Towards metformin prodrugs

Article abstract of DOI:10.1055/s-0028-1083603

The first examples of possible metformin prodrugs have been synthesized and characterized by spectroscopic methods. These five different types of chemically stable prodrugs were prepared from metformin and the corresponding halogenated promoieties. The products from unstable derivatives were also identified and reasons for the rearrangements are discussed. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0028-1083603

PAPER
3619
Towards Metformin Prodrugs
Towards
M
etform
r
u
i
gs stiina M. Huttunen,*a Jukka Leppänen,a Eeva Kemppainen,a Piia Palonen,a Jarkko Rautio,a Tomi Järvinen,a
Jouko Vepsäläinenb
a
Department of Pharmaceutical Chemistry, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland
Fax +358(17)162456; E-mail: Kristiina.Huttunen@uku.fi
Department of Biosciences, University of Kuopio, P.O. Box 1627, 70211 Kuopio, Finland
b
Received 6 June 2008; revised 13 August 2008
A typical solution for the above mentioned problems is to
use the prodrug approach, in which an applicable bio-
reversible promoiety is attached to a parent drug. These
pharmacologically inactive derivatives require a chemical
Abstract: The first examples of possible metformin prodrugs have
been synthesized and characterized by spectroscopic methods.
These five different types of chemically stable prodrugs were pre-
pared from metformin and the corresponding halogenated promoi-
eties. The products from unstable derivatives were also identified and/or enzymatic degradation after the delivery to release
and reasons for the rearrangements are discussed.
the parent drug.
Key words: metformin, prodrug, heterocycle, self-immolative
spacer group, urethane
To our knowledge, prodrugs of metformin have not been
reported, which is rather surprising, since typical require-
ments for prodrug derivatives of metformin are fulfilled:
1) widely used generic drug, 2) pharmacokinetic proper-
Metformin (1, Met, N,N-dimethylimidodicarbonimidic
diamide) is a synthetic antidiabetic drug widely used to
treat type 2 diabetes. Its precursor, galegine (isoamylene
guanidine), was isolated in the late 1920’s from Goat’s rue
(Galega officinalis), a plant which was used in medieval
times as a prescription for intense urination that can ac-
company the disease nowadays called diabetes.1,2 Met-
formin was clinically used for the first time in Europe in
the 1950’s together with two other structurally analogous
antidiabetic drugs, phenformin and buformin. However,
phenformin was withdrawn from the market in the 1970’s
due to its side effect of frequent lactic acidosis and in-
creased cardiac mortality while a less lipophilic metform-
in with a short plasma half-life, unsubstantial metabolism,
and protein binding proved to be safer. Metformin was fi-
nally approved for the treatment of diabetes by FDA
(United States Food and Drug Administration) in the mid
1990s. At the moment, metformin is the most popular an-
tidiabetic drug in both the United States and Europe with
ca. 35 million prescriptions filled in the US only in 2006.
ties are far from perfect, and 3) there is functionality (here
the NH2 group) to attach promoieties. Most probably this
is due to the complex chemistry of metformin with several
nucleophilic electron pairs leading to susceptible forma-
tion of heteroaromatic rings after formation of classical N-
substituted prodrugs, like N-acyls, N-alkoxycarbonyls or
N-acyloxymethylcarbonyls.6–10 Also characterization by
NMR techniques is challenging, since metformin itself
lacks protons bound to carbon atoms and protons at nitro-
gen atoms are to some extent pH dependent broad signals
without any coupling information. Moreover, measuring
of 13C and 15N NMR spectra is time consuming due to
long relaxation times and 15N also suffers from low natu-
ral abundance.
In this paper, we report for the first time chemically stable
metformin derivatives to be used as prodrugs of metform-
in. Our strategy here was to demonstrate which kinds of
metformin prodrug structures are stable, and not to pre-
pare a series of compounds in which only one substituent
is varied. The prepared compounds, collected in
Scheme 1, are synthesized from metformin and the corre-
sponding halogenated promoieties under basic conditions.
According to our previous studies (described later in this
paper) we realized that rigid spacer groups, like in the
compounds 2 and 3, or enough long alkyl chain, as in the
compound 4, are needed to avoid the above mentioned cy-
clization reactions. However, we were quite surprised
when we found that the derivatives 5 and 6 were also sta-
ble crystalline compounds.
NH NH2
NH2 NH
N
NH
NH
Me2N
N
NH2
Me2N
NH2
Me2N
N
H
NH2
1
Figure 1 Tautomeric stabilization of protonated metformin
Metformin is fully protonated under physiological condi-
tions (Figure 1), and therefore slowly and incompletely
absorbed from the upper intestine after oral administra-
tion. Together with a rapid kidney excretion, metformin
suffers from the poor bioavailability and causes uncom-
fortable gastrointestinal side effects at effective doses.3–5
Nowadays, the most prominent rigid self-immolative
spacer groups are based on an 1,6-elimination reaction.
These spacer prodrugs release after the enzymatic hydro-
lysis of the amide or ester bond an intermediate that be-
comes electron-donating and initiates an electronic
cascade, which leads to an elimination of either p-amino-
or p-hydroxybenzylic leaving group, respectively, and the
parent drug.11–15 We used this prodrug method to prepare
SYNTHESIS 2008, No. 22, pp 3619–3624
x
x.
x
x
.
2
0
0
8
Advanced online publication: 30.10.2008
DOI: 10.1055/s-0028-1083603; Art ID: Z11208SS
© Georg Thieme Verlag Stuttgart · New York

3620
K. M. Huttunen et al.
PAPER
O
NH NH2
NH NH2
O
NH NH2
NH2
NHCO2t-Bu
Me2N
BrH2C
Cl
OR
O
Me2N
N
N
H
O
Me2N
N
N
OR
N
1
H
2
N
H
Ot-Bu
O
Ph
5a, R = Bn
5b, R = CH2CCl3
P
O2CR
O
Cl
O
Cl
O
3
O
NH NH2
BrH2C
NH NH2
O
X
O
Me2N
N
N
O
NH NH2
Ph
P
Me2N
N
N
H
O
X
O
R
Me2N
N
N
O
H
3a, X = H, R = (CH2)6Me
3b, X = H, R = NEt2
3c, X = OH, R = t-Bu
6
4
H
Scheme 1 Designed and synthesized prodrugs of metformin (1)
the aminocarbamic acid ester 2 and the esters 3a and 3b Scheme 3) is not favored, since the alkyloxy units are
from selected halogenated benzyl derivatives and met- poor leaving groups. However, if the route 1 → 12 is pre-
formin in 20, 26 and 35% yield, respectively. In the case dominant, the chloride or bromide anion must be the leav-
of the compound 3c, the situation was more complex.
ing group being formed after elimination of H2CO from
ClCH2O group or an unusual ethyloxide from the corre-
sponding ethyl derivative. Based on the result from the re-
action with 1-chloroethyl pivalate (Scheme 4) the
formation of the intermediate 12 via 13 or 14 is more lia-
ble.
Surprisingly, the reduction of the aldehyde 8 to the alco-
hol 9 has not been previously described in the literature,
not even with simple ester substituents, like acetyl and
pivaloyl (Scheme 2). We realized the possible reason for
that was probably the unexpected rearranged product 11
when we tried to reduce the aldehyde 8 with NaBH4.16 The
correct product 9 was obtained by using Adam’s catalyst
(PtO2) as the hydrogenation catalyst as shown in
Scheme 2. Moreover, the coupling reaction of 10 to met-
formin afforded the monoester 3c, possible due to anchi-
meric assistance of neighboring amino group of
metformin.
O
R
X
+
NH NH2
Me2N
N
NH2
1
CMe3
X
O
t-Bu
O
Me
CHO
O
X = Cl, OCH2Cl
or O(CH2)2Br
• •
OH
OR
O
NH HN
NaBH4
PtO2
[H]
NH
NH
O
NH2
O
Me2N
N
••
Me2N
N
NH2
14
Me
R
13
O
OR
OR
O
t-Bu
O
– H2CO
–
••
NH
9, X = OH
10, X = Br
11
7, R = H
NH
O
8, R = pivaloyl
Me2N
N
Scheme 2 Reduction of 4-formylphenyl ester 8 with NaBH4 yielded
the compound 11 whereas desired compound 9 was obtained by hy-
drogenation with PtO2 as catalyst
NH2
t
R
12a, R = t-Bu
12b, R = Me
12c, R = Ot-Bu
HN
O
N
N
Ot-Bu
H2N
N
The prepared compound 4 is an uncommon choice for a
prodrug due to poor leaving properties of the propyl chain.
Actually, the propyl moiety was the first stable alkyl de-
rivative in the series, which we managed to attach to met-
formin. This is because both acetyloxymethyl ester, which
is a frequently used promoiety,17 and acetyloxyethyl ester
derivatives yielded in our hands aromatic 1,3,5-triazines
16a and 16b, respectively, as shown in Scheme 3. Ac-
cording to this and since compound 4 was stable, we ex-
pected that the corresponding methyl and ethyl
intermediates 13 and 14 were formed first and rearranged
further to the intermediate 12, which was finally cyclized
to the energetically favored aromatic end products 16a
and 16b, respectively. On the other hand, direct attack of
nucleophilic nitrogen of metformin to electrophilic carbo-
nyl group of prodrug promoiety (route 1 → 12 in
Me2N
N
NH2
H
15
16a, R = t-Bu
16b, R = Me
16c, R = OH
Scheme 3 Proposed reaction mechanisms of rearrangement of met-
formin derivatives to 1,3,5-triazines 16a–c
t-Bu
Me
O
O
Me
t-Bu
Me
NH
H
N
NH
Cl
O
O
N
– t-BuCO2H
Me2N
1
N
NH2
Me2N
N
17
NH2
18
Scheme 4 Proposed reaction mechanism of metformin and 1-chlo-
roethyl pivalate to 1,3,5-triazine 18
Synthesis 2008, No. 22, 3619–3624 © Thieme Stuttgart · New York

PAPER
Towards Metformin Prodrugs
3621
OEt
The cyclic products were also obtained with 1-chloroethyl
pivalate (Scheme 4), which is currently the most ideal
acyloxyalkyl promoiety due to its safe ethanal elimination
product compared to formaldehyde obtained from the cor-
responding methyl derivative. In this case, the formed cy-
clic end product 18 was explained only by the expected
alkylation product 17 synthesized from metformin and 1-
chloroethyl pivalate.18 However, the product 17 was not
stable due to elimination of pivalic acid, which in this case
was a good leaving group. The same elimination reaction
was also observed in the reaction of the dimethylcyan-
amide and boc-guanidine 15 (Scheme 3) which led to the
heteroaromatic product 16c.19
O
P
OEt
OEt
O
OEt
N
OEt
OEt
P
Cl
H
P
N
NH
N
1
– H2O
Me2N
N
NH2
Me2N
N
NH2
20
19
Scheme 5 Proposed reaction mechanism of metformin and diethyl
chlorophosphate to 1,3,5,2-triazaphosphinine 20
RO
N
Na+
NH2
N
O
MeONH2⋅HCl
HONH2⋅HCl
–N
CN
CN
N
N
H
NH2
H2N
CN
21
22
23
Unexpectedly, we were able to prepare also the traditional
urethane prodrugs 5a,b (Scheme 1) in 26 and 23% yields,
respectively, while the amides 12 prepared from metfor-
min and an acid derivative, for example, X = Cl, were un-
stable as shown in Scheme 3. In the case of urethanes
5a,b, our first trials afforded also the heteroaromatic prod-
uct 16 (R = OH) before we realized that cyclization oc-
curred gradually during and after the chromatographic
separation when the urethane derivative was in the base
form. However, white crystalline products were obtained
with reasonable yields when the crude product was first
acidified with HCl gas and then purified using normal
phase flash chromatography.
Me2NH⋅HCl
Me2NH
– Me2NH
OH
OR
MeONH2⋅HCl
NH
N
HONH2⋅HCl NH
NH
N
CN
Me2N
N
NH2 Me2N N
Me2N
N
NH2
H
H
H
24
25
26
Scheme 6 Proposed reaction mechanisms of the compounds 22 and
24
In conclusion, the traditional methods to prepare more li-
pophilic prodrugs of metformin, like N-acetylation and N-
alkoxycarbonylation, lead to rearrangement and internal
cyclization of metformin derivatives producing five or
six-membered heterocycles. To avoid the cyclization be-
havior of metformin derivatives, cyclic phosphate and
urethane prodrugs were synthesized. Long and rigid spac-
er groups, like self-immolative 1,6-elimination spacer
groups, were also used successfully in the syntheses of
metformin prodrugs. Although prodrugs of metformin
have not been described in literature previously, probably
due to distinctive chemistry of metformin, novel ap-
proaches to enhance the bioavailability and to decrease
variable adverse effects of this widely used antidiabetic
drug are unquestionably needed.
The last example, the compound 6 in the Scheme 1, be-
longs to the prodrug family of cyclic phosphates. The cy-
clic phosphates are cytochrome P450 (CYP)-selective
prodrugs designed to be activated predominantly in the
liver.20,21 The cyclic phosphate 6 was synthesized from
metformin and the corresponding cyclic chlorophosphate
with 33% yield. Interestingly, it seems that only the cyclic
phosphate derivatives, like the compound 6, are stable
since we tried to prepare also acyclic derivatives with
poor success. As mentioned previously, the tendency for
the stable, energetically favored heteroaromatic ring was
strong since normally an extremely stable P=O bond from
product 19 was broken up and the 1,3,5,2-triazaphosphin-
ine 20 was formed after elimination of water (Scheme 5).
Commercially available reagents and solvents were used without
further purification. Reactions were monitored by using 60 F245
TLC plates. Chromatographic separations were performed on Shi-
madzu preparative HPLC system (Shimadzu Corporation, Kyoto,
Japan) attached to UV-VIS detector (wavelength 254 nm) by using
a Kromasil® 100 Å column (C8, 150 × 20 mm, 5 mM), at a flow rate
Prodrugs of metformin were also tried to prepare from se-
lected nitriles as shown in Scheme 6 and previously in
Scheme 3. In most cases we obtained either cyclic end
products, a complicate mixture of products, or the starting
materials did not react. Actually, the only prodrug candi-
date of metformin, which we managed to prepare by fol-
lowing this strategy, was the compound 25 (R = Me)
either starting from sodium cyanamide (21) via N1-cyano-
N2-methoxyguanidine (22) or N1,N1-dimethyl-N3-cyano-
guanidine (24) as shown in Scheme 6. In the case of O-
pivaloylhydroxylamine (R = t-Bu) the reaction was ex-
tremely slow and with hydroxylamine, according to 13C
NMR (d = 170.1, 168.5) and mass (m/z = 100.9, M + H+)
spectra results, the heteroaromatic 3,5-diamino-1,2,4-
oxadiazole 2322,23 was obtained.
of 12 mL/min. H, 13C, and 31P NMR spectra were recorded on a
1
Bruker Avance 500 spectrometer (Bruker Biospin, Fällanden, Swit-
zerland) operating at 500, 126, and 202 MHz, respectively. TMS
was used as an internal standard. Elemental analyses (C, H, N) were
performed on a ThermoQuest CE Instruments EA 1110-CHNSO
elemental analyzer (CE Instruments, Milan, Italy). Mass spectra
were obtained on a Finnigan LCQ quadropole ion trap mass spec-
trometer (Finnigan MAT, San Jose, CA, USA) equipped with an
electrospray ionization source. The analytical and spectroscopical
data for the known compounds 8,24 15,25 16a,26 16b, 18,27,28 and 2429
were not provided. The benzyl halides used in syntheses of the com-
pounds 2 and 3a,b were prepared in a similar manner to that of the
benzyl bromide 10 given below.
Synthesis 2008, No. 22, 3619–3624 © Thieme Stuttgart · New York

3622
K. M. Huttunen et al.
PAPER
1,1-Dimethylimidodicarbonimidic Diamide (1)
4-[(3-(N,N-Dimethylcarbamimidoyl)guanidino)methyl]-3-hy-
droxyphenyl Pivalate (3c)
Prepared from 4-(bromomethyl)-1,3-phenylene bis(2,2-dimethyl-
propanoate) (10; 0.33 g, 0.89 mmol) in DMF (2 mL) and metformin
(1; 0.14 g, 1.07 mmol) in DMF (8 mL) with an 18 h reaction time.
After flash chromatography eluting with MeOH–CH2Cl2 (1:20),
compound 3c was obtained as a yellowish oil; yield: 30 mg (8%).
1H NMR (DMSO-d6): d = 1.33 (s, 3 H), 3.02 (s, 6 H), 4.39 (s, 2 H),
6.48–6.53 (m, 2 H), 7.18–7.21 (m, 1 H).
13C NMR (DMSO-d6): d = 27.43 (q, 3 C), 39.98 (q, 2 C), 41.42 (t),
43.33 (s), 109.78 (d), 113.20 (d), 123.09 (s), 131.10 (d), 152.81 (s),
157.34 (s), 158.47 (s), 161.49 (s), 178.63 (s).
A mixture of 1,1-dimethylimidodicarbonimidic diamide hydrochlo-
ride (4.20 g, 25.5 mmol) in aq 1 M NaOH (30 mL) was stirred at r.t.
for 30 min. H2O was evaporated in vacuo and the residue was dis-
solved in MeOH (50 mL). The solvent was evaporated and the res-
idue was redissolved in MeOH (20 mL). NaCl formed was filtered
out of the solution and the filtrate was evaporated to yield basic met-
formin as a white solid compound 1; yield: 3.25 g (99%).
Benzyl Derivatives of Metformin; General Procedure
A solution of the bromo derivative in anhyd DMF or MeCN was
added dropwise to a solution of metformin (1) in anhyd DMF at 0
°C under argon. The reaction mixture was stirred at r.t. for 2–18 h
and the solvent was evaporated in vacuo. The residue was purified
by flash chromatography eluting with MeOH–CH2Cl2 (1:10).
ESI-MS: m/z = 336.4 (M + H)+.
Anal. Calcd for C16H25N5O3·1.0MeOH·1.5CH2Cl2·0.5H2O: C,
44.10; H, 6.30; N, 13.90. Found: C, 44.08; H, 6.43; N, 14.19.
tert-Butyl 4-[(3-(N,N-Dimethylcarbamimidoyl)guanidi-
no)methyl]phenyl Carbamate (2)
Prepared from tert-butyl 4-(bromomethyl)phenylcarbamate (0.4 g,
1.4 mmol) in DMF (2 mL) and metformin (1; 0.27 g, 2.10 mmol) in
DMF (8 mL) with an 18 h reaction time. Compound 2 was obtained
as a yellowish oil; yield: 90 mg (20%).
1H NMR (DMSO-d6): d = 1.47 (s, 3 H), 2.94 (s, 6 H), 4.24 (s, 2 H),
7.18–7.20 (m, 2 H), 7.39–7.41 (m, 2 H).
13C NMR (DMSO-d6): d = 28.09 (q, 3 C), 38.03 (q, 2 C), 45.21 (t),
78.83 (s), 117.94 (d), 127.62 (d), 132.21 (d), 138.40 (s), 152.71 (s),
156.48 (s), 159,65 (s).
3-[3-(N,N-Dimethylcarbamimidoyl)guanidino]propyl Acetate
(4)
A mixture of the plain metformin (1; 0.1 g, 0.77 mmol), NaI (0.23
g, 1.55 mmol), and 3-chloropropyl acetate (0.19 mL, 1.55 mmol) in
anhyd acetone (10 mL) was refluxed for 6 h under argon. The sol-
vent was removed under reduced pressure and the residue was puri-
fied by flash chromatography eluting with MeOH–CH2Cl2 (0.5:10)
to afford the compound 4 as a yellowish oil; yield: 130 mg (73%).
1H NMR (DMSO-d6): d = 1.89–1.99 (m, 2 H), 2.07 (s, 3 H), 3.13 (s,
6 H), 3.33–3.39 (m, 2 H), 4.13–4.18 (m, 2 H).
13C NMR (DMSO-d6): d = 21.19 (q), 29.18 (t), 37.35 (q, 2 C), 37.92
(t), 65.68 (t), 156.25 (s), 157.22 (s), 171.55 (s).
ESI-MS: m/z = 335.2 (M + H)+.
Anal. Calcd for C16H26N6O2·4.8DMSO·2.8H2O: C, 30.04; H, 5.61;
N, 13.14. Found: C, 29.91; H, 6.06; N, 13.45.
ESI-MS: m/z = 230.1 (M + H)+.
Anal. Calcd for C9H19N5O2·0.87MeOH·1.1CH2Cl2·0.81HCl: C,
31.19; H, 6.18; N, 18.42. Found: C, 31.19; H, 6.18; N, 18.42.
4-{[3-(N,N-Dimethylcarbamimidoyl)guanidino]methyl}phenyl
Octanoate (3a)
Prepared from 4-(bromomethyl)phenyl octanoate (0.3 g, 0.96
mmol) in MeCN (4 mL) and metformin (1; 0.19 g, 1.44 mmol) in
MeCN (16 mL) with an 18 h reaction time. Compound 3a was ob-
tained as a yellowish oil; yield: 90 mg (26%).
[(N¢,N¢-Dimethylguanidino)iminomethyl]carbamic Acid Benzyl
Ester (5a)
Benzyl chloroformate (0.85 g, 5.0 mmol) was added dropwise to a
solution of plain metformin (1; 1.13 g, 8.8 mmol) in anhyd MeCN
(50 mL) at 0 °C under argon. The mixture was stirred at 0 °C for 2.5
h. The mixture was filtered and the half of the filtrate was treated
with dry HCl gas for few min and the solvent was removed under
reduced pressure. The residue was purified by flash chromatogra-
phy eluting with MeOH–CH2Cl2 (1:10) to afford the compound 5a
as a white solid; yield: 195 mg (26%).
1H NMR (CD3OD): d = 3.09 (s, 6 H), 5.23 (s, 2 H), 7.37 (m, 5 H).
13C NMR (CD3OD): d = 38.32 (q), 39.09 (q), 68.94 (t), 129.33 (d),
129.54 (d), 129.57 (d), 136.56 (s), 153.72 (s), 155.42 (s), 160.30 (s).
3
1H NMR (DMSO-d6): d = 0.87 (t, JH,H = 7.0 Hz, 3 H), 1.21–1.38
(m, 8 H), 1.63 (q, 3JH,H = 7.2 Hz, 2 H), 2.56 (t, 3JH,H = 7.4 Hz, 2 H),
2.92 (s, 6 H), 4.19 (s, 2 H), 7.09–7.13 (m, 2 H), 7.20–7.26 (m, 2 H).
13C NMR (DMSO-d6): d = 14.39 (q), 22.49 (t), 24.80 (t), 28.80 (t, 2
C), 31.56 (t), 37.93 (t), 38.49 (q), 45.76 (t), 115.45 (d), 122.04 (d),
129.10 (d), 129.96 (d), 156.93 (s), 160.16 (s).
ESI-MS: m/z = 362.2 (M + H)+.
Anal. Calcd for C19H31N5O2·2.1DMSO: C, 43.42; H, 5.95; N, 13.03.
Found: C, 42.83; H, 5.90; N, 13.03.
ESI-MS: m/z = 263.3 (M)+.
4-{[3-(N,N-Dimethylcarbamimidoyl)guanidino]methyl}phenyl
Diethylcarbamate (3b)
Prepared from 4-(Bromomethyl)phenyl diethylcarbamate (0.87 g,
3.05 mmol) and metformin (1; 0.79 g, 6.10 mmol) in anhyd MeCN
(20 mL) with a 2 h reaction time to obtain compound 3b; yield: 295
mg (29%).
1H NMR (CDCl3–CD3OD): d = 0.97 (t, 3JH,H = 7.0 Hz, 3 H), 1.03 (t,
3JH,H = 7.0 Hz, 3 H), 2.81 (s, 6 H), 3.14 (q, 3JH,H = 7.0 Hz, 2 H), 3.23
(t, 3JH,H = 7.0 Hz, 2 H), 4.12 (s, 2 H), 6.81 (d, 3JH,H = 8.52 Hz, 2 H),
7.09 (m, 2 H).
Anal. Calcd for C12H17N5O2·1.0HCl·0.5H2O: C, 46.68; H, 6.20; N,
22.68. Found: C, 46.86; H, 6.20; N, 22.30.
[(N¢,N¢-Dimethylguanidino)iminomethyl]carbamic Acid 2,2,2-
Trichloroethyl Ester (5b)
2,2,2-Trichloroethyl chloroformate (1.63 g, 7.8 mmol) was added
dropwise to a solution of plain metformin (1; 2.0 g, 15.5 mmol) in
anhyd MeCN (50 mL) at 0 °C under argon. The mixture was stirred
at 0 °C for 2 h. The mixture was filtered and the filtrate was treated
with dry HCl gas for few min and the solvent was removed under
reduced pressure. The residue was purified by flash chromatogra-
phy eluting with MeOH–CH2Cl2 (1:10) to afford the compound 5b
as a white solid; yield: 610 mg (23%).
13C NMR (CDCl3–CD3OD): d = 12.63, 13.49, 37.84, 41.61, 41.91,
45.65, 121.43, 128.27, 134.82, 150.15, 154.37, 155.65, 159.80.
ESI-MS: m/z = 335.2 (M + H)+.
1H NMR (CD3OD): d = 3.10 (s, 6 H), 4.94 (s, 2 H).
13C NMR (CD3OD): d = 37.94 (q), 38.82 (q), 75.70 (t), 96.03 (s),
153.04 (s), 154.44 (s), 161.65 (s).
Synthesis 2008, No. 22, 3619–3624 © Thieme Stuttgart · New York

PAPER
Towards Metformin Prodrugs
3623
Anal. Calcd for C7H12Cl3N5O2·1.0HCl·0.4MeOH: C, 46.68; H,
6.20; N, 22.68. Found: C, 46.86; H, 6.20; N, 22.30.
mixture was stirred at r.t. for 2 h and quenched by the addition of
hexane (100 mL). The mixture was filtered and the filtrate was
evaporated under reduced pressure. The residue was purified by
flash chromatography eluting with EtOAc–hexane (1:5) to yield the
compound 10 as a yellowish oil, yield: 0.35 (97%).
[(N1,N1-Dimethylcarbamimidoyl)guanidino]-4-phenyl-1,3,2-di-
oxaphosphoramidate (6)
Freshly distilled POCl3 (0.12 mL, 1.31 mmol) in cold anhyd Et2O
(15 mL) was added dropwise to a solution of 1-phenylpropane-1,3-
diol (0.2 g, 1.31 mmol) and Et3N (0.36 mL, 2.60 mmol) in anhyd
Et2O (5 mL) at 0 °C under argon. The mixture was stirred at r.t. for
2 h. The precipitate was filtered out of the mixture and washed thor-
oughly with Et2O (3 × 10 mL). The filtrate was concentrated in vac-
uo to obtain the cyclic phosphoryl chloride as a brownish oily
mixture of diastereomers (55:45, 0.34 g). Due to partial decomposi-
tion, the product was not purified further.
1H NMR (CDCl3): d = 1.34 (s, 9 H), 1.41 (s, 9 H), 4.38 (s, 2 H),
6.92–6.96 (m, 2 H), 7.39 (d, 3JH,H = 8.3 Hz, 1 H).
13C NMR (CDCl3): d = 27.18 (q), 27.27 (q), 29.46 (t), 39.26 (s),
39.55 (s), 116.72 (d), 119.34 (d), 127.11 (d), 131.24 (s), 149.83 (s),
151.82 (s), 176.18 (s), 176.58 (s).
4-Amino-6-dimethylamino[1,3,5]triazin-2-ol (16c)
Boc-guanidine 1525 (0.8 g, 5 mmol) and dimethylcyanamide (5 mL,
61.3 mmol) were heated at 90 °C for 6 h. The mixture was filtered
and washed with EtOAc (2 × 10 mL) to give the white solid product
16c; yield: 0.18 g (23%).
1H NMR (DMSO-d6): d = 3.01 (s, 6 H), 7.06 (br s, 2 H), 10.53 (br
s, 1 H).
13C NMR (DMSO-d6): d = 36.15 (q), 156.60 (s), 159.10 (s), 165.08
(s).
ESI-MS: m/z = 156.2 (M + H)+.
1H NMR (CDCl3): d (major diastereomer) = 2.04–2.13 (m, 1 H),
3
2.47–2.76 (m, 1 H), 4.52–4.77 (m, 2 H), 5.57 (td, JH,H = 11.9 Hz,
3JH,P = 2.3 Hz, 1 H), 7.34–7.50 (m, 5 H); d (minor diastereomer) =
2.17–2.47 (m, 2 H), 4.52–4.77 (m, 2 H), 5.74–5.80 (m, 1 H), 7.34–
7.50 (m, 5 H).
31P NMR (CDCl3): d (major diastereomer) = –1.85; d (minor dias-
tereomer) = –2.37.
The plain metformin (1; 1.51 mmol) in anhyd MeCN (10 mL) was
added dropwise to a solution of the above prepared cyclic phospho-
ryl chloride (0.36 g, 1.56 mmol) and 1-methylimidazole (0.1 mL,
1.25 mmol) in anhyd MeCN (10 mL) at 0 °C under argon. The mix-
ture was stirred at r.t. overnight, the solvent was removed under re-
duced pressure, and the residue was purified by flash
chromatography eluting with MeOH–CH2Cl2 (1:10) solution to af-
ford the compound 6 as a yellowish oil; yield: 165 mg (33%).
1H NMR (DMSO-d6): d = 1.92–2.11 (m, 2 H), 2.96 (s, 6 H), 4.36–
4.15 (m, 2 H), 5.28–5.33 (m, 1 H), 6.42 (s, 1 H), 6.89 (s, 1 H), 7.09
(s, 1 H), 7.31–7.42 (m, 5 H), 7.57 (s, 1 H).
2,2-Diethoxy-N4,N4-dimethyl-[1,3,5,2]triazaphosphinine-4,6-di-
amine (20)
Freshly distilled POCl3 (0.52 mL, 3.61 mmol) in cold anhyd MeCN
(5 mL) was added dropwise to a solution of plain metformin (1; 0.31
g, 2.39 mmol) and pyridine (0.29 mL, 3.59 mmol) in anhyd MeCN
(20 mL) at –10 °C under argon. The mixture was stirred at r.t. for
1 h and the solvent was evaporated under reduced pressure. The res-
idue was purified by flash chromatography eluting with MeOH–
CH2Cl2 (0.5:10) to yield the compound 20 as a yellowish solid;
yield: 0.30 g (47%).
3
13C NMR (DMSO-d6): d = 34.45 (dt, JC,P = 4.5 Hz), 36.71 (q),
3
1H NMR (CDCl3): d = 1.35 (t, JH,H = 7.3 Hz, 6 H), 3.10 (s, 3 H),
2
2
66.53 (dt, JC,P = 7.0 Hz), 79.25 (dt, JC,P = 7.0 Hz), 125.92 (d),
3.34 (s, 3 H), 4.02 (q, 2JH,H = 15.0 Hz, 3JH,H = 7.3 Hz, 4 H).
128.42 (d), 128.87 (d), 140.99 (s), 159.69 (s), 162.48 (s).
13C NMR (CDCl3): d = 15.78 dq (3JC,P = 7.8 Hz), 37.76 (dq,
4JC,P = 3.8 Hz), 38.03 (dq, 4JC,P = 3.8 Hz), 64.25 (dt, 2JC,P = 5.9 Hz),
155.58 (s), 158.67 (ds, 2JC,P = 5.8 Hz).
31P NMR (DMSO-d6): d = 2.65.
ESI-MS: m/z = 326.1 (M + H)+.
Anal. Calcd for C13H20N5O3P·1.0MeOH·0.5CH2Cl2·0.2H2O: C,
43.17; H, 6.22; N, 17.36. Found: C, 43.53; H, 6.57; N, 17.00.
31P NMR (CDCl3): d = 22.53.
ESI-MS: m/z = 248.1 (M + H)+.
4-(Hydroxymethyl)-1,3-phenylene Bis(2,2-dimethylpropan-
oate) (9)
1-Cyano-2-methoxyguanidine (22)
Sodium dicyanamide (1.1 g, 12.36 mmol) and methoxylamine hy-
drochloride (1.03 g, 12.36 mmol) in anhyd EtOH (20 mL) were
stirred at r.t. overnight. NaCl was filtered out of the solution and the
solvent was evaporated in vacuo. The product 22 (1.25 g, 98%) was
recrystallized from H2O.
1H NMR (DMSO-d6): d = 3.48 (s, 3 H), 7.44 (br s, 2 H), 10.52 (br
s, 1 H).
To a solution of 824 (1.22 g, 4 mmol) in propan-2-ol (50 mL) was
added 10% Pd/C (100 mg) and the mixture was hydrogenated at 1.0
bar pressure for 3 h. Catalyst was filtered off through Celite, washed
with propan-2-ol (10 mL) and the filtrates were evaporated to dry-
ness in vacuo to give 9 (1.03 g, 84%) as a colorless oil. This product
was used without further purification in the next step.
1H NMR (CDCl3): d = 1.34 (s, 9 H), 1.37 (s, 9 H), 4.54 (s, 2 H), 6.82
13C NMR (DMSO-d6): d = 64.24 (q), 117.43 (s), 162.85 (s).
ESI-MS: m/z = 113.4 (M – H)–.
4
4
(d, JH,H = 2.1 Hz, 1 H), 6.98 (dd, JH,H = 2.1 Hz, 1 H), 7.47
(d, 3JH,H = 8.4 Hz, 1 H).
13C NMR CDCl3): d = 27.14 (q), 27.20 (q), 39.17 (s), 39.36 (s),
60.12 (t), 115.88 (d), 119.39 (d), 129.9 (d), 149.09 (s), 151.18 (s),
176.72 (s), 177.30 (s).
Anal. Calcd for C3H6N4O·0.38H2O: C, 29.79; H, 5.63; N, 46.32.
Found: C, 30.16; H, 5.19; N, 45.90.
[1,2,4]Oxadiazole-3,5-diamine (23)
Method A: N1,N1-Dimethyl-N3-cyanoguanidine (24;29 0.4 g, 3.57
mmol), NH2OH·HCl (0.37 g, 5.35 mmol) and Et3N (0.75 mL, 5.35
mmol) were stirred in anhyd EtOH (10 mL) at r.t. overnight. The so-
lution was filtered and the solvent was evaporated in vacuo. The res-
idue was purified by preparative HPLC on reversed phase
Kromasil® 100 Å (C8) column using MeCN–H2O (3:7) as eluent to
obtain the white solid compound 23; yield: 0.28 g (78%).
4-(Bromomethyl)-1,3-phenylene Bis(2,2-dimethylpropanoate)
(10)
The conversion of the alcohol 9 into the corresponding bromide 10
was performed using a modification of the method reported by Lee
and Hwang.30 A solution of Ph3P (0.31 g, 1.17 mmol) in anhyd
CH2Cl2 (10 mL) was added to a solution of 9 (0.30 g, 0.97 mmol) in
anhyd CH2Cl2 (10 mL) at –20 °C under argon followed by N-bro-
mosuccinimide (0.21 g, 1.17 mmol) in anhyd CH2Cl2 (10 mL). The
1H NMR (DMSO-d6): d = 5.52 (br s, 2 H), 7.24 (br s, 2 H).
Synthesis 2008, No. 22, 3619–3624 © Thieme Stuttgart · New York

3624
K. M. Huttunen et al.
PAPER
13C NMR (DMSO-d6): d = 168.46 (s), 170.08 (s).
ESI-MS: m/z = 100.9 (M + H)+.
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11, 223.
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Rohlf, J. L. Am. J. Med. 1997, 103, 491.
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Hansel, S. B.; Vyas, D. M. Bioorg. Med. Chem. Lett. 1994,
4, 1985.
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Sci. 1997, 5, 79.
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Lett. 2001, 43, 57.
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L.; Cedillo, R.; Castillo, R. Bioorg. Med. Chem. Lett. 2001,
11, 1359.
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Milhous, W. K.; Skillman, D. R.; Lin, A. J. Bioorg. Med.
Chem. 2005, 13, 699.
Anal. Calcd for C2H4N4O·1.85EtOH·0.1Me2NH: C, 36.07; H, 8.02;
N, 29.52. Found: C, 36.12; H, 7.61; N, 29.21.
Method B:28,29 Sodium dicyanamide (21; 0.5 g, 5.62 mmol) and
NH2OH·HCl (0.39 g, 5.62 mmol) were stirred in anhyd EtOH (10
mL) at r.t. overnight. NaCl was filtered out of the solution and the
solvent was evaporated under reduced pressure to obtain the prod-
uct 23; yield: 0.26 g (45%). The chemical shifts of the NMR spectra
as well as the ESI-MS spectrum were identical to spectra of com-
pound 23 obtained by the Method A.
Anal. Calcd for C2H4N4O·0.27HCl: C, 21.85; H, 3.92; N, 50.97.
Found: C, 22.25; H, 4.02; N, 50.81.
N1,N1-Dimethylcarbamimidoyl-N4-methoxyguanidine (25)
Method A: N1,N1-dimethyl-N3-cyanoguanidine (24;29 0.5 g, 4.46
mmol), methoxyamine hydrochloride (0.74 g, 8.92 mmol), and py-
ridine (0.72 mL, 8.92 mmol) were refluxed in anhyd EtOH (10 mL)
for 6 h. The solvent was evaporated under reduced pressure and the
residue was purified by preparative HPLC on reversed phase Kro-
masil® 100 Å (C8) column using MeCN–H2O (3:7) as eluent to ob-
tain 25 (0.47 g, 66%) as a light yellow oil.
1H NMR (DMSO-d6): d = 2.95 (s, 6 H), 3.69 (s, 3 H), 7.52 (br s, 3
H), 8.41 (br s, 1 H).
13C NMR (DMSO-d6): d = 38.32 (q), 64.40 (q), 157.17 (s), 157.61
(s).
(11) Carl, P. L.; Chakravarty, P. K.; Katzenellenbogen, J. A.
J. Med. Chem. 1981, 24, 479.
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L. W. A.; Busscher, G. F.; Seelen, A. E.; Albrecht, C.; de
Bruijn, P.; Scheeren, H. W. J. Org. Chem. 2001, 66, 8815.
(15) Toki, B. E.; Cerveny, C. G.; Wahl, A. F.; Senter, P. D.
J. Org. Chem. 2002, 67, 1866.
ESI-MS: m/z = 160.0 (M + H)+.
(16) van de Wateer, R. W.; Magdziak, D. J.; Chau, J. N.; Pettus,
T. R. R. J. Am. Chem. Soc. 2000, 122, 6502.
(17) Rautio, J.; Kumpulainen, H.; Heimbach, T.; Oliyai, R.; Oh,
D.; Järvinen, T.; Savolainen, J. Nat. Rev. Drug Discovery
2008, 7, 255.
(18) Davidsen, S. K.; Summers, J. B.; Albert, D. H.; Holms, J. H.;
Heyman, H. R.; Magoc, T. J.; Conway, R. G.; Rhein, D. A.;
Carter, G. W. J. Med. Chem. 1994, 37, 4423.
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J.; Juvonen, R. O.; Raunio, H.; Kumpulainen, H.; Järvinen,
T.; Rautio, J. Pharm. Res. 2007, 24, 679.
Anal. Calcd for C5H13N5O·1.0MeOH·0.6pyridine: C, 30.19; H,
7.18; N, 29.34. Found: C, 30.49; H, 7.13; N, 29.14.
Method B: N1-Cyano-N2-methoxyguanidine (22; 0.5 g, 4.38 mmol),
dimethylamine hydrochloride (0.36 g, 4.38 mmol), and Et3N (0.61
mL, 4.38 mmol) were refluxed in anhyd EtOH (10 mL) overnight.
The solvent was evaporated in vacuo and the residue was purified
by preparative HPLC on reversed phase Kromasil® 100 Å (C8) col-
umn using MeCN–H2O (3:7) as eluent to obtain the impure com-
pound 25 (0.32 g, 46%) as a light yellow oil. The chemical shifts of
the NMR spectra as well as the ESI-MS spectrum were identical to
the spectra of compound 25 obtained by Method A.
(21) Erion, M. D.; Bullough, D. A.; Lin, C.-C.; Hong, Z. Curr.
Opin. Invest. Drugs 2006, 7, 109.
(22) Roemer, J. J.; Kaiser, D. W. American Cyanamid Co., US
Patent 2648669, 1953; Chem. Abstr. 1953, 47, 66237.
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Part A1 1971, 9, 1117.
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M.; Muñoz-Dorado, M.; Rodríguez-García, I. Chem. Eur. J.
2007, 13, 557.
(25) Feichtinger, K.; Sings, H. L.; Baker, T. J.; Matthews, K.;
Goodman, M. J. Org. Chem. 1998, 63, 8432.
(26) Chen, W.; Du, C.; Guo, J.-P.; Wei, X.-H.; Liu, D.-S.
J. Organomet. Chem. 2002, 655, 89.
Acknowledgment
We thank Mrs. Katja Hötti, Mrs. Miia Reponen, and Mrs. Maritta
Salminkoski for precious technical assistance as well as Docent Ta-
pio Nevalainen for the great scientific contribution. The work was
financially supported by the National Graduate School of Organic
Chemistry and Chemical Biology, the Finnish Funding Agency for
Technology and Innovation (Tekes), the Academy of Finland
(grants #106313, J. L.; #204905, J. R.; #204755, J. V.), Research
and Science Foundation of Farmos, the Diabetes Research Funding
Foundation, the Emil Aaltonen Foundation, the Alfred Kordelin
Foundation (the Gust. Komppa fund), and the Finnish Konkordia
Foundation.
(27) Shapiro, S. L.; Isaacs, E. S.; Parrino, V. A.; Freedman, L.
J. Org. Chem. 1961, 26, 68.
(28) Nishigaki, S.; Yoneda, F.; Matsumoto, H.; Morinaga, K.
J. Med. Chem. 1969, 12, 39.
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Synthesis 2008, No. 22, 3619–3624 © Thieme Stuttgart · New York