 64.9, 64.5, 43.3, 26.8, 21.3,
19.2, 9.8; ESI-MS (m/z) 383.2 (M + H+), 405.1 (M + Na+). Anal.
Calcd for C23H30O3Si: C, 72.27; H, 7.90. Found: C, 71.80; H, 7.82.
(1R,4R,5R)-4-tert-Butyldiphenylsilyloxymethyl-2-oxabicyclo-
[3.1.0]hexane-1-carboxylic Acid (7). Method A. Oxalyl chloride
(206 μL, 2.36 mmol) was dissolved in dry CH2Cl2 (10 mL) under
argon, and the solution was cooled to −78 °C. Dimethyl sulfoxide
(DMSO, 335 μL, 4.72 mmol) was added dropwise, and the reaction
was stirred for 5 min at that temperature. The primary alcohol 16 (450
mg, 1.18 mmol) was added to the mixture (disolved in 5.0 mL of dry
CH2Cl2), and the reaction was stirred for 1 h at −78 °C.
Triethylamine (987 μL, 7.08 mmol) was added, and the mixture was
warmed to room temperature and stirred for 1 h. Satd aqueous
ammonium chloride solution (50 mL) was added, and the aqueous
phase was extracted with Et2O (3 × 20 mL). The combined organic
washings were dried (MgSO4) and concentrated. The crude aldehyde
18 was sufficiently pure to be used in the next oxidation step without
any further purification: 1H NMR (400 MHz, CDCl3) δ 9.42 (s, 1 H),
Benzyl (1R,4R,5R)-4-tert-Butyldiphenylsilyloxymethyl-2-
oxabicyclo[3.1.0]hexan-1-ylcarbamate (6). The carbocyclic acid
7 (450 mg, 1.13 mmol) was dissolved in dry toluene (10 mL) and
cooled to 0 °C under argon. Triethylamine (236 μL, 1.70 mmol) and
diphenylphosphoryl azide (DPPA, 319 μL, 1.48 mmol) were added,
and the mixture was warmed to room temperature and stirred for 2 h.
The mixture was heated to 80 °C for 2 h and then cooled to 0 °C
again. Benzyl alcohol (293 μL, 2.83 mmol) was added, and the mixture
was stirred at room temperature for 30 min and then warmed to 80 °C
for 2 h. After being cooled to room temperature, the solvent was
evaporated and the crude was directly purified by flash chromatog-
raphy on silica gel (EtOAc in hexanes 50−70%) to yield the Cbz-
protected amine 6 (414 mg, 73%) as a colorless foam: [α]20D = 13.32
(c 1.0, CHCl3); IR (neat) 3321, 3069, 2930, 2857, 1733, 1589, 1489,
1
1427, 1389, 1227, 1190, 1106, 1084, 960, 822, 803, 737 cm−1; H
NMR (400 MHz, CDCl3) δ 7.65−7.60 (m 4 H), 7.42−7.20 (m, 11
H), 5.58 (bs, 1 H), 5.06 (s, 2 H), 3.99 (dd, 1 H, J = 9.5, 2.1 Hz), 3.84−
3.74 (m, 2 H), 3.55 (dd, 1 H, J = 9.5, 7.0 Hz), 2.39 (dddd, 1 H, J = 7.4,
821
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7.4, 7.0, 2.1 Hz), 1.55−1.47 (m, 1 H), 1.27−1.20 (m, 1 H), 1.03 (s, 9
H), 1.03−1.00 (m, 1 H); 13C NMR (100 MHz, CDCl3) δ 156.0,
136.1, 135.6, 133.7, 133.6, 129.7, 128.7, 128.6, 128.5, 128.1, 127.7,
73.5, 69.1, 66.9, 65.2, 44.4, 26.8, 23.5, 19.2, 16.3; ESI-MS (m/z) 502.2
(M + H+), 524.2 (M + Na+), 540.1 (M + K+); HR-FABMS m/z calcd
for C30H36NO4Si (M + H) 502.2414, found 502.2391.
hexanes 5−30%) over a short pad of silica gel. The clean acyl azide was
subsequently dissolved in dry toluene (6.0 mL), heated to 90 °C for 4
h, and then cooled to −78 °C. To this crude isocyanate, was added
dropwise a solution of freshly prepared lithium salt of (E)-3-
methoxyacrylamide [prepared by the dropwise addition of n-butyl
lithium (1.6 M in hexanes, 1.01 mmol, 630 μL) to a solution of (E)-3-
methoxyacrylamide (102 mg, 1.01 mmol) in dry THF (5.0 mL) at
−78 °C under argon and stirring for 0.5 h at this temperature], and the
mixture was stirred for 0.5 h at this temperature. The reaction was
quenched by the careful addition of satd aqueous ammonium chloride
solution (50 mL) and 1 N HCl (2 mL). The aqueous phase was
extracted with CH2Cl2 (3 × 50 mL), and the combined organic
extracts were dried (MgSO4) and concentrated. The crude product
was purified by chromatography on silica gel (EtOAc in hexanes 50−
70%) to yield the acyl urea 26 (85.0 mg, 34%) as a colorless foam:;
1-((1R,4R,5R)-4-tert-Butyldiphenylsilyloxymethyl-2-
oxabicyclo[3.1.0]hexan-1-yl)urea (22). The carbocyclic acid 7
(260 mg, 0.656 mmol) was dissolved in dry CH2Cl2 (6.0 mL) and
cooled to 0 °C under argon. Triethylamine (137 μL, 0.984 mmol) and
diphenylphosphoryl azide (DPPA, 184 μL, 0.852 mmol) were added,
and the mixture was warmed to room temperature and stirred for 24 h.
After evaporation of the solvent, the crude acyl azide was purified by
flash chromatography (EtOAc in hexanes 5−30%) over a short pad of
silica gel. The clean acyl azide was subsequently dissolved in dry
toluene (6.0 mL), heated to 90 °C for 4 h, and then cooled to 0 °C.
Concentrated aqueous ammonia solution (500 μL) was added, and the
mixture was stirred at room temperature for 2 h. The solvent was
evaporated, and the crude was directly purified by flash chromatog-
raphy on silica gel (EtOAc in hexanes 40−70%) to yield the urea 22
(194 mg, 72%) as a colorless foam: [α]20D = 45.90 (c 1.0, CHCl3); IR
(neat) 3482, 3318, 3209, 2929, 2857, 1681, 1589, 1427, 1360, 1307,
1194, 1109, 1025, 966, 933, 822, 790, 734 cm−1; 1H NMR (400 MHz,
CDCl3) δ 7.65−7.60 (m, 4 H), 7.45−7.30 (m, 7 H), 6.50 (bs, 1 H),
5.90 (bs, 1 H), 4.08 (dd, 1 H, J = 9.5, 2.3 Hz), 3.68 (dd, 1 H, J = 9.5,
6.7 Hz), 3.59 (dd, 1 H, J = 10.0, 6.5 Hz), 3.46 (dd, 1 H, J = 10.0, 8.0
Hz), 2.46 (dddd, 1 H, J = 6.7, 6.7, 6.7, 2.3 Hz), 1.85 (dd, 1 H, J = 9.5,
5.9 Hz), 1.56 (dd, 1 H, J = 9.5, 5.9 Hz), 1.18 (dd, 1 H, J = 5.9, 5.9 Hz),
1.03 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ 173.6, 135.5, 135.5,
133.5, 133.3, 129.2, 127.7, 127.7, 70.9, 67.7, 65.0, 44.0, 28.2, 26.8, 19.2,
16.3; HRFAB-MS m/z calcd for C23H31N2O3Si (M + H+) 411.2104,
found 411.2120.
[α]20 = 30.15 (c 1.0, CHCl3); IR (neat) 3260, 2931, 1702, 1660,
D
1
1616, 1532, 1470, 1428, 1359, 1241, 1109, 984, 805 cm−1; H NMR
(400 MHz, CDCl3) δ 9.42 (bs, 1 H), 9.20 (bs, 1H), 7.66−7.60 (m, 4
H), 7.58 (d, 1 H, J = 12.2 Hz), 7.40−7.32 (m, 6 H), 5.19 (d, 1 H, J =
12.2 Hz), 4.03 (dd, 1 H, J = 9.5, 2.5 Hz), 3.78 (dd, 1 H, J = 9.0, 7.5
Hz), 3.74 (dd, 1 H, J = 9.0, 6.5 Hz), 3.60 (dd, 1 H, J = 9.5, 7.1 Hz),
3.56 (s, 3 H), 2.40 (dddd, 1 H, J = 7.5, 7.3, 7.3, 2.5 Hz), 1.57 (dd, 1 H,
J = 9.6, 5.0 Hz), 1.25 (dd, 1 H, J = 6.5, 5.0 Hz), 1.08 (dd, 1 H, J = 9.6,
6.5 Hz), 1.02 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ 167.6, 163.7,
155.2, 135.5, 133.6, 133.6, 129.6, 127.7, 97.3, 72.5, 69.5, 65.2, 57.7,
44.9, 26.8, 23.4, 19.2, 16.6; APCI-MS m/z 495.2 (M + H+). Anal.
Calcd for C27H34N2O5Si: C, 65.56; H, 6.93; N, 5.66. Found: C, 65.55;
H, 6.67; N, 5.34.
1-((1R,4R,5R)-4-tert-Butyldiphenylsilyloxymethyl-2-
oxabicyclo[3.1.0]hexan-1-yl)pyrimidine-2,4(1H,3H)-dione
(28). The urea 26 (80.0 mg, 0.162 mmol) was dissolved in ethanol
(3.0 mL) and treated with concentrated aqueous ammonium
hydroxide solution (3.0 mL) at room temperature. The solution was
heated in a sealed pressure tube at 100 °C for 5 h. After the solution
was cooled to room temperature, the solvent was evaporated and the
residue was purified by chromatography on a chromatotron (MeOH in
CH2Cl2 0−5%) to yield the uracil derivative 28 (68.8 mg, 92%) as a
(E)-3-Methoxyacrylamide (24). (E)-3-Methoxyacryloyl chloride
(1.00 g, 8.3 mmol) was dissolved in dry CH2Cl2 (90 mL) under argon
and cooled to −20 °C. Aqueous concentrated ammonia solution (1.90
mL, 33.2 mmol) was added dropwise, and the mixture was vigorously
stirred at −20 °C for 1 h and then warmed to room temperature. The
solvent was evaporated, and the residue was purified by flash
chromatography on silica gel (MeOH in CH2Cl2 0−10%) to yield
the acrylamide 24 (720 mg, 86%) as a colorless solid: mp 126−129
°C; IR (neat) 3318, 3143, 1639, 1580, 1437, 1408, 1326, 1262, 1218,
colorless foam: [α]20 = 24.26 (c 1.0, CHCl3); IR (neat) 3061, 2931,
D
1
2856, 1692, 1427, 1385, 1290, 1206, 1107, 955, 821, 741 cm−1; H
NMR (400 MHz, CDCl3) δ 9.15 (bs, 1 H), 7.66−7.60 (m, 4 H),
7.45−7.34 (m, 6 H), 7.18 (d, 1 H, J = 8.0 Hz), 5.56 (dd, 1 H, J = 8.0,
2.2 Hz), 4.11 (dd, 1 H, J = 9.4, 3.4 Hz), 3.87 (dd, 1 H, J = 10.1, 7.4
Hz), 3.83 (dd, 1 H, J = 10.1, 7.4 Hz), 3.72 (dd, 1 H, J = 9.4, 7.5 Hz),
2.52 (dddd,, 1 H J = 7.3, 7.3, 7.3, 3.4 Hz), 1.86 (dd, 1 H, J = 10.1, 5.3
Hz), 1.45 (dd, 1 H, J = 7.0, 5.3 Hz), 1.25 (dd, 1 H, J = 10.1, 7.0 Hz),
1.05 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ 163.5, 150.1, 144.7,
135.6, 135.5, 133.4, 133.4, 129.8, 127.6, 102.6, 79.2, 71.2, 64.9, 44.8,
26.9, 24.4, 19.3, 17.9; APCI-MS m/z 463.2 (M + H+), 385.1 (M −
C6H6)+. Anal. Calcd for C26H30N2O4Si: C, 67.50; H, 6.54; N, 6.06.
Found: C, 67.56; H, 6.67; N, 5.84.
1
1175, 1121, 964, 933, 820 cm−1; H NMR (400 MHz, DMSO-d6) δ
7.29 (d, 1 H, J = 12.5 Hz), 7.10−7.00 (bs, 1 H), 6.60−6.50 (bs, 1 H),
5.25 (d, 1 H, J = 12.5 Hz), 3.55 (s, 3 H); 13C NMR (100 MHz,
DMSO-d6) δ 167.8, 159.6, 99.4, 57.4; ESI-MS (m/z) 102.1 (M + H+),
124.0 (M + Na+). Anal. Calcd for C4H7NO2·0.25H2O: C, 45.49; H,
7.16; N, 13.26. Found: C, 45.46; H, 6.94; N, 13.14.
(E)-3-Methoxy-2-methylacrylamide (25). (E)-3-Methoxy-2-
methylacryloyl chloride (1.00 g, 7.43 mmol) was dissolved in dry
CH2Cl2 (90 mL) under argon and cooled to −20 °C. Aqueous
concentrated ammonia solution (1.90 mL, 33.2 mmol) was added
dropwise, and the mixture was vigorously stirred at −20 °C for 1 h and
then warmed to room temperature. The solvent was evaporated, and
the residue was purified by flash chromatography on silica gel (MeOH
in CH2Cl2 0−10%) to yield the acrylamide 25 (625 mg, 83%) as a
colorless solid: mp 105−107 °C; IR (neat) 3332, 3161, 1656, 1590,
1-((1R,4S,5R)-4-Hydroxymethyl-2-oxabicyclo[3.1.0]hexan-1-
yl)pyrimidine-2,4(1H,3H)-dione (29). The silylated nucleoside 28
(40.0 mg, 86.5 μmol) was dissolved in THF (2.0 mL) at 0 °C, and a 1
M solution of tetrabutylammonium fluoride (TBAF, 173 μL, 0.173
mmol) was added dropwise. The cooling bath was removed, and the
mixture was stirred for 6 h at room temperature. The solvent was
evaporated, and the crude nucleoside was purified by chromatography
on a chromatotron (MeOH in CH2Cl2 0−10%) to yield the title
compound 29 (17.1 mg, 88%) as a colorless foam. The nucleoside was
redissolved in a mixture of acetonitrile/water (1:1, 3.0 mL) and was
1
1429, 1383, 1359, 1238, 1177, 1144, 971, 885, 813 cm−1; H NMR
(400 MHz, DMSO-d6) δ 7.04 (q, 1 H, J = 1.2 Hz), 6.90−6.50 (bs, 2
H), 3.65 (s, 3 H), 1.55 (d, 3 H, J = 1.2 Hz); 13C NMR (100 MHz,
DMSO-d6) δ 169.9, 154.9, 108.3, 60.8, 9.9; ESI-MS (m/z) 116.1 (M +
H+), 138.0 (M + Na+). Anal. Calcd for C5H9NO2: C, 52.16; H, 7.88;
N, 12.17. Found: C, 52.33; H, 7.97; N, 12.05.
(E)-N-((1R,4R,5R)-4-tert-Butyldiphenylsilyloxymethyl-2-
oxabicyclo[3.1.0]hexan-1-ylcarbamoyl)-3-methoxyacrylamide
(26). The carboxylic acid 7 (200 mg, 0.505 mmol) was dissolved in
dry CH2Cl2 (4.0 mL) and cooled to 0 °C under argon. Triethylamine
(120 μL, 0.857 mmol) and diphenylphosphoryl azide (DPPA, 164 μL,
0.758 mmol) were added, and the mixture was warmed to room
temperature and stirred for 24 h. After evaporation of the solvent, the
crude acyl azide was purified by flash chromatography (EtOAc in
lyophilized to yield the uracil derivative 29 as a colorless cotton: [α]20
D
= 42.68 (c 1.0, MeOH); IR (neat) 3398, 3035, 2853, 1671, 1455,
1
1390, 1294, 1203, 1051, 989, 974, 879, 850 cm−1; H NMR (400
MHz, CD3OD) δ 7.66 (d, 1 H, J = 8.0 Hz), 5.64 (d, 1 H, J = 8.0 Hz),
4.05 (dd, 1 H, J = 9.3, 2.8 Hz), 3.75−3.68 (m, 3 H), 2.46 (ddd, 1 H, J
= 13.8, 6.4, 2.8 Hz), 1.91 (dd, 1 H, J = 10.0, 5.3 Hz), 1.43 (dd, 1 H, J =
6.9, 5.3 Hz), 1.27 (dd, 1 H, J = 10.0, 6.9 Hz); 13C NMR (100 MHz,
CD3OD) δ 164.8, 150.9, 145.8, 101.6, 79.4, 70.4, 63.1, 44.8, 24.3, 15.9;
APCI-MS m/z 225.0 (M + H+); 242.1 (M + NH4 ). HRFAB-MS m/z
+
calcd for C10H13N2O4 (M + H+) 225.0875, found 225.0868.
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The Journal of Organic Chemistry
Featured Article
(E)-N-((1R,4R,5R)-4-(tert-Butyldiphenylsilyloxymethyl)-2-
oxabicyclo[3.1.0]hexan-1-ylcarbamoyl)-3-methoxy-2-methyla-
crylamide (27). The carboxylic acid 7 (200 mg, 0.505 mmol) was
dissolved in dry CH2Cl2 (4.0 mL) and cooled to 0 °C under argon.
Triethylamine (120 μL, 0.857 mmol) and diphenylphosphoryl azide
(DPPA, 164 μL, 0.758 mmol) were added, and the mixture was
warmed to room temperature and stirred for 24 h. After evaporation of
the solvent, the crude acyl azide was purified by flash chromatography
(EtOAc in hexanes 5−30%) over a short pad of silica gel. The clean
acyl azide was subsequently dissolved in dry toluene (6.0 mL), heated
to 90 °C for 4 h, and then cooled to −78 °C. To the crude isocyanate
was added dropwise a solution of the freshly prepared lithium salt of
(E)-3-methoxy-2-methylacrylamide [prepared by dropwise addition of
n-butyl lithium (1.6 M in hexanes, 1.01 mmol, 630 μL) to a solution of
(E)-3-methoxy-2-methylacrylamide (116 mg, 1.01 mmol) in dry THF
(5.0 mL) at −78 °C under argon and stirring for 0.5 h at this
temperature], and the mixture was stirred for 0.5 h at this temperature.
The reaction was quenched by the careful addition of satd aqueous
ammonium chloride solution (50 mL) and 1 N HCl (2 mL). The
aqueous phase was extracted with CH2Cl2 (3 × 50 mL), and the
combined organic extracts were dried (MgSO4) and concentrated. The
crude product was purified by chromatography on silica gel (EtOAc in
hexanes 40−60%) to yield the acyl urea 27 (105 mg, 41%) as a
mixture was allowed to warm to room temperature and stirred
overnight. The reaction was quenched by the addition of satd aqueous
sodium bicarbonate solution (50 mL), and the aqueous phase was
extracted with CH2Cl2 (3 × 50 mL). The combined organic extracts
were dried (MgSO4) and concentrated. The crude product was
purified by chromatography on a chromatotron (MeOH in CH2Cl2,
0−5%) to yield the title compound 32 (84.4 mg, 95%) as a colorless
foam: [α]20 = 23.20 (c 1.0, CHCl3); IR (neat) 3074, 2931, 2865,
D
1
1696, 1630, 1540, 1507, 1464, 1401, 1303, 1109, 954, 781 cm−1; H
NMR (400 MHz, CDCl3) δ 9.18 (s, 1 H), 8.05 (s, 1 H), 7.79 (d, 1 H, J
= 7.2 Hz), 7.64−7.59 (m, 4 H), 7.40−7.30 (m, 6 H), 6.81 (d, 1 H, J =
7.2 Hz), 4.21 (dd, 1 H, J = 9.3, 3.8 Hz), 3.95 (dd, 1 H, J = 10.1, 7.4
Hz), 3.88 (dd, 1 H, J = 10.1, 7.0 Hz), 3.79 (dd, 1 H, J = 9.3, 7.6 Hz),
2.56 (dddd, 1 H, J = 7.3, 7.3, 7.3, 3.8 Hz), 1.93 (dd, 1 H, J = 10.1, 5.4
Hz), 1.49 (dd, 1 H, J = 7.0, 5.4 Hz), 1.31 (dd, 1 H, J = 10.1, 7.0 Hz),
1.02 (s, 9 H); 13C NMR (100 MHz, CDCl3) δ 159.5 (C-4); 154.1 (C-
2); 154.0 (C-5, triazole); 151.1 (C-3, triazole); 143.4, 135.5, 135.5,
133.5, 129.8, 129.8, 127.8, 94.9, 81.4, 72.5, 65.1, 45.2, 26.9, 25.0, 19.1,
18.4; APCI-MS m/z 514.2 (M + H+); 436.1 ((M − C6H6)+); HRFAB-
MS m/z calcd for C28H32N5O3Si (M + H+) 514.2274, found 514.2252.
4-Amino-1-((1R,4S,5R)-4-hydroxymethyl-2-oxabicyclo-
[3.1.0]hexan-1-yl)pyrimidin-2(1H)-one (34). Compound 32 (75.0
mg, 0.146 mmol) was dissolved in dioxane (4.0 mL), and concd
aqueous ammonium hydroxide solution (1.0 mL) was added at room
temperature. The solution was stirred at room temperature overnight.
The solvent was evaporated, and the residue was dried under high
vacuum. Subsequently, the crude cytidine analogue (67.5 mg, 0.146
mmol) was dissolved in THF (3.0 mL) at 0 °C, and a 1 M solution of
tetrabutylammonium fluoride (TBAF, 292 μL, 0.292 mmol) was
added dropwise. The cooling bath was removed, and the mixture was
stirred at room temperature overnight. The solvent was evaporated
and the crude nucleoside was purified by chromatography on a
chromatotron (MeOH in CH2Cl2 0−15%) to yield the title compound
34 (25.0 mg, 77%) as a colorless foam: [α]20D = 48.68 (c 1.0, MeOH);
IR (neat) 3295, 2957, 1645, 1488, 1379, 1045, 865, 786, 715 cm−1; 1H
NMR (400 MHz, CD3OD) δ 7.63 (d, 1 H, J = 7.4 Hz), 5.85 (d, 1 H, J
= 7.4 Hz), 4.09 (dd, 1 H, J = 9.2, 2.8 Hz) 3.79 (dd, 1 H, J = 10.9, 5.4
Hz), 3.73−3.67 (m, 2 H), 2.48 (dddd, 1 H, J = 8.0, 5.5, 5.5, 2.8 Hz),
1.88 (dd, 1 H, J = 10.0, 5.2 Hz), 1.41 (dd, 1 H, J = 6.7, 5.2 Hz), 1.20
(dd, 1 H, J = 10.0, 6.7 Hz); 13C NMR (100 MHz, CD3OD) δ 166.5,
157.2, 146.2, 95.3, 80.4, 70.0, 63.4, 44.4, 24.7, 15.8; APCI-MS m/z
224.1 (M + H+); HRFAB-MS m/z calcd for C10H14N3O3 (M + H+)
224.1035, found 224.1029.
colorless foam: [α]20 = 28.87 (c 1.0, CHCl3); IR (neat) 3260, 2931,
D
2857, 1702, 1660, 1616, 1532, 1470, 1428, 1294, 1241, 1195, 1108,
905 cm−1; 1H NMR (400 MHz, CDCl3) δ 9.45 (bs, 1 H), 7.85 (bs, 1
H), 7.65−7.60 (m, 4 H), 7.45−7.35 (m, 6 H), 7.28 (q, 1 H, J = 1.0
Hz), 4.07 (dd, 1 H, J = 9.4, 2.2 Hz), 3.82 (s, 3 H), 3.76 (d, 2 H, J = 7.6
Hz), 3.58 (dd, 1 H, J = 9.4, 7.0 Hz), 2.38 (dddd, 1 H, J = 7.6, 7.6, 7.5,
2.2 Hz), 1.72 (d, 3 H, J = 1.0 Hz), 1.53 (dd, 1 H, J = 9.5, 5.0 Hz), 1.28
(dd, 1 H, J = 6.5, 5.0 Hz), 1.06 (dd, 1 H, J = 9.5, 6.5 Hz), 1.03 (s, 9
H); 13C NMR (100 MHz, CDCl3) δ 169.0, 158.8, 153.9, 135.5, 133.7,
133.71, 129.6, 127.7, 106.7, 72.4, 69.3, 65.2, 61.6, 44.8, 26.8, 23.2, 19.2,
16.4, 8.8; ESI-MS m/z 509.2 (M + H+), 531.2 (M + Na+). Anal. Calcd
for C28H36N2O5Si: C, 66.11; H, 7.13; N, 5.51. Found: C, 66.27; H,
7.12; N, 5.42
1-((1R,4S,5R)-4-Hydroxymethyl-2-oxabicyclo[3.1.0]hexan-1-
yl)-5-methylpyridine-2,4(1H,3H)-dione (31). The urea 27 (100
mg, 0.197 mmol) was dissolved in ethanol (3.0 mL) and treated with
concentrated aqueous ammonium hydroxide solution (3.0 mL) at
room temperature. The solution was heated in a sealed pressure tube
at 100 °C for 24 h. After the mixture was cooled to room temperature,
the solvent was evaporated and the residue was dried under high
vacuum. Subsequently, the crude silylated nucleoside 30 was dissolved
in THF (2.0 mL) at 0 °C, and a 1 M solution of tetrabutylammonium
fluoride (TBAF, 393 μL, 0.393 mmol) was added dropwise. The
cooling bath was removed, and the mixture was stirred for 6 h at room
temperature. The solvent was evaporated, and the crude nucleoside
was purified by chromatography on a chromatotron (MeOH in
CH2Cl2 0−10%) to yield the title compound 31 (41.3 mg, 88%) as a
colorless foam. The nucleoside was redissolved in a mixture of
acetonitrile/water (1:1, 3.0 mL) and was lyophilized to yield the
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org/.
AUTHOR INFORMATION
■
thymine derivative 31 as a colorless cotton: [α]20 = 45.28 (c 1.0,
D
Corresponding Author
MeOH); IR (neat) 3439, 3059, 2905, 1668, 1446, 1389, 1290, 1196,
1019, 950, 862, 758 cm−1; 1H NMR (400 MHz, CD3OD) δ 7.49 (q, 1
H, J = 1.1 Hz), 4.05 (dd, 1 H, J = 9.3, 2.8 Hz), 3.78−3.67 (m, 3 H),
2.45 (ddd, 1 H, J = 13.7, 6.2, 2.8 Hz), 1.91 (dd, 1 H, J = 10.0, 5.3 Hz),
1.83 (d, 3 H, J = 1.1 Hz), 1.42 (dd, 1 H, J = 6.8, 5.3 Hz), 1.25 (dd, 1 H,
J = 10.0, 6.8 Hz); 13C NMR (100 MHz, CD3OD) δ 165.0, 151.2,
141.4, 110.4, 79.2, 70.2, 63.2, 44.4, 24.3, 15.9, 10.7; APCI-MS m/z
239.1 (M + H+). Anal. Calcd for C11H14N2O4·0.80 H2O: C, 52.29; H,
6.22; N, 11.09. Found: C, 52.68; H, 5.94; N, 10.72.
1-((1R,4R,5R)-4-(tert-Butyldiphenylsilyloxymethyl)-2-
oxabicyclo[3.1.0]hexan-1-yl)-4-(1H-1,2,4-triazol-1-yl)-
pyrimidin-2(1H)-one (32). Triazole (358 mg, 5.19 mmol) was
dissolved in dry acetonitrile (10 mL) at 0 °C. Phosphoryl chloride
(47.5 μL, 0.591 mmol) was added dropwise, followed by a slow
addition of triethylamine (796 μL, 5.71 mmol), and the mixture was
stirred for 1 h at 0 °C. A solution of the uracil derivative 28 (80.0 mg,
0.173 mmol) in dry acetonitrile (2.0 mL) was slowly added, and the
*E-mail: marquezv@mail.nih.gov.
ACKNOWLEDGMENTS
■
We express our gratitude to Drs. Stephen H. Hughes and B.
Christie Vu of the HIV Drug Resistant Program at NCI for the
biological testing. This research was supported by the
Intramural Research Program of the NIH, National Cancer
Institute, Center for Cancer Research.
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