Stereoselective approach to the Z-isomers of methylenecyclopropane analogues of nucleosides: A new synthesis of antiviral synguanol

Article abstract of DOI:10.1080/15257770902831060

Stereoselective synthesis of antiviral synguanol (1) is described. Reaction of 6-benzyloxy-2-(dimethylaminomethyleneamino)purine (10) with ethyl (cis,trans)-2-chloro-2-(chloromethyl) cyclopropane-1-carboxylate (2c) under the conditions of alkylation-elimi

Full text of DOI:10.1080/15257770902831060

Nucleosides, Nucleotides and Nucleic Acids, 28:165–174, 2009
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1525-7770 print / 1532-2335 online
DOI: 10.1080/15257770902831060
STEREOSELECTIVE APPROACH TO THE Z-ISOMERS OF
METHYLENECYCLOPROPANE ANALOGUES OF NUCLEOSIDES:
A NEW SYNTHESIS OF ANTIVIRAL SYNGUANOL
Zhimeng Wu, Shaoman Zhou, and Jiri Zemlicka
Developmental Therapeutics Program, Barbara Ann Karmanos Cancer Institute, Wayne
State University School of Medicine, Detroit, Michigan, USA
Stereoselective synthesis of antiviral synguanol (1) is described. Reaction of 6-benzyloxy-
2
2-(dimethylaminomethyleneamino)purine (10) with ethyl (cis,trans)-2-chloro-2-(chloromethyl)
cyclopropane-1-carboxylate (2c) under the conditions of alkylation-elimination gave (Z)-6-
benzyloxy-2-formylamino-9-[(2-carbethoxycyclopropylidene)methyl]purine (11) but no E,N9-isomer.
Minor amounts of (Z)-6-benzyloxy-2-formylamino-7-[(2-carbethoxy-cyclopropylidene)methyl]purine
(13) were also obtained. Hydrolysis of compounds 11 and 13 in 80% acetic acid
afforded (Z)-9-[2-(carbethoxycyclopropylidene)methyl]guanine (14) and (Z)-7-[2-(carbethoxy-
cyclopropylidene)methyl]guanine (15). Reduction of 14 furnished synguanol (1). Reaction of
N4-acetylcytosine (7) with ester 2c led to (Z,E)-1-(2-carbethoxycyclopropropylidenemethyl)cytosine (8,
Z/E ratio 6.1:1). Basicity of purine base, lower reactivity of alkylation intermediates as well as
interaction of the purine N3 or cytosine O2 atoms with the carbonyl group of ester moiety seem to
be essential for the observed high stereoselectivity of the alkylation-elimination. The Z-selectivity is
interpreted in terms of E1cB mechanism leading to a transitory “cyclic” cyclopropenes which undergo
a cyclopropene-methylenecyclopropane rearrangement.
Keywords Methylenecyclopropanes; nucleoside analogues; alkylation-elimination; Z-
stereoselectivity; antivirals; synguanol
INTRODUCTION
Methylenecyclopropane analogues of nucleosides have received much
attention owing to their significant antiviral effects.[1–3]This biological
activity resides in the Z-isomers of purine derivatives whereas the E-
isomers and pyrimidines are effective only in a few instances. In the
Received 30 October 2008; accepted 3 February 2009.
We thank Hongwang Zhang for some initial experiments and L. M. Hrihorczuk from the Central
Instrumentation Facility (D. M. Coleman, Director) for mass spectra. The work described herein was
supported by SBIR Grant 5 R44 AI0554135 to Microbiotix, Inc., Worcester, MA, USA and Research Grant
1 RO1 CA32779 from the National Cancer Institute, National Institutes of Health, Bethesda, MD, USA.
Address correspondence to Jiri Zemlicka, Barbara Ann Karmanos Cancer Institute, 110 E. Warren
Ave., Detroit, MI, 48201-1379, USA. E-mail: zemlicka@karmanos.org
165

166
Z. Wu et al.
SCHEME 1 Synthesis of methylenecyclopropane analogues by alkylation-elimination.
first generation series, the Z-isomer synguanol (1) is among the most
potent. It is active in vivo against human and murine cytomegalovirus
(HCMV and MCMV).[4]Synthesis of methylenecyclopropane analogues of
nucleosides is based on alkylation-elimination[5–8]method (Scheme 1). The
reaction of dibromocyclopropanes 2a or 2b and nucleic acid bases or
appropriate precursors 3 is usually carried out under base catalysis at an
elevated temperature in DMF. Under these conditions, the products of
alkylation 4a or 4b are not usually isolated but they are converted in situ
to Z- and E-isomeric mixtures of methylenecyclopropanes 5a or 5b. After
reduction[5,6](5a) or deacetylation[7] (5b), the Z- and E-isomers of 6 are
separated by chromatography. Difficult isomer separation coupled with the
fact that a considerable amount of intermediates 4a or 4b is converted
to E-isomers of very limited use constitutes a drawback of this protocol.
Selective synthesis of the Z-isomers of purine methylenecyclopropanes 6
can therefore be of a significant value. An example of such a procedure,
synthesis of synguanol (1), is described in this communication.
RESULTS AND DISCUSSION
Synthesis
Alkylation-elimination procedure with esters 2a gave a somewhat im-
proved Z/E ratio (2:1)[5,6] over that employing reagents 2b (1:1 or
1:2).[7,8]We have now found that this ratio was significantly increased by

Z-Methylenecyclopropane Nucleoside Analogues
167
SCHEME 2 Stereoselective alkylation-elimination of N4-acetylcytosine (7) with ester 2c.
using dichloro ester 2c in the reaction with N4-acetylcytosine (7) employing
K2CO3/Cs2CO3 as a base in DMF at 100–105◦C to give ester 8 (Z/E 6.1:1,
56% yield, Scheme 2). Compound[9]2c was obtained in 65% yield by addi-
tion of ethyl diazoacetate on 2,3-dichloro-1-propene catalyzed by Rh2(OAc)4
using the previously described[5] protocol for the corresponding dibromo
derivative6 2a. It should be noted that reaction of 7 with dibromo ester
2a under similar conditions (K2CO3, DMF, 100◦C) gave 47% yield of the
ester[6]9 (Et = Me) with the Z/E ratio of only 2.3:1. As indicated above,
purine Z-methylenecyclopropanes are in the center of our current interest.
In contrast to dibromo ester[5]2a, reaction of dichloro ester 2c with 2-amino-
6-chloropurine (9a) led to a mixture of products even after a prolonged
reaction time. We have reasoned that lower reactivity of 2c may be offset by
employing a more basic nucleobase precursor.
To increase the basicity of a purine base, we turned our attention to
N-dimethylaminomethylene derivatives of purine bases.[10]The 2-amino-6-
benzyloxypurine (9b) was converted to N-dimethylaminomethylene com-
pound 10 using N,N-dimethylformamide dimethyl acetal in DMF[10] (85%,
Scheme 3). Alkylation-elimination of 10 with dichloro ester 2c in DMF
(K2CO3, 110–115◦C, 40 hours) followed by chromatography gave the Z,N9-
isomer of N-formylmethylenecyclopropane 11 (via intermediary chloro
ester 12) in 41–47% yield. The Z,N7-isomer Z-13 (3.4%) and unresolved
mixture of Z,N7- and E,N7-isomers E, Z-13 (8.6%) were also obtained. The
E-isomer of 11 was not detected. As expected,[11]the long-wavelength UV
maximum of the N7-isomer Z-13a was bathochromically shifted relative
to that of N9-isomer 11. Likewise, the chemical shifts of 11 and Z-13
/(δH8(N9) < δH8(N7) and C8(N9) < C8(N7)/ reflected the pattern of
other purine methylenecyclopropanes[11]and they were in line with N7- and
N9-alkylpurines.[12]The strong NOE enhancements between the H4 - H8
ꢁ
7
ꢁ
ꢁ
(5.81%) and H1 - H3 (3.37%) of Z-13 provided final support for the Z,N
isomeric structure while they ruled out the Z,N3- or E,N3-isomers.

168
Z. Wu et al.
SCHEME 3 Stereoselective synthesis of synguanol (1).
A smooth transformation of the N-dimethylaminomethylene group
into N-formyl function is surprising. In a study of hydrolysis of N-
dimethylaminomethylene derivatives of nucleosides[13]at different pH val-
ues, the N-formyl intermediates have not been detected. Treatment of 11
with 80% acetic acid at 75–85◦C resulted in removal of the both N-formyl
and O-benzyl groups to give guanine methylenecyclopropane ester (14,
85%). A similar hydrolysis of the Z,N7-isomer Z-13 furnished ester 15 in
71% yield. The UVmax and H8 (C8) chemical shifts patterns of 14 and 15
corresponded to those observed for compounds 11 and Z-13. Reduction of
14 with DIBALH in THF furnished synguanol (1, 79%).
Reaction Course
Two sites of proton abstraction and two mechanisms of elimination can
be considered to explain the reaction course observed in the β-elimination
of halide (Br or Cl) from the Z(trans)- and E(cis) intermediates 4a-4c. The
E2 mechanism initiated by a proton abstraction at the methylene group

Z-Methylenecyclopropane Nucleoside Analogues
169
adjacent to a heterocyclic base can lead to a direct formation of both Z-
and E-methylenecyclopropane isomers 5a–5c. Depending on the base used,
the E(cis)-isomeric esters 4a (B = adenine) gave the Z,E isomers of 5a (B
= adenine) in the ratios[5]of 1.5–2.5:1 whereas the Z(trans)-isomers 4a (B =
adenine) led to ratios 0.5–1.5:1. It is important to note that a preponderance
of the Z-isomers 5a (B = adenine) may possibly be explained by an
interaction of the carbonyl group with the N3 of purine ring in a syn
conformation (structure 16). A similar structure 17, with the carbethoxy
function interacting with the 2-keto group of cytosine of the E(cis)-isomer
4a (B = cytosine), can also be visualized whereas in the Z(trans)-isomers 4a
(B = adenine or cytosine) such a seven-membered “cyclic” arrangement is
not possible (structures 18 and 19). Interestingly, the IR spectra provided
an evidence for such an interaction in the E(cis)-isomer 16 (νCO 1745 cm−1)
as opposed to the Z(trans)-isomer 18 (νCO 1725 cm−1).[5]
In an alternate E1cB mechanism (Scheme 4), proton abstraction by a
base A(-) at the cyclopropane carbon attached to the ester moiety of 12 can
generate carbanion 20 from both Z- and E-isomers of 12. The elimination
of chloride leads then to the cyclopropene intermediate 21 with a cis ar-
rangement of the heterocyclic portion and carbethoxy group. Because of an
increased rigidity of 21 and necessary cis-configuration of the cyclopropene
moiety, interaction between the carbonyl group of the ester function
and the N3 can be stronger than that observed in the Z-isomer[5]16.
This “cyclic” arrangement (seven-membered “ring”) of the cyclopropene
21 may then facilitate a subsequent cyclopropene-methylenecyclopropane
rearrangement[14]to form preferentially a Z-methylenecyclopropane system
of 11 via carbanion 22. Similar rearrangement was observed in methylened-
ifluorocyclopropane series of purine nucleoside analogues.[15]In this in-
stance, a thermodynamically controlled mixture of Z- and E-isomers
and difluorocyclopropene analogue related to the proposed intermediate
21 was obtained. It is likely that the N3- CO (ester) interaction will
be strengthened in the presence of more basic heterocycles. The N-
dimethylaminomethylene group can increase the electron density at the N3
of purine ring (structures 23 and 24). In a similar fashion, intermediate 25
may explain high Z-selectivity in the formation of ester 8. Interestingly, in
this case, the N-acetyl function which decreases the basicity of the cytosine
ring does not significantly interfere with the Z-stereoselectivity of the
process.
Difference between the degree of stereoselectivity of β-elimination of
bromo and chloro substituted derivatives 4a and 4c is more difficult to
explain. A mechanistic dichotomy at two different reaction sites, E2 for
bromides at a methylene group carrying the nucleic acid base and E1cB for
chlorides at cyclopropane CH α to the carbethoxy group, may be invoked.
Although bromides appear to be better leaving groups than chlorides
for both nucleophilic substitution and β-elimination,[16]kinetic studies of

170
Z. Wu et al.
SCHEME 4 Proposed mechanism of stereoselective alkylation-elimination.
β-elimination in systems with different leaving groups including bromide
and chlorides are not clear-cut. In some cases, the chlorides preferred
E1cB mechanisms[17]whereas bromides followed E2 but in other[18] no
definite conclusion was possible. Nevertheless, in these instances, only a
single possible reaction site was present in the molecule.
EXPERIMENTAL SECTION
General Methods
The UV spectra were measured in ethanol and NMR spectra were
determined at 300 or 400 MHz (1H) and 75 or 100 MHz (13C) in CD3SOCD3

Z-Methylenecyclopropane Nucleoside Analogues
171
unless stated otherwise. Mass spectra were determined in electron-impact
(EI-MS), chemical ionization (CI-MS, 2-methylpropane) or electrospray ion-
ization (ESI-MS, methanol-NaCl) mode. Thin-layer chromatography (TLC)
was performed on Analtech aluminum foils coated with silica gel F254. For
acronyms of common reagents, solvents and protecting groups see J. Org.
Chem. 2006, 71, 28A-29A.
Starting Materials
2-Amino-6-benzyloxypurine (9b) was prepared as described.[19]
Ethyl (cis,trans)-2-Chloro-2-(chloromethyl)cyclopropane-1-carboxylate
(2c). A solution of ethyl diazoacetate (11.4 g, 0.1 mol) in CH2Cl2 (10 mL)
was added into a stirred mixture of 2,3-dichloro-1-propene (15.9 mL, 10.17
mol) and Rh2(OAc)4 (11.5 mg, 0.025 mmol) in CH2Cl2 (2 mL) with the aid
of a syringe pump at a rate of 1 mL/h at room temperature. The solvent
and unreacted alkene were distilled off and trapped at −78◦C. The water
(50 mL) was added to the residue followed by a solution of KMnO4 (15 g)
in water (60 mL) with external ice-cooling and stirring. The stirring was
continued for 2 hours and excess of KMnO4 was removed by addition of
solid Na2S2O3. The mixture was extracted with ethyl ether (3 × 25 mL),
the organic phase was washed successively with saturated NaHCO3, water
and brine. It was dried (MgSO4) and the solvent was evaporated to give
1
compound 2c as a yellow oil (12.8 g, 65%). H NMR (CDCl3) δ 1.29 (two
overlapped t, 3H, J = 7.2 Hz, CH3), 1.51 (dd, J = 9.3, 6.9 Hz), 1.71 (d, J
= 8.1 Hz), 1.85 (t, J = 6.9 Hz, total 2H, H3), 2.18 (dd, J = 9.3, 6.9 Hz),
2.35 (t, J = 8.3 Hz, 1H, H1), 3.76, 3.80 (AB, J = 12.3 Hz), 3.97, 4.08 (AB,
J = 11.9 Hz, 2H, CH2Cl), 4.20 (two overlapped q, J = 7.2 Hz, CH2 of Et).
13C NMR 14.3, 14.5 (CH3), 20.6, 24.6 (C3), 27.9, 30.1 (C1), 47.0, 48.7 (C2),
48.8, 51.9 (CH2Cl), 61.7, 61.8 (CH2 of Et), 168.1, 169.7 (C=O). EI-MS 196,
198 (M, 2.5, 1.8), 133 (100.0), HRMS calcd for C7H1035Cl2O2 196.0058,
found 196.0052. CI-MS (2-methylpropane) 197, 199 (M + H, 100.0, 64.0).
(Z,E)-1-(2-Carbethoxycyclopropropylidenemethyl)cytosine (8). A mix-
ture of N4-acetylcytosine (7, 1.53 g, 10 mmol), dichloro ester 2c (2.2 g,
11 mmol), K2CO3 (4.14 g, 30 mmol) and Cs2CO3 (9.75 g, 30 mmol) was
heated in DMF (50 mL) with stirring at 100–105◦C for 7 hours. The reaction
mixture was cooled to 80◦C, ethanol (15 mL) was added and the stirring was
continued for 1 hour. The solvents were evaporated in vacuo and the crude
product was chromatographed on a silica gel column in CH2Cl2-MeOH
(40:1 to 20:1) and then in CH2Cl2-MeOH (40:1) to give the Z,E-isomers (8,
1.31 g, 56%, ratio 6.1:1). 1H NMR corresponded to compound6 8 (Et = Me)
except the Z/E ratio. 1H NMR δ 1.11–1.23 (m, 3H, CH3), 1.79, 1.72 (t and m,
ꢁ
ꢁ
Z-isomer), 2.03, 1.94 (2m, total 2H, H3 , E-isomer), 2.76, 2.43 (2m, 1H, H4 ),
ꢁ
4.08 (m, 2H, CH2 of Et), 5.81 (d, J = 7.2 Hz, 2H, H5,), 7.37 (s, 1H, H1 ), 7.41
(s, 2H, NH2), 7.65, 7.95, (2d, 1H, J = 7.6 Hz, Z- and E-isomer, ratio 6.1:1,

172
Z. Wu et al.
13
ꢁ
ꢁ
H6), C NMR (Z-isomer) 9.3 (C3 ), 14.7 (CH3), 18.8 (C4 ), 61.4 (CH2 of
ꢁ
ꢁ
Et), 109.3, 116.7 (C1 , C2 ), 96.1, 140.5, 154.4, 166.1 (cytosine), 171.0 (C=O,
ester).
6-Benzyloxy-2-(dimethylaminomethyleneamino)purine
(10).
A
mixture of 2-amino-6-benzyloxypurine (9b, 5.6 g, 23 mmol) and
N,N-dimethylformamide dimethyl acetal (9.0 mL, 66.7 mmol) in DMF
(100 mL) was stirred at 40◦C for 16 hours. The solvent was evaporated in
vacuo and the crude product was crystallized from EtOAc (40 mL) to give
compound 10 (5.79 g, 85%) as a white solid, mp 244–245◦C. UV λmax 293
1
nm (ε 26,400), 233 (ε 14,200), 202 (ε 23,800). H NMR δ 2.99, 3.10 (2s,
6H, CH3), 5.54 (s, 2H, CH2 of Bn), 7.31–7.40 (m, 3H), 7.47 (d, 2H, J = 8.0
Hz, Ph), 8.01 (s, 1H, H8), 8.57 (s, 1H, N2=CH), 12.74 (bs, 1H, NH). 13C
NMR 35.2, 41.0 (CH3), 67.7 (CH2 of Bn), 128.6, 128.9, 129.1, 137.7 (Ph),
140.5, 142.9, 155.4, 158.6, 160.0, 162.3 (purine, N2=CH). EI-MS 296 (M,
5.2), 93 (100.0). HRMS calcd M 296.1386, found 296.1383. Anal. Calcd for
C15H16N6O: C, 60.80; H, 5.44; N, 28.36. Found: C, 60.81; H, 5.55; N, 28.20.
(Z)-6-Benzyloxy-2-formylamino-9-[(2-carbethoxycyclopropylidene)
methyl]purine (11) and (Z)-6-Benzyloxy-2-formylamino-7-[(2-carbethoxy-
cyclopropylidene)methyl]purine (13). A mixture of compound 10 (450
mg, 1.49 mmol), dichloro ester 2c (600 mg, 3.0 mmol) and K2CO3 (1.24
g, 8.93 mmol) in DMF (8 mL) was stirred at 110–115◦C under N2 for
40 hours. After cooling, the insoluble portion was filtered off and it
was washed with DMF. The filtrate was concentrated and residue was
chromatographed on a silica gel column using CH2Cl2-MeOH (100:1 to
20:1) to give the Z,N9-isomer 11 (239 mg, 41%), Z,N7-isomer Z-13 (20 mg,
3.4%) and mixture of the Z- and E-N7-isomer 13 (50 mg, 8.6%). In another
experiment run on a 4-mmol scale, compound 11 was obtained in 47%
yield.
Z,N9-Isomer 11: Mp 162–163◦C. UV λmax 271 (ε 18,000), 232 (ε 32,200),
210 nm (ε 32,800). 1H NMR δ 1.07 (t, J = 6.8 Hz, 3H, CH3), 1.92 (m, 1H),
ꢁ
ꢁ
2.03 (t, 1H, J = 8.8 Hz, H3 ), 2.98 (m, 1H, H4 ), 4.02 (q, 2H, J = 6.8 Hz, CH2
ꢁ
of Et), 5.59 (s, 2H, CH2 of Bn), 7.34–7.41, 7.49–7.53 (2m, 6H, Ph and H1 ),
8.29 (s, 1H, H8), 9.45 (d, J = 9.6 Hz, 1H, CH=O), 10.90 (d, J = 9.6 Hz,
1H, NH). 13C NMR 11.2 (C3 ), 14.6 (CH3), 20.2 (C4 ), 61.4 (CH2 of Et), 69.0
ꢁ
ꢁ
ꢁ
ꢁ
(CH2 of Bn), 111.9, 114.5, 118.1, 140.1, 152.3, 153.5, 160.8 (C1 , C2 , purine),
129.0, 129.2, 129.4, 136.7 (Ph), 164.5 (CH=O), 170.8 (C=O, ester). EI-MS
393 (M, 15.6), 91 (100.0). HRMS calcd M 393.1437, found 393.1437. Anal.
Calcd for C20H19N5O4: C, 61.06; H, 4.87; N, 17.80. Found: C, 61.07; H, 4.76;
N, 17.79.
Z,N7-Isomer Z-13: Mp 153–154◦C. UV λmax 285 (ε 7,000), 250 (ε 33,300),
1
206 (ε 23,300). H NMR δ 1.10 (t, J = 6.4 Hz, 3H, CH3), 1.92–1.95 (m,
ꢁ
ꢁ
1H), 2.01 (t, J = 8.8 Hz, 1H, H3 ), 2.94 (t, J = 5.6 Hz, 1H, H4 ), 4.04–4.07
(m, 2H, CH2 of Et), 5.62 (s, 2H, CH2 of Bn), 7.34–7.41 (m, 3H), 7.55 (d,

Z-Methylenecyclopropane Nucleoside Analogues
173
ꢁ
J = 7.6 Hz, 3H, Ph overlapped with H1 ), 8.51 (s, 1H, H8), 9.39 (d, J = 10.0
Hz, 1H, CH=O), 10.86 (d, J = 10.0 Hz, 1H, NH), 13C NMR 10.8 (C3 ), 14.7
ꢁ
ꢁ
(CH3), 19.4 (C4 ), 61.6 (CH2 of Et), 69.2 (CH2 of Bn), 109.1, 113.9, 114.3,
ꢁ
ꢁ
143.8, 153.2, 157.3, 162.7 (C1 , C2 , purine), 164.3 (CH=O), 129.0, 129.1,
129.3, 136.6 (Ph), 170.4 (C=O, ester), ESI-MS (MeOH + AcOK) 432 (M +
K, 100.0), 394 (M + H, 66.7). Anal. Calcd for C20H19N5O4: C, 61.06; H, 4.87;
N, 17.80. Found: C, 60.87; H, 4.93; N, 17.81.
(Z)-9-[2-(Carbethoxycyclopropylidene)methyl]guanine (14). A solution
of compound 11 (230 mg, 0.59 mmol) in 80% acetic acid (10 mL) was
heated at 75–85◦C for 5 hours. The solvent was removed and the residue
was chromatographed on a silica gel column using CH2Cl2-MeOH (20:1 to
8:1) to give compound 14 (137 mg, 85%), mp 247–249◦C (decomp). UV
1
λmax 273 (ε 10,500). 231 nm (ε 23,800), H NMR δ 1.13 (split t, J = 6.4
ꢁ
Hz, 3H, CH3), 1.87–1.90 (m, 1H, ), 1.97 (t, J = 8.0 Hz, 1H, H3), 2.85 (dt,
ꢁ
J = 4.8 Hz, 2.4 Hz, 1H, H4 ), 4.05–4.11 (m, 2H, CH2 of Et), 6.54 (s, 2H,
NH2), 7.24 (s, 1H, H1 ), 7.78 (s, 1H, H8), 10.75 (s, 1H, NH). 13C NMR 10.8
ꢁ
ꢁ
ꢁ
(C3 ), 14.7 (CH3), 19.5 (C4 ), 61.5 (CH2 of Et), 111.5, 112.7, 116.5, 134.0,
ꢁ
ꢁ
150.6, 154.8, 157.3 (C1 , C2 , guanine), 170.9 (C=O, ester). EI-MS 275 (M,
17.7), 55 (100.0). HRMS calcd M 275.1018, found 275.1020. Anal. Calcd for
C12H13N5O3 × 0.3 CH2Cl2: C, 49.12; H, 4.56; N, 23.29. Found: C, 49.07; H,
4.61; N, 23.07.
(Z)-7-[2-(Carbethoxycyclopropylidene)methyl]guanine (15). A mixure
of the Z,N7-isomer Z-13 (20 mg, 0.051 mmol) in 80% AcOH (1 mL) was
heated at 85◦C for 16 hours. The solvent was evaporated and residue was
chromatographed on a silica gel column using CH2Cl2-MeOH (20:1 to 8:1)
to give product 15 (10 mg, 71%). Mp 255◦C (decomp). UV λmax 304 nm (ε
1
2,900), 233 (ε 8,100). H NMR δ 1.13 (t, J = 6.4 Hz, 3H, CH3), 1.89 (m,
ꢁ
ꢁ
1H), 1.96 (t, J = 8.0 Hz, 1H, H3 ), 2.86 (poorly resolved t, C4 ), 4.08 (m,
ꢁ
2H, CH2 of Et), 6.49 (s, 2H, NH2), 7.70 (s, 1H, H1 ), 8.05 (s, 1H, H8), 11.20
(bs, 1H, NH), C NMR 10.5 (C3 ), 14.7 (CH3), 19.3 (C4 ), 61.5 (CH2 of
13
ꢁ
ꢁ
ꢁ
ꢁ
Et), 107.6, 112.1, 113.8, 139.5, 154.1, 155.3, 160.3 (C1 , C2 ., guanine), 170.7
(C=O, ester), ESI-MS 276 (M + H, 83.3), 298 (M + Na, 100.0). ESI-HRMS
calcd for M + H 276.1113, found 276.1110.
Synguanol (1). DIBALH in THF (1.8 M, 0.5 mL, 0.9 mmol) was added
dropwise into a solution of ester 15a (73 mg, 0.27 mmol) in THF (10 mL)
at 0◦C with stirring under N2. After 0.5 hours, another portion of DIBALH
(0.5 mL, 0.9 mmol) was added and the stirring was continued for 0.5 hours.
Aqueous methanol (50%, 1 mL) was added and the mixture was stirred
overnight at room temperature. The solvents were evaporated and crude
product was chromatographed on silica gel column using CH2Cl2-MeOH
(8:1 to 4:1) to give synguanol (1, 49.5 mg, 79%) identical (1H a and 13C
NMR) to an authentic sample.[5]

174
Z. Wu et al.
REFERENCES
1. Zemlicka, J. In Recent Advances in Nucleosides: Chemistry and Chemotherapy; C. K. Chu, Ed.; Elsevier;
Amsterdam, 2002, pp. 327–357.
2. Zemlicka, J.; Chen, X. In Frontiers in Nucleosides and Nucleic Acids; R. F. Schinazi, D. C., Liotta, Eds.;
IHL Press; Tucker, Georgia, 2004, pp. 267–307.
3. Zemlicka, J. In Advances in Antiviral Drug Design; Vol. 5, De Clercq, E., Ed.; Elsevier, Amsterdam, The
Netherlands, 2007, pp. 113–165.
4. Rybak, R. J.; Zemlicka, J.; Qiu, Y.-L.; Harline, C.B.; Kern, E.R. Effective treatment of murine
cytomegalovirus infections with methylenecyclopropane analogues of nucleosides. Antiviral Res.
1999, 43, 175–188.
5. Qiu, Y.-L.; Ksebati, M.B.; Ptak, R.G.; Fan, B.Y.; Breitenbach, J.M.; Lin, J.-S.; Cheng, Y.-C.; Kern,
E.R.; Drach, J.C.; Zemlicka, J. (Z)- and (E)-2-(Hydroxymethylcyclopropylidene)methyladenine and
guanine. New nucleoside analogues with a broad spectrum antiviral activity. J. Med. Chem. 1998, 41,
10–23.
6. Qiu, Y.-L.; Ptak, R.G.; Fan, B.Y.; Breitenbach, J.M.; Lin, J.-S.; Cheng, Y.-C.; Kern, E.R.; Drach, J.C.;
Zemlicka, J. (Z)- and (E)-2-(Hydroxymethylcyclopropylidene)-methylpurines and pyrimidines as
antiviral agents. Antiviral Chem. Chemother. 1998, 9, 341–352.
7. Qiu, Y.-L.; Zemlicka, J. New efficient synthesis of antiviral methylenecyclopropane analogs of purine
nucleosides. Synthesis 1998, 1447–1452.
8. Ambrose, A.; Zemlicka, J.; Kern, E.R.; Drach, J.C.; Gullen, ; Cheng, Y.-C. Phosphoralaninate
pronucleotides of pyrimidine methylenecyclopropane analogues of nucleosides: synthesis and
antiviral activity. Nucleosides, Nucleotides & Nucleic Acids 2005, 24, 1763–1775.
9. Previously, compound 2c (cis and trans isomers) was synthesized using CuSO4 catalysis and
it was characterized only by analysis of Cl: Dyakonov, I.A.; Domareva, T.V. Reactions of ethyl
diazo-compounds with unsaturated compounds. XIX. Reaction of ethyl diazoacetate with 2,3-
dichloropropene in the presence of cupric sulfate. J. Gen. Chem. USSR (Engl. Transl.) 1955, 25,
1435–1440. Ester 2c was first prepared in our laboratory by Yao-Ling Qiu.
10. Zemlicka, J.; Holy, A. Preparation of N-Dimethylaminomethylene Derivatives—A new method for a
selective substitution of nucleoside amino groups. Collect. Czech. Chem. Commun. 1967, 32, 3159–3168.
11. Wang, R.; Chen, X.; Drach, J.C.; Kern, E.R.; Zemlicka, J. Synthesis and biological activity of 2-
aminopurine methylenecyclopropane analogues of nucleosides. Nucleosides, Nucleotides & Nucleic
Acids 2003, 22, 135–144.
12. Kjellberg, J.; Johansson, N.G. Characterization of N7 and N9 alkylated purine analogues by 1H and
13C NMR. Tetrahedron 1986, 42, 6541–6544.
13. Holy, A.; Zemlicka, J. Oligonucleotidic Compounds. XXXIII. A study on hydrolysis of n-
dimethylaminomethylenecytidine, -adenosine, -guanosine, and related 2ꢁ-deoxy compounds. Collect.
Czech. Chem. Commun. 1969, 34, 2449–2458.
14. Halton, B.; Banwell, M.G. Cyclopropenes. In: The Chemistry of the Cyclopropyl Group; Rappoport, Z, Ed.;
Part 2, 1987, pp 1223–1339, loc. cit. 1257–1258.
15. Wang, R.; Ksebati, M.B.; Corbett, T.H.; Kern, E.R.; Drach, J.C.; Zemlicka, J. Methylene-gem-
difluorocyclopropane analogues of nucleosides: Synthesis, cyclopropene-methylenecyclopropane
rearrangement, and biological activity. J. Med. Chem. 2001, 44, 4019–4022.
16. Stirling, C.J.M. Leaving groups and nucleofugality in elimination and other organic reactions. Acc.
Chem. Res. 1979, 12, 198–203.
17. Marshall, D.R.; Thomas, P.J.; Stirling, C.J.M. Elimination and addition reactions. Part 32. Discrim-
ination between concerted and stepwise processes in activated elimination reactions. J. Chem. Soc.
Perkin II 1977, 1914–1919.
18. Alunni, S.; De Angelis, F.; Ottavi, L.; Papavasileiou, M.; Tarantelli, F. Evidence for a boderline region
between E1cb and E2 Elimination reaction mechanisms: A combined experimental and theoretical
study of systems activated by the pyridine ring. J. Am. Chem. Soc. 2005, 127, 15151–15160.
19. Bowles, W.A.; Schneider, F.H.; Lewis, L.R.; Robins, R.K. Synthesis and antitumor activity of 9-
(tetrahydro-2-furyl)purine analogs of biologically important deoxynucleosides. J. Med. Chem. 1963,
6, 471–480.