Facile preparation of nucleoside-5'-carboxylic acids

Full text of DOI:10.1021/jo981316g

J . Org. Chem. 1999, 64, 293-295
293
Sch em e 1
F a cile P r ep a r a tion of
Nu cleosid e-5′-ca r boxylic Acid s
J effrey B. Epp and Theodore S. Widlanski*
Department of Chemistry, Indiana University,
Bloomington, Indiana 47405-4001
Received J uly 7, 1998
Oxidation of the 5′-hydroxymethylene of nucleosides
to 5′-carboxylates is an essential step in the preparation
of a number of biologically active molecules.1 There are
relatively few methods describing the general preparation
of such nucleoside-5′-carboxylic acids. Herein, we report
a mild procedure for the oxidation of 2′,3′-isopropylidene-
protected purine- and pyrimidine-containing nucleosides
to their respective 5′-carboxylic acids. This method gives
high yields and has a very simple isolation procedure.
One of the most widely applied methods for affecting
the oxidation of the 5′-hydroxyl of unprotected nucleo-
sides employs molecular oxygen and a platinum catalyst.2
However, this method affords relatively low yields when
applied to 2′,3′-isopropylidene-protected nucleosides.3
Instead, these nucleosides are most often oxidized using
potassium permanganate under strongly alkaline reac-
tion conditions.4 This limits the potassium permanganate
methodology to purine-containing nucleosides. Two other
systems used to affect these conversions are CrO3/acetic
acid5 and a two-step method involving the generation of
the aldehyde followed by oxidation with m-CPBA.6 How-
ever, these methods have not been generally applied.
Recently, a method utilizing ruthenium trichloride and
sodium periodate under Sharpless conditions was used
to obtain the 5′-carboxylic acids of 2′,3′-isopropylidenepu-
rine-containing nucleosides in high yield.7 Unfortunately,
the methodology cannot be used with pyrimidine-
containing nucleosides, as the reaction conditions cause
loss of the nucleoside base. Extension of ruthenium
trichloride-mediated oxidation to 2′,3′-isopropylidenepy-
rimidine-containing nucleosides requires the use of both
alkaline conditions and potassium persulfate.8 Thus,
many available methods for the generation of nucleoside-
5′-carboxylic acids require relatively basic conditions that
limit their utility.
A recent publication described the oxidation of alcohols
to ketones and aldehydes using catalytic amounts of
2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) and sto-
ichiometric amounts of an organic oxidant, [bis(acetoxy)-
iodo]benzene (BAIB).9 The method drew our attention
because of its mildness and efficiency. The active oxidant
is an N-oxoammonium salt generated by dismutation of
TEMPO; BAIB is necessary to regenerate TEMPO by
oxidizing the corresponding hydroxylamine of TEMPO.
The reaction generates acetic acid and iodobenzene as
byproducts and is different from most other TEMPO-
mediated oxidations in that it avoids inorganic salt
contaminants.9 In addition, N-oxoammonium salt-medi-
ated oxidations are compatible with double and triple
bonds, esters, ethers, acetals, epoxides, amides, halides,
and azides. Finally, protecting groups such as TBDMS,
THP, MOM, Boc, Cbz, benzyl, and acetyl are also stable
to the reaction conditions.10
In the presence of high concentrations of water, N-
oxoammonium salts convert aliphatic alcohols to their
respective carboxylic acids.10 Since the TEMPO-BAIB
system seemed to be a particularly convenient method
for N-oxoammonium salt oxidations, we hoped to obtain
the desired 5′-carboxylates using the TEMPO-BAIB
system. The original TEMPO-BAIB reference cited a
single example of oxidation in an aqueous system,
namely, the oxidation of an allylic alcohol to an allylic
aldehyde in 1:1 acetonitrile-aqueous buffer (pH 7).9 Since
the presence of large amounts of buffer complicates the
isolation of carboxylic acids in other TEMPO-mediated
oxidations,10 we hoped to utilize the TEMPO-BAIB
oxidation system in a 1:1 acetonitrile-water solvent
system without the use of large amounts of buffer
(Scheme 1).
(1) (a) Prasad, R. N.; Bariana, D. S.; Fung, A.; Savic, M.; Tietje, K.;
Stein, H. H.; Brondyk, H. D.; Egan, R. S. J . Med. Chem. 1980, 23,
313. (b) Bridges, A. J .; Bruns, R. F.; Ortwine, D. F.; Priebe, S. R.;
Szotek, D. L.; Trivedi, B. K. J . Med. Chem. 1988, 31, 1282. (Source
Tetrahedron Lett. 1996, 37(10), 1567.) (c) Gallo-Rodriguez, C.; J i, X.;
Melman, N., Siegman, B. D.; Sanders, L. H.; Orlina, J .; Fischer, B.,
Pu, Q.; Olah, M. E.; van Galen, P. J . M.; Stiles, G. L.; J acobson, K. A.
J . Med. Chem. 1994, 37, 636. (d) Wnuk, S. F.; Liu, S.; Yuan, C. S.;
Borchardt, R. T.; Robins, M. J . J . Med. Chem. 1996, 39, 4162.
(2) Moss, G. P.; Reese, C. B.; Schofield, K.; Shapiro, R.; Todd, L. J .
Chem. Soc. 1963, 1149.
(3) Harper P. J .; Hampton A. J . Org. Chem. 1970, 35, 1688.
(4) (a) Harmon, R. E.; Zenarosa, C. V.; Gupta, C. V. Chem. Ind.
(London) 1969, 1141. (b) Hutchison, A. J .; Williams, M.; de J esus, R.;
Yokoyama, R.; Oei, H. H.; Ghai, G. R.; Webb, R. L.; Zoganas, H. C.;
Stone, G. A.; J arvis, M. F. J . Med. Chem. 1990, 33, 1919.
(5) Olsson, R. A.; Kusachi S.; Thompson, R. D.; Ukena D.; Padgett
W.; Daly, J . W. J . Med. Chem. 1986, 29, 1683.
Indeed, in our first attempt, the 5′-carboxylic acid of
2′,3′-isopropylideneadenosine, 2b, precipitated from the
reaction solution. After 3 h of stirring at room temper-
ature, the precipitated carboxylic acid was filtered and
triturated sequentially with diethyl ether and acetone.
This simple procedure yielded 2b in 90% yield and high
purity.11 Similarly, the 5′-carboxylic acid of N-benzoyl-
2′,3′-isopropylideneadenosine, 1b, precipitated from solu-
(9) De Mico, A.; Margarita, R.; Parlanti, L.; Vescovi, A.; Piancatelli,
G. J . Org. Chem. 1997, 62, 6974.
(10) De Nooy, A. E. J .; Besemer, A. C.; van Bekkum, H. Synthesis
1996, 1153.
(6) Barton, D. H. R.; Gero, S. D.; Quiclet-Sire, B.; Samadi M. J .
Chem. Soc., Perkin. Trans. 1 1991, 981.
(7) Singh, A. K.; Varma, R. S. Tetrahedron Lett. 1992, 33(17), 2307.
(8) Varma R. S.; Hogan, M. E. Tetrahedron Lett. 1992, 33(50), 7719.
(11) The 1H NMR data matched that previously reported except for
the higher order coupling observed at 400 MHz. The 13C NMR data
also match previously reported frequencies, although the previously
reported spectrum is offset by 3.9 ppm due to a referencing error.
10.1021/jo981316g CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/09/1998

294 J . Org. Chem., Vol. 64, No. 1, 1999
Notes
tion. Filtration and sequential trituration with diethyl
ether and acetone gave 1b in 70% yield and high purity.12
We found that the diethyl ether wash removes the
catalytic amounts of TEMPO and reaction byproducts
and gives material that is sufficiently pure for all
subsequent applications. However, trituration with ac-
etone was necessary to obtain analytically pure samples.
Since carboxylates 1b and 2b are essentially insoluble
in acetone, the acetone trituration did not reduce the
yields of these products. The ease of product isolation in
these reactions makes the method ideal for large-scale
preparations.
In contrast to the reaction behavior of the first two 5′-
carboxylates, the 5′-carboxylate of 2′,3′-isopropylidene-
uridine, 3b, oiled out of solution instead of precipitating
as a solid. Consequently, after 3 h of stirring at room
temperature, the reaction solution was concentrated to
dryness, causing the product to solidify. The material was
then triturated with diethyl ether and the product
isolated by filtration. This alternative workup resulted
in 76% yield of 3b in good purity.13,14 Thus, the catalytic
amounts of TEMPO and reaction byproducts may be
easily removed by diethyl ether trituration, even when
the product does not crystallize from the reaction solu-
tion.
The extension of the methodology to 2′,3′-isopropy-
lidene derivatives of cytidine and guanosine required
some slight procedural modifications. Reaction of 2′,3′-
isopropylidenecytidine under the previously stated condi-
tions resulted in poor yields of the desired product. Since
the starting material used was the hydrochloride salt,
we added 1 equiv of sodium bicarbonate to achieve
conditions similar to those used to obtain products 1b-
3b. Under these conditions, the desired 5′-carboxylic acid
of 2′,3′-isopropylidenecytidine, 4b, crystallized from the
reaction solution. After filtration and trituration with
diethyl ether, 30% of the expected 4b was obtained in
high purity.15
We were able to increase this yield by adding a second
equivalent of sodium bicarbonate. With these modified
reaction conditions, 4b still precipitated from solution
and was isolated in 57% yield, with no loss in product
purity. Since the reaction of 4a appears to be sensitive
to the pH of the reaction solution, the second equivalent
of sodium bicarbonate may be necessary to neutralize 1
equivt of the acetic acid generated as a reaction byprod-
uct. Finally, since N-oxoammonium salts are known to
be substantially more stable at 0 °C than at room
temperature,16 we sought a further increase in yield by
carrying out the reaction at ice-bath temperatures. With
this final modification, 4b was produced in 72% yield and
in high purity.
in poor yields. As in the case of 2′,3′-isopropylidenecyti-
dine, the addition of an equivalent of sodium bicarbonate
dramatically altered the reaction outcome. In the case
of product 5b, however, it was found that the sodium salt
of the 5′-carboxylic acid precipitated from solution. This
material was filtered and triturated sequentially with
diethyl ether and acetone to yield 72% of 5b. However,
we found that the product was contaminated with a small
amount of an impurity that could not be removed by
simple trituration. In an attempt to remedy this, the
reaction conditions were modified by adding a second
equivalent of sodium bicarbonate. Under these modified
reaction conditions, the sodium salt of the 5′-carboxylic
acid was generated in 75% yield, free from the impurity
generated under the previous reaction conditions.17
In summary, we have developed a mild, general
procedure for the production of 5′-carboxylic acids of
nucleosides. The method was used to generate the
5′-carboxylic acids of five 2′,3′-isopropylidene-protected
nucleosides: adenosine, N-benzoyladenosine, uridine,
cytidine, and guanosine. In all cases, the desired products
were obtained in good yields and separated from TEMPO
and reaction byproducts by trituration with diethyl ether.
The mildness of this reaction and its tolerance of acid
sensitive, base sensitive, and oxidatively labile functional
groups should make it an attractive method for the
oxidation of primary alcohols to carboxylic acids for both
nucleosides and non-nucleosides. Finally, the method’s
facility and efficiency make it suitable for large-scale
reactions.
Exp er im en ta l Section
Gen er a l. Melting points are uncorrected. 1H and 13C NMR
spectra were recorded at 400 and 100.6 MHz, respectively. The
chemical shifts are expressed in parts per million (δ) relative to
tetramethylsilane for 1H NMR and relative to DMSO-d6 for 13C
NMR. Chemicals 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO),
bis(acetoxy)iodo]benzene (BAIB), 2′,3′-isopropylideneuridine, and
2′,3′-isopropylideneadenosine were purchased from Aldrich Chemi-
cal Co; 2′,3′-isopropylidenecytidine (hydrochloride salt) and 2′,3′-
isopropylideneguanosine were purchased from Sigma Chemical
Co. Finally, N-benzoyl-2′,3′-isopropylideneadenosine was ob-
tained by the method of Chladek and Smrt.18
Gen er a l P r oced u r e for Gen er a tion of Nu cleosid e-5′-
ca r boxyla tes. BAIB (709 mg, 2.2 mmol), TEMPO (32 mg, 0.2
mmol), and a 2′,3′-isopropylidene-protected nucleoside (1 mmol)
were combined in a reaction vessel, and to this mixture was
added 2 mL of a 1:1 acetonitrile-water solution. The reaction
mixtures were stirred for 3 h before the respective products were
isolated as individually described below.
N6-Ben zoyl-2′,3′-isop r oylid en ea d en osin e-5′-ca r b oxylic
Acid (1b). The resulting precipitate was filtered, triturated
sequentially with diethyl ether and acetone, and dried in vacuo.
Yield: 70%. Mp: 208-209 °C. 1H NMR (DMSO-d6, 50 °C): δ
11.0 (1H, br s, NH); 8.68 and 8.64 (2 × 1H, 2 × s, H-2 and H-8);
8.04, 7.62 and 7.54 (5H, m, Ph); 6.45 (1H, s, H-1′); 5.54 (1H, d,
J ) 6.0 Hz, H-2′); 5.51 (1H, dd, J ) 6.0 and 1.7 Hz, H-3′); 4.72
(1H, d, J ) 1.7 Hz, H-4′); 1.55 and 1.38 (2 × 3H, 2 × s, CMe2).
13C NMR (DMSO-d6, 20 °C): δ 170.61, 165.55, 151.93, 151.18,
150.12, 143.85, 133.43, 132.21, 128.32, 128.28, 125.25, 112.68,
89.91, 85.74, 83.7, 83.47, 26.45, 24.89. Anal. Calcd for
C20H19N5O6: C, 56.47; H, 4.50; N, 16.46. Found: C, 56.25; H,
4.59; N, 16.38.
Initial attempts to obtain the 5′-carboxylic acid of 2′,3′-
isopropylideneguanosine, 5b, under the original reaction
conditions used to obtain products 1b-3b also resulted
(12) Although we report the 1H and 13C NMR’s in DMSO-d6, a 1H
NMR spectra of the product in pyridine-d5 was consistent with a
previous report (see ref 6).
(13) Kondo, K.; Inoue, I. J . Org. Chem. 1979, 44(25), 4713.
(14) Although the material was of good purity, recrystallization from
acetone was necessary to obtain an analytically pure sample. The 1H
NMR of the product was spectroscopically identical to previous 60 MHz
data, except for the higher order coupling observed at 400 MHz.
(15) Although there was a report of this compound in the literature,
it appears that we are the first to report physical data for this
compound (see ref 8).
(17) Although the material was of good purity, recrystallization from
acetone-water was necessary to obtain an analytically pure sample.
To the best of our knowledge, there are no prior reports of this
compound in the literature. The 1H and 13C NMR spectra of the isolated
product are in good agreement with the proposed structure.
(18) Chladek, S.; Smrt, J . Collect. Czech. Chem. Commun. 1964, 29,
214.
(16) Anelli, P. L.; Biffi, C.; Montanari, F.; Quici, S. J . Org. Chem.
1987, 52, 2559.

Notes
2′,3′-O-Isop r oylid en ea d en osin e-5′-ca r boxylic Acid (2b).
J . Org. Chem., Vol. 64, No. 1, 1999 295
7.34 (2H, br d, NH2); 5.70 (1H, d, J ) 7.4 Hz, H-5); 5.65 (1H, s,
H-1′); 5.24 (1H, dd, J ) 6.0 and 2.0 Hz, H-3′); 5.05 (1H, d, J )
6.2 Hz, H-2′); 4.48 (1H, d, J ) 1.9 Hz, H-4′); 1.45 and 1.29 (2 ×
3H, 2 × s, CMe2). 13C NMR (DMSO-d6, 50 °C): δ 170.71, 165.96,
155.03, 144.64, 111.90, 96.04, 93.75, 86.53, 84.09, 83.91, 26.45,
24.73. Anal. Calcd for C12H15N3O6: C, 48.49; H, 5.09; N, 14.14.
Found: C, 48.49; H, 5.10; N, 14.07.
The resulting precipitate was filtered, triturated sequentially
with diethyl ether and acetone, and dried in vacuo. Yield: 90%.
Mp: 246-249 °C dec. 1H NMR (DMSO-d6, 50 °C): δ 8.23 and
8.10 (2 × 1H, 2 × s, H-2 and H-8); 7.15 (2H, s, NH2); 6.32 (1H,
s, H-1′); 5.52 (1H, dd, J ) 6.0 and 1.9 Hz, H-3′); 5.48 (1H, d, J
) 6.0 Hz, H-2′); 4.67 (1H, d, J ) 1.9 Hz, H-4′); 1.53 and 1.36 (2
× 3H, 2 × s, CMe2). 13C NMR (DMSO-d6, 50 °C): δ 170.36,
155.84, 152.15, 149.03, 140.11, 118.70, 112.66, 89.48, 85.14,
83.52, 83.27, 26.39, 24.86. Anal. Calcd for C13H15N5O5: C, 48.60;
H, 4.71; N, 21.80. Found: C, 48.43; H, 4.77; N, 21.62.
2′,3′-O-Isop r oylid en eu r id in e-5′-ca r boxylic Acid (3b). The
product separated from the reaction solution as an oil. Therefore,
the solvents were removed in vacuo, and the resulting residue
was triturated with diethyl ether, filtered, and dried in vacuo.
Yield: 76%. An analytically pure sample was obtained by
recrystallization from acetone. Mp: 233-235 °C. 1H NMR
(DMSO-d6, 50 °C): δ 12.6 (1H, br s, COOH); 11.22 (1H, br s,
NH); 7.76 (1H, d, J ) 7.9 Hz, H-6); 5.77 (1H, s, H-1′); 5.59 (1H,
dd, J ) 7.9 and 2.2 Hz, H-5); 5.20 (1H, dd, J ) 6.2 and 2.1 Hz,
H-3′); 5.15 (1H, d, J ) 6.2 Hz, H-2′); 4.54 (1H, d, J ) 2.1 Hz,
H-4′); 1.46 and 1.31 (2 × 3H, 2 × s, CMe2). 13C NMR (DMSO-d6,
50 °C): δ 170.50, 163.14, 150.57, 144.23, 112.11, 101.24, 95.29,
86.45, 83.80, 83.52, 26.33, 24.69. Anal. Calcd for C12H14N2O7:
C, 48.33; H, 4.73; N, 9.39. Found: C, 48.12; H, 4.74; N, 9.43.
2′,3′-O-Isopr oyliden ecytidin e-5′-car boxylic Acid (4b). Two
equivalents of sodium bicarbonate were added to the reagents
listed in the general procedure. The reaction was conducted at
ice-bath temperature for the first 2 h and then allowed to warm
to room temperature over the third hour. The resulting precipi-
tate was filtered, triturated sequentially with diethyl ether and
acetone, and dried in vacuo. Yield: 72%. Mp: 244-246 °C dec.
1H NMR (DMSO-d6, 20 °C): δ 7.78 (1H, d, J ) 7.4 Hz, H-6);
2′,3′-O-Isop r oylid en egu a n osin e-5′-ca r boxylic Acid (5b).
Two equivalents of sodium bicarbonate were added to the
reagents listed in the general procedure. The resulting precipi-
tate was filtered, triturated sequentially with diethyl ether and
acetone, and dried in vacuo. Yield: 75%. An analytically pure
sample was obtained by recrystallization from acetone-water.
1
Mp: 246-248 °C dec. H NMR (DMSO, 50 °C): δ 11.04 (1H, br
s, N-H); 8.32 (1H, s, H-8); 6.65 (2H, s, NH2); 5.95 (1H, d, J )
2.6 Hz, H-1′); 5.06 (1H, dd, J ) 6.0 and 1.7 Hz, H-3′); 4.95 (1H,
dd, J ) 6.0 and 2.1 Hz, H-2′); 4.32 (1H, d, J ) 1.7 Hz, H-4′);
1.50 and 1.30 (2 × 3H, 2 × s, CMe2). 13C NMR (DMSO, 50 °C):
δ 172.38, 157.33, 154.13, 150.83, 136.29, 116.16, 112.21, 89.22,
87.09, 84.55, 83.80, 26.87, 25.03. Anal. Calcd for C13H14N5NaO6‚
2H2O: C, 39.5; H, 4.59; N, 17.72; Na, 5.82. Found: C, 39.64; H,
4.58; N, 17.37; Na, 6.09.
Ackn owledgm en t. WegratefullyacknowledgeProme-
ga Corporation for financial support and Stephen M.
Antonelli for helpful discussions.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra of compounds 1b-5b (10 pages). See any current
masthead page for ordering and Internet access information.
J O981316G