Purine derivatives are widely found in nature. They have a lot of biological and pharmaceutical activities. Among them, C-2, N-9 substituted 6-benzylaminopurine derivatives are cyclin-dependent kinase inhibitor . 6-Alkylamino-9-benzyl-9H-purines are a class of anticonvulsant agents [103a]. 9-benzyl-6-dimethylamino-9H-purines have antirhinovirus activity [103b]. 2,6,9-Trisubstituted purines are selective CDK1 inhibitors . 9-Benzylpurines are active against macobacterium tuberculosis . 6-Alkenyl- and 6-alkynylpurines have cytostatic activity [106, 107]. 2-Alkynylpurines have inhibitory activity on platelet aggregation  and potent antihypertensive effects . 9-Substituted purines are potent antiparasitic agents , high selective sulfotransferase inhibitors , and exhibit HIV-1 infectivity . Furthermore, modified purines containing carbon substituents in the 2-, 6-, or 8-position are associated with interesting biological properties.
Purine is composed of a pyrimidine ring and an imidazole ring. The pyrimidine part is π-electron-deficient, and then nucleophilic displacement takes place more readily at 6-position than at 2-position. The same is observed in the oxidative addition to Pd(0), even purines with the otherwise weakll reactive chloro leaving group at position 6 can undergo Pd-catalyzed cross-coupling reactions. 8-halo-groups could also be substituted in this way. In our present work, Pd-catalyzed cross-coupling reactions were used to introduce aryl and in particular aminoalkynyl substituent into the purine skeleton in order to establish compounds of the general structure 8 as potential calcineurin inhibitors.
Using metal-catalyzed coupling reactions of halopurines is the best choice to realize C-C bond formations in different positions. There are many reports about the coupling reactions of organozinc, organotin, organoboronic and Grignard reagents.
(1) Suzuki coupling
Halopurines can undergo cross-coupling reactions with arylboronic acid, to introduce aryl groups into 2-, 6- or 8-position. When 9-benzyl-6-bromopurine 132 and 6-amino-9-benzyl-8-bromopurine 133 reacted with a series of aryl boronic acid , 134 and 135 were obtained. (Scheme 3.1)
2,6-Dihalopurine 136, 137 and 6,8-dihalopurine 138 can undergo regioselective Suzuki cross-coupling reactions, when equal equivalents of phenylboronic acid was used [113, 114]. 139, 140, 141 were obtained in good yields. (Scheme 3.2)
(2) Stille coupling
Halopurines can undergo cross-coupling reactions with organotin reagents. Aryl, alkenyl, alkynyl and alkyl groups were introduced into 2-, 6- or 8-position in this way[115-117], giving access to products 144, 145, and 146.(Scheme 3.3)
2,6-Dihalopurine and 6,8-dihalopurine can undergo regioselective Stille cross-coupling reactions, when one equivalent of organotin reagent was used [118-120]. (Scheme 3.4 and Scheme 3.5) Again, the regioselectivity depends on either the type of halo-leaving groups or the positions.
(3) Negishi coupling
9-Benzyl-6-chloropurine and 7-benzyl-6-chloropurine were shown to undergo Negishi cross-coupling reactions with phenyl-, benzyl- and alkylzinc reagents, and introduce phenyl-, benzyl- and alkyl group into the 6-position. 8-Chloro-6-phenyl-9-(tetrahydropyran-2-yl)purine reacted with benzylzinc chloride at 8-position [114, 115]. (Scheme 3.6)
2,6-Dihalopurine and 6,8-dihalopurine can undergo regioselective Negishi cross-coupling reactions, when one equivalent of organozinc reagent was used . (Scheme 3.7)
(4) Kumada coupling
Ni- and Pd-catalyzed cross-coupling reactions of halopurines with Grignard reagents were used to introduce aryl or alkyl group into 6- or 8-position [121, 122]. (Scheme 3.8)
6,8-Dichloro-9-(tetrahydropyran-2-yl)purine 238 reacted with one equivalent of methyl magnesium chloride by introducing a methyl group into the 8-position rather than into the 6-position. This violates the normal positional reactivity sequence . (Scheme 3.9)
(5) Coupling with alkylaluminium reagents
Halopurines can undergo cross-coupling reactions with trialkylalumiunium by introducing an alkyl group into 2-, 6- or 8-position . (Scheme 3.10):
|Scheme 3.10 Pd-catalyzed couplings of halopurines with trimethylaluminium|
(6) Sonogashira coupling
Alkynyl groups were introduced in into 2-, 6- or 8-position of halopurines by Sonogashira reactions with alkynes [108, 109, and 124]. (Scheme 3.11) Generally, high yields of products were obtained.
6-Chloro-2-iodopurine can undergo Sonogashira cross-coupling with alkynes regioselectively, and the 2-alkynyl substituted products 178were obtained . (Scheme 3.12)
Our strategy of synthesizing potential calcineurin inhibitors of the general structure 8 with purine as central heterocycle, was based on the assembly of halopurines with two aryl substituents, followed by the introduction of a functionalized side chain by Sonogashira coupling or nucleophilic substitution. The aryl groups were either directly connected to the purine rings or separated by a methylene group, i.e. a benzyl group.
Synthesis of purine derivatives started with 2,6-dichloropurine179 and commercially available 6-chloropurine180.
The former compound was obtained starting from adenine, which was readily oxidized with hydrogen peroxide in aqueous acetic acid to give adenine-1-N-oxide 181. The hydroxylation of the N-oxide was carried out with sodium nitrite in acetic acid (diazo-hydrolysis) to give hypoxanthine-1-N-oxide 182. 2,6-Dichloropurine 179 was obtained in low yield by treating 182 with phosphoryl chloride in the presence of catalytic amount of N,N-dimethyl aniline . (Scheme 3.13)
In order to introduce aryl groups into purine skeletons connected by a CH2- spacer, 2,6-dichloropurine 179 was N-alkylated with benzyl chloride in DMF in the presence of K2CO3. The corresponding 9-benzyl r137 and 7-benzyl isomer 183 were isolated  in yields of 44 % and 21 %, respectively. Similarly, 6-chloropurine 180 led to the two isomers 132and 155 in yields of 50 % and 23 %, respectively, after benzylation. (Scheme 3.14)
The N-benzylated chloropurines 137, 183 and 132 were further submitted to Suzuki reactions in order to introduce the second aryl group of the general target structure 8.
9-Benzyl-2,6-dichloro-purine 137 and 7-benzyl-2,6-dichloro-purine 183 reacted with one equivalent of phenylboronic acid in the presence of Pd(PPh3)4 and anhydrous K2CO3 to give the corresponding 6-phenyl products  140 and 184 in yields of 70 % and 66 %, respectively. (Scheme 3.15)
Using the same Suzuki coupling conditions, and starting from 9-benzyl-6-chloro-purine 132, 9-benzyl-6-phenylpurine 134a was obtained in 75% yield [70, 113]. (Scheme3.16)
In order to create a second reactive site at the purine skeleton, the chloropurine 132 was iodinated with NIS to give the 9-benzyl-6-chloro-8-iodo-purine 152 in a 46 % yield . Unexpectedly, this product 152 resisted regioselective Suzuki cross-coupling with one equivalent of phenylboronic acid to the envisaged 9-benzyl-6-chloro-8-phenylpurine 154. (Scheme 3.17)
We further introduced a bromo-substituent into the position-8 of the 9-benzyl-6-phenyl-purine 134a by bromination with NBS. While this reaction failed in DMF at room temperature, refluxing in THF for a long time was appropriate. (Scheme 3.18)
Introduction of a 3-dimethylaminopropyl chain into the 8-bromopurine 185 could be implemented by a two-steps sequence, i.e. first, by Sonogashira coupling of 185 with N,N-dimethyl propargylamine, followed by catalytic hydrogenation. Both steps gave good yields and provided an interesting product 187. (Scheme 3.19) 187 could help to answer the question, whether the separation of one of the two aryl groups from the core heterocycle by a CH2- spacer in the general structure 8 would effect the calcineurin inhibiting activity.
Attempts to introduce the 3-dimethylaminopropynyl chain into the 2-chloropurine 184 failed even under forcing Sonogashira conditions (Pd(PPh3)4, CuI, TEA, DMF, 100°C, 24 h). (Scheme 3.20)
As a structural alternative, a 3-dimethylaminopropylamino group could be readily introduced into the 2-position of the 2-chloropurine 184 by uncatalyzed nucleophilic substitution providing 189 in high yield. (Scheme 3.21)
In an analogous way, the 2-(3-dimethylaminopropylamino)-purine 190 and the 2-(2-dimethyl-aminoethoxy)-purine 191 could be obtained starting from the isomeric 2-chloropurine 140 and 3-dimethylaminopropylamine or 2-dimethylaminoethanol, respectively.Treatment of 140 with 3-dimethyl-amino-propane-1-thiol, did not lead to the desired product 192. (Scheme 3.22)
Quinoxaline and pteridine derivatives are very important nitrogen-containing heterocycles and have been widely used as pharmaceuticals. Quinoxaline derivatives have also been used as photoelectrochemical materials. Pyrido[2,3-b]pyrazine (5-azaquinoxaline) derivatives are the analogues of pteridine and quinoxaline, and have potential pharmaceutical activities and other applications [128-130].
We chose the pyrido[2,3-b]pyrazine system as core heterocycle in potential calcineurin inhibitors of the general structure 8.
A starting material 193 with two aryl substituents, suitable for the introduction of the side chains by Pd-catalyzed coupling, was easily on hand. Condensation of 5-bromo-2,3-diaminopyridine with benzil (1,2-diphenyl-ethane-1,2-dione) in refluxing ethanol, in the presence of hydrochloric acid, provided 7-bromo-2, 3-diphenylpyrido[2,3-b]pyrazine 193 in 73 % yield [131, 133]. (Scheme 3.23)
7-Bromo-2,3-diphenylpyrido[2,3-b]pyrazine 193 was used in uncatalyzed nucleophilic substitution before. For instance, Kumari  reported the nucleophilic substitution of 193 with secondary amines. A mixture of 7-substituted and 8-substituted products were obtained. Vinot  reported the reactions of 193 with organomagnesium reagents. When 193 reacted with phenylmagnesium bromide, 2,3,7-triphenylpyrido[2,3-b]pyrazine was obtained in 28 % yield, while 193 reacted with ethylmagnesium bromide, providing 6-ethyl-4,6-dihydro compound in 55 % by addition reaction. Armoand  reported the reduction of pyrido[2,3-b]pyrazine derivatives with NaBH4 to the corresponding 5,6-dihydro compound. To the best of our knowledge, there is no report about the cross-coupling [page 56↓]reaction of pyrido[2,3-b]pyrazine derivatives. Therefore, it is interesting to do some research in this field.
ω-Functionalized side chains could be introduced into 7-bromo-2,3-diphenylpyrido[2,3-b]pyrazine 193 by Sonagashira coupling with3-butyn-1-ol or N,N-dimethylpropargyl amine, catalyzed by Pd(PPh3)2Cl2 and CuI. 4-(2,3-Diphenyl-pyrido[2,3-b]pyrazin-7-yl)-but-3-yn-1-ol 194a,and[3-(2,3-diphenyl-pyrido[2,3-b]pyrazin-7-yl)-prop-2-ynyl]-dimethyl-amine 194b were obtained in high yields. Further catalytic hydrogenation of 194b provided [3-(2,3-diphenyl-pyrido[2,3-b]pyrazin-7-yl)-propyl]-dimethyl-amine 195 in 54 % yield. The aromatic heterocyclic ring was not affected under these reductive conditions. (Scheme 3.24)
As an alternative to the aforementioned Sonogashira/hydrogenation sequence, the introduction of aminoalkyl or hydroxyalkyl chains into purines was envisaged by Heck coupling followed by reduction. Thus, 7-bromo-2, 3-diphenylpyrido[2,3-b]pyrazine 193 could be coupled withmethyl acrylate, acrylonitrile or N-allyl-phthalimide, catalyzed by Pd(OAc)2/P(o-toly)3 in MeCN in the presence of TEA at 100 °C. The corresponding trans-alkene products 200, 201 and 202 were obtained in variable yields depending on the type of alkene. (Scheme 3. 25)
Since the known reaction of 7-bromo-2,3-diphenylpyrido[2,3-b]pyrazine with phenyl magnesium bromide afforded low yield of the corresponding substitution product 203 , we checked the suitability of the Suzuki reaction to introduce aryl groups into pyrido[2,3-b]pyrazine series.
7-Bromo-2, 3-diphenylpyrido[2,3-b]pyrazine 193 reacted with arylboronic acids, in the presence of Pd(PPh3)4 and K2CO3 in toluene, affording 2,3,7-triphenylpyrido[2,3-b]pyrazine 203 and 1-[4-(2,3-diphenyl-pyrido[2,3-b]pyrazin-7-yl)-phenyl]-ethanone 204 in yields of 96% and 50%, respectively. Thus, the Suzuki cross-coupling ismuch better than the aforementioned uncatalyzed reaction with phenyl Grignard reagent. (Scheme 3.26)
|Scheme 3.26 Suzuki cross-couplings of 7-bromo-2, 3-diphenylpyrido[2,3-b]pyrazine|
Kumuri  reported the substitution of 7-bromo-5-azaquinoxaline 193 with secondary amines. In case of morpholine, two isomers of products 196 and 197 were isolated, probably formed via elimination/addition mechanism. (Scheme 3.27)
We tried to achieve regioselective introduction of functionalized amines by Pd-catalysis. Using typical reaction conditions of Buchwald-Hartwig amination, 7-bromo-2,3-diphenylpyrido[2,3-b]pyrazine 193 was treated with 1-methyl-piperazine, catalyzed by Pd2(dba)3 and BINAP. Only one isomeric of amination product 7-(4-methyl-piperazin-1-yl)-2,3-diphenylpyrido[2,3-b] pyrazine 198 was obtained in 68 % yield. Similarly, when N,N-dimethyl-1,3-propan-diamine was used, N'-(2,3-diphenyl-pyrido[2,3-b]pyrazin-7-yl)-N,N-dimethylpropane-1,3-diamine 199 was obtained in 73 % yield. (Scheme 3.28)
Both products 198 and 199 fit into the general structure 8, where the side chain is connected to the core heterocycle via a nitrogen bridge. In the former case, the side chain is part of a saturated ring.
Imidazo[1,2-a]pyridine and imidazo[1,2-b]pyridazine derivative are analogues of purine and have potential pharmaceutical and biological activities [134-137].
Similar to purine, they are composed of a 6-membered N-heterocycle and an imidazole ring. The pyridine (pyrazine) ring is π-electron deficient, and so nucleophilic displacement takes place more readily. The imidazole ring is π-electron excessive and can easily undergo electrophilic substitution.
There are some reports about the nucleophilic substitutions at imidazo[1,2-a]pyridines and imidazo[1,2-b]pyridazines, however, only a few cross-coupling reactions are known.
Thus, 3-iodoimidazo[1,2-a]pyridine derivative 205  and 6-bromoimidazo[1,2-a]-pyridine derivative 206  underwent Suzuki cross-coupling reactions with phenylboronic acid to give 207 and 208. (Scheme 3.29)
3-Bromo-2-n-butylimidazo[1,2-a]pyridine 210 was coupled in a Negishi reaction with aryl methylzinc bromide to give 212, catalyzed by Pd2(dba)3/P(o-toyl)3 . Similarly, 6-chloro-2-substituted imidazo[1,2-b]pyridazine 211 was transformed into 213 with 4-chloro-1-butylzinc iodide under Pd(PPh3)2Cl2 catalysis . (Scheme 3.30 )
Furthermore, 6-bromoimidazo[1,2-a]pyridine 214 underwent Buchwald-Hartwig aminations with secondary or primary amines in the presence of Pd2(dba)3 and BINAP , providing a series of substituted 6-aminoimidazo[1,2-a]pyridines 215. (Scheme 3.31)
Gudmundson  reported the Heck coupling of 3-iodoimidazo[1,2-a]pyridines such as 216 with dihydrofuran in the presence of Pd(OAc)2-PhAs3-Ag2CO3 systems. (Scheme 3.32)
6-Bromo-2, 3-diphenylimidazo[1,2-a]pyridine 218 and 6-chloro-2,3-diphenylimidazo[1,2-b]pyridazine 219 were chosen as starting materials. These compounds possess purine analogous heterocycles with two peripheral phenyl groups. In order to achieve compounds of the general structure 8, the ω-functionalized side chains were introduced by Pd-catalyzed cross-couplings.
These two heterocycles 218 and 219 were prepared by condensation of 2-aminopyridine or 3-aminopyridazine with desyl bromide in a one-step procedure . (Scheme 3.33)
Further following up our strategies to introduce aminoalkyl groups into heterocycles by the sequence Sonogashira coupling/hydrogenation, we tried to submit the imidazo[1,2-a]pyridine 218 and imidazo[1,2-b]pyridazine 219 to this synthetic methodology.
6-Bromo-2,3-diphenylimidadazo[1,2-a]pyridine 218 reacted with N,N-dimethylpropargyl amine to give [3-(2,3-diphenyl-imidazo[1,2-a]pyrimidin-6-yl)-prop-2-ynyl]-dimethyl-amine 220 in 76% yield, using Pd(PPh3)2Cl2/CuI catalysis. When the reaction was catalyzed by Pd/C, PPh3 and CuI, the desired product could not be observed. Further catalytic hydrogenation of 220 led to the [3-(2,3-diphenyl-imidazo[1,2-a]pyrimidin-6-yl)-propyl]-dimethyl-amine 221 in 73 % yield. (Scheme 3.34)
The same sequence could be applied to 6-chloro-2,3-diphenylimidazo[1,2-b]pyridazine 219,giving access to the [3-(2,3-diphenyl-imidazo[1,2-a]pyrimidin-6-yl)-propyl]-dimethyl-amine 224 in high yield via [3-(2,3-diphenyl-imidazo[1,2-a]pyrimidin-6-yl)-prop-2-ynyl]-dimethyl-amine 223. (Scheme 3.35)
Using the catalytic system Pd2(dba)3/BINAP, 6-bromo-2,3-diphenylimidazo[1,2-a]pyridine 218 could react with morpholine, 1-methyl-piperazine or N,N-dimethyl-1,3-propan-diamine to give 6-morpholin-4-yl-2,3-diphenylimidazo[1,2-a]pyridine 225, 6-(4-methyl-piperazin-1-yl)-2,3-diphenylimidazo[1,2-a]pyridine 226,and N'-(2,3-diphenyl-imidazo[1,2-a]pyridin-6-yl)-N,N-dimethyl-propane-1,3-diamine 227 (Scheme 3.36). Unfortunately, the most interesting product 227, which fits best into the general structure 8, gave only lower yield.
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