Synthesis of aza-analogues of Ganciclovir

Article abstract of DOI:10.1016/j.tet.2006.08.072

Aza-analogues of ganciclovir have been prepared via coupling of nucleobases with N-[2-pivaloyloxy-1-(pivaloyloxymethyl)ethyl]methanesulfonamide or 3-mesyl-4-(benzoyloxymethyl)-1,3-oxazolidine.

Full text of DOI:10.1016/j.tet.2006.08.072

Tetrahedron 62 (2006) 10325–10331
Synthesis of aza-analogues of Ganciclovir
a
Mariola Koszytkowska-Stawinska, Wojciech Sas and Erik De Clercq
a,
b
*
ꢀ
aFaculty of Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warszawa, Poland
bRega Institute for Medical Research, Katholieke Universiteit Leuven, Minderbroedersstraat 10, B-3000 Leuven, Belgium
Received 27 June 2006; revised 8 August 2006; accepted 23 August 2006
Available online 14 September 2006
Abstract—Aza-analogues of ganciclovir have been prepared via coupling of nucleobases with N-[2-pivaloyloxy-1-(pivaloyloxymethyl)-
ethyl]methanesulfonamide or 3-mesyl-4-(benzoyloxymethyl)-1,3-oxazolidine.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
N-(a-acetyloxyalkyl)amides (which, are derivatives of
amino acids or peptides) with the nucleobase or its silylated
derivative in the presence of triethylamine (or sodium
hydride)6e,f,k or tin(IV) chloride.6d The acyclic aza-ana-
logues have also been synthesized by the coupling of
silylated nucleobases, natural or synthetic, with N-(acetoxy-
methyl)amides,6m N-(chloromethyl)sulfonamides6g or N-
(chloromethyl)amides.6i Other methods, such as Curtius
rearrangement, have also been reported.6a Acyclic aza-
nucleosides containing a nitrogen atom at the 30-position
are also known; they disclose biological activity.8
The synthesis of acyclic nucleosides1,2 has attracted consid-
erable attention due to their antiviral3 or anticancer activity.4
Some of them (Fig. 1) have been approved for the clinical
treatment of herpes virus infections (e.g. acyclovir, ganci-
clovir, penciclovir, cidofovir, famciclovir) or hepatitis B
virus infections (e.g. adefovir).3 In order to improve their
efficacy, the parent antiviral drugs are also used in the
form of prodrugs, usually esters (e.g. valacyclovir or valgan-
ciclovir, Fig. 1).5 The synthesis of new nucleoside analogues
bearing molecular modifications on the sugar mimic is one
of the approaches towards promising antiviral agents.1
Some of these modifications involve replacement of the
oxygen or carbon atom at the 20-position with sulfur or
nitrogen.1a,6 Acyclic analogues with a nitrogen atom at the
20-position (termed acyclic azanucleosides) are much less
known than the corresponding oxa or carbo derivatives.7
Recently we have shown that acyclic azanucleosides can be
readily obtained by the coupling of N-(pivaloyloxymethyl)-
amides or sulfonamides with silylated nucleobases in the
presence of Lewis acids. Using this approach we have
synthesized the aza-analogues of ganciclovir9 and acyclo-
vir10 protected with mesyl or tosyl group, respectively,
at a nitrogen atom of the sugar mimic.11 Herein we
describe two methods for the first synthesis of the aza-
analogues of ganciclovir: (i) from N-[2-pivaloyloxy-1-
Most of them are amino acid or peptide derivatives of
5-fluorouracil. They are obtained by the condensation of
NH2
O
R2
N
N
N
HN
N
N
N
O
O
N
O
N
HO
HO
H2N
R2O
N
P
R1
X
2'
1'
R1
R3
OH
X
R1
R2
H
L-Val
H
R1
H
R2
R3
Acyclovir
O
O
O
O
H
H
Adefovir
NH2 (HO)2P(O)CH2OCH2
Cidofovir
Valacyclovir
Ganciclovir
CH2OH
Famciclovir
NH2
H
(AcOCH2)2CHCH2
Valganciclovir
Penciclovir
CH2OH L-Val
CH2 CH2OH
H
Figure 1.
Keywords: N-Pivaloyloxymethyl sulfonamides; N-Mesyl-5-hydroxymethyl-1,3-oxazolidine; Acyclic azanucleosides; Ganciclovir analogues.
* Corresponding author. Tel.: +48 22 628 0763; fax: +48 22 628 2741; e-mail: sas@ch.pw.edu.pl
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.08.072

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M. Koszytkowska-Stawinꢀska et al. / Tetrahedron 62 (2006) 10325–10331
Ms
N
(pivaloyloxymethyl)ethyl]-methanesulfonamide or (ii) from
3-mesyl-4-(benzoyloxymethyl)-1,3-oxazolidine. These both
substrates are easily accessible from 3-nitro-1,5-dioxa-
spiro[5.5]undecane 1.12 Antiviral activities of aza-analogues
of ganciclovir are also reported.
O
O
U
Ms
N
HN
Ms
O
OPiv
ii
9
i
O
10
O
O
O
9
11
iii
13
12
1
2
8
N
O
7
6
3
Ms
4
5
2
8
10
2. Results and discussion
Scheme 2. Reagents and conditions: (i) NaH, DMF, ClCH2OPiv, rt, 72 h,
72%; (ii) uracil (U), BSA, TMSOTf, MeCN, rt, 24 h and (iii) Lewis acid
(TMSOTf or AlCl3 or BF3$Et2O), MeCN, rt, 24 h, 39%.
Nitrodioxane 1 was converted into the serinol derivative 4 in
four steps (Scheme 1): (i) palladium catalyzed hydrogena-
tion, (ii) reaction of the resulting amine with methane-
sulfonyl chloride (MsCl) in the presence of pyridine, (iii)
acidic hydrolysis of 2, (iv) O-pivaloylation of 3 with pivaloyl
chloride in the presence of pyridine. The alkylation of 4 with
chloromethyl pivaloate in the presence of sodium hydride
in dry DMF gave N-(pivaloyloxymethyl)sulfonamide 5
required for the nucleoside coupling.
A plausible pathway for formation of 10 from 8 in the pres-
ence of TMSOTf is shown in Scheme 3. According to the
literature data,14 we assume that 8 reacts with TMSOTf to
give the intermediate enol ether A.
N-(Pivaloyloxymethyl)sulfonamide group is then converted
into iminium cation B (Scheme 3), which undergoes intra-
molecular Mannich type reaction with the enol ether moiety.
The process is completed with closure of the acetal ring to
yield tricyclic derivative 10.
A one-pot base silylation/nucleoside coupling procedure
was employed for the syntheses of protected azanucleosides
6CBz and 6ACbz (Scheme 1). Compound 5 was coupled with
silylated N4-Bz-cytosine (CBz) or N6-Cbz-adenine (ACbz) in
the presence of trimethylsilyl trifluoromethanesulfonate
(TMSOTf) or tin(IV) chloride, respectively.13 Thus, these
nucleobases were silylated with N,O-bis(trimethylsilyl)acet-
amide (BSA) in acetonitrile and subsequently 5 and a cata-
lyst (Lewis acid) was added to afford 6CBz or 6ACbz in
72% or 44% yield, respectively. Heating of 6CBz with
ammonium hydroxide in methanol furnished deprotected
azanucleoside 7C in 83% yield. Deprotection of 6ACbz
was completed in two steps: (i) Cbz protecting group was
removed by palladium catalyzed hydrogenolysis and (ii)
hydroxy groups were deprotected by heating of the corre-
sponding O-pivaloyl derivative with ammonium hydroxide
to afford 7A in 73% overall yield.
The compound 10 was also obtained in the same yield in the
presence of aluminum(III) chloride or boron trifluoride
etherate.
Unexpectedly,
N-(pivaloyloxymethyl)-N-dioxanesulfon-
amide 8 underwent transformation into 3-mesyl-4-(hydroxy-
methyl)-1,3-oxazolidine 11 in 90% yield in the presence of a
Brønsted acid [p-toluenesulfonic acid (PTSA) or Dowex-
50(H+)] in methanol (Scheme 4). Compound 11 was fully
analyzed as O-benzoyl derivative 12 (Scheme 5).
Considering the literature data concerning the hydrolysis of
N-(pivaloyloxymethyl)sulfonamides,15 we assume that pro-
tonation of the carbonyl oxygen of the pivaloyl group with
a Brønsted acid takes place. Then, loss of pivalic acid gives
iminium cation C, which undergoes intramolecular rear-
rangement to give 1,3-oxazolidine intermediate D. The
methanolysis of D gives 11.
We also tried to obtain the uracil azanucleoside 9 from
N-(pivaloyloxymethyl)-N-dioxanesulfonamide 8 (resulting
from alkylation of 2 with chloromethyl pivaloate) (Scheme
2). However, attempts to couple 8 with silylated uracil (U)
in the presence of TMSOTf failed. Instead of the expected
derivative 9 the tricyclic derivative 10 was isolated in 39%
yield from the complex reaction mixture.
Some acyclic azanucleosides were prepared from 3-tosyl-
oxazolidine-5-ones,6l obtained from N-tosyl derivatives of
O
PivO
PivO
HO
HO
O
i - ii
iii
iv
NO2
NHMs
NHMs
NHMs
O
O
3
4
1
2
v
NHR
NHR
N
N
N
N
Ms
N
Ms
N
Ms
N
R'O
R'O
PivO
viii
vi
N
N
O
OPiv
OR'
OR'
OPiv
6CBz R = Bz, R' = Piv
7C R = R' = H
6ACbz R = Cbz, R' = Piv
7A R = R' = H
5
vii
ix, vii
Scheme 1. Reagents and conditions: (i) H2, 10% Pd/C, EtOH, rt, 60 bar, 24 h, quantitative; (ii) MeSO2Cl (MsCl), pyridine, CH2Cl2, rt, 67%; (iii) MeOH,
Dowex-50 (H+), rt, 48 h, 67%; (iv) PivCl, pyridine, rt, 24 h, 42%; (v) NaH, DMF, ClCH2OPiv, rt, 72 h, 72%; (vi) N4-Bz-cytosine (CBz), BSA, TMSOTf,
MeCN rt, 24 h, 71%; (vii) NH4OHconcd, MeOH, sealed tube, 70 ꢀC, 1 d; (viii) N6-Cbz-adenine (ACbz), BSA, SnCl4, MeCN, 24 h, 44% and (ix) H2 (balloon),
10% Pd/C, MeOH, rt, 1 d.

M. Koszytkowska-Stawinꢀska et al. / Tetrahedron 62 (2006) 10325–10331
10327
MsN
O
MsN
OPiv
OPiv
CH2
O
TfO MsHN
TMSOTf
-TfOH
TfOH
-TMSOTf
O
O
O
10
O
TMS
-PivOH
TMS
8
A
B
Scheme 3.
CH2
O
MsN
O
MsHN
O
OPiv
Ms
N
Ms
N
H
MeOH
-H
HO
O
O
-PivOH
O
O
8
C
D
11
Scheme 4. Dowex-50(H+) MeOH, rt, 1 d; 90% (crude).
amino acids. Consequently we envisaged that oxazolidine 11
could be used for the synthesis of aza-analogues of
ganciclovir as well (Scheme 5, Table 1).
were added to the acetonitrile solution of the corresponding
silylated nucleobase to yield compounds 13 (Table 1). Py-
rimidine nucleosides 13 (T, FU or CBz) were obtained in sat-
isfactory yields, which were higher than that of the adenine
derivative 13ABz. Attempts to obtain the guanine derivative
using this procedure as well the original Robins’ procedure16
(nucleoside coupling was conducted in hot toluene in the
presence of TMSOTf) were unsuccessful.17 In the case of
5-fluorouracil and adenine derivatives (13FU and 13ABz,
respectively) the yield was improved when TMSOTf was
replaced by tin(IV) chloride (Table 1, entries 2–3 and 5–6).
The N-1 or N-9 regioselectivity of the coupling was proved
Crude 11 was treated with benzoyl chloride in the presence
of pyridine in dry dichloromethane to afford O-benzoyl
derivative 12 in 72% overall yield. The aforementioned
one-pot base silylation/nucleoside coupling procedure was
employed for the coupling of oxazolidine 12 with thymine
(B¼T), 5-fluorouracil (B¼FU), N4-Bz-cytosine (B¼CBz),
N6-Bz-adenine (B¼ABz) or N2-acetyl-O6-(diphenylcarba-
moyl)guanine (B¼GPAC). Accordingly, 12 and TMSOTf
Ms
BzO
N
i
11
O
12
ii (B = ABz
)
ii (B = T or FU)
ii (B = CBz
)
NHR
N
O
R
NHR
N
N
N
NH
Ms
Ms
N
Ms
N
HO
HO
HO
N
O
N
O
N
N
OR'
OR'
OR'
13T (or 13FU) R = CH3(or F), R' = Bz
7T (or 7FU) R = CH3(or F), R' = H
13CBz R = R' = Bz
7C R = R' = H
13ABz R = R' = Bz
7A R = R' = H
iii
iii
iii
Scheme 5. Reagents and conditions: (i) BzCl, pyridine, CH2Cl2, rt, 1 d, 72% (from 8); (ii) B, BSA, catalyst (TMSOTf or SnCl4), MeCN, rt, 2 d (for details see
Table 1) and (iii) NH4OHconcd, MeOH, rt, 1 d.
Table 1. Syntheses of aza-analogues of Ganciclovir 13 and 7
Aza-analogues of Ganciclovir 13
Aza-analogues of Ganciclovir 7
Entry
Base (B)
Catalyst
Nucleoside
Yield [%]
Entry
Base
Nucleoside
Yield [%]
1
2
3
4
5
6
Thymine (T)
TMSOTf
TMSOTf
SnCl4
TMSOTf
TMSOTf
SnCl4
13T
67
44
56
66
14
26
7
8
Thymine (T)
7T
99
80
74
67
5-Fluorouracil (FU)
N4-Bz-cytosine (CBz
N6-Bz-Adenine (ABz
13FU
13CBz
13ABz
5-Fluorouracil (FU)
Cytosine (C)
7FU
7C
)
9
)
10
Adenine (A)
7A

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M. Koszytkowska-Stawinꢀska et al. / Tetrahedron 62 (2006) 10325–10331
by 1H–13C HMBC correlations observed at thymine deriva-
77.16 ppm or DMSO-d6, dH (residual DMSO) 2.50 ppm,
dC 39.52 ppm; signals are quoted as ‘s’ (singlet), ‘d’ (dou-
blet), ‘t’ (triplet), ‘dt’ (doublet of triplets), ‘m’ (multiplet)
and ‘br s’ (broad singlet). Coupling constants (J) are re-
ported in Hertz. 13C-NMR APT (Attached Proton Test) spec-
tra were recorded on a Varian Gemini 200 spectrometer at
50 MHz; signals quoted as (–) indicate signals of carbon
atoms from methyl (CH3) or methylidene group (CH). Pre-
coated Merck silica gel 60 F254 (0.2 mm) plates were used
for thin-layer chromatography (TLC), and the spots were
detected under UV light (254 nm). Column chromatography
was performed using silica gel (200–400 mesh, Merck).
Solvents were purified by routine methods. The anhydrous
MgSO4 was employed as a drying agent. Solvents were dis-
tilled off under reduced pressure on a rotating evaporator.
tive 13T or adenine derivative 13ABz, respectively.
Treatment of compounds 13 with concd ammonium hydrox-
ide in methanol at room temperature for 1 d afforded the
azanucleosides 7 in high yields (Table 1, entries 7–10).
2.1. Antiviral activity
The antiviral activities of compounds 7 (T, FU, C or A), 13T
and 13FU were evaluated in vitro against a variety of viruses.
The following viruses and host cells were used for the
evaluation:
(a) Vero cell cultures: parainfluenza-3 virus, reovirus-1,
Sindbis virus, Coxsackie B4 virus and Punta Toro virus;
(b) E6SM cell cultures: herpes simplex virus-1 (KOS),
herpes simplex virus-2 (G), herpes simplex virus-1
(TKꢁ KOS ACVr), vaccinia virus and vesicular sto-
matitis virus;
4.1.1. 3-(Mesylamino)-1,5-dioxaspiro[5.5]undecane (2).
3-Nitro-1,5-dioxaspiro[5.5]undecane (1, 20.1 g, 0.1 mol)12
in ethanol (100 cm3) was hydrogenated under 60 bar
pressure in the presence of 10% Pd/C (0.5 g) at room temper-
ature for 24 h. The mixture was filtered through a Celite pad
and the solvent was distilled off from the filtrate. The residue
was dried in vacuum desiccator (over P2O5) for 24 h. The
solution of the crude amine and pyridine (20.24 g, 0.2 mol,
27.9 cm3) in DCM (160 cm3) was cooled in ice water and
the solution of methanesulfonyl chloride (17.2 g, 0.15 mol,
11.7 cm3) in DCM (40 cm3) was added dropwise. When
the addition was completed, stirring was continued for
10 min. The mixture was washed subsequently with water
(3ꢂ100 cm3), diluted (5%) hydrochloric acid, water and
dried. Solvent was distilled off and the residue was crystal-
lized (ethyl acetate–hexane, 9/1, v/v) to yield 1 as white
crystals (16.9 g, 67%), mp 101–103 ꢀC. dH (CDCl3) 1.52
(m, 8H), 1.84 (m, 2H), 3.01 (s, 3H), 3.41 (m, 1H), 3.75
(c) HeLa cell cultures: vesicular stomatitis virus, Coxsackie
B4 virus and respiratory syncytial virus.
Brivudin, (S)-9-(2,3-dihydroxypropyl)adenine ((S)-DHPA),
ribavirin, acyclovir and ganciclovir were used as the refer-
ence compounds. Among them, only thymine derivative
13T showed very low activity in test against respiratory syn-
cytial virus in HeLa cell cultures (IC50¼49 mg/ml). In the
same test ribavirin displayed IC50 of 9.6 mg/ml. The follow-
ing minimum cytotoxic concentration values were estimated
for tested azanucleosides:18 (i) Vero cell cultures: 200 mM;
(ii) E6SM cell cultures: >200 mM and (iii) HeLa cell
cultures: >200 mM.19
3
(m, 2H), 4.14 (m, 2H), 5.30 (d, 1H, J 9.0). dC (CDCl3)
3. Conclusions
22.47, 22.56, 25.63, 28.15, 36.81, 42.51, 48.22, 63.57,
99.00. IR: 3324, 2944, 1312, 1164, 1136, 1104. HRMS
(EI, 70 eV) m/z calcd for C10H19NO4S (M+) 249.1032, found
249.1035.
We have shown that the aza-analogues of ganciclovir can be
readily obtained from N-[2-pivaloyloxy-1-(pivaloyloxy-
methyl)ethyl]methanesulfonamide 5 or 3-mesyl-4-(benzoyl-
oxymethyl)-1,3-oxazolidine 11 by employing the one-pot
base silylation/nucleoside coupling procedure. The oxazo-
lidine 11 is useful for the synthesis of mono O-substituted
aza-analogues of ganciclovir. Further studies on the im-
provement and extension of the methodologies described
for the syntheses of various acyclic azanucleosides are in
progress.
4.1.2. N-[2-Pivaloyloxy-1-(pivaloyloxymethyl)ethyl]-
methanesulfonamide (4). A mixture of 2 (1.0 g, 4 mmol)
and Dowex-50(H+) (1 g) in methanol (20 cm3) was shaken
at room temperature for 24 h. The ion exchange resin was fil-
tered off and the solvent was distilled off. The residue was
dried in vacuum desiccator (over P2O5) for 24 h, and then
this was dissolved in dry pyridine (6 cm3) and cooled in
water bath. Pivaloyl chloride (1.16 g, 9.6 mmol, 1.2 cm3)
was added to this solution in one portion. The mixture was
kept at room temperature for 24 h. The reaction was
quenched by addition of water (16 cm3) and DCM (4 cm3).
The organic phase was separated, washed with water, brine
and dried. The solvent was distilled off and the residue
was purified by flash chromatography (chloroform) to yield
4 (oil, 0.57 g, 42%). dH (CDCl3) 1.27 (s, 18H), 3.02 (s, 3H),
3.98 (m, 1H), 4.17 (m, 4H), 4.81 (br s, 1H). dC (CDCl3)
22.92, 39.06, 42.33, 52.11, 63.43 (ꢂ2), 97.14, 178.34. IR:
3288, 2976, 1728, 1320, 1156. HRMS m/z calcd for
C14H27NO6NaS (M+Na)+ 360.1451, found 360.1463.
4. Experimental
4.1. General
High Resolution Mass Spectra (Electrospray Ionization,
ESI) were performed on a MarinerÔ spectrometer in posi-
tive ionization mode unless otherwise indicated. IR spectra
were recorded on a Specord M80 (Carl-Zeiss Jena) spec-
trometer in KBr disc unless otherwise indicated; absorption
maxima (nmax) are given in cmꢁ1. 1H and 13C NMR spectra
were recorded on a Varian Gemini 200 spectrometer at
200 MHz and 50 MHz, respectively. H and 13C chemical
shifts are reported in parts per million relative to the solvent
signals: CDCl3, dH (residual CHCl3) 7.26 ppm, dC
4.1.3. N-(Pivaloyloxymethyl)-N-[2-pivaloyloxy-1-(piva-
loyloxymethyl)ethyl]methanesulfonamide (5). A mixture
of sodium hydride (63% suspension in mineral oil, 2.8 g,
1

M. Koszytkowska-Stawinꢀska et al. / Tetrahedron 62 (2006) 10325–10331
10329
70 mmol) and 4 (15.8 g, 35 mmol) in dry DMF (20 cm3) was
stirred at room temperature for 1 h. Then chloromethyl piv-
aloate (15.8 g, 105 mmol, 11.2 cm3) was added in one por-
tion. After 3 d of stirring at room temperature the mixture
was poured into cold water (40 cm3) followed by extraction
with ethyl acetate (3ꢂ20 cm3). The combined extracts were
washed with water and dried. The low boiling solvents were
distilled off under reduced pressure and residual DMF was
removed in vacuum at 80 ꢀC (0.05 mmHg). The residue
was purified by flash chromatography (hexane–ethyl ace-
tate, 3/1, v/v) to yield 5 as an oil (88%, 13.9 g). dH
(CDCl3) 1.21 (s, 27H), 3.09 (s, 3H), 4.27 (m, 5H), 5.53 (s,
2H). dC (CDCl3) 27.15, 27.27, 38.95, 42.41, 54.26, 62.32,
70.58, 177.55, 178.04. IR: 2976, 1728, 1340, 1148.
HRMS m/z calcd for C20H37NO8NaS (M+Na)+ 474.2132,
found 474.2126.
(0.18 cm3, 1.5 mmol) were added, successively. The resulted
mixture was worked-up as described for 6CBz. The residue
was purified by flash chromatography (CHCl3–acetone, 98/
2, v/v) to give 6ACbz as amorphous foam, yield 44%
(158 mg). dH (CDCl3) 1.08 (m, 18H), 2.96 (s, 3H), 4.27
(m, 5H), 5.28 (s, 2H), 5.75 (s, 2H), 7.45 (m, 5H), 8.43 (s,
1H), 8.74 (s, 1H), 9.26 (br s, 1H). dC (CDCl3) 27.07,
38.72, 41.98, 52.05, 56.08, 61.93, 67.83, 121.36, 128.51,
128.62, 135.46, 143.54, 149.87, 151.02, 151.16, 153.17,
177.84. IR: 2976, 1732, 1616, 1588, 1472, 1340, 1284,
1212, 1148. HRMS m/z calcd for C28H38N6O8NaS
(M+Na)+ 641.2364, found 641.2380.
4.1.7. 6-Amino-9-{N-[2-hydroxy-1-(hydroxymethyl)-
ethyl]mesylaminomethyl}-9H-purine (7A). The mixture
of 6ACbz (141 mg, 22.8 mmol) and 10% Pd/C (20 mg) in
MeOH (3 cm3) was hydrogenated under atmospheric pres-
sure (balloon) at room temperature for 2 d. The catalyst
was filtered off through a short pad of Celite. Concd
NH4OH (1 cm3) was added to the filtrate and the mixture
was heated in a sealed tube at 70 ꢀC for 24 h. The solvent
was distilled off and the residue was purified by flash chro-
matography (chloroform–methanol, 9/1, v/v) to give 7A as
white crystals, yield 75% (54 mg), mp 240–242 ꢀC. dH
(DMSO-d6) 3.10 (s, 3H), 3.47 (m, 4H), 3.82 (m, 1H), 5.00
(m, 2H), 5.65 (s, 2H), 7.36 (br s, 2H), 8.13 (s, 1H), 8.19 (s,
1H). dC (DMSO-d6) 39.97, 51.47, 59.50, 61.93, 118.34,
140.29, 149.05, 152.67, 156.08. IR: 3324, 3160, 1664,
1600, 1332, 1152. HRMS m/z calcd for C10H17N6O4S
(M+H)+ 317.1027, found 317.1042.
4.1.4. 4-(Benzoylamino)-1-{N-[2-hydroxy-1-(hydroxy-
methyl)ethyl]mesylaminomethyl}-1H-pyrimidin-2-one
(6CBz). A mixture of N4-benzoylcytosine (B¼CBz, 430 mg,
2.0 mmol) and BSA (814 mg, 4.0 mmol, 1.0 cm3) in dry ace-
tonitrile (10 cm3) was stirred at room temperature under
argon for 1 h. Then a solution of 5 (363 mg, 1.0 mmol) in
acetonitrile (1 cm3) and TMSOTf (0.3 cm3, 1.66 mmol)
were added, successively. After 24 h the reaction mixture
was quenched by the addition of CHCl3 (50 cm3) and a satu-
rated solution of sodium bicarbonate (1 cm3) and the result-
ing mixture was stirred for 1 h. Insoluble material was
removed by filtration through a Celite pad. The organic layer
was separated, washed with water, brine and dried. The sol-
vent was distilled off and the residue was purified by flash
chromatography (CH2Cl2/MeOH, 98/2, v/v) to give 6CBz
as amorphous foam, yield 71% (400 mg). dH (CDCl3) 1.18
(s, 18H), 3.06 (s, 3H), 4.32 (m, 5H), 5.49 (s, 2H), 7.55 (m,
4.1.8. 3-[N-Mesyl-N-(pivaloyloxymethyl)amino]-1,5-di-
oxaspiro[5.5]undecane (8). A mixture of sodium hydride
(63% suspension in mineral oil, 2.8 g, 70 mmol) and 2
(8.7 g, 35 mmol) in dry DMF (20 cm3) was stirred at room
temperature for 1 h. Then, chloromethyl pivaloate (15.8 g,
105 mmol, 11.2 cm3) was added to the resulted suspension
in one portion. After 3 d stirring at room temperature the
reaction mixture was poured into ice water (40 cm3). The
mixture was extracted with ethyl acetate (3ꢂ20 cm3) and
the combined extracts were washed with water and dried.
The solvents were distilled off and residual DMF was re-
moved in vacuum (80 ꢀC, 0.05 mmHg), successively. The
residue was purified by flash chromatography (hexane–ethyl
acetate, 3/1, v/v) to yield 8 as white crystals (9.2 g, 72%), mp
91–93 ꢀC. dH (CDCl3) 1.21 (s, 9H), 1.60 (m, 10H), 3.00 (s,
3H), 3.89 (m, 3H), 4.14 (m, 2H), 5.75 (s, 2H). dC (CDCl3)
22.44, 22.65, 25.62, 27.22, 30.65, 34.08, 38.82, 42.27,
50.31, 62.33, 72.62, 98.77, 177.46. IR: 2936, 1712, 1332,
1164, 1156. HRMS m/z calcd for C16H29NO6NaS
(M+Na)+ 386.1608, found 386.1627.
3
4H), 7.90 (m, 2H), 8.17 (d, J 7.6, 1H), 8.78 (br s, 1H). dC
(CDCl3) 27.26, 38.89, 41.85, 56.67, 58.42, 62.04, 97.77,
127.74, 129.22, 132.94, 133.51, 148.41, 155.75, 163.00,
178.05. IR: 3308, 2976, 2932, 1732, 1696, 1676, 1556,
1484, 1340, 1312, 1276, 1252, 1148. HRMS m/z calcd for
C26H37N4O8S (M+H)+ 565.2327, found 565.2346.
4.1.5. 4-Amino-1-{N-[2-hydroxy-1-(hydroxymethyl)-
ethyl]mesylaminomethyl}-1H-pyrimidin-2-one (7C). A
mixture of 6CBz (305 mg, 0.54 mmol), concd NH4OH
(1 cm3) and MeOH (2 cm3) was heated in a sealed tube at
70 ꢀC for 24 h. The solvent was distilled off and the residue
was purified by flash chromatography (acetone–methanol–
NH3 aq, 6/1/0.4, v/v/v) to give 7C as white crystals, yield
74% (117 mg), mp 212–222 ꢀC (dec). dH (DMSO-d6) 3.06
(s, 3H), 3.44 (m, 4H), 3.79 (m, 1H), 4.90 (m, 1H), 5.19 (s,
3
2H), 5.76 (d, J 7.4, 1H), 7.18 (s, 1H), 7.24 (s, 1H), 7.63
(d, J 7.4 1H). dC (DMSO-d6) 39.94, 55.49, 59.57, 62.04,
3
94.60, 143.98, 155.63, 165.81. IR: 3504, 3428, 3348,
3140, 1680, 1616, 1512, 1320, 1148. HRMS m/z calcd for
C9H17N4O5S (M+H)+ 293.0914, found 293.0925.
4.1.9. 8-Mesyl-11,12-dioxa-8-aza-tricyclo[7.2.2.01,6]tri-
decane (10). A mixture of 8 (100 mg, 0.28 mmol) and
a Lewis acid (TMSOTf, AlCl3 or BF3$Et2O) (0.3 mmol) in
acetonitrile (4 ml) was kept at room temperature for 24 h
and then poured into a saturated solution of sodium bicar-
bonate. The product was extracted with ethyl acetate
(2ꢂ2 cm3). The combined extracts were washed with brine
and dried. The solvent was distilled off and the residue
was purified by flash chromatography (DCM) to give 10 as
oil. dH (CDCl3) 1.32–1.90 (m, 8H), 2.16 (m, 1H), 2.98 (s,
4.1.6. 6-(Benzyloxycarbonylamino)-9-{N-[2-hydroxy-1-
(hydroxymethyl)ethyl]mesylaminomethyl}-9H-purine
(6ACbz). A mixture of the N6-(benzyloxycarbonyl)adenine
(B¼ACbz 270 mg, 1.0 mmol) and BSA (814 mg, 4.0 mmol,
1.0 cm3) in dry acetonitrile (10 cm3) was stirred at room
temperature under argon for 1 h, and then a solution of 5
(260 mg, 0.58 mmol) in acetonitrile (1 cm3) and SnCl4
3
2
3
3H), 3.13 (dd, 1H, J 9.5, J 14.2), 3.75 (dd, 1H, J 7.4,

10330
M. Koszytkowska-Stawinꢀska et al. / Tetrahedron 62 (2006) 10325–10331
3
3
2J 14.2), 3.94 (dd, 1H, J 2.0, J 10.4), 4.01–4.09 (m, 2H),
4.19–4.24 (m, 2H). dC (CDCl3, APT) 22.37, 24.61, 29.55,
38.97(–), 39.25, 46.50, 46.74(–), 51.67(–), 64.33, 65.02,
99.20. IR: 2932, 2860, 1320, 1256, 1196, 1144, 1112,
1052, 956. HRMS m/z calcd for C11H19NO4NaS (M+Na)+
284.0927, found 284.0929.
4.2.2. 1-{N-[2-Benzoyloxy-1-(hydroxymethyl)ethyl]mesyl-
aminomethyl}-5-fluoro-1H,3H-pyrimidin-2,4-dione
(13FU). Chromatographic purification (chloroform–metha-
nol, 93/7, v/v) afforded 13FU, yield 44% (177 mg,
TMSOTf) or 56% (230 mg, SnCl4), mp 183–187 ꢀC. dH
(DMSO-d6) 3.15 (s, 3H), 3.71 (m, 2H), 4.27 (m, 3H), 5.28
(m, 3H), 7.72 (m, 6H), 11.90 (br s, 1H). dC (DMSO-d6)
2
4.1.10. 3-Mesyl-4-(benzoyloxymethyl)-1,3-oxazolidine
(12). A mixture of 8 (2.08 g, 5.5 mmol) and Dowex-
50(H+) (2.4 g) in methanol (50 cm3) was shaken at room
temperature for 24 h. The ion exchange resin was filtered
off and the filtrate was concentrated to dryness. An analytical
sample of 11 was purified by flash chromatography (methyl-
ene chloride). dH (CDCl3) 2.69 (m, 1H), 2.89 (s, 3H), 3.68
40.20, 55.84, 58.19, 58.86, 62.06, 127.08 (d, JC–F 33.8),
128.72, 129.13, 133.48, 139.77 (d, JC–F 230.3), 149.84,
157.02 (d, JC–F 26.2), 165.35. IR: 3504, 3012, 2856,
1
2
1712, 1692, 1660, 1328, 1272, 1148, 1100. HRMS m/z
calcd for C16H18N3O7FNaS (M+Na)+ 438.0742, found
438.0738.
3
(m, 3H), 3.88 (m, 1H), 4.26 (m, 1H), 4.52 (d, J 7.2, 1H),
4.2.3. 4-(Benzoylamino)-1-{N-[2-benzoyloxy-1-(hydroxy-
methyl)ethyl]mesylaminomethyl}-1H-pyrimidin-2-one
(13CBz). Chromatographic purification (chloroform–metha-
nol, 95/5, v/v) gave 13CBz, yield 66% (324 mg), foam. dH
(DMSO-d6) 3.19 (m, 3H), 3.73 (m, 2H), 4.35 (m, 3H),
5.43 (m, 3H), 7.20–8.24 (m, 12H), 11.20 (br s, 1H). dC
(DMSO-d6) 40.17, 57.43, 58.62, 59.21, 62.64, 96.74,
128.47, 128.64, 129.19, 129.30, 129.38, 129.70, 132.78,
133.12, 133.35, 148.17, 155.33, 163.38, 167.23. IR: 3410,
3030, 1721, 1690, 1660, 1338, 1147. HRMS m/z calcd for
C23H25N4O7S (M+H)+ 501.1438, found 501.1451.
5.11 (d, 3J 7.2, 1H). dC (CDCl3) 35.90, 60.13, 63.10,
68.56, 81.13. IR: (neat) 3504, 3380, 3016, 2936, 2888,
1332, 1164, 1060.
The crude 11 was dissolved in dry DCM (4 cm3). Dry pyri-
dine (0.9 cm3, 0.87 g, 11 mmol) and benzoyl chloride
(0.7 cm3, 0.84 g, 6 mmol) were added to that solution, sub-
sequently. The mixture was left at room temperature for
24 h. Water (30 cm3) was added and the mixture was stirred
for 1 h. The organic layer was separated and the aqueous
phase was extracted with DCM (2ꢂ10 cm3). The organic
phases were combined and washed with water and dried.
The solvent was distilled off and the residue was crystallized
from hexane–ethyl acetate mixture (1/1, v/v) to give 12 as
white crystals, yield 72% (1.13 g), mp 102–103 ꢀC. dH
(CDCl3) 2.91 (s, 3H), 3.78 (m, 1H), 4.29 (m, 2H), 4.44 (m,
2H), 4.62 (d, J 7.1, 1H), 5.20 (d, J 7.1, 1H), 7.45 (m,
2H), 7.59 (m, 1H), 8.04 (m, 2H). dC (CDCl3) 37.04, 57.06,
64.70, 69.00, 80.97, 128.68, 129.52, 129.84, 133.55,
166.26. IR: (neat) 3012, 2932, 2892, 1708, 1456, 1340,
1284, 1160, 1128. HRMS m/z calcd for C12H15NO5NaS
(M+Na)+ 308.0563, found 308.0571.
4.2.4. 6-(Benzoylamino)-9-{N-[2-benzoyloxy-1-(hydroxy-
methyl)ethyl]mesylaminomethyl}-9H-purine (13ABz).
Chromatographic purification (chloroform–acetone, 98/2,
v/v) afforded 13ABz, yield 14% (74 mg, TMSOTf) or 26%
(138 mg, SnCl4), foam. dH (CDCl3) 2.74 (s, 3H), 4.06 (m,
2H), 4.39 (m, 1H), 4.55 (m, 2H), 4.94 (m, 1H), 5.88 (s,
2H), 7.52 (m, 6H), 7.98 (m, 4H), 8.36 (s, 1H), 8.77 (s,
1H), 9.17 (br s, 1H). dC (CDCl3) 42.90, 53.96, 60.32,
61.30, 63.38, 123.33, 128.67, 129.34, 129.72, 129.94,
130.39, 133.81, 134.11, 134.24, 145.29, 150.82, 151.80,
153.54, 165.33, 166.86. IR: 3410, 1716, 1623, 1527, 1325,
1143. HRMS m/z calcd for C24H24N6O6NaS (M+Na)+
547.1370, found 547.1383.
3
3
4.2. Synthesis of azanucleosides 13
Azanucleosides 13 were obtained according to the procedure
described for the syntheses of 6CBz and 6ACbz (for details
see Table 1). For the syntheses of 13T and 13CBz a molar
ratio of nucleobase (B)–BSA–Lewis acids–12 was 2.0/4.0/
1.66/1.0, respectively. In the case of the preparation of
13FU and 13ABz the above ratio was 2.0/4.0/3.0/1.0. The
reaction mixtures were purified by flash chromatography to
give the corresponding azanucleosides 13; eluting solvents
are given in parentheses below.
4.3. Deprotection of 13
Nucleosides 13 were treated with concd ammonium hydrox-
ide in MeOH at room temperature for 24 h. The ratio of
13–NH4OHconcd–MeOH was 0.25 mmol/4 cm3/4 cm3, re-
spectively. The reaction mixtures were evaporated to dryness
under reduced pressure and the residues were purified by
flash chromatography to give the corresponding azanucleo-
sides: 7T, 7FU, 7C or 7A from 13T, 13FU, 13CBz or
13ABz, respectively (Table 1). The nucleosides 7C and 7A
obtained from corresponding 13CBz and 13ABz, respec-
tively, are identical with those obtained from 5.
4.2.1. 1-{N-[2-Benzoyloxy-1-(hydroxymethyl)ethyl]mesyl-
aminomethyl}-5-methyl-1H,3H-pyrimidin-2,4-dione
(13T). Chromatographic purification (chloroform–metha-
nol, 95/5, v/v) afforded 13T, yield 67% (273 mg), mp
182–186 ꢀC. dH (DMSO-d6) 1.42 (s, 3H), 3.14 (s, 3H),
4.3.1. 1-{N-[2-Hydroxy-1-(hydroxymethyl)ethyl]mesyl-
aminomethyl}-5-methyl-1H,3H-pyrimidin-2,4-dione
(7T). Yield 99% (74 mg), (chloroform–methanol, 9/1, v/v,
2
3.70 (m, 2H), 4.20 (m, 3H), 5.14 (d, J 15.1, 1H), 5.27 (m,
1H), 5.41 (d, 2J 15.1, 1H), 7.49 (m, 2H), 7.54 (s, 1H), 7.63
(m, 1H), 7.91 (m, 2H), 11.35 (br s, 1H). dC (DMSO-d6)
11.80, 40.37, 54.82, 57.94, 58.43, 61.92, 109.77, 128.70,
129.14, 129.26, 133.39, 138.41, 151.14, 163.62, 165.26.
IR: 3408, 3036, 1724, 1692, 1668, 1336, 1280, 1148.
HRMS m/z calcd for C17H21N3O7NaS (M+Na)+ 434.0992,
found 434.1000.
mp 160–187 ꢀC). dH (DMSO-d6) 1.77 (d, J 1.2, 3H), 3.08
4
(s, 3H), 3.47 (m, 4H), 3.78 (m, 1H), 4.92 (m, 2H), 5.18 (s,
2H), 7.50 (q, 4J 1.2, 2H). dC (DMSO-d6) 12.30, 40.20,
54.87, 59.59, 62.04, 109.55, 138.69, 150.98, 163.96. IR:
3424, 3364, 1704, 1656, 1340, 1280, 1164, 1128. HRMS
m/z calcd for C10H17N3O6NaS (M+Na)+ 330.0730, found
330.0744.

M. Koszytkowska-Stawinꢀska et al. / Tetrahedron 62 (2006) 10325–10331
10331
Goloshchapov, N. M.; Goloshchapova, E. N.; Michurina,
A. E.; Shipov, A. G.; Baukov, Yu. I. Zh. Obshch. Khim. 1998,
68, 502–504; Russ. J. Gen. Chem. (Engl. Transl.) 1998, 469–
471; (m) Madec-Lougerstay, R.; Florent, J.-C.; Monneret, C.
J. Chem. Soc., Perkin Trans. 1 1999, 1369–1376; (n) Gilchrist,
T. L.; Mendonca, R. Synlett 2000, 1843–1845.
4.3.2. 1-{N-[2-Hydroxy-1-(hydroxymethyl)ethyl]mesyl-
aminomethyl}-5-fluoro-1H,3H-pyrimidin-2,4-dione
(7FU). Yield 80% (137 mg), chloroform–methanol–NH3 aq,
7/3/0.5 and then 6/4/0.8, v/v/v, mp 172–190 ꢀC (dec). dH
(200 MHz, DMSO-d6) 3.10 (s, 3H), 3.49 (m, 4H), 3.78 (m,
1H), 4.96 (m, 2H), 5.18 (s, 3H), 7.87 (d, 3JF–H 6.6, 1H). dC
(50 MHz, DMSO-d6) 39.67, 55.52, 59.48, 62.22, 127.17
(d, JC–F 33.8), 139.70 (d, JC–F 229.5), 149.61, 157.12 (d,
2JC–F 25.8). IR: 3512, 3436, 3064, 2836, 1700, 1348,
1316, 1260, 1144. HRMS m/z calcd for C9H14N3O6FNaS
(M+Na)+ 334.0480, found 334.0496.
7. In the review on acyclic nucleosides1 no acyclic azanucleoside
is described.
2
1
8. (a) Bergmeier, S. C.; Fundy, S. L.; Drach, J. C. Nucleosides
Nucleotides Nucleic Acids 1999, 18, 227–238; (b) Sheikha,
G. A.; La Colla, P.; Loi, A. G. Nucleosides Nucleotides
Nucleic Acids 2002, 10, 619–635.
ꢀ
9. Koszytkowska–Stawinska, M.; Sas, W. Tetrahedron Lett. 2004,
Acknowledgements
45, 5437–5440.
10. Sas, W.;Koszytkowska-Stawinska, M.;Kaleta, K.;DeClercq, E.
ꢀ
Nucleosides Nucleotides Nucleic Acids, in press.
This work was financially supported by Warsaw University
of Technology.
11. Literature data show that the synthesis of N-unprotected
acyclic azanucleosides is impossible; it was found out that
attempts at removal of N-acyl protecting group from ‘cyclic’
azanucleosides caused their rapid decomposition. Altmann,
K.-H. Tetrahedron Lett. 1993, 34, 7721–7724; Rassau, G.;
Pinna, L.; Spanu, P.; Ulgheri, F.; Casiraghi, G.; Altmann,
K.-H. Tetrahedron Lett. 1994, 35, 4019–4022; Recently, simi-
lar observation has been made by us for acyclic N-acetyl
analogues of ganciclovir; an intramolecular migration of
N-acetyl group from nitrogen atom to oxygen, causing depro-
tection of the nitrogen atom of sugar mimic, is responsible
for a decomposition of the azanucleosides in a basic medium
(unpublished results).
References and notes
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17. In the light of successful synthesis of N-acetyl9 and N-tosyl10
acyclic aza-analogues of guanosine as well as adenosine (inde-
pendently of kind of N-substituent), the fiasco of preparation of
the N-mesyl analogue guanosine by various methods (Ref. 9
and this paper) is somewhat puzzling, indeed. Studies on solu-
tion of this problem are in progress.
18. Required to cause a microscopically detectable alteration of
normal cell morphology.
19. The minimum cytotoxic concentrations of reference com-
pounds were as follows: brivudin and ribavirin (Vero, E6SM
or HeLa cells, >400 mM); (S)-DHPA (Vero or HeLa cells,
>400 mM); acyclovir (E6SM cells, >400 mM); and ganciclovir
(E6SM cells, >100 mM).