A Convenient Method for the Preparation of N2-Ethylguanine Nucleosides and Nucleotides

Full text of DOI:10.1021/jo990500e

J . Org. Chem. 1999, 64, 5719-5721
5719
Sch em e 1
A Con ven ien t Meth od for th e P r ep a r a tion
of N2-Eth ylgu a n in e Nu cleosid es a n d
Nu cleotid es
Magoichi Sako,* Hiroyoshi Kawada, and Kosaku Hirota
Gifu Pharmaceutical University, 5-6-1, Mitahora-higashi,
Gifu 502-8585, J apan
Received March 22, 1999
Recently, it has been documented that N2-ethyl-dGTP
is efficiently incorporated in place of dGTP as a substrate
for Escherichia coli DNA-polymerase I with a template-
primer system to give the corresponding full-length
oligodeoxyribonucleotides.1 And also, an N2-ethyl-3′-
dGMP moiety has been reported to be involved in the
granulocyte and lymphocyte DNA of alcohol abusers at
a high level as the ultimate DNA adduct of acetaldehyde
which is formed as a primary product during the meta-
bolic oxidation of ethanol by NAD-dependent alcohol
dehydrogenase in the liver.2 These facts are of interest
in relation to the occurrence of cancers of the pharynx,
esophagus, and liver in alcohol abusers. To study the
molecular mechanisms explaining the carcinogenic effects
of the primary metabolite of ethanol, the preparation of
a variety of N2-ethylated guanosine derivatives was
required.
sides and nucleotides. Characteristics of this synthetic
method are the use of commercially available starting
materials and the reactions proceeding efficiently even
without any protection of the functional groups in the
sugar moiety.
The synthetic methods for N2-ethylguanine nucleosides
and nucleotides previously reported involve (1) NaBH4
reduction of N2-[1-(p-tolylthio)-ethyl]guanosine formed in
the thermal condensation of guanosine with acetaldehyde
in the presence of p-thiocresol,3a (2) oxidative deprotection
of 5-ethyl-4-desmethylwyosine and its 2′-deoxy derivative,
prepared by the cyclocondensation of guanosine and 2′-
deoxyguanosine with bromoacetone followed by regio-
selective ethylation,3b (3) nucleophilic substitution at the
2-position in 2-bromo-2′-deoxyinosine by ethylamine
under thermal conditions,1 and (4) thermal condensation
of 3′-dGMP with acetaldehyde followed by NaBH4
reduction.2d,e AICA Riboside3c,d and the O6-mesyl-N2-
benzoylguanosine derivative3e are also useful as the
starting materials for the preparation of N2-ethyl-
guanosine. These methods, however, required many steps
from commercially available starting materials and are
inadequate in overall yields for the preparation of the
desired N2-ethylguanosine derivatives.
A mixture of guanosine and excess acetaldehyde was
heated at 50 °C for 2 days in aqueous methanol contain-
ing NaBH3CN. After pH adjustment to pH 4, chromato-
graphic separation of the reaction mixture with the
reversed-phase column afforded N2-ethylguanosine (1a )
in 71% yield, together with the unchanged guanosine
(13%) and N2,N2-diethylguanosine (2a ) (10%). Analogous
results were obtained using neutral or alkaline buffered
solution in place of the aqueous methanol. On the other
hand, the use of acidic buffered solution as the solvent
or the use of NaBH4 in place of NaBH3CN as the reducing
agent caused a decrease in the formation of (1a ) (27-
36%) but recovery of most of the starting guanosine even
after prolonged reaction times, e.g., 1 week. The structure
1
of (1a ) was confirmed by comparison with the H NMR
spectral data previously reported.2a,3a,b For example, its
1H NMR spectrum showed a broad triplet signal assign-
able to the NH group at δ 6.35 (1H, J ) 5 Hz) ppm and
characteristic signals for the N2-ethyl group at δ 3.28 (2H,
br dq, J ) 5, 7, and 14 Hz) and 1.12 (3H, t, J ) 7 Hz)
ppm. In a similar manner, N2-ethyl-2′-deoxyguanosine
(1b),2c-e,3b N2-ethyl-3′(and/or 2′)-GMP (1c), N2-ethyl-5′-
GMP (1d ), N2-ethyl-3′-dGMP (1e),2e and N2-ethyl-5′-
dGMP (1f) 1 were obtained in high conversion yields. The
present methodology was applicable to the syntheses of
other N2-alkylated guanosines, i.e., N2-n-propylguanosine
(1g), N2-n-butylguanosine (1h ),2b N2-n-propyl-2′-deoxy-
guanosine (1i), and N2-n-butyl-2′-deoxyguanosine (1j).
The structural proofs of the N2-alkylated guanosines
(1b-j and 2a ,b) rest upon their spectral data (see
Experimental Section).
We now report that the reductive ethylation using an
acetaldehyde-NaBH3CN system in aqueous methanol
and subsequent purification with a reversed-phase col-
umn at atmospheric pressure is a convenient method for
the preparation of a variety of N2-ethylguanine nucleo-
(1) Freese, S.; Hoheisel, J .; Lehrach, H.; Wright, G. Nucleosides
Nucleotides 1991, 10, 1507-1524.
(2) (a) Hemminki, K.; Suni, R. Arch. Toxicol. 1984, 55, 186-190.
(b) Noonan, T.; Brown, N.; Dudycz, L.; Wright, G. J . Med. Chem. 1991,
34, 1302-1307. (c) Dong, Z.; J effrey, A. M. Carcinogenesis 1991, 12,
1125-1128. (d) Vaca, C. E.; Fang, J .-L.; Schweda, E. K. H. Chem.-
Biol. Interact. 1995, 98, 51-67. (e) Fang, J .-L.; Vaca, C. E. Carcino-
genesis 1995, 16, 2177-2185; 1997, 18, 627-632.
(3) (a) Kemal, O.; Reese, C. B. Synthesis 1980, 1025-1028. (b)
Boryski, J .; Ueda, T. Nucleosides Nucleotides 1985, 4, 595-606. (c)
Yamazaki, A.; Kumashiro, I.; Takenishi, T. J . Org. Chem. 1967, 32,
3032-3038. (d) Okutsu, M.; Yamazaki, A. Nucleic Acids Res. 1976, 3,
237-250. (e) Bridson, P. K.; Markiewicz, W.; Reese, C. B. J . Chem.
Soc., Chem. Commun. 1977, 447-448.
The direct alkylations of guanosine derivatives using
alkyl halides and diazoalkanes have been known to occur
10.1021/jo990500e CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/07/1999

5720 J . Org. Chem., Vol. 64, No. 15, 1999
Notes
Hz), 7.88 (1H, s), 10.48 (1H, br); HR-FAB MS m/z [M + H]+
296.1353 (calcd for C12H18N5O4 [M + H]+ 296.1359).
at the N1- or N7-position of the guanine ring.4 The present
method is based on the thermal condensation of the
aldehydes with the exocyclic amino group in the guanine
ring and subsequent reduction of the resulting imino
group by a moderately active reducing agent. It should
be noted that no detectable formation of any products
were observed in the reactions of adenosine, cytidine,
thymidine, and uridine with acetaldehyde in the presence
of NaBH3CN under the conditions employed, suggesting
that the present method is, in principle, applicable to the
regioselective modification of guanine residues in oligo-
nucleotides.
For N2,N2-diethyl-2′-deoxyguanosine (2b): from fractions
eluted with 40% methanol-containing water; mp 103-105 °C;
IR (KBr) 1685, 1585 cm-1; UV (ꢀ) λmax 280 (sh, 7.6 × 103), 261
(1.5 × 104) nM in H2O; 1H NMR (DMSO-d6) δ 1.10 (6H, t, J ) 7
Hz), 2.18 and 2.61 (each 1H, each m), 3.52 (6H, m), 3.79 (1H,
m), 4.33 (1H, m), 4.85 (1H, br t, J ) 6 Hz), 5.26 (1H, br d, J )
4 Hz), 6.12 (1H, t, J ) 7 Hz), 7.88 (1H, s), 10.59 (1H, br); HR-
FAB MS m/z [M + H]+ 324.1676 (calcd for C14H22N5O4 [M +
H]+ 324.1672).
N2-Eth ylgu a n osin e-3′(a n d /or 2′)-p h osp h a te (1c): from
fractions eluted with 10-20% methanol-containing water; 13.1
mg (66% yield) [with recovery of the starting 3′-GMP (2.4 mg,
13%); the ratios of 2′- and 3′-isomers were not determined]; IR
(KBr) 1688, 1612 cm-1; UV (ꢀ) λmax 275 (sh, 8.2 × 103), 254 (1.5
× 104) nM in H2O; 1H NMR (DMSO-d6) δ 1.12 (3H, t, J ) 7 Hz),
3.29 (2H, br dq, J ) 5, 7, and 14 Hz), 3.51 and 3.57 (each 1H,
each Q, each J ) 4 and 12 Hz), 3.95 (1H, m), 4.53 (1H, m), 4.54
(1H, br), 5.69 (1H, br d, J ) 6 Hz), 6.41 (1H, br t), 7.90 (1H, s),
10.53 (1H, br); HR-FAB MS m/z [M + H]+ 392.0966 (calcd for
Exp er im en ta l Section
Melting points are uncorrected. 1H NMR spectra were ob-
tained at 400 MHz. Column chromatography was performed
with Sep-Pak [Waters, Vac 35 cm3 (10 g), C18 Cartridges].
Unless otherwise noted, materials obtained from commercial
suppliers were used without further purification.
C
12H19N5O8P [M + H]+ 392.0972).
N2-Eth ylgu a n osin e-5′-p h osp h a te (1d ): from fractions elut-
P r ep a r a tion of N2-Eth ylgu a n osin e Der iva tives (1). Gen -
er a l P r oced u r e. To a suspension of the appropriate guanosine
derivative (Sigma, >98% purity) (0.05 mmol) and NaBH3CN
(Aldrich, 95% purity) (20 mg, 0.30 mmol) in 50% aqueous
methanol (3 mL) was added acetaldehyde (Nacalai tesque, >95%
purity) (100 µL, 1.7 mmol) in one portion, and the mixture was
heated at 50 °C for 2 days in a test tube equipped with an argon
balloon. After removal of the solvent under reduced pressure,
the residue was dissolved in water (3 mL) and acidified to pH 4
with 1 N HCl. The resulting solution was subjected to the
reversed-phase column eluting with 0, 10, 20, 30, 40, and 50%
methanol-containing water (each 50 mL). The UV-positive
fractions were collected, evaporated to dryness, and triturated
with diethyl ether to obtain the desired products (1) as a powder
in a pure state.
ed with 10-20% methanol-containing water; 14.4 mg (73% yield)
[with recovery of the starting 5′-GMP (2.2 mg, 12%)]; mp 260
°C (dec.); IR (KBr) 1685, 1611 cm-1; UV (ꢀ) λmax 275 (sh, 8.9 ×
103), 254 (1.5 × 104) nM in H2O; 1H NMR (DMSO-d6-D2O) δ
1.09 (3H, t, J ) 7 Hz), 3.26 (2H, q, J ) 7 and 14 Hz), 3.76 (2H,
m), 4.00 (1H, m), 4.13 (1H, q, J ) 3 and 4 Hz), 4.53 (1H, t, J )
5 Hz), 5.70 (1H, d, J ) 7 Hz), 7.99 (1H, s); HR-FAB MS m/z [M
+ H]+ 392.0980 (calcd for C12H19N5O8P [M + H]+ 392.0971).
N2-Eth yl-2′-d eoxygu a n osin e-3′-p h osp h a te (1e): from frac-
tions eluted with 10-20% methanol-containing water; 12.2 mg
(65% yield) [with recovery of the starting 3′-dGMP (3.0 mg,
17%)]; mp 190-192 °C (dec.); IR (KBr) 1688, 1610 cm-1; UV (ꢀ)
λmax 274 (sh, 1.1 × 104), 250 (1.5 × 104) nM in H2O; 1H NMR
(DMSO-d6) δ 1.16 (3H, t, J ) 7 Hz), 2.48 and 2.65 (each 1H,
each m), 3.29 (2H, m), 3.54 (2H, m), 4.01 (1H, m), 4.80 (1H, br),
6.11 (1H, t, J ) 7 Hz), 6.42 (1H, br t), 7.88 (1H, s), 10.54 (1H,
br); HR-FAB MS m/z [M + H]+ 376.1031 (calcd for C12H19N5O7P
[M + H]+ 376.1022).
N2-Eth ylgu a n osin e (1a ): from fractions eluted with 30%
methanol-containing water; 11.1 mg (71% yield) [with recovery
of the starting guanosine (1.8 mg, 13%), together with N2,N2-
diethylguanosine (2a ) (1.7 mg, 10%)]; mp 237-239 °C (dec.) (lit.3b
mp 229-230 °C); IR (KBr) 1686, 1610 cm-1; UV (ꢀ) λmax 275 (sh,
8.6 × 103), 254 (1.5 × 104) nM in H2O; 1H NMR (DMSO-d6) δ
1.12 (3H, t, J ) 7 Hz), 3.28 (2H, br dq, J ) 5, 7, and 14 Hz),
3.49 (1H, br dq, J ) 5, 5, and 12 Hz), 3.58 (1H, dq, J ) 4, 5, and
12 Hz), 3.86 (1H, br dt, J ) 4 and 4 Hz), 4.10 (1H, br dq, J ) 3,
4, and 5 Hz), 4.50 (1H, br dq, J ) 5, 5, and 6 Hz), 4.90 (1H, br
t, J ) 5 Hz), 5.13 (1H, br d, J ) 5 Hz), 5.35 (1H, br d, J ) 6 Hz),
5.69 (1H, d, J ) 6 Hz), 6.35 (1H, br t, J ) 5 Hz), 7.90 (1H, s),
10.51 (1H, br); HR-FAB MS m/z [M + H]+ 312.1306 (calcd for
N2-Eth yl-2′-d eoxygu a n osin e-5′-p h osp h a te (1f): from frac-
tions eluted with 10-20% methanol-containing water; 14.2 mg
(76% yield) [with recovery of the starting 5′-dGMP (3.0 mg,
17%)]; mp 267-270 °C (dec.); IR (KBr) 1689, 1615 cm-1; UV (ꢀ)
λmax 275 (sh, 7.8 × 103), 255 (1.5 × 104) nM in H2O; 1H NMR
(DMSO-d6) δ 1.12 (3H, t, J ) 7 Hz), 2.17 and 2.61 (each 1H,
each m), 3.26 (2H, br q, J ) 7 and 14 Hz), 3.76 (2H, m), 3.86
(1H, m), 4.46 (1H, m), 6.13 (1H, t, J ) 7 Hz), 6.45 (1H, br), 7.92
(1H, s), 10.52 (1H, br); HR-FAB MS m/z [M + H]+ 376.1022
(calcd for C C12H19N5O7P [M + H]+ 376.1022).
C
12H18N5O5 [M + H]+ 312.1308).
N2-n -P r op ylgu a n osin e (1g): from fractions eluted with 40%
methanol-containing water; 13.2 mg (81% yield) [with recovery
of the starting guanosine (1.8 mg, 13%)]; mp 129-132 °C; IR
(KBr) 1686, 1610 cm-1; UV (ꢀ) λmax 275 (sh, 8.8 × 103), 255 (1.5
× 104) nM in H2O; 1H NMR (DMSO-d6) δ 0.89 (3H, t, J ) 7 Hz),
1.53 (2H, q, J ) 7 and 14 Hz), 3.22 (2H, br dt, J ) 6, 7, and 7
Hz), 3.50 (1H, br dq, J ) 5, 5, and 12 Hz), 3.60 (1H, br dq, J )
4, 5, and 12 Hz), 3.86 (1H, dt, J ) 4 and 4 Hz), 4.09 (1H, dt, J
) 4, 5, and 5 Hz), 4.50 (1H, dt, J ) 5, 6, and 6 Hz), 4.90 (1H, br
t, J ) 5 Hz), 5.13 (1H, br d, J ) 5 Hz), 5.37 (1H, br d, J ) 6 Hz),
5.69 (1H, d, J ) 6 Hz), 6.38 (1H, br t, J ) 5 Hz), 7.89 (1H, s),
10.44 (1H, br); HR-FAB MS m/z [M + H]+ 326.1459 (calcd for
For N2,N2-diethylguanosine (2a ): from fractions eluted with
40% methanol-containing water; mp 118-120 °C; IR (KBr) 1686,
1588 cm-1; UV (ꢀ) λmax 280 (sh, 7.9 × 103), 261 (1.5 × 104) nM in
H2O; 1H NMR (DMSO-d6) δ 1.10 (6H, t, J ) 7 Hz), 3.51 (4H, m),
3.55 (2H, m), 3.84 (1H, dt, J ) 4 and 4 Hz), 4.09 (1H, br dt, J )
4, 5, and 5 Hz), 4.50 (1H, br q, J ) 5 and 6 Hz), 4.90 (1H, br t,
J ) 5 Hz), 5.13 (1H, br d, J ) 5 Hz), 5.35 (1H, br d, J ) 6 Hz),
5.67 (1H, d, J ) 6 Hz), 7.90 (1H, s), 10.60 (1H, br); HR-FAB MS
m/z [M + H]+ 340.1624 (calcd for C14H22N5O5 [M + H]+
340.1621).
N2-Eth yl-2′-d eoxygu a n osin e (1b): from fractions eluted
with 30% methanol-containing water; 11.8 mg (80% yield) [with
recovery of the starting 2′-deoxyguanosine (1.1 mg, 8%), together
with N2,N2-diethyl-2′-deoxyguanosine (2b) (1.3 mg, 8%)]; mp
C
13H20N5O5 [M + H]+ 326.1464).
N2-n -Bu tylgu a n osin e (1h ): from fractions eluted with 40%
240-242 °C (dec.) (lit.3b 236-237 °C); IR (KBr) 1692, 1606 cm-1
;
methanol-containing water; 13.6 mg (80% yield) [with recovery
of the starting guanosine (1.1 mg, 8%)]; mp 121-122 °C (lit.2b
mp 188-190 °C); IR (KBr) 1686, 1610 cm-1; UV (ꢀ) λmax 275 (sh,
8.9 × 103), 254 (1.5 × 104) nM in H2O; 1H NMR (DMSO-d6) δ
0.90 (3H, t, J ) 7 Hz), 1.33 (2H, m), 1.50 (2H, m), 3.26 (2H, br
dt, J ) 5, 7, and 7 Hz), 3.50 and 3.60 (each 1H, each br dq, each
J ) 4, 5, and 12 Hz), 3.86 (1H, br dt, J ) 4 and 4 Hz), 4.09 (1H,
br dq, J ) 3, 4, and 5 Hz), 4.50 (1H, br dt, J ) 5, 6, and 6 Hz),
4.90 (1H, br t, J ) 5 Hz), 5.13 (1H, br d, J ) 4 Hz), 5.37 (1H, br
d, J ) 6 Hz), 5.69 (1H, d, J ) 6 Hz), 6.37 (1H, br t), 7.89 (1H, s),
10.42 (1H, br); HR-FAB MS m/z [M + H]+ 340.1614 (calcd for
UV (ꢀ) λmax 275 (sh, 8.9 × 103), 254 (1.5 × 104) nM in H2O; H
NMR (DMSO-d6) δ 1.12 (3H, t, J ) 7 Hz), 2.18 and 2.61 (each
1H, each m), 3.28 (2H, br q, J ) 7 and 14 Hz), 3.46 (1H, br dq,
J ) 5, 6, and 12 Hz), 3.54 (1H, br dq, J ) 4, 5, and 12 Hz), 3.80
(1H, m), 4.34 (1H, m), 4.85 (1H, br t, J ) 5 Hz), 5.26 (1H, br d,
J ) 4 Hz), 6.14 (1H, q, J ) 6 and 7 Hz), 6.33 (1H, br t, J ) 5
1
(4) Haines, J . A.; Reese, C. B.; Todd, L. J . Chem. Soc. 1962, 5281-
5288. J ones, J . W.; Robins, R. K. J . Am. Chem. Soc. 1963, 85, 193-
201. Broom, A. D.; Townsend, L. B.; J ones, J . W.; Robins, R. K.
Biochemistry 1964, 3, 494-500.
C
14H22N5O5 [M + H]+ 340.1621).

Notes
N2-n -P r op yl-2′-d eoxygu a n osin e (1i): from fractions eluted
J . Org. Chem., Vol. 64, No. 15, 1999 5721
the conditions described in the general procedure. TLC densi-
tometric analyses of the reaction mixtures with the developing
solvent chloroform-methanol-acetic acid (16/6/3) showed the
presence of guanosine, N2-ethylguanosine (1a ), and/or N2,N2-
diethylguanosine (2a ) in the following yields: guanosine (60%)
and 1a (36%) in pH 4.25; guanosine (62%) and 1a (26%) in pH
5.06; guanosine (53%) and 1a (45%) in pH 6.00; guanosine (10%),
1a (71%), and 2a (9%) in pH 7.03; 1a (73%) and 2a (19%) in pH
7.96; 1a (75%) and 2a (18%) in pH 9.05.
Effect of th e Na BH3CN Qu a n tity on th e Red u ctive N2-
Eth yla tion of Gu a n osin e. The reaction of guanosine (0.05
mmol) with acetaldehyde (1.7 mmol) in the presence of NaBH3-
CN [10 mg (0.15 mmol), 20 mg (0.30 mmol), 30 mg (0.45 mmol),
or 40 mg (0.60 mmol)] was carried out in aqueous methanol (3
mL) under the conditions described in the general procedure.
TLC densitometric analyses of the reaction mixtures with the
developing solvent chloroform-methanol-acetic acid (16/6/3)
showed the presence of guanosine, 1a , and 2a in the following
yields: guanosine (23%), 1a (72%), and 2a (5%) in the case of
NaBH3CN (3 equiv); guanosine (15%), 1a (71%), and 2a (10%)
in the case of NaBH3CN (6 equiv); guanosine (9%), 1a (72%),
and 2a (15%) in the case of NaBH3CN (9 equiv); guanosine (9%),
1a (73%), and 2a (14%) in the case of NaBH3CN (12 equiv).
with 40% methanol-containing water; 10.5 mg (68% yield) [with
recovery of the starting 2′-deoxyguanosine (3.9 mg, 29%)]; mp
252-255 °C (dec.); IR (KBr) 1687, 1610 cm-1; UV (ꢀ) λmax 275
(sh, 8.8 × 103), 254 (1.5 × 104) nM in H2O; 1H NMR (DMSO-d6)
δ 0.89 (3H, t, J ) 7 Hz), 1.53 (2H, m), 2.18 and 2.62 (each 1H,
each m), 3.22 (2H, br dt, J ) 5, 7, and 7 Hz), 3.47 (1H, br q, J
) 5 and 12 Hz), 3.55 (1H, br q, J ) 4 and 12 Hz), 3.80 (1H, dq,
J ) 4, 4, and 5 Hz), 4.34 (1H, m), 4.85 (1H, br), 5.26 (1H, br d,
J ) 3 Hz), 6.13 (1H, t, J ) 7 Hz), 6.38 (1H, br t, J ) 5 Hz), 7.88
(1H, s), 10.43 (1H, br); HR-FAB MS m/z [M + H]+ 310.1506
(calcd for C13H20N5O4 [M + H]+ 310.1515).
N2-n -Bu tyl-2′-d eoxygu a n osin e (1j): from fractions eluted
with 40% methanol-containing water; 11.8 mg (73% yield) [with
recovery of the starting 2′-deoxyguanosine (2.7 mg, 20%)]; mp
285-287 °C (dec.); IR (KBr) 1687, 1610 cm-1; UV (ꢀ) λmax 275
(sh, 8.9 × 103), 254 (1.5 × 104) nM in H2O; 1H NMR (DMSO-d6)
δ 0.90 (3H, t, J ) 7 Hz), 1.31 (2H, m), 1.50 (2H, m), 2.18 and
2.62 (each 1H, each m), 3.26 (2H, br dt, J ) 5, 7, and 7 Hz), 3.48
and 3.55 (each 1H, each br dq, each J ) 5, 5, and 12 Hz), 3.80
(1H, br dq, J ) 3, 4, and 5 Hz), 4.33 (1H, m), 4.85 (1H, br t, J )
5 Hz), 5.26 (1H, br d, J ) 4 Hz), 6.13 (1H, t, J ) 7 Hz), 6.35 (1H,
br t, J ) 5 Hz), 7.88 (1H, s), 10.42 (1H, br); HR-FAB MS m/z [M
+ H]+ 324.1664 (calcd for C14H22N5O4 [M + H]+ 324.1672).
p H Dep en d en cy d u r in g th e Red u ctive N2-Eth yla tion of
Gu a n osin e Usin g th e Aceta ld eh yd e-Na BH3CN System .
The reactions of guanosine (0.05 mmol) with acetaldehyde (1.7
mmol) in the pH range of 4.25-9.05 of 0.5-mol phosphate buffers
(3 mL) containing NaBH3CN (0.30 mmol) were carried out under
Su p p or tin g In for m a tion Ava ila ble: Spectral data and
spectra for compounds prepared. This material is available free
of charge via the Internet at http://pubs.acs.org.
J O990500E