HETEROCYCLES

Paper | Regular issue | Vol. 78, No. 1, 2009, pp. 71-91
Received, 6th July, 2008, Accepted, 27th August, 2008, Published online, 28th August, 2008.
DOI: 10.3987/COM-08-11482

■ Diversity-oriented Synthesis of Substituted Pyrazolo[4,3-d][1,2,3]triazin-4-ones

Daniel Meibom,* Marcus Bauser, Heinrich Meier, Dirk Schneider, Nils Svenstrup, and Dieter Haebich

Medical Chemistry, Bayer Health Care AG, D-42096 Wuppertal, Germany

Abstract

An efficient synthesis of 3,7-disubstituted 3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-ones (2), especially suited for late-stage decoration, is described. Diverse 3-benzyl analogs were obtained by amidation-cyclization in one pot and various pyrazolotriazinone-7-bromides were employed in Suzuki and Sonogashira cross-coupling reactions.

INTRODUCTION
In medicinal chemistry heterocyclic scaffolds represent building blocks of paramount importance, since they provide an assorted selection of varying geometries, polarities, and hydrogen-bonding properties, useful for the elaboration of structure-activity relationships and the optimization of pharmacokinetic and physicochemical parameters. In this respect and also in terms of intellectual property, underutilized heterocyclic systems such as the pyrazolotriazinones offer a variety of new opportunities.
Purine base analogs have been the subject of numerous medicinal chemistry programs as their structure inherently bears the key to many beneficial biological properties.1,2 Surprisingly, only rare literature precedent exists on pyrazolo[4,3-d][1,2,3]triazin-4-ones. For example, Baraldi et al. have described them as adenosine A1- and A2a-receptor antagonists3 and Manfredini et al. showed pyrazolotriazinone nucleosides to display cytostatic properties.4 Previously, these heterocycles were accessed by a six-step linear sequence.3-6 However, for elaboration of structure-activity relationships, this approach is hampered by the early introduction of substituents R and R’. A recent medicinal chemistry program required a general, straightforward, and diversity-oriented synthesis of 3,7-disubstituted 3,5-dihydro-4H- pyrazolo[4,3-d][1,2,3]triazin-4-ones 2 (Scheme 1). Herein, we report two novel synthetic strategies to pyrazolotriazinones 2, designed for rapid and convenient “analoging” allowing the late-stage introduction of substituents R or R’.

RESULTS AND DISCUSSION
Published syntheses of 3,7-disubstituted 3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-ones 2, followed a diazonium mediated ring closing strategy starting from pyrazole carboxamides of type 107,8 (Scheme 2).

Accordingly, the incorporation of a new substituent R at position 3 required a three-step sequence starting from nitrocarboxylic acid 8. Efforts to establish a convenient and chemically feasible one pot procedure failed as reactants and solvents were incompatible for this purpose. As a consequence, the isolation and purification of all intermediates was required. In our search for an improved method we got inspired by the proficient work of Braese et al.,9 who converted a resin-bound 2-carboxy-phenyltriazene into a benzotriazinone by amide coupling followed by TFA-mediated cleavage and simultaneous cyclization. We adapted Braese’s solid-phase methodology to our needs for producing a solution-phase library, using triazene carboxylate 14 as a key intermediate (Scheme 3). In this route, the N,N-dimethyltriazene was acting as a protected form of the diazonium ion intermediate. This route was readily amenable to parallel synthesis.

Esters 11 were prepared according to literature procedures10,11 from commercially available starting materials 7. Hydrogenation of 11 yielded the amines 12 which were treated with sodium nitrite. The intermediate diazonium ions could be captured by addition of dimethylamine to form the stable triazenes 13. Saponification of 13 gave the dipotassium salts 14 on a multi-gram scale. Triazenes 14 reacted with a broad variety of primary amines under standard amide coupling conditions with (Benzotriazol-1-yloxy)tripyrrolidinophosphonium hexafluorophosphate (PyBOP) as the coupling reagent. The triazene amides could be isolated, however, subsequent addition of acetic acid to the reaction mixture induced clean cyclization to the desired pyrazolotriazinones 16a-i in one pot from triazenes 14, conceivably via the intermediate 15. Starting from esters 11 the complete sequence required just a single final chromatography to obtain the pyrazolotriazinones 16a-i in acceptable yields (Table 1).

Aromatic as well as heterocyclic benzylic amines with different substitution patterns have successfully been utilized following the amidation-cyclization-procedure (Table 1). In addition, branched substrates with R’’ = alkyl could be employed. In all, by the procedure sketched in Scheme 3, we rapidly prepared over one hundred structurally diverse analogs.12
Position 7 in the pyrazolotriazinone-scaffold could successfully be addressed by Pd-catalyzed methodologies starting from bromo derivatives of type 18 (Scheme 4) or 22 (Scheme 5). The novel bromides 17 and 21 were synthesized in good yield by heating 16f or 20 with NBS in acetonitrile. The same procedure could be employed for the preparation of iodo- or chloro-substituted congeners by using NIS (Scheme 7) or NCS12 instead of NBS. Suzuki and Sonogashira cross-coupling reactions were readily feasible (Scheme 4, Scheme 5, Table 2, Table 3). However, attempts to perform coupling reactions with N-5 H-pyrazolotriazinones failed due to decomposition of starting material. Representative examples selected from about 70 diverse analogs synthesized by a Suzuki protocol are listed in Table 2. Low yields were observed in the cases 19a-h where unpurified starting material was employed.

Sonogashira couplings were preferentially performed on SEM-protected substrates such as 22 and 24 (Scheme 5, Table 3). Compounds 24 were obtained analogously to the procedure sketched in Scheme 5 (see experimental section). The reactions reproducibly gave the desired products in moderate to good yield (Table 3). In some cases Glaser-type alkynyl homo coupling products were isolated along the target compounds. Functionalities like esters of benzoic acid, ether-, dialkylamino- and silyl-groups were tolerated while the conversion of alkynylesters met with failure.

The deprotection of the pyrazole nitrogen was achieved in a stepwise manner (Scheme 6). Treatment of 23a with trifluoroacetic acid yielded mixtures of hemiaminal 2513 and the desired NH-pyrazolotriazinone 26a. However, pure 26a could be obtained by treatment of the intermediate mixture under basic conditions. Yields varied considerably when this procedure was transferred to other substrates (Table 4). Attempts to achieve complete deprotection with tetrabutylammonium fluoride failed due to decomposition of the starting material.

In an alternative approach to introduce structural diversity in the 7-position we were able to capitalize of iodide 28 by applying a palladium catalyzed methoxycarbonylation using carbon monoxide under atmospheric pressure (Scheme 7). In two consecutive steps the acid 31 was prepared from ester 29. Building blocks of type 31 open the door to a wide variety of drug like compounds (e.g. amides) by applying standard protocols.

In summary, we have designed an efficient late-stage decorating procedure for the pyrazolotriazinone core 2. A one pot amidation-cyclization procedure was applied in broad scope and gave access to diverse analogs with benzylic substituents in the 3-position. Furthermore, pyrazolotriazinone-7-bromides were employed in a series of Suzuki and Sonogashira cross-coupling reactions. Likewise, also Stille or Heck-type Pd-catalyzed cross-couplings should be feasible with these valuable novel halogenated building-blocks.

EXPERIMENTAL
Methods and materials:
Unless otherwise noted, all materials were obtained from commercial suppliers and used without further purification. Flash chromatography was done on silica gel (0.063 - 0.200 mm) from Merck KgaA, Darmstadt, Germany. Preparative HPLC chromatography was done on a 250 mm x 30 mm column packed with YMC gel ODS-AQ S-5 / 15 µM, with MeCN / water as eluent and UV detection. Spin multiplicities of NMR spectra are given as s (singlet), d (doublet), t (triplet), q (quartet) m (multiplet) as well as br (broad).
LC-MS method 1 for Table 1, 2, a and b: instrument MS Waters ZQ 2000; Instrument HPLC Agilent 1100, 2-column-circuit, autosampler HTC PAL; column YMC-ODS-AQ, 50 mm x 4.6 mm, 3.0 µm; eluent A: water + 0.1% formic acid, eluent B: MeCN + 0.1% formic acid; gradient: 0.0 min 100%A ➔ 0.2 min 95%A ➔ 1.8 min 25%A ➔1.9 min 10%A ➔ 2.0 min 5%A ➔ 3.2 min 5%A ➔ 3.21 min 100%A ➔ 3.35 min 100%A; oven: 40 °C; flow: 3.0 mL/min; UV-detection: 210 nm.
LC-MS method 2: instrument MS Micromass ZQ; instrument HPLC Waters Alliance 2795; column Phenomenex Synergi 2µ Hydro-RP Mercury 20 mm x 4 mm; eluent A: 1 l water + 0.5 mL 50% formic acid, eluent B: 1 l MeCN + 0.5 mL 50% formic acid; gradient: 0.0 min 90%A ➔ 2.5 min 30%A ➔ 3.0 min 5%A ➔ 4.5 min 5%A; flow: 0.0 min 1 mL/min, 2.5 min/3.0 min/4.5 min 2 mL/min; oven: 50 °C; UV-detection: 210 nm.
LC-MS method 3: Instrument Micromass Quattro LCZ with HPLC Agilent series 1100; column Phenomenex Synergi 2 µ Hydro-RP Mercury 20 mm x 4 mm; eluent A: 1 l water + 0.5 mL 50% formic acid, eluent B: 1 l MeCN + 0.5 mL 50% formic acid; gradient: 0.0 min 90% A ➔ 2.5 min 30% A ➔ 3.0 min 5% A ➔ 4.5 min 5% A; flow: 0.0 min 1 mL/min, 2.5 min/3.0 min/4.5 min 2 mL/min; oven: 50 °C; UV-detection: 208- 400 nm..
LC-MS method 4: Instrument MS Micromass ZQ; Instrument HPLC HP 1100 Series; UV DAD; column Phenomenex Synergi 2 µ Hydro-RP Mercury 20 mm x 4 mm; eluent A: 1 l water + 0.5 mL 50% formic acid, eluent B: 1 l MeCN + 0.5 mL 50% formic acid; gradient: 0.0 min 90% A ‡ 2.5 min 30% A ‡ 3.0 min 5% A ‡ 4.5 min 5% A; flow: 0.0 min 1 mL/min, 2.5 min/3.0 min/4.5 min. 2 mL/min; oven: 50 °C;
UV-detection: 210 nm..
HPLC method 1: Instrument HP 1100 with DAD-detection; column Kromasil 100 RP-18, 60 mm x 2.1 mm, 3.5 µm; eluent: A = 5mL HClO4 (70 %)/L H2O, B = MeCN, gradient: 0 min 2% B ➔ 0.5 min 2% B ➔ 4.5 min 90% B ➔ 6.5 min 90% B ➔ 6.7 min 2% B ➔ 7.5 min 2 % B, flow: 0.75 mL/min, column temperature: 30 °C, detection: UV 210 nm.
HPLC method 2: Instrument: HP 1100 with DAD-detection; column Kromasil 100 RP-18, 60 mm x 2.1 mm, 3.5 µm; eluent: A = 5 mL HClO4 (70 %ig)/L H2O, B = MeCN; gradient: 0 min 2% B ➔ 0.5 min 2% B ➔ 4.5 min 90% B ➔ 15 min 90% B ➔ 15.2 min 2% B ➔ 16 min 2 % B, flow: 0.75 mL/min, column temperature: 30 °C, detection: UV 210 nm.
HPLC method 3: Instrument HP 1100 with DAD-detection; column Kromasil 100 RP-18, 60 mm x 2.1 mm, 3.5 µm; eluent: A = 5 mL HClO4 (70 %ig)/L H2O, B = MeCN; gradient: 0 min 2% B ➔ 0.5 min 2% B ➔ 4.5 min 90% B ➔ 9 min 90% B ➔ 9.2 min 2% B ➔ 10 min 2 % B, flow: 0.75 mL/min, column temperature: 30 °C, detection: UV 210 nm.

Synthetic procedures:
4-[3,3-Dimethyltriaz-1-en-1-yl]-3-methyl-1H-pyrazol-5-carboxylic acid methyl ester (13) (R’ = Me)
3-Methyl-4-nitro-1H-pyrazol-5-carboxylic acid methyl ester 11 (13.10 g, 71 mmol) was dissolved in MeOH (150 mL) and hydrogenated (4 bar H2) for 2 h using 10 % palladium on charcoal (1.00 g) as catalyst. The resulting suspension was filtered through Celite, the solvent was removed in vacuo and the residue was taken up in water (40 mL) and concentrated hydrochloric acid (18 mL). After cooling to 0 °C a solution of sodium nitrite (4.90 g, 71 mmol) in water (10 mL) was added dropwise. The reaction was stirred for 10 min at 0 °C. Then a 40 % aqueous solution of dimethylamine was added dropwise until pH = 9 was reached. The ice bath was removed and the reaction mixture was stirred for additional 10 min. The product was collected by suction, washed with water and dried overnight at 60 °C under vacuum. 7.32 g (48%) of the target compound were obtained as a solid.
MS (ESIpos): m/z = 212 (M+H)+.
1H-NMR (300 MHz, DMSO-d6): 2 sets of signals in a ratio of 1.5:1. Signal set 1: δ = 2.23 (s, 3H), 3.0 - 3.4 (s br, 6H), 3.72 (s, 3H), 13.03 (s br, 1H). Signal set 2: δ = 2.20 (s, 3H), 3.0 - 3.4 (s br, 6H), 3.78 (s, 3H), 13.24 (s br, 1H).
4-[3,3-Dimethyltriaz-1-en-1-yl]-1H-pyrazol-5-carboxylic acid methyl ester (13) (R’ = H) was synthesized analogously.

Dipotassium 4-[3,3-dimethyltriaz-1-en-1-yl]-3-methyl-1H-pyrazol-1-yl-5-carboxylate (14) (R’=Me)
Water (330 µL) was added dropwise to a suspension of potassium tert-butylate (2.06 g, 18 mmol) in dry
THF (20 mL). Then the mixture was stirred for 10 min at rt. 4-[3,3-Dimethyltriaz-1-en-1-yl]-3- methyl-1H-pyrazol-5-carboxylic acid methyl ester (1.29 g, 6.0 mmol) was added and stirring was continued for additional 10 min. The solvent was evaporated, and the resulting product was used without further purification.
Dipotassium 4-[3,3-dimethyltriaz-1-en-1-yl]-1H-pyrazol-1-yl-5-carboxylate (14) (R’ = H) was synthesized analogously.

3-(4-Chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (16f)
Dipotassium 4-[3,3-dimethyltriaz-1-en-1-yl]-1H-pyrazol-1-yl-5-carboxylate (32 mg, 0.1 mmol) and triethylammonium chloride (41 mg, 3.0 mmol) were dissolved in DMF (0.8 mL) and the mixture was stirred for 15 min. PyBOP (62 mg, 0.12 mmol) was added followed by 1-(4-chlorophenyl)methanamine (42 mg, 0.3 mmol) and the solution was stirred for 2 h at rt. Acetic acid (0.5 mL) was added, and the reaction mixture was stirred for additional 16 h at ambient temperature. The suspension was diluted with DMF, filtered and purified by preparative HPLC. The target compound was obtained as a solid: 8 mg (30 %).
LC/MS method 4: Rt = 2.13 min, m/z = 262 (M+H)+.
1H-NMR (400 MHz, DMSO-d6): δ = 5.58 (s, 2H), 7.37 (d, 2H), 7.41 (d, 2H), 8.81 (s br, 1H), 15.01 (s br, 1H).
All examples in Table a were prepared analogously and characterized by LC/MS method 1.

7-Bromo-3-(4-chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (17)
3.5 g (13.3 mmol) 3-(4-chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one 16f and 2.6 g (14.6 mmol) N-bromosuccinimid were suspended in 50 mL anhydrous MeCN and the reaction mixture was stirred for 1 h at 60 °C. The reaction was quenched by addition of 25 mL water and the solvent was removed in vacuo partially causing the product to crystallize. The solid was filtered off and washed with water. 4.16 g (92%) of the target compound was obtained.
LC/MS method 2: Rt = 2.16 min, m/z = 341 (M+H)+.
1H-NMR (300 MHz, DMSO-d6): δ = 5.60 (s, 2H), 7.34 - 7.44 (m, 4H), 15.40 (s br, 1H).

7-Bromo-3-(4-chlorobenzyl)-5-propyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (18)
7-Bromo-3-(4-chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (97 mg, 0.3 mmol), 1-iodopropane (49 mg, 0.3 mmol) and potassium carbonate (79 mg, 0.6 mmol) were stirred overnight in DMF (4 mL) at rt. The reaction mixture was separated by preparative HPLC. 65 mg (59%) of the target compound were obtained as a solid.
LC/MS method 2: Rt = 2.77 min, m/z = 384 (M+H)+.
1H-NMR (400 MHz, DMSO-d6): δ = 7.42 (d, 2H), 7.39 (d, 2H), 5.57 (s, 2H), 4.51 (t, 2H), 1.86 (d tr, 2H), 0.84 (t, 3H).

7-(3-Nitrophenyl)-3-(4-chlorobenzyl)-5-propyl-3,5-dihydro-4H-pyrazolo[4,3d][1,2,3] triazin-4-one (19i)
7-Bromo-3-(4-chlorobenzyl)-5-propyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (151 mg, 0.394 mmol), 3-nitrophenylboronic acid (82 mg, 0.493 mmol), tetrakis(triphenylphosphino)palladium (28.5 mg, 0.025 mmol) and sodium carbonate (104 mg, 0.985 mmol) were mixed in dioxane (2.5 mL) and water (0.5 mL). The reaction mixture was stirred overnight at 85 °C. Yield according to LC/MS method 2 49 %.
All examples in Table b were prepared analogously and characterized by LC/MS method 1.

3-(2,3-dichlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (20)
9.97 g (63.5 mmol) 4-nitro-1H-pyrazole-5-carboxylic acid, 10.16 g (57.7 mmol) 1-(2,3-dichlorophenyl)- methanamine, 14.38 g (75.0 mmol) N-[3-(dimethylamino)propyl]-N'-ethylcarbodiimide hydrochloride and 9.36 g (69.2 mmol) 1H-benzotriazol-1-ol were dissolved in 380 mL of anhydrous DMF. The reaction was stirred over night before the solvent was removed in vacuo. After the residue has been taken up with water, the resulting suspension was poured onto a mixture of ice in water acidified by concentrated hydrochloric acid. The suspension was allowed to reach rt, then the solid was collected by filtration. After washing with water, recrystallization from EtOH and washing with Et2O, 16.34 g (89 %) of N-(2,3-dichlorobenzyl)-4-nitro-1H-pyrazole-5-carboxamide was obtained.
1.167 g 10 % palladium on charcoal was suspended in 86 mL water and cooled with an ice bath before being treated with a suspension of 3.924 g (103.7 mmol) sodium borohydride in 86 mL water and 86 mL MeOH. 16.34 g (51.9 mmol) N-(2,3-dichlorobenzyl)-4-nitro-1H-pyrazole-5-carboxamide were added portionwise over a period of 10 min to the reducing mixture. The reaction was slowly allowed to reach rt, stirred for 1 h then again cooled to 0 °C. 1.962 g (51.9 mmol) sodium borohydride was added and the ice bath was removed. After the mixture has reached rt, it was filtered through celite. The solvent of the filtrate was removed in vacuo and the residue was washed with water. 10.87 g (74 %) 4-amino-N-(2,3-dichlorobenzyl)-1H-pyrazole-5-carboxamide was obtained.
10.87 g (38.1 mmol) 4-amino-N-(2,3-dichlorobenzyl)-1H-pyrazole-5-carboxamide was dissolved in 61 mL DMF, 61 mL water and 122 mL acetic acid before being cooled to 0 °C. 3.288 g (47.7 mmol) sodium nitrite was added and the reaction was stirred over night at ambient temperature. The solvent was removed in vacuo and the residue was taken up with water. The solid was collected by filtration and washed with water. 10.23 g (91 %) of the title compound was obtained.
HPLC method 1: Rt = 4.31 min
MS (CIpos): m/z = 313 (M+NH4)+.
1H-NMR (400 MHz, DMSO-d6): 4.85 (s, 2H), 6.25 (d, 1H), 6.45 (t, 1H), 6.74 (d, 1H), 7.93 (s br, 1H)

7-Bromo-3-(2,3-dichlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (21)
9.00 g (30.4 mmol) 3-(2,3-dichlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one and 5.95 g (33.4 mmol) N-bromosuccinimid were suspended in 115 mL anhydrous MeCN and the reaction mixture was stirred for 2 h at 60 °C. The reaction was quenched by addition of 57 mL water and the solvent was removed in vacuo. The solid was filtered off and washed with water. 10.4 g (91%) of the target compound was obtained.
HPLC method 1: Rt = 4.75 min
1H-NMR (300 MHz, DMSO-d6): δ = 5.70 (s, 2H), 7.17 (d, 1H), 7.31 (t, 1H), 7.63 (d, 1H), 15.46 (s br, 1H).

7-Bromo-3-(2,3-dichlorobenzyl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-pyrazolo[4,3-d]- [1,2,3]triazin-4-one (22)
1.00 g (2.67 mmol) 7-bromo-3-(2,3-dichlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one 21 and 1.39 g (8.36 mmol) [2-(chloromethoxy)ethyl](trimethyl)silane were dissolved in 8.40 mL anhydrous DMF followed by a portionwise addition of 304 mg (7.60 mmol) sodium hydride (60% w/w) at 0 °C. The reaction mixture was then stirred at rt for 30 min before being quenched with saturated aqueous sodium hydrogen carbonate solution. EtOAc was added, the organic phase was separated, washed with water and brine, dried over magnesium sulfate, and evaporated in vacuum. Purification by preparative HPLC yielded 904 mg (67%) of the target compound.
MS (CIpos): m/z = 521 (M+NH4)+.
1H-NMR (300 MHz, DMSO-d6): δ = -0.11 (s, 9H), 0.82 (t, 2H), 3.60 (t, 2H), 5.71 (s, 2H), 5,80 (s, 2H), 7.22 (dd, 1H), 7.30 (t, 1H), 7.62 (dd, 1H).
7-(3-{[tert-Butyl(dimethyl)silyl]oxy}prop-1-in-1-yl)-3-(2,3-dichlorobenzyl)-5-{[2(trimethylsilyl)-ethoxy]- methyl}-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (23a)
500 mg (0.99 mmol) 7-bromo-3-(2,3-dichlorobenzyl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro- 4H-pyrazolo[4,3-d][1,2,3]triazin-4-one 22 were dissolved in 6.20 mL of anhydrous DMF together with 75 mg (0.40 mmol) copper(I) iodide and successively treated with 1.69 g (9.90 mmol, 2.01 mL) tert-butyl(dimethyl)(prop-2-in-1-yloxy)silane, 200 mg (1.98 mmol, 0.28 mL) triethylamine and 229 mg (0.20 mmol) tetrakis(triphenylphosphin)palladium(0). The mixture was stirred for 6 days at rt before being quenched with water and diluted with EtOAc. The organic phase was separated, washed with brine, dried over magnesium sulfate, filtered and evaporated to dryness. Purification by preparative HPLC yielded 301 mg (51%) of the target compound.
MS (ESIpos): m/z = 594 (M+H)+.
1H-NMR (300 MHz, DMSO-d6): δ = -0.13 (s, 9H), 0.15 (s, 6H), 0.81 (t, 2H), 0.89 (s, 9H), 3.60 (t, 2H), 4.69 (s, 2H), 5.71 (s, 2H), 5.81 (s, 2H), 7.20 (dd, 1H), 7.30 (t, 1H), 7.62 (dd, 1H).

3-Benzyl-7-(phenylethynyl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]- triazin-4-one (23b)
Preparation as in the case of 23a. 43 mg (42%) of the target compound were obtained.
MS (CIpos): m/z = 458 (M+H)+.
HPLC method 2: Rt = 5.99 min
1H-NMR (300 MHz, DMSO-d6): δ = -0.01 (s, 9H), 0.94 (t, 2H), 3.73 (t, 2H), 5.74 (s, 2H), 5.95 (s, 2H), 7.38 - 7.49 (m, 5H), 7.57 - 7.61 (m, 3H), 7.76 - 7.81 (m, 2H).

3-Benzyl-5-{[2-(trimethylsilyl)ethoxy]methyl}-7-[(trimethylsilyl)ethinyl]-3,5-dihydro-4H-pyrazolo-
[4,3-d][1,2,3]triazin-4-one (23c)
Preparation as in the case of 23a. 370 mg (61%) of the target compound were obtained.
MS (CIpos): m/z = 454 (M+H)+.
HPLC method 2: Rt = 5.88 min
1H-NMR (300 MHz, DMSO-d6): δ = -0.12 (s, 9H), 0.29 (s, 9H), 0.81 (t, 2H), 3.58 (t, 2H), 5.60 (s, 2H), 5.80 (s, 2H), 7.26 - 7.34 (m, 5H).

3-Benzyl-7-(3-{[tert-butyl(dimethyl)silyl]oxy}prop-1-yn-1-yl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5- dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (23d)
Preparation as in the case of 23a. 1.35 g (56%) of the target compound were obtained.
HPLC method 2: Rt = 6.20 min
1H-NMR (300 MHz, DMSO-d6): δ = 0.00 (s, 9H), 0.28 (s, 6H), 0.93 (t, 2H), 1.02 (s, 9H), 3.73 (t, 2H), 4.81 (s, 2H), 5.73 (s, 2H), 5.93 (s, 2H), 7.42 - 7.47 (m, 5H).

3-(4-Chlorobenzyl)-7-[(4-methoxyphenyl)ethinyl]-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (23e)
Preparation as in the case of 23a. 276 mg (50%) of the target compound were obtained.
LC/MS method 3: Rt = 3.44 min, m/z = 521
1H-NMR (400 MHz, DMSO-d6): δ = -0.12 (s, 9H), 0.82 (t, 2H), 3.60 (t, 2H), 3.82 (s, 3H), 5.61 (s, 2H), 5.82 (s, 2H), 7.05 (d, 2H), 7.39 (s, 4H), 7.62 (d, 2H).

3-(4-Chlorobenzyl)-7-(cyclohexylethinyl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-
pyrazolo[4,3-d][1,2,3]triazin-4-one (23f)
Preparation as in the case of 23a. 491 mg (93%) of the target compound were obtained.
MS (MSES+): m/z = 499 (M+H)+.
HPLC method 2: Rt = 6.80 min
1H-NMR (300 MHz, DMSO-d6): δ = -0.14 (s, 9H), 0.80 (t, 2H), 1.25 - 1.84 (m, 11H), 3.56 (t, 2H), 5.58 (s, 2H), 5.77 (s, 2H), 7.38 (s, 4H).

Methyl 2-{[3-(4-chlorobenzyl)-4-oxo-5-{[2-(trimethylsilyl)ethoxy]methyl}-4,5-dihydro-3H-pyrazolo- [4,3-d][1,2,3]triazin-7-yl]ethinyl}benzoate (23g)
Preparation as in the case of 23a. 354 mg (61%) of the target compound were obtained.
LC/MS method 4: Rt = 3.44 min, m/z = 549
1H-NMR (400 MHz, DMSO-d6): δ = 0.00 (s, 9H), 0.95 (t, 2H), 3.73 (t, 2H), 4.03 (s, 3H), 5.74 (s, 2H), 5.96 (s, 2H), 7.51 (s, 4H), 7.75 (dt, 1H), 7.83 (dt,1H), 7.94 (d br, 1H), 8.11 (d br, 1H).

Methyl 2-{[3-(2,3-dichlorobenzyl)-4-oxo-5-{[2-(trimethylsilyl)ethoxy]methyl}-4,5-dihydro-3H-pyrazolo- [4,3-d][1,2,3]triazin-7-yl]ethinyl}benzoate (23h)
Preparation as in the case of 23a. 270 mg (62%) of the target compound were obtained.
LC/MS method 3: Rt = 3.46 min, m/z = 583
1H-NMR (400 MHz, DMSO-d6): δ = 0.00 (s, 9H), 0.95 (t, 2H), 3.74 (t, 2H), 4.02 (s, 3H), 5.84 (s, 2H), 5.97 (s, 2H), 7.33 (d br, 1H), 7.41 (t, 1H), 7.75 (dt, 2H), 7.81 (dt, 1H), 7.93 (d br, 1H), 8.10 (d br, 1H).

3-(4-chlorobenzyl)-7-[3-(dimethylamino)prop-1-yn-1-yl]-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-
dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (23i)
Preparation as in the case of 23a. 317 mg (63%) of the target compound were obtained.
MS (CIpos): m/z = 473.
HPLC method 2: Rt = 4.74 min
1H-NMR (300 MHz, DMSO-d6): δ = 0.12 (s, 9H), 0.92-1.01 (m, 2H), 2.41 (s, 6H), 3.67 - 3.74 (m, 2H), 3.75 (s, 2H), 5.72 (s, 2H), 5.92 (s, 2H), 7.51 (s, 4H).

7-Bromo-3-benzyl-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4- one (24) (Ar = Ph)
6g (38.2 mmol) commercially available 4-nitro-3-pyrazolcarboxylic acid and 19.887 g (38.2 mmol) PyBOP were dissolved in 100 mL DMF before being treated with 4.1 g (38.2 mmol) benzylamine and N,N-diisopropylethylamine. The reaction was stirred over night, concentrated in vacuo and poured on a mixture of concentrated HCl, ice and water. The precipitate was collected and washed with water. Recrystallization from ethanol afforded 7.83 g (77%) of N-benzyl-4-nitro-1H-pyrazole-5-carboxamide.
0.715 g 10 % palladium on charcoal was suspended in 50 mL water and cooled with an ice bath before being treated with a suspension of 2.4 g (63.4 mmol) sodium borohydride in 50 mL water and 50 mL MeOH. 7.8 g (31.7 mmol) N-benzyl-4-nitro-1H-pyrazole-5-carboxamide were added portionwise over a period of 10 minutes to the reducing mixture. The reaction was slowly allowed to reach rt, stirred for 1.5 h and filtered through celite. The solvent of the filtrate was removed in vacuo and the residue was washed with water. 7.55 g of the residue were obtained which were taken in the subsequent reaction without further drying. 7.55 g (34.914 mmol) of the solid were taken up with a mixture of 30 mL of water, 30 mL of DMF and 60 mL of acetic acid before being cooled to 0°C. 3.011 g (43.6 mmol) sodium nitrite was added and the reaction was stirred for 30 min. The ice bath was removed and stirring continued for 3 h at rt. The solvent was removed in vacuo and the residue was taken up with water. The solid was collected by filtration and washed with water. 6.6 g (81 %) 3-benzyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-　4-one was obtained.
6.72 g (29.6 mmol) 3-benzyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one and 5.8 g (32.5 mmol) N-bromosuccinimid were suspended in acetonitrile and heated to 60 °C for 45 min. Water was added and solvents were reduced in vacuo until precipitation occurs. The precipitate was collected, washed with water and dried at 60 °C in vacuo. 7.67 g (85 %) of 7-Bromo-3-benzyl-3,5-dihydro-4H-pyrazolo- [4,3-d][1,2,3]triazin-4-one were obtained.
2.67 g (8.72 mmol) 7-bromo-3-benzyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one and 4.8 g (28.8 mmol) [2-(chloromethoxy)ethyl](trimethyl)silane were dissolved in 29 mL anhydrous DMF followed by a portionwise addition of 1.05 g (26.2 mmol) sodium hydride (60% w/w) at 0 °C. The reaction mixture was then stirred at rt for 30 min before being quenched with saturated aqueous sodium hydrogen carbonate solution at 0 °C. EtOAc was added, the organic phase was separated, washed with water and brine, dried over magnesium sulfate, and evaporated in vacuum. Purification by preparative HPLC yielded 2.74 g (72%) of the target compound.
MS (CIpos): m/z = 436 (M+NH4)+.
1H-NMR (300 MHz, DMSO-d6): δ = -0.11 (s, 9H), 0.82 (t, 2H), 3.60 (t, 2H), 5.60 (s, 2H), 5.80 (s, 2H), 7.23 – 7.43 (m, 5H).

7-Bromo-3-(4-chlorobenzyl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-pyrazolo[4,3-d]-
[1,2,3]triazin-4-one (24) (Ar = p-ClPh)
3.5 g (8.72 mmol) 7-bromo-3-(4-chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one 17 and 5.65 g (33.9 mmol) [2-(chloromethoxy)ethyl](trimethyl)silane were dissolved in 34 mL anhydrous DMF followed by a portionwise addition of 1.233 g (30.8 mmol) sodium hydride (60% w/w) at 0 °C. The reaction mixture was then stirred at rt for 30 min before being quenched with saturated aqueous sodium hydrogen carbonate solution at 0 °C. EtOAc was added, the organic phase was separated, washed with water and brine, dried over magnesium sulfate, and evaporated in vacuum. Purification by preparative HPLC yielded 3.66 g (76%) of the target compound.
MS (CIpos): m/z = 487 (M+NH4)+.
1H-NMR (300 MHz, DMSO-d6): δ = -0.12 (s, 9H), 0.81 (t, 2H), 3.58 (t, 2H), 5.59 (s, 2H), 5.79 (s, 2H), 7.38 (s, 4H).

3-(2,3-Dichlorobenzyl)-7-(3-hydroxyprop-1-in-1-yl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (26a)
293 mg (0.49 mmol) 7-(3-{[tert-butyl(dimethyl)silyl]oxy}prop-1-in-1-yl)-3-(2,3-dichlorobenzyl)-5- {[2(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one 23a were dissolved in 4.10 mL CH­­­2Cl2 at 0 °C and treated with 4.10 mL trifluoroacetic acid. The mixture was stirred overnight at rt before being evaporated to dryness. A solution of 202 mg (2.46 mmol) sodium acetate in 0.75 mL water and 7.5 mL EtOH was added to the residue. The mixture was again stirred overnight, reduced and purified by preparative HPLC. 95 mg (55%) of the target compound were obtained.
MS (ES+): m/z = 351 (M+H)+.
HPLC method 3: Rt = 4.21 min
1H-NMR (400 MHz, DMSO-d6): δ = 4.44 (d, 2H), 5.59 (t, 1H), 5.70 (s, 2H), 7.14 (d, 1H), 7.31 (t, 1H), 7.62 (d, 1H).

3-Benzyl-7-phenylethinyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (26b)
For preparation see example 26a. 112 mg (70%) of the target compound were obtained.
MS (CIpos): m/z = 345 (M+NH4)+.
1H-NMR (400 MHz, DMSO-d6): δ = 5.62 (s, 2H), 7.27 - 7.39 (m, 5H), 7.47 - 7.56 (m, 3H), 7.65 - 7.70 (m, 2H), 15.44 (s br, 1H).

3-Benzyl-7-trimethylsilylethinyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (26c)
For preparation see example 26a. 95 mg (53%) of the target compound were obtained.
MS (ES+): m/z = 324 (M+H)+.
HPLC method 2: Rt = 5.02 min
1H-NMR (300 MHz, DMSO-d6): δ = 0.30 (s, 9H), 5.60 (s, 1H), 7.30 - 7.40 (m, 5H), 15.39 (s br, 1H).

3-Benzyl-7-(3-hydroxyprop-1-in-1-yl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (26d)
For preparation see example 26a. 300 mg (62%) of the target compound were obtained.
MS (ES+): m/z = 282 (M+H)+.
HPLC method 1: Rt = 3.73 min
1H-NMR (300 MHz, DMSO-d6): δ = 4.44 (d, 2H), 5.58 (t, 1H), 5.60 (s, 2H), 7.25 - 7.39 (m, 5H), 15.33 (s br, H).

3-(4-Chlorobenzyl)-7-cyclohexylethinyl-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (26e)
For preparation see example 26a. 50 mg (14%) of the target compound were obtained.
MS (ES+): m/z = 368 (M+H)+.
HPLC method 3: Rt = 5.03 min
1H-NMR (300 MHz, DMSO-d6): δ = 1.30 – 1.46 (m, 3H), 1.46 – 1.62 (m, 3H), 1.63 – 1.78 (m, 2H), 1.82 – 1.94 (m, 2H), 2.82 (tt, 1H), 5.57 (s, 2H), 7.32 - 7.44 (AA’BB’-system, 4H), 15.18 (s br, 1H).

Methyl 2-{[3-(4-chlorobenzyl)-4-oxo-4,5-dihydro-3H-pyrazolo[4,3-d][1,2,3]triazin-7-yl]ethinyl}- benzoate (26f)
For preparation see example 26a. 102 mg (39%) of the target compound were obtained.
MS (ES+): m/z = 420 (M+H)+.
HPLC method 3: Rt = 4.92 min
1H-NMR (300 MHz, DMSO-d6): δ = 3.92 (s, 3H), 5.62 (s, 2H), 7.36 - 7.46 (AA’BB’-system, 4H), 7.64 (td, 1H), 7.73 (td, 1H), 7.83 (dd, 1H’), 8.01 (dd, 1H), 15.49 (s br, 1H).

Methyl 2-{[3-(2,3-dichlorobenzyl)-4-oxo-4,5-dihydro-3H-pyrazolo[4,3-d][1,2,3]triazin-7-yl]ethinyl}- benzoate (26g)
For preparation see example 26a. 135 mg (67%) of the target compound were obtained.
MS (ES+): m/z = 456 (M+2H)+.
HPLC method 3: Rt = 5.04 min
1H-NMR (400 MHz, DMSO-d6): δ = 3.93 (s, 3H), 5.72 (s, 2H), 7.18 (d, 1H), 7.63 (d, 1H), 7.63 (t, 1H), 7.73 (t, 1H), 7.83 (d, 1H), 8.01 (d, 1H), 15.53 (s br, 1H).

3-(4-Chlorobenzyl)-7-(4-methoxyphenylethinyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (26h)
For preparation see example 26a. 41 mg (20%) of the target compound were obtained.
MS (ES+): m/z = 392 (M+H)+.
HPLC method 1: Rt = 4.96 min
1H-NMR (400 MHz, DMSO-d6): δ = 3.83 (s, 3H), 5.61 (s, 2H), 7.06 (d, 2H), 7.36 - 7.44 (AA’BB’-System, 4H), 7.62 (d, 2H), 15.38 (s br, 1H).

7-Iodo-3-(4-chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one (27)
500 mg (1.911 mmol) 3-(4-chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one 16f and 1.12 g (4.97 mmol) N-iodosuccinimide were suspended in 68 mL anhydrous MeCN, and the reaction mixture was stirred overnight at 80 °C. The solvent was evaporated and the residue was taken up in EtOAc. The solution was washed with water and brine before being concentrated in vacuo. Purification of the solid by preparative HPLC yielded 183 mg (25%) of the target compound.
LC/MS method 2: Rt = 2.01 min, m/z = 388 (M+H)+.
1H-NMR (400 MHz, DMSO-d6): δ = 5.60 (s, 2H), 7.39 (s, 4H).

7-Iodo-3-(4-chlorobenzyl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]- triazin-4-one (28)
750 mg (1.94 mmol) 7-iodo-3-(4-chlorobenzyl)-3,5-dihydro-4H-pyrazolo[4,3-d][1,2,3]triazin-4-one 27 and 720 mg (4.26 mmol) [2-(chloromethoxy)ethyl](trimethyl)silane were dissolved in 6.3 mL anhydrous DMF followed by a portionwise addition of 155 mg (3.87 mmol) sodium hydride (60% w/w) at 0 °C. The reaction mixture is then stirred at rt for 30 min before being quenched with saturated aqueous sodium hydrogen carbonate solution. EtOAc and water were added. The organic phase was separated and extracted additional twice with EtOAc. The combined organic extracts were washed with brine, dried over magnesium sulfate, and the solvent was distilled off under reduced pressure. Purification of the
crude product by preparative HPLC yielded 540 mg (54%) of the target compound.
MS (ES+): m/z = 518 (M+H)+.
HPLC method 3: Rt = 5.88 min
1H-NMR (400 MHz, DMSO-d6): δ = 0.00 (s, 9H), 0.93 (t, 2H), 3.70 (t, 2H), 5.71 (s, 2H), 5.92 (s, 2H), 7.51 (s, 4H).

Methyl 3-(4-chlorobenzyl)-4-oxo-5-{[2-(trimethylsilyl)ethoxy]methyl}-4,5-dihydro-3H-pyrazolo[4,3-d]- [1,2,3] triazine-7-carboxylate (29)
160 mg (0.309 mmol) 7-iodo-3-(4-chlorobenzyl)-5-{[2-(trimethylsilyl)ethoxy]methyl}-3,5-dihydro-4H- pyrazolo[4,3-d][1,2,3]triazin-4-one 28, 188 mg (1.85 mmol) N,N-diethylethanamine, 1.19 g (37.0 mmol) dry MeOH, 6 mg (0.028 mmol) palladium(II) acetate and 15 mg (0.056 mmol) triphenylphosphine were dissolved in 1.3 mL anhydrous DMF. Carbon monoxide was bubbled slowly through the mixture for 2 min before a balloon containing carbon monoxide was put on the reaction vessel. The mixture was stirred overnight, evaporated to dryness and purified by preparative HPLC. 108 mg (78%) of the target compound was obtained.
MS (ES+): m/z = 450 (M+H)+.
HPLC method 2: Rt = 5.39 min
1H-NMR (400 MHz, DMSO-d6): δ = -0.13 (s, 9H), 0.81 (t, 2H), 3.59 (t, 2H), 3.96 (s, 3H), 5.62 (s, 2H), 5.89 (s, 2H), 7.39 (s, 4H).

Methyl 3-(4-chlorobenzyl)-4-oxo-4,5-dihydro-3H-pyrazolo[4,3-d][1,2,3]triazine-7-carboxylate (30)
102 mg (0.227 mmol) methyl 3-(4-chlorobenzyl)-4-oxo-5-{[2-(trimethylsilyl)ethoxy]methyl}-4,5- dihydro-3H-pyrazolo[4,3-d][1,2,3]triazine-7-carboxylate 29 were dissolved in 1.9 mL CH2Cl2 and 1.9 mL trifluoroacetic acid at 0 °C. The mixture was stirred for 2 h at rt before being concentrated in vacuo. The residue was taken up with a solution of 93 mg (1.1 mmol) sodium acetate in 3.6 mL EtOH and 0.36 mL water. After stirring overnight the reaction was purified by preparative HPLC. 31 mg (43%) of the target compound were obtained.
MS (MSES+): m/z = 319 (M+H)+.
HPLC method 1: Rt = 4.19 min
1H-NMR (400 MHz, DMSO-d6): δ = 3.11 (s, 3H), 4.76 (s, 2H), 6.54 (s, 4H).

3-(4-Chlorobenzyl)-4-oxo-4,5-dihydro-3H-pyrazolo[4,3-d][1,2,3]triazine-7-carboxylic acid (31)
15 mg (0.047 mmol) Methyl 3-(4-chlorobenzyl)-4-oxo-4,5-dihydro-3H-pyrazolo[4,3-d][1,2,3]triazine-7- carboxylate 30 were dissolved in 0.4 mL THF and 0.3 mL water. After addition of 12 mg (0.206 mmol) potassium hydroxide the mixture was stirred overnight, diluted with water, acidified with 2N HCl to pH = 2 and chromatographed by preparative HPLC. 6 mg (42%) of the target compound was obtained.
HPLC method 3: Rt = 3.83 min
1H-NMR (400 MHz, DMSO-d6): δ = 5.59 (s, 2H), 7.39 (s, 4H).

ACKNOWLEDGEMENTS
We thank L. Baerfacker and H. Schirok for valuable scientific discussion. Furthermore, we thank S. Vogt for her contribution to the experimental section of the manuscript.

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