Synthesis and anti-HIV evaluation of 2',3'-dideoxy imidazo- and ν- triazolo[4,5-d]pyridazine nucleosides

Article abstract of DOI:10.1016/S0968-0896(99)00184-4

The syntheses of the 2'-deoxy and 2',3'-dideoxynucleosides of 2,8-diaza- 3-deazainosine and the 2',3'-dideoxynucleoside of 2-aza-3-deazainosine were achieved and the pathways leading to these novel nucleosides are described. The preparation of the 2',3'-dideoxynucleoside (1) of 2-aza-3-deazainosine involved deoxygenation of the 2'-deoxy-3'-imidazolide intermediate with n- Bu₃SnH and AIBN. The latter nucleoside was synthesized from the known 2'- deoxy derivative of 2-aza-3-deazainosine. The three-step synthesis of 1 from the 2'-deoxy analogue was accomplished in 40% overall yield. Rather than synthesize the corresponding 2',3'-dideoxynucleoside (2) of 2,8-diaza-3- deazainosine in the same manner, i.e. deoxygenation of the 2'- deoxynucleoside, a more cost-effective route was chosen. This pathway involved reductive cleavage of the 5'-protected, 2',3'-thiocarbonate derivative to furnish a mixture of the 2'- and 3'-deoxy isomers. This mixture was not separated, but was deoxygenated by the aforementioned imidazolide method. Using this methodology, 2 was prepared in 23% overall yield from 2,8- diaza-3-deazainosine. Nucleosides 1 and 2 were evaluated for antiretroviral activity and were found to be inactive. (C) 1999 Elsevier Science Ltd.

Full text of DOI:10.1016/S0968-0896(99)00184-4

Bioorganic & Medicinal Chemistry 7 (1999) 2373±2379
Synthesis and Anti-HIV Evaluation of 20,30-Dideoxy Imidazo- and
ꢀ-Triazolo[4,5-d]pyridazine Nucleosidesy
Jacqueline C. Bussolari and Raymond P. Panzica*
Departments of Biomedical Sciences and Chemistry, University of Rhode Island, Kingston, RI 02881-0809, USA
Received 18 January 1999; accepted 10 May 1999
AbstractÐThe syntheses of the 20-deoxy and 20,30-dideoxynucleosides of 2,8-diaza-3-deazainosine and the 20,30-dideoxynucleoside
of 2-aza-3-deazainosine were achieved and the pathways leading to these novel nucleosides are described. The preparation of the
20,30-dideoxynucleoside (1) of 2-aza-3-deazainosine involved deoxygenation of the 20-deoxy-30-imidazolide intermediate with n-
Bu3SnH and AIBN. The latter nucleoside was synthesized from the known 20-deoxy derivative of 2-aza-3-deazainosine. The three-
step synthesis of 1 from the 20-deoxy analogue was accomplished in 40% overall yield. Rather than synthesize the corresponding
20,30-dideoxynucleoside (2) of 2,8-diaza-3-deazainosine in the same manner, i.e. deoxygenation of the 20-deoxynucleoside, a more
cost-e€ective route was chosen. This pathway involved reductive cleavage of the 50-protected, 20,30-thiocarbonate derivative to
furnish a mixture of the 20- and 30-deoxy isomers. This mixture was not separated, but was deoxygenated by the aforementioned
imidazolide method. Using this methodology, 2 was prepared in 23% overall yield from 2,8-diaza-3-deazainosine. Nucleosides 1
and 2 were evaluated for antiretroviral activity and were found to be inactive. # 1999 Elsevier Science Ltd. All rights reserved.
Introduction
allowed and does not alter inhibitory activity. On the
other hand, the HN-1 is required for inhibition9 and
removal of this pyrrole-type nitrogen results in a drastic
loss of activity. The 20,30-dideoxyinosine counterparts of
the title ring systems satisfy the aforementioned struc-
tural requirements deemed necessary for potential inhi-
bitory activity. Furthermore, 2-aza-3-deazainosine and
2,8-diaza-3-deazainosine are resistant to phosphorolytic
cleavage by purine nucleoside phosphorylase (PNP).7
This is an important feature, since ddI has been reported
to be a substrate for PNP.13 With these factors in mind,
we prepared 20,30-dideoxy-2-aza-3-deazainosine (1) and
20,30-dideoxy-2,8-diaza-3-deazainosine (2) and evaluated
their activity against human immunode®ciency virus.
The design and synthesis of nucleoside analogues as
anti-HIV agents continues to provide potential clinical
candidates and useful drugs (for reviews see refs 1±5).To
date, the dideoxynucleosides have provided some of the
most active agents in inhibiting reverse transcriptase.
Among those approved for the treatment of AIDS are
AZT (Retrovir), ddC (Hivid), d4T (Zerit), and ddI
(Videx).1±5 In this group, the dideoxynucleoside of par-
ticular interest to us was ddI. We have been actively
engaged in the synthesis of imidazo- and ꢀ-triazolo[4,5-
d]pyridazine nucleosides6±8 and in both series the
respective inosine analogue served as the key inter-
mediate. Seela and co-workers have prepared a variety
of modi®ed purine-ring 20,30-dideoxynucleosides9±11 and
selected derivatives have shown signi®cant inhibitory
activity against HIV-1 reverse transcriptase as their O-
50-triphosphates.12 Their pivotal contributions have
demonstrated that modi®cation in the ®ve-membered
ring portion of the purine moiety, i.e. removal of the N-
7 atom and replacement with a methine9,10 or replace-
ment of the HC-8 methine with a nitrogen atom,11 is
Chemistry and Discussion
Key words: Imidazopyridazines; ꢀ-triazolopyridazines; deoxynucleo-
sides; ddI analogues; HIV.
* Corresponding author
Presented in part at The VIth International Antiviral Symposium,
Nice, France, June 1994, Poster No. 32.
Recently we reported6 an ecient synthesis of 1-(2-
deoxy-b-d-erythro-pentofuranosyl)imidazo[4,5-d]pyrid-
azine-4(5H)-one (4) which can be used to prepare 20,30-
y
0968-0896/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0968-0896(99)00184-4

2374
J. C. Bussolari, R. P. Panzica / Bioorg. Med. Chem. 7 (1999) 2373±2379
dideoxy-2-aza-3-deazainosine (1) in three steps.
Nucleoside 4 was prepared by the ring closure of 3 with
anhydrous hydrazine in ethanol as depicted in Scheme
1. The reaction proceeded in good yield,14 but was
always accompanied by a small amount of the a-anomer
5. In our initial study,6 we carefully checked the
anomeric con®guration of the nucleoside intermediates
leading to 4 and established that both were beta (b).
Therefore, we examined the reaction leading to 4 and
found that anomerization was occurring after ring clo-
sure. Once cyclized, 4 is isolated as the hydrazino salt.
Neutralization is then carried out using Amberlite IR-
120 H+ resin.15 If the temperature is not controlled
during neutralization, anomerization occurs and can be
as high as 50%. This process can be minimized by
rapidly neutralizing the salt at 5ꢀC. The anomers were
easily separated as their respective 50-O-tert-butyldi-
methylsilyl (TBDMS) ethers (6 and 7) by fractional
crystallization from methanol. The mixture of anomers
(4 and 5) was regiospeci®cally 50-silylated with tert-
butyldimethylsilyl chloride (TBDMSCl) in dimethylfor-
mamide (DMF) to furnish 1-(5-O-tert-butyldimethyl-
silyl-2-deoxy-b-d-ribofuranosyl)imidazo[4,5-d]pyridazin-
4(5H)-one (6) and 1-(5-O-tert-butyldimethylsilyl-2-
deoxy-a-d-ribofuranosyl)imidazo[4,5-d]pyridazin-4(5H)-
one (7) in a 70% yield. On standing in methanol, the
a-anomer 7 readily crystallized out of solution. Nucleo-
side 7 was assigned as the a-anomer based on its char-
Next, the b-nucleoside 6, isolated by fractional crystal-
lization, was thioacylated6 using phenyl chlor-
othionoformate to provide the 30-O-phenoxythiocarbonyl
analogue 8 (Scheme 2). This intermediate was then sub-
jected to the Barton reduction16 which furnished the 30-
deoxygenated nucleoside 10 in good yield. An alternate
route to 10 involves the treatment of 6 with 1,10-thio-
carbonyldiimidazole (TCDI) in DMF to give the 30-O-
(1-imidazolylthiocarbonyl) analogue 9. This derivative
was used immediately without puri®cation. It was trea-
ted with tri-n-butyltin hydride (for a deoxygenation
procedure which avoids the use of n-Bu3SnH or AIBN,
see refs 17±19) in the presence of AIBN in dry toluene at
re¯ux. This two-step, one-pot procedure provided 10
from 6 in an overall 61% yield. The 50-O-TBDMS pro-
tected nucleoside (10) was deblocked with tetra-
butylammonium ¯uoride (TBAF) in THF at room
temperature for 1 h and the target nucleoside 1-(2,3-
dideoxy-b-d-glycero-pentofuranosyl)imidazo[4,5-d]pyr-
idazin-4 (5H)-one (1) was obtained in a 94% yield. This
synthetic sequence a€orded 1 in an overall 40% yield.
We now turned our attention to the synthesis of 20,30-
dideoxy-2,8-diaza-3-deazainosine (2). Two pathways
were envisaged. The ®rst approach we adopted was
similar to the synthetic sequence depicted in Scheme 2.
This approach called for the preparation of the hitherto
unknown 20-deoxy analogue (15) of 1-(b-d-ribofur-
anosyl)-v-triazolo[4,5-d]pyridazin-4-(5H)-one (11). The
1
acteristic splitting pattern in H NMR spectrum.6
Scheme 1. Reagents: (a) 97% NH2NH2, abs. EtOH; (b) t-BuSi(CH3)2Cl, imidazole, DMF.
Scheme 2. Reagents: for compd. 8, (a) C6H5OC(S)Cl, DMAP, TEA, CH3CN; for compd. 9, (b) TCDl, DMF; (c) n-Bu3SnH, AIBN, toluene; (d)
TBAF, THF.

J. C. Bussolari, R. P. Panzica / Bioorg. Med. Chem. 7 (1999) 2373±2379
2375
latter nucleoside, i.e. the proposed starting material for 15,
was recently synthesized in our laboratory.7 The pre-
parative pathway leading to 1-(2-deoxy-b-d-erythro-pen-
tofuranosyl)-v-triazolo[4,5-d]pyridazin-4(5H)-one (15) is
illustrated in Scheme 3. The 50-OH and 30-OH of 11 are
simultaneously protected using 1,3-dichloro-1,1,2,2-tetra-
isopropyl-disiloxane (TIPDSiCl2) in pyridine to give 12 in
an 83% yield. Thioacylation of 12 using phenyl chlor-
othionoformate in acetonitrile a€orded the 2-O-phe-
noxthiocarbonyl 12 in good yield. Deoxygenation of 13
using tri-n-butyltin hydride in re¯uxing toluene in the pre-
sence of AIBN a€orded the silylated 20-deoxy nucleoside
14. Standard deprotection of the silyl ether with TBAF
a€orded 15 in an overall 50% yield from 11. This repre-
sents the ®rst disclosure of the 20-deoxy nucleoside in this
series. At this point, we decided to select an alternate route
to 2 which was shorter, i.e. avoided the generation and
isolation of 15, and more cost-e€ective. This procedure is
illustrated in Scheme 4 and again employs nucleoside 11 as
starting material. Treatment of 11 with tert-butyldi-
methylsilyl chloride in DMF provided the silyl ether 16 in
good yield. Heating 16 with 1,10-thiocarbonyldiimidazole
(TCDI) in dry DMF at 80ꢀC formed the desired cyclic
thiocarbonate 17. The reductive cleavage of 17 furnished
an ca. 1:1 mixture of the 20-deoxy (18) and 30-deoxy (19)
isomers. This pattern is consistent with the orginal report
of Barton and Subramanian16 on the radical-initiated
reduction of nucleoside 20,30-thiocarbonates with n-
Discovery Program and screened against HIV-1 grown
in CEM-SS cells. Their degree of activity was determined
by the XTT assay20. Both of these nucleosides were
4
found to be inactive at concentrations up to 2.00Â10
M. It is worth mentioning that 1 and 2 were not cyto-
toxic in the concentration range, i.e. 6.00Â10
8
to
2.00Â10±4, which they were tested.
Conclusion
In conclusion, we have prepared the 20,30-dideoxy-
nucleosides of 2-aza-3-deazainosine and 2,8-diaza-3-
deazainosine. Our future research plans in this area
include preparing the 50-phosphonates21 and the 50-
(di¯uoromethyl)phosphonates22,23 of 1 and 2. Phospho-
nate replacement is attractive since the carbon-phos-
phorus bond in phosphonates is not susceptible to
enzymatic degradation by phosphatases, thus enhancing
their physiological stability. Phosphonate esters are also
less polar and this chemical feature gives rise to better
cell permeability24. We anticipate that the 50-phospho-
nate analogues of 1 and 2 will function as substrates for
the nucleotide kinases and eventually be converted, in
vivo, to their active 50-triphosphate counterparts.
Experimental
1
Bu3SnH in the presence of AIBN. The characteristic H
NMR spin patterns of the anomeric protons of these two
nucleosides con®rmed the assignment of this mixture as 1-
(5-O-tert-butyldimethylsilyl-2-deoxy-b-d-erythro-pento-
furanosyl)-ꢀ-triazolo[4,5-d]pyridazin-4(5H)-one (18) and 1-
5-O-tert-butyl-dimethylsilyl-3-deoxy-b-d-erythro-pento-
furanosyl)-ꢀ-triazolo[4,5-d]pyridazin-4(5H)-one (19). The
isomeric mixture can be treated in the same manner as 6
(Scheme 2) with TCDI. Like 9, the respective inter-
mediates 20 and 21 were not isolated or puri®ed. Reduc-
tion and deprotection then gave 2 in 23% overall yield.
Melting points were determined on a Buchi 535 melting
point apparatus and are uncorrected. 1H and 13C NMR
spectra were recorded on either a Varian EM 390 or a
Bruker AM-300 spectrometer, as indicated, using Me4Si
(TMS) as an internal standard. Optical rotations were
measured on a Perkin±Elmer Model 141 automatic
digital readout polarimeter. All moisture-sensitive reac-
tions were performed using ¯ame-dried glassware.
Methylene chloride (CH2Cl2) and acetonitrile were
dried over CaH2 and distilled. Anhydrous THF was
obtained by distillation over sodium benzophenone
ketyl. Evaporations were performed under diminished
pressure using a Buchi Rotary Evaporator unless stated
otherwise. Davison silica gel (grade H, 60±200 mesh),
purchased from Fisher Scienti®c, was used for ¯ash
column chromatography. A Chromatotron (centri®cally
Biological Evaluation
The 20,30-dideoxynucleosides 1 and 2 were evaluated in
the National Cancer Institute in vitro Anti-AIDS Drug
Scheme 3. Reagents: (a) TIPSiCl, pyridine; (b) C6H5OC(S)Cl, DMAP, TEA, CH3CN; (c) n-Bu3SnH, AIBN, toluene; (d) TBAF, THF.

2376
J. C. Bussolari, R. P. Panzica / Bioorg. Med. Chem. 7 (1999) 2373±2379
Scheme 4. Reagents: (a) t-BuSi(CH3)2Cl, imidazole, DMF; (b) TCDI, DMF; (c) n-Bu3SnH, AIBN, toluene; (d) TBAF, THF.
accelerated, preparative thin-layer, radial chromato-
graph), Model 7924 T was used to complete various
separations as indicated. The 1.0 and 2.0 mm plates
used were coated with silica gel PF254 containing
CaSO4. Thin-layer chromatography was performed on
precoated silica gel plates (60-F254, 0.2 mm) manu-
factured by E.M. Science, Inc. and short-wave ultravio-
let light (254 nm) was used to detect the UV absorbing
compounds. All solvent proportions are by volume
unless otherwise indicated. Elemental analyses were
performed by MHW Laboratories, Phoenix, AR.
3.82 (m, 2H, H-50), 3.92 (q, 1H, J=6.94 Hz, J=3.53 Hz,
H-40), 4.32±4.37 (m, 1H, H-30), 5.41 (d, 1H, J=4.06 Hz,
D2O exchangeable, OH), 6.36 (t, 1H, J=6.39 Hz, H-10),
8.46 (s, 1H, H-2), 8.54 (s, 1H, H-7), 12.70 (s, 1H, D2O
exchangeable, NH). Anal. calcd for C16H26N4O4Si: C,
52.44; H, 7.15; N, 15.29. Found: C, 52.38; H, 7.27; N,
15.64.
1-(5-O-tert-Butyldimethylsilyl-3-O-phenoxythiocarbonyl-
2-deoxy-ꢁ-D-erythro-pentofuranosyl)-imidazo[4,5-d]pyri-
dazin - 4(5H) - one (8). A solution of 6 (0.40 g, 1.09
mmol), dimethylaminopyridine (0.15 g, 1.19 mmol),
phenylchlorothionoformate (0.45 mL, 3.26 mmol), and
triethylamine (2 mL) in anhyd acetonitrile (10 mL) was
stirred at room temperature for 5 h. The reaction mixture
was poured over cracked ice/water (50 mL), and the water
solution extracted with ethyl acetate (3Â60 mL). The
organic layers were combined, dried over anhyd like
Na2SO4, and concentrated. The residue was puri®ed by
¯ash column chromatography using ethyl acetate:CH2Cl2
(3:2) as the eluent to give 8 (0.22 g, 40% yield) as a yellow
1-(5-O-tert-Butyldimethylsilyl-2-deoxy-ꢁ-D-erythro-pen-
tofuranosyl)imidazo[4,5-d]pyridazin-4(5H)-one (6).
A
mixture containing the anomers 4,56 (2.20 g, 8.62
mmol), imidazole (1.29 g, 18.9 mmol), and tert-butyldi-
methylsilyl chloride in dry DMF (10 mL) was stirred under
N2 for 12 h. The reaction mixture was concentrated under
vacuum and the residue was puri®ed by ¯ash column chro-
matography using ethyl acetate:methanol (19:1) as the elu-
ent to give a mixture of compounds 6 and 7 (2.27 g, 71%) as
a white solid. Recrystallization of this intimate mixture
from methanol gave pure 6 (2.00 g, 67%): mp 171±172ꢀC;
1H NMR (CDCl3, 300 MHz) d 0.01 (s, 6H, Si(CH3)2),
0.81 (s, 9H, Si(CH3)3), 2.35±2.55 (m, 2H, H-20), 3.65±
ꢀ
1
foam: mp 84±86 C; H NMR (CDCl3) d 0.15 (s, 6H,
Si(CH3)2), 0.90 (s, 9H, Si(CH3)3), 2.68±2.90 (m, 2H, H-20),
3.89±4.10 (m, 2H, H-50), 4.40±4.54 (m, 1H, H-40), 5.73±
5.90 (m, 1H, H-30), 6.30 (dd, 1H, J=5.7 Hz, J=8.8 Hz
H-10), 6.93±7.48 (m, 5H, C6H5), 8.12 (s, 1H, H-2), 8.49 (s,

J. C. Bussolari, R. P. Panzica / Bioorg. Med. Chem. 7 (1999) 2373±2379
2377
1H, H-7), 11.73 (s, 1H, D2O exchangeable, NH). Anal.
calcd for C23H30 N4O5SSi; C, 54.96; H, 6.02; N, 11.14.
Found: C, 55.05; H, 5.87; N, 10.91.
isopropyldisiloxane (2.70 mL, 8.6 mmol). The reaction
mixture was stirred at room temperature under N2 for 6 h.
Cold water (50 mL) was then added to the reaction mixture
and the aq solution was extracted with CH2Cl2 (3Â60 mL).
The organic layers were combined, dried over anhydrous
Na2SO4, ®ltered, and concentrated. The crude product was
puri®ed by ¯ash column chromatography using CH2Cl2:
methanol (19:1) as the eluent to a€ord 12 (3.31 g, 83%
yield) as a white foam: mp 80 C sinters, 87±88 C; H
NMR (CDCl3) d 0.98, 1.07 (2s, 28H, i-Pr), 3.85±4.35 (m,
3H, H-50, H-40), 4.63±4.91 (m, 2H, H-30, H-20), 6.32 (s, 1H,
H-10), 8.55 (s, 1H, H-7), 12.09 (bs, 1H, D2O exchangeable,
NH). Anal. calcd for C21H37N5O6Si2: C, 49.29; H, 7.29; N,
13.68. Found: C, 49.50; H, 7.47; N, 13.36.
1-(5-O-tert-Butyldimethylsilyl-2,3-dideoxy-ꢁ-D-glycero-
pentofuranosyl)imidazo[4,5-d]pyridazin-4(5H)-one (10).
Method A. Compound 8 (0.10 g, 0.20 mmol) was dis-
solved in dry toluene (5 mL) and heated to re¯ux as a solu-
tion of tri-n-butyltin hydride (0.5 mL) and a,a0-
azobisisobutyronitrile (AIBN, 5 mg) in toluene (2 mL)
was added dropwise. After the addition was completed, the
reaction was stirred at re¯ux for 4 h and then cooled to
room temperature and concentrated. Puri®cation of the
crude product by ¯ash column chromatography using
CH2Cl2:ethyl acetate (1:1) as the eluent a€orded 10 (0.06 g,
81% yield) as a white solid: mp 49±50 ꢀC sinters, 92±94ꢀC;
1H NMR (CDCl3) d 0.17 (s, 6H, Si (CH3)2), 0.88 (s, 9H,
Si(CH3)2), 1.90±2.61 (m, 4H, H-20, H-30), 3.49±3.91 (m, 2H,
H-50), 4.02±4.30 (m, 1H, H-40), 6.02 (dd, 1H, J=4.0 Hz,
J=6.0 Hz, H-10), 8.13 (s, 1H, H-2), 8.23 (s, 1H, H-7), 11.32
(bs, 1H, D2O exchangeable, NH). Anal. calcd for C16H26
N4O3Si: C, 54.83; H, 7.48; N, 15.99. Found: C, 54.49; H,
7.55; N, 15.73.
ꢀ
ꢀ
1
1-{2-O-Phenoxythiocarbonyl-3,5-O-(1,1,3,3-tetraisopro-
pyldisiloxanyl)-ꢁ-D-ribofuranosyl}-ꢀ-triazolo[4,5-d]pyri-
dazin-4(5H)-one (13). A solution containing 12 (1.74 g,
3.40 mmol), dimethylaminopyridine (0.46 g, 3.7 mmol),
phenyl chlorothionoformate (1.00 mL, 7.0 mmol), tri-
ethylamine (5 mL) and anhyd acetonitrile (30 mL) was
stirred at room temperature for 5 h as for the preparation
of 8. The usual work up followed by ¯ash column chro-
matography using CH2Cl2:methanol (9:1) as the eluent
a€orded 13 (1.76 g, 80% yield) as a white solid: mp 83±84
Method B. A solution of 6 (0.34 g, 0.87 mmol) and 1,10-
thiocarbonyldiimidazole (0.47g, 2.63 mmol) and dry DMF
(5 mL) was stirred at room temperature for 20 h under N2.
The mixture was poured into cold H2O (ca. 5 mL) and the
aq solution was extracted with ethyl acetate (2Â20 mL).
The organic layers were combined, dried over Na2SO4,
®ltered, and then concentrated. Crude 9 was dissolved in
dry toluene (6 mL) and heated to re¯ux while a solution of
tri-n-butyltin hydride (0.5 mL) and AIBN (5 mg) in tolu-
ene (2 mL) was added dropwise. The reaction was stirred
at re¯ux for 5 h, cooled to room temperature, and con-
centrated. The oily residue was puri®ed by ¯ash column
chromatography using ethyl acetate as the eluent to give 10
(0.19 g, 61% yield) having physical data identical in all
respects to that obtained by Method A.
ꢀ
1
C; H NMR (CDCl3) d 0.99, 1.09 (2s, 28H, i-Pr), 3.91±
4.34 (m, 3H, H-40, H-50), 4.88±5.08 (m, 1H, H-30), 6.47 (d,
1H, J=4.5 Hz, H-20), 6.52 (s, 1H, H-10), 6.98±7.51 (m, 5H,
C6H5), 8.51 (s, 1H, H-7), 11.73 (bs, 1H, D2O exchangeable,
NH). Anal. calcd for C28H41N5O7SSi2: C, 51.91; H, 6.38;
N, 10.81. Found: C, 52.08; H, 6.27; N, 10.82
1-{2-Deoxy-3,5-O-(1,1,3,3-tetraisopropyldisiloxanyl)-ꢁ-
D -erythro-pentofuranosyl)-ꢀ -triazolo[4,5-d]pyridazin-4
(5H)-one (14). To a solution of 13 (0.32 g, 0.49 mmol)
and anhyd toluene (15 mL) stirred at room temperature,
under N2, was added tri-n-butyltin hydride (1.5 mL) and
AIBN (15 mg). The mixture was heated at re¯ux for 4 h,
cooled to room temperature and then concentrated. Pur-
i®cation of the residue by ¯ash column chromatography
using CH2Cl2:ethyl acetate (9:1) as the eluent gave 14 (0.22
1-(2,3-dideoxy-ꢁ-D-glycero-pentofuranosyl)imidazo[4,5-
d]pyridazin - 4(5H) - one (1). Tetrabutylammonium ¯uo-
ride (TBAF, 0.2 mL, 1 M solution in THF) was added
dropwise to a solution of 10 (0.10 g, 0.29 mmol) in dry
THF (5 mL) and stirred at room temperature under N2
for 2 h. The solvent was evaporated to give the crude
product which was dissolved in H2O (3 mL), washed with
CH2Cl2 (2Â10 mL), and then lyophilized. The product
was puri®ed by ¯ash column chromatography using ethyl
acetate as the eluent to give 1 (0.05 g, 94% yield) as a white
ꢀ
1
g, 89% yield) as a white foam: mp 80±85 C; H NMR
(CDCl3) d 0.87±1.09 (m, 28H, i-Pr), 2.57±3.33 (m, 2H, H-
20), 3.80±4.07 (m, 3H, H-50, H-40), 4.63±4.93 (m, 1H, H-
30), 6.50 (dd, 1H, J=6.5 Hz, H-10) 8.53 (s, 1H, H-7), 11.40
(s, 1H, D2O exchangeable, NH). Anal. calcd for C21
H3N5O5Si2: C, 50.88; H, 7.52; N, 14.13. Found: C, 51.07;
H, 7.58; N, 14.20.
1-(2-Deoxy-ꢁ-D-erythro-pentofuranosyl)-ꢀ-triazolo[4,5-
d]pyridazin-4(5H)-one (15). To a solution of 14 (0.45 g,
0.91 mmol) in dry THF (10 mL) was added TBAF (1.0
mL, 1 M solution in THF) and the resulting mixture was
stirred at room temperature for 40 min. The mixture was
then poured over cracked ice/water (50 mL), the aq
solution washed with CH2Cl2 (3Â60 mL) and then lyo-
philized. Puri®cation of the resulting residue on a Chro-
matotron (2.0 mm plate) using ethyl acetate:methanol
(9:1) as the eluent followed by recrystallization from
methanol a€orded pure 15 (0.190 g, 82% yield) as crys-
ꢀ
1
solid: mp 252 C dec; [a]2d5 30.6ꢀ (c 0.88, DMF); H
NMR (300 MHz, DMSO-d6) d 1.93±2.03 (m, 2H, H-30),
2.33±2.50 (m, 2H, H-20), 3.41±3.48 (m, 1H, H-50), 3.54±
3.61 (m, 1H, H-50), 4.11±4.19 (m, 1H, H-40), 4.97 (t, 1H,
J=5.28 Hz, D2O exchangeable, OH), 6.27 (dd, 1H,
J=6.37 Hz, J=3.31 Hz, H-10), 8.53 (s, 1H, H-2), 8.55 (s,
1H, H-7). Anal. calcd for C10H12N4O3: C, 50.67; H, 5.44;
N, 23.64. Found: C, 50.48; H, 5.21; N, 23.46.
1-{3,5-O-(1,1,3,3-Tetraisopropyldisiloxanyl)-ꢁ-D-ribofur-
anosyl}-ꢀ-triazolo[4,5-d]pyridazin-4(5H)-one (12). To a
solution containing 117 (2.10 g, 7.8 mmol) and anhyd
pyridine (20 mL) was added 1,3-dichloro-1,1,3,3-tetra-
talline prisms: mp 145±146ꢀC; H NMR (DMSO-d6) d
1
2.27±2.60, 2.73±3.07 (m, 2H, H-20, H-200), 3.23±3.50 (m,
2H, H-50, H-500), 3.82±4.00 (m, 1H, H-40), 4.28±4.50 (m,

2378
J. C. Bussolari, R. P. Panzica / Bioorg. Med. Chem. 7 (1999) 2373±2379
1H, H-30), 4.72 (t, 1H, J=5.0 Hz, D2O exchangeable,
OH), 5.33 (d, 1H, J=4.5 Hz, D2O exchangeable, OH),
6.20 (t, 1H, J=6 Hz, H-10), 8.69 (s, 1H, H-7), 11.95 (s,
1H, D2O exchangeable, NH). Anal. calcd for C9H11
N5O4: C, 42.69; H, 4.38; N, 27.66. Found: C, 42.73; H,
4.53; N, 27.76.
1-(5-O-tert-Butyldimethylsilyl-2,3-dideoxy-ꢁ-D-glycero-
pentofuranosyl) - ꢀ - triazolo[4,5 - d]pyridazin - 4(5H) - one
(22). A solution of isomers 18 and 19 (0.22 g, 0.61
mmol) and TCDI (0.32 g, 1.18 mmol) in dry DMF (5
mL) was stirred for 20 h at room temperature under N2.
The mixture was concentrated in vacuo, dissolved in
CH2Cl2 (20 mL), and washed with H2O (2Â10 mL). The
aqueous phase was washed with ethyl acetate (10 mL) and
the organic phases were combined, dried over Na2SO4,
®ltered, and concentrated. The residue was dissolved in
dry toluene (5 mL) and heated to re¯ux as a solution of n-
Bu3SnH (0.5 mL) and AIBN (5 mg) in dry toluene (2 mL)
was added dropwise. The reaction was stirred at re¯ux for
3 h, cooled, and concentrated. The oily residue was pur-
i®ed by ¯ash column chromatography using CH2Cl2:
ethyl acetate (4:1) as the eluent to provide 22 (0.12 g, 54%
1-(5-O-tert-Butyldimethylsilyl-ꢁ-D-ribofuranosyl)-ꢀ-tria-
zolo[4,5-d]pyridazin-4(5H)-one (16). To a solution con-
taining 11 (1.07 g, 3.97 mmol) and imidazole (0.68 g, 9.92
mmol), in anhyd DMF (10 mL) under N2 was added tert-
butyldimethylsilyl chloride (0.72 g, 4.76 mmol). The reac-
tion mixture was then stirred at room temperature for 20
h. The solvent was removed in vacuo and the resulting
residue puri®ed by ¯ash column chromatography using
CH2Cl2:methanol (19:1) as the eluent to yield 16 (1.06 g,
70% yield) as a white foam: mp 88±89 ꢀC; 1H NMR (300
MHz, DMSO-d6) d 0.02 (s, 6H, Si(CH3)2), 0.72 (s, 9H,
Si(CH3)3), 3.63±3.88 (dq, 2H, H-50, H-500), 4.06±4.26 (m,
3H, H-40, H-30), 4.69±4.77 (m, 1H, H-20), 5.34 (d, 1H, J=6
Hz, D2O exchangeable, OH), 5.79 (d, 1H, J=6.0 Hz, D2O
exchangeable, OH), 6.31 (d, 1H, J=4.4 Hz, H-10), 8.77 (s,
1H, H-7). Anal. calcd for C15H25N5O5Si: C, 46.98; H,
6.57; N, 18.26. Found: C, 46.90; H, 6.65; N, 17.87.
ꢀ
1
yield) as a waxy solid: mp 92±94 C; H NMR (CDCl3,
300 MHz) d ±0.09 (s, 6H, Si(CH3)2), 0.76 (s, 9H,
Si(CH3)3), 2.03±2.18 (m, 2H, H-30), 2.55±2.68 (m, 1H, H-
0
00
0
2 ), 3.21±3.30 (m, 1H, H-2 ), 3.51±3.87 (dq, 2H, J5 ,5 =
00
0
00
0
0
00
0
11.42, J5 ,4 =3.15, J5 ,4 =4.39 Hz, H-5 , H-5 ), 4.40±
4.48 (m, 1H, H-40), 6.40 (dd, 1H, J=6.3 Hz, J=1.9 Hz, H-
10), 8.65 (s, 1H, H-7), 10.41 (s, 1H, D2O exchangeable,
NH). Anal. calcd for C15H25N5O3Si; C, 51.25; H, 7.17;
N, 19.93. Found: C, 51.46; H, 7.27; N, 19.71.
1-(5-O-tert-Butyldimethylsilyl-2,3-O-thionocarbonyl-ꢁ-D-
ribofuranosyl)- ꢀ-triazolo[4,5-d]pyridazin-4(5H)-one (17).
A solution containing 16 (1.03 g, 2.69 mmol) and 1,10-
thiocarbonyldiimidazole (0.62 g, 3.49 mmol) in anhyd
DMF (10 mL) was heated to 80 ꢀC under N2 for 4 h. The
solvent was removed in vacuo and the resulting residue
was puri®ed by ¯ash column chromatography using
CH2Cl2:ethyl acetate (4:1) as the eluent. Recrystallization
of the product from CH2Cl2 a€orded pure 17 (1.14 g, 91%
yield) as a white solid: mp 279±280 C; H NMR (300
MHz, DMSO-d6) d 0.22 (s, 3H, Si(CH3)2), 0.17 (s, 3H,
Si(CH3)2), 0.67 (s, 9H, Si(CH3)3), 3.80 (dq, 2H, H-50, H-
500), 4.79 (t, 1H, H-40), 5.84 (d, 1H, J=6.6 Hz, H-30), 6.53
(d, 1H,J=7.2 Hz, H-20), 7.16 (s, 1H, H-10), 8.78 (s, 1H, H-
7). Anal. calcd for C16H23N5O5SiS: C, 45.16; H, 5.45; N,
16.46; S, 73. Found: C, 44.97; H, 5.29; N, 16.20; S, 7.23.
1-(2,3-dideoxy-ꢁ-D-glycero-pentofuranosyl)-ꢀ-triazolo[4,
5-d]pyridazin-4(5H)-one (2). Tetrabutylammonium ¯uo-
ride (TBAF, 0.5 mL, 1 M solution in THF) was added
dropwise to a solution of 22 (0.10 g, 0.29 mmol) in dry
THF (5 mL) and stirred at room temperature under N2
for 2 h. Reaction work up and puri®cation as for the
synthesis of 1 gave 2 (0.05 g, 93% yield) as a white solid:
mp 140±142 ꢀC dec.; [a]2d5 44.7ꢀ (c 0.98, abs EtOH); 1H
NMR (MeOH-d4) d 1.96±2.30 (m, 2H, H-30, H-300), 2.55±
2.70, 3.00±3.15 (2m, 2H, H-20, H-200), 3.40±3.74 (m, 2H,
H-50), 4.30±4.43 (m, 1H, H-40), 6.54 (dd, 1H, J=6.6 Hz,
J=1.6 Hz, H-10), 8.69 (s, 1H, H-7). Anal. calcd for
C10H12N4O3: C, 45.57; H, 4.67; N, 29.52. Found: C, 45.35;
H, 4.71; N, 29.47.
ꢀ
1
1-(5-O-tert-Butyldimethylsilyl-3-deoxy-ꢁ-D-erythro-pento-
furanosyl)-ꢀ-triazolo[4,5-d]pyridazin-4(5H)-one (18) and 1
-(5-O-tert-butyldimethylsilyl-2-deoxy-ꢁ-D-erythro-pento-
furanosyl)-ꢀ -triazolo[4,5-d]pyridazin-4(5H)-one (19).
Compound 17 (0.91 g, 2.15 mmol) was heated to re¯ux in
dry toluene (20 mL) while a solution containing tri-n-
butyltin hydride (1.00 mL) and AIBN (30 mg) in toluene
(2 mL) was added dropwise. The reaction was stirred at
re¯ux for 4 h, cooled, concentrated and then puri®ed by
¯ash column chromatography using CH2Cl2:ethyl acetate
(3:1) as the eluent to give ca. 1:1 intimate mixture of 18
Acknowledgements
We thank Professor John C. Drach for helpful discus-
sions. We also thank Mr. David J. Guerin and Dr. Hui-
Fang (Amy) Chang for technical assistance.
References and Notes
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and 19 (0.58 g, 73% yield) as a white solid: H NMR
(CDCl3, 300 MHz) d 0.07 (s, 6H, Si(CH3)2), 0.04 (s,
3H, Si(CH3)2), 0.03 (s, 3H, Si(CH3)2), 0.76 (s, 9H,
Si(CH3)3), 0.79 (s, 9H, Si(CH3)3), 2.15±2.22 (m, 1H), 2.29±
2.38 (m, 1H), 2.73±2.81 (m, 1H), 3.08±3.16 (m, 2H), 3.60±
3.94 (m, 4H), 4.25 (q, 1H ), 4.71±4.78 (m, 1H), 5.36 (d,
1H), 6.35 (s, 1H, H-10), 6.64 (t, 1H, J=5.7 Hz, H-10), 8.67
(s, 1H), 8.70 (s, 1H), 11.14 (bs, 1H), 11.22 (bs, 1H). Anal.
calcd for C15H25N5O4Si; C, 49.03; H, 6.86; N, 19.06.
Found: C, 49.23; H, 6.95; N, 18.95.

J. C. Bussolari, R. P. Panzica / Bioorg. Med. Chem. 7 (1999) 2373±2379
2379
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