An efficient stereospecific method for the synthesis of 8-aza-3-deazaguanine nucleosides from glycosyl azides

Article abstract of DOI:10.1055/s-1999-2755

8-aza-3-deazaguanine nucleosides 8 were prepared from glycosyl azides 1 via a stereospecific Dimroth reaction with dimethyl 3-oxoglutarate, followed by selective manipulation of the 1,2,3-triazole C-4 and 5 substituents. Of the two methods evaluated for t

Full text of DOI:10.1055/s-1999-2755

LETTER
1069
An Efficient Stereospecific Method for the Synthesis of 8-Aza-3-deazaguanine
Nucleosides from Glycosyl Azides
Anton Štimac,a1 Ivan Leban,b and Joûe Kobe*a
aNational Institute of Chemistry, Hajdrihova 19, 1115 Ljubljana, Slovenia
Fax 386 61 1259 244; E-mail: joze.kobe@ki.si
bFaculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerèeva 5, 1000 Ljubljana, Slovenia
Received 8 April 1999
site number of C-atoms was utilized, however, it still suf-
Abstract:8-aza-3-deazaguanine nucleosides 8 were prepared from
fered from slow reactivity of 1a with the allenic ester as
glycosyl azides 1 via a stereospecific Dimroth reaction with dime-
well as the high instability of the dipolarophile.
thyl 3-oxoglutarate, followed by selective manipulation of the
1,2,3-triazole C-4 and 5 substituents. Of the two methods evaluated
In continuation of our efforts in the synthesis of 8-aza-3-
for the model synthesis of 8-aza-3-deazaguanosine, that involving
deazaguanine nucleosides with various glycosyl residues,
selective dehydration of the diamides 4 and subsequent ring closure
retrosynthetic analysis suggested a more efficient ap-
and deprotection of cyanoamides 7 afforded target nucleosides of
proach to this class of compounds. The Dimroth reaction10
superior purity.
of glycosyl azides 111 with dimethyl 3-oxoglutarate
(DOG), possessing the appropriate number of C-atoms,
was expected to produce the desired regioisomers 2 exclu-
sively. The Dimroth reaction usually requires strongly al-
kaline medium, however, the application to the base-
Key words: glycosyl azides, Dimroth reaction, 1,2,3-triazoles,
1,2,3-triazolo[4,5-c]pyridines, nucleoside analogues
Several 8-aza- and 3-deazapurines as well as the appropri-
sensitive glycosyl azides required some adaptation of the
ate nucleosides display interesting antiviral and antitu-
reaction conditions. In this regard, reaction of the azide 1a
mour activity.2 However, little is known about the
with DOG in a K2CO3/DMSO system12 provided the opti-
chemistry, and still less about the biological effects, of the
mal conditions. The success of the reaction critically de-
hybrid 8-aza-3-deazapurine counterparts.2,3 The replace-
pended on the stoichiometry of the reactants. At the
optimal molar ratio of 1a : DOG : K2CO3 = 1:2:1, it was
ment of 2’-deoxyguanosine by a 3-deaza modified build-
ing block within oligonucleotides results in a minor
complete within 15 h at room temperature resulting in a
groove modification of the duplex and it possibly reduces
single regioisomer 2a,13 which was isolated after workup
its binding capacity.4 However, the recently reported spe-
and crystallization from methanol in an excellent (95%)
cific structure requirements for quadruplex-forming hexa-
yield.
deoxyribonucleotides with high anti-HIV-1 activity5
In this way, a variety of other glycosyl azides 1 were con-
verted to the corresponding methyl 5-methoxycarbonylm-
suggested a more detailed investigation of the possible
self-assembled 8-aza-3-deazaguanine nucleoside iono-
ethyl-1-(D-glycosyl)-1,2,3-triazole-4-carboxylates
2,
phore possessing appropriate donor-acceptor properties as
the natural guanosine while lacking minor groove accep-
tor ability.6 The synthesis of 8-aza-3-deazaguanosine
(8b), the only known representative of the 1,2,3-triazo-
lo[4,5-c]pyridine nucleoside analogue isosteric to gua-
nine, has been previously prepared by the ammonolysis of
the O-protected methyl 5-cyanomethyl-1-(b-D-ribofura-
nosyl)-1,2,3-triazole-4-carboxylate.7-9 Although the cy-
clization proceeded almost quantitatively, the purification
of the highly colored nucleoside led to a substantial loss
of material (29-37% reported yields). The requisite pre-
cursor has been in turn prepared either directly via glyco-
sylation of a suitable 1,2,3-triazole derivative (convergent
approach)7 or in few steps using linear methodologies.8,9
In view of the poor regioselectivity of the glycosylation,
linear approaches greatly improved the yield of the regio-
isomeric intermediates suitable for further conversion to
8b. These include the 1,3-dipolar cycloaddition of the
azide 1a with methyl 4-hydroxy-2-butynoate8 or dimethyl
1,3-allenedicarboxylate9 as the key steps. Our method us-
ing dimethyl 1,3-allenedicarboxylate proved to be more
efficient than the former since a dipolarophile with requi-
usually in more than 80% yield after purification (see step
a in Scheme 2 and Table 1). Reactions, which were too
slow at room temperature, were realized successfully at
45 °C. Most importantly, all reactions proceeded with
complete retention of the original anomeric configuration
of the starting glycosyl azides and irrespective of the rel-
ative 1,2-configuration.14 For example, the b-azide 1e was
transformed into a non-crystalline nucleoside 2e and its
5'-O-deacylated derivative 2d. The coupling constant (J1’2’
~ 0 Hz) of 2e is indicative of the b-configuration, while
the latter compound was identical with products of the cy-
clocondensation of 1d with DOG and isopropylidenation
of 2b. However, the a-anomeric azide 1f was converted
into a completely different set of products 2f and 2g as de-
termined on the basis of the 1H and 13C NMR spectra as
well as the optical rotation values. The structure of 2f, a
crystalline solid with mp 204-206 °C, was unambiguously
confirmed by X-ray analysis,15 while the identity of 2g
was established by the 5'-O-deacylation of 2f in boiling
methanolic triethylamine. Similarly, each of the anomeric
arabino azides 1u and 1w was transformed to a corre-
Synlett 1999, No. 07, 1069–1073 ISSN 0936-5214 © Thieme Stuttgart · New York

1070
A. ätimac et al.
LETTER
to furnish the cyanoester 5c (86%). Ring closure with si-
multaneous deprotection by heating with liquid ammonia
at 100 °C yielded the target 8-aza-3-deazaguanosine (8b)
in 77% yield after purification by chromatography as a
brownish yellow foam (Scheme 1).
sponding triazole arabinoside 2u or 2w, respectively, as
the only nucleosidic product.
The 1,2,3-triazoles prepared by a Dimroth reaction with
DOG were designed to possess suitable functional groups
for further processing to the condensed 1,2,3-triazolo[4,5-
c]pyridine nucleosides. The target 8-aza-3-deazaguanine
nucleosides were accessed first by working out the syn-
thesis of the known riboside 8b. Thus, the protected di-
ester 2a was first deprotected with NaOMe in methanol to
give 2b (98%), which was subjected to a controlled selec- Scheme 1. (a) NaOMe, MeOH, 23 °C, 2 h; (b) NH3, MeOH, 0 °C,
4.5-5 h; (c) Ac2O (3.6 eq.), Et3N (4 eq.), cat. DMAP, MeCN, 23 °C,
1 h; (d) (CF3CO)2O (1 eq.), pyridine (2 eq.), THF, 5 °C, 8 min, then
warming to 23 °C; (e) NH3 liq., 100 °C, 17 h.
tive ammonolysis of the aliphatic ester group with metha-
nolic ammonia16 at 0 °C. The reaction was stopped after
4.5-5 h at ~85% conversion, when the maximum yield
(~65%) of the monoamide 3b was achieved as estimated
by HPLC analysis. Isolation and separation of the desired
intermediate was aided by the prior acetylation17 of the
crude mixture of O-deprotected ammonolysis products.
The monoamide 3c (59%) was then dehydrated18 with tri-
fluoroacetic anhydride in the presence of pyridine at 5 °C
An alternative approach involved the complete am-
monolysis of the deblocked diester 2b by methanolic am-
monia for 1 d at room temperature and subsequent
acetylation to give the diamide 4c (91%). This was selec-
tively dehydrated by a limited amount (1.3 eq.) of trifluo-
Synlett 1999, No. 07, 1069–1073 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
An Efficient Stereospecific Method for the Synthesis of 8-Aza-3-deazaguanine Nucleosides
1071
roacetic anhydride in the presence of pyridine (2.6 eq.) in
THF at -12 to -2 °C, predominately at the aliphatic carb-
oxamide group to furnish the cyanoamide 7c (65%) along
with a small amount of the dinitrile 6c, which did not in-
terfere with the isolation of 7c. It is interesting to note, that
neither the controlled ammonolysis of the diester 2b nor
dehydration of the diamide 4c were chemospecific for the
aliphatic carboxylic ester derivative when compared to
the corresponding reactions of other azoles, containing
less than three N-atoms within the ring.19 This effect can
be attributed to the increased electron-withdrawing ability
of the triazole ring, increasing the reactivity of the aromat-
ic carboxylic acid derivative relative to the aliphatic one.
Final cyclization of 7c and concomitant deprotection with
1 M triethylamine in aq. methanol afforded the crude 8-
aza-3-deazaguanosine20 (8b, 72%) as an analytically pure
ivory colored powder. The overall yield of 8b from 1a via
the cyanoamide ring closure21 was slightly higher (42%)
vs. that of the cyanoester method (38%), yet, more impor-
tantly, provided the pure target nucleoside, free of highly
colored impurities. For this very advantage, the cyanoam-
ide method was adopted for the synthesis of other 8-aza-
3-deazaguanine nucleosides without further optimization
(Scheme 2, Table 2).
Thus, starting from b-D-glycosyl azides 1h,k,n,r,u the
first four steps: Dimroth reaction with DOG, deacylation,
ammonolysis and acetylation were carried out without pu-
Scheme 2 (a) dimethyl 3-oxoglutarate (2 eq.), K2CO3 (1 eq.), DM-
SO, conditions as in Table 1; (b) i: NaOMe, MeOH, 23 °C, 2-16 h; ii:
rification of intermediates, except for the unprotected dia- NH3, MeOH, 23 °C, 1 d; iii: Ac2O, Et3N, cat. DMAP, MeCN, 23 °C,
1-3 h (for 2u only step ii necessary); (c) (CF3CO)2O (1.3 eq.), pyridi-
ne (2.6 eq.), THF (DMF for 4s), -12 to -2 °C, 35-60 min, then war-
ming to 23 °C; (d) 1 M Et3N in 80% aq. MeOH, 23 °C, up to 6 d and/
or reflux, 6.5-24 h, (e) cyclohexene, EtOH, 20% Pd(OH)2/C, reflux,
mide 4m, which crystallized with ease. Pure per-O-
acetylated diamides 4 were obtained after chromatogra-
phy or crystallization in 50-57% overall yields. The arabi-
no-diamide 4u with O-benzyl protecting groups was
45 h.
prepared similarly in 56% overall yield from 1u without
the need for deprotection and reprotection steps. Selective
dehydration of the O-protected diamides 4 was performed
in THF as described for the ribo derivative 4c above, ex-
20% Pd(OH)2 on carbon. The b-configuration of com-
cept for 4s, where DMF was used instead due to its low
solubility in THF. Cyanoamides 7 were isolated as major
products (44-83%) along with dinitriles 6 (5-10%) by
chromatography on silica gel. Syntheses of 8-aza-3-de-
azaguanine nucleosides 8 were accomplished by ring clo-
sure of cyanoamides 7 in basic medium. Per-O-acetates
were simultaneously deprotected, while 8-aza-3-deaza-
guanine arabinoside 8v was obtained after hydrogenolytic
debenzylation22 of 8u with cyclohexene in the presence of
pounds 8b,j,m,p,t,v and, consequently, all intermediates
derived from the starting b-D-glycosyl azides was deter-
mined by the 1H NOE difference NMR spectroscopy. The
key NOE enhancements were between H-7 and H-3' and
H-2' on the b-side and between H-1' and H-4' on the α-
side of the sugar moieties. In addition, the C(1')-F cou-
pling constants in 13C NMR spectra of 2'-deoxy-2'-fluoro-
b-D-arabinofuranosyl compounds 1r, 2r, 4s, 6s, 7s, and 8t
Synlett 1999, No. 07, 1069–1073 ISSN 0936-5214 © Thieme Stuttgart · New York

1072
A. ätimac et al.
LETTER
(2JC(1’),F = 17.1 ± 0.0 Hz) was found to be highly diagnostic
for the b-configuration.23
(13) Typical experimental procedure for the Dimroth reaction of
1a with DOG: To a solution of 1a (24.40 g, 50 mmol) and
DOG (17.4 g, 100 mmol) in DMSO (60 mL) was added dried
and finely ground K2CO3 (6.9 g, 50 mmol). The
In conclusion, we have developed an efficient method for
the stereospecific formation of single N-1(3)-glycosylated
1,2,3-triazole regioisomers, convertible to a variety of 8-
aza-3-deazapurine nucleosides. In this work, an efficient
route for the preparation of 8-aza-3-deazaguanine nucleo-
sides was demonstrated, however, by a proper choice of
synthetic methods, derivatives with diverse substitution
pattern (8-aza-3-deazaadenines, -xanthines, -hypoxan-
thines, etc.) should be accessible. The dinitrile 6c, for ex-
ample, which was more conveniently prepared by
dehydration of both carboxamide groups of 4c, was found
to be a useful precursor to 8-aza-3-deazaisoguanosine.24
heterogeneous mixture was stirred at room temperature for 15
h and poured into an ice-water mixture (0.5 L). The solid was
filtered and washed with water (1 L). The filter cake was
dissolved in ethyl acetate (300 mL), the organic phase was
washed with 1 M Na2CO3 (2 x 200 mL), 0.1 M HCl (200 mL)
and brine (200 mL) and dried (Na2SO4). Concentration
afforded almost colorless foam (32.6 g), which was
crystallized from methanol to give 2a (30.48 g, 95%) as very
small needles with mp 110-113 °C; 1H NMR(300 MHz,
CDCl3): d 3.69, 3.94 (2 s, 6, 2 CH3), 4.18, 4.39 (2d, 2, CH2;
J
gem 17.1 Hz), 4.51 (dd, 1, H-5'a; J5’a,5’b 12.2, J4’,5’a 4.6 Hz), 4.80
(dd, 1, H-5'b; J4’,5’b 3.5 Hz), 4.90 (ddd, 1, H-4'), 6.24 (dd, 1, H-
3'; J2’,3’5.1, J3’,4’7.1 Hz), 6.36 (d, 1, H-1'; J1’,2’1.95 Hz), 6.58
(dd, 1, H-2'), 7.34-7.63 and 7.92-8.05 (2m, 15, Ar-H). 13
C
NMR(75.4 MHz, CDCl3): d 28.7 (CH2), 52.0, 52.8 (2 CH3),
63.0 (C-5'), 71.4 (C-3'), 74.9 (C-2'), 81.3 (C-4'), 89.1 (C-1'),
128.3-133.8 (Ph-C), 135.8, 137.5 (C-4, 5), 161.3, 164.9,
165.0, 165.9, 167.7 (5 CO). IR (film):1732 (C=O), 1268
(benzoate C-O), 1124 (PhCOO-C), 712 cm-1 (benzoate). MS
(EI): m/z 643 (M+), 445 ([M-B]+, 9%), 105 (100). Anal. calcd
for C33H29N3O11 (643.61): C, 61.58; H, 4.54; N, 6.53. Found:
C, 61.53; H, 4.54; N, 6.50.
Acknowledgement
This investigation was supported by the Ministry of Science and
Technology of Slovenia (Grant No. 33-030 and CI-01513) and the
EU (BIOMED1, Agreement No. ERBCIPDCT 930194, PL 93-
1112). We thank dr. Bogdan Kralj at the Mass Spectrometry Centre
at Joûef Stefan Institute (Ljubljana) and dr. Janez Plavec at the
Slovenian National NMR Centre.
(14) The stereochemistry of the Dimroth reaction of glycosyl
azides with DOG is in sharp contrast to that observed with
cyanoacetamide, which was reported to yield 1,2-trans
nucleosides exclusively or predominately from either 1,2-
trans or 1,2-cis glycosyl azides: Tolman, R. L.; Smith, C. W.;
Robins, R. K. J. Am. Chem. Soc. 1972, 94, 2530; Smith, C. W.;
Sidwell, R. W.; Robins, R. K.; Tolman, R. L. J. Med. Chem.
1972, 15, 883; Chretien, F.; Gross, B. Tetrahedron 1982, 38,
103. This difference may be attributed to much more
favourable intramolecular triazene N-atom to carbonyl group
cyclization in the glycosyl azide-DOG adduct as opposed to
the cyclization to cyano group in the glycosyl azide-
cyanoacetamide adduct. For this reason, the competitive
anomerization process in the former case should be effectively
supressed.
(15) All new compounds were characterized by spectroscopic
(NMR, IR, and MS) and analytical methods (microanalysis
and/or HRMS) including an X-ray analysis of compound 2f.
Details will be provided in a full paper.
(16) Ammonia, saturated in methanol at 0 °C, was used.
(17) Matsuda, A.; Shinozaki, M.; Suzuki, M.; Watanabe, K.;
Miyasaka, T. Synthesis 1986, 385.
References and Notes
(1) Present address: Krka, Pharmaceutical and Chemical Works,
Šmarješka c. 6, 8501 Novo mesto, Slovenia.
(2) Revankar, G. R.; Robins, R. K. In Chemistry of Nucleosides
and Nucleotides; Townsend, L. B., Ed.; Plenum Press: New
York, Vol. 2, 1991, p 161.
(3) Franchetti, P.; Messini, L.; Cappellacci, L.; Grifantini, M.;
Nocentini, G.; Guarracino, P.; Marongiu, M. E.; La Colla P.
Antiviral Chem. Chemother. 1993, 4, 341.
(4) Seela, F.; Lampe, S. Helv. Chim. Acta 1991, 74, 1790.
(5) Hotoda, H.; Koizumi, M.; Koga, R.; Kaneko, M.; Momota, K.;
Ohmine, T.; Furukawa, H.; Agatsuma, T.; Nishigaki, T.; Sone,
J.; Tsutsumi, S.; Kosaka, T.; Abe, K.; Kimura, S.; Shimada, K.
J. Med. Chem. 1998, 41, 3655.
(6) Morales, J. C.; Kool, E. T. J. Am. Chem. Soc. 1999, 121, 2323.
(7) Meyer, R. B., Jr.; Revankar, G. R.; Cook, P. D.; Ehler, K. W.;
Schweizer, M. P.; Robins, R. K. J. Heterocycl. Chem. 1980,
17, 159.
(8) Earl, R. A.; Townsend, L. B. Can. J. Chem. 1980, 58, 2550.
(9) Štimac, A.; Townsend, L. B.; Kobe, J. Nucleosides,
Nucleotides 1991, 10, 727.
(18) Campagna, F.; Carotti, A.; Casini, G. Tetrahedron Lett. 1977,
1813.
(10) L'abbe, G. Ind. Chim. Belg. 1971, 36, 3.
(19) For the chemospecific ammonolysis of similar diesters, see for
the imidazole series: Robins, R. K.; Horner, J. K.; Greco, C.
V.; Noell, C. W.; Beames, C. G., Jr. J. Org. Chem. 1963, 28,
3041; the pyrazole series: Ehler, K. W.; Robins, R. K.; Meyer,
R. B., Jr. J. Med. Chem. 1977, 20, 317; the pyrrole series:
Schneller, S. W.; Hosmane, R. S.; MacCartney, L. B.;
Hessinger, D. A. J. Med. Chem. 1978, 21, 990. However, the
chemospecific ammonolysis of 1(3)-unsubstituted methyl
4(5)-methoxycarbonylmethyl-1,2,3-triazole-5(4)-carboxylate
could only be achieved with conc. NH4OH at 0 °C, see ref. 7.
(20) 8b: mp 245 °C (dec.), [a]D25 -117 (c 1.00, DMSO), {lit.8 mp
239-241 °C (dec.), [a]D25 -123.2}; 1H NMR(300 MHz,
DMSO-d6): d 3.42-3.62 (m, 2, H-5'a,5'b), 3.97 (ddd, 1, H-4'),
4.19 (ddd, 1, H-3'), 4.63 (ddd, 1, H-2'), 4.86 (t, 1, OH; J 5.4
Hz), 5.27 (d, 1, OH; J 5.2 Hz), 5.49 (s, 1, H-7), 5.55 (d, 1, OH;
J 5.9 Hz), 5.82 (d, 1, H-1'; J1’,2’5.1 Hz), 6.08 (br s, 2, NH2),
10.71 (br s, 1, NH). 13C NMR(75.4 MHz, DMSO-d6): d 62.34
(11) Preparation of glycosyl azides 1a, 1e, and 1f: Štimac, A.;
Kobe, J. Carbohydr. Res. 1992, 232, 359; 1b: Baddiley, J.;
Buchanan, J. G.; Hodges, R.; Prescott, J. F. J. Chem. Soc.
1957, 4769; 1c, by the isopropylidenation of 1b with Me2CO/
I2 according to: Kartha, K. P. R. Tetrahedron Lett. 1986, 27,
3415; 1h and 1k, by the azidotrimethylsilane-SnCl4 method as
reported for 1a above; 1n, by substitution of 3,5-di-O-(p-
toluoyl)-2-deoxy-a-D-erythropentofuranosyl chloride with
KN3 or CsN3 in DMSO at room temperature for 1 h; 1r, 1u,
and 1w, by the liquid-liquid phase transfer catalytic method
according to: Tropper, F. D.; Andersson, F. O.; Braun, S.;
Roy, R. Synthesis 1992, 618. Details of the preparation of
previously unknown substrates as well as those prepared by
more stereoselective routes will be given in separate
publications.
(12) Cottrell, I. F.; Hands, D.; Houghton, P. G.; Humphrey, G. R.;
Wright, S. H. B. J. Heterocycl. Chem. 1991, 28, 301.
Synlett 1999, No. 07, 1069–1073 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
An Efficient Stereospecific Method for the Synthesis of 8-Aza-3-deazaguanine Nucleosides
1073
(C-5’), 68.31 (C-7), 71.03 (C-3’), 73.44 (C-2’), 86.06 (C-4’),
liquid NH3: Cook, P. D.; Rousseau, R. J.; Mian, A. M.; Dea,
P.; Meyer, R. B., Jr.; Robins, R. K. J. Am. Chem. Soc. 1976,
98, 1492.
90.27 (C-1’), 130.15 (C-3a), 143.30 (C-7a), 150.98 (C-6),
156.36 (C-4). IR (KBr):3376, 3343, 3186, 1685, 1622, 1590,
1388, 1124, 1106, 1047 cm-1. 1H-, 13C NMR, and IR data were
in complete agreement with the reported values.7,8 MS (FAB):
m/z 284 (MH)+. UV (pH 11): λmax 218, 286 nm. Anal. calcd
for C10H13N5O5 (283.25): C, 42.41; H, 4.63; N, 24.73. Found:
C, 42.15; H, 4.64; N, 24.71.
(22) Hanessian, S.; Liak, T. J.; Vanasse, B. Synthesis 1981, 396.
(23) Mansuri, M. M.; Krishnan, B.; Martin, J. C. Tetrahedron Lett.
1991, 32, 1287.
(24) Ješelnik, M.; Kobe, J., in preparation.
(21) Similarly, the ring closure of 5-cyanomethyl-1-(ß-D-
ribofuranosyl)imidazole-4-carboxamide was also reported to
give 3-deazaguanosine of superior purity and in much higher
yield than the cyclization of the cyanoester analogue with
Article Identifier:
1437-2096,E;1999,0,07,1069,1073,ftx,en;G11299ST.pdf
Synlett 1999, No. 07, 1069–1073 ISSN 0936-5214 © Thieme Stuttgart · New York