Symmetrically and Unsymmetrically Bridged Methylenebis(allopurinols): Synthesis of Dimeric Potential Anti-Gout Drugs

Liquid-liquid phase transfer alkylation of 4-methoxy-pyrazolo[3,4-d]-pyrimidine (1a) with a dichloromethane/dibromomethane mixture (3:1, v/v) gave the regioisomeric methylenebis(heterocycles) 3a-5a. These were converted by dilute aqueous sodium hydroxide containing dimethylsulfoxide (DMSO) at concentrations between 0 and 60 vol-% into the methylenebis(allopurinols) 3b-5b by nucleophilic SNAr reactions at C(4). The effect of DMSO on the reaction kinetics was investigated.

In this paper we now report the synthesis of methylenebis(allopurinols) 3b-5b applying nucleophilic substitution reactions (S N Ar) on the corresponding 4-methoxy precursors 3a-5a, as well as the influence of dimethylsulfoxide on the reaction kinetics.
The methoxy group can subsequently undergo S N Ar nucleophilic displacement reactions, which are particularly advantageous for the synthesis of N(1)-and N(2)-glycosylated allopurinol nucleosides [13,16,17].Cleavage of the methoxy function with acid always bears the risk of concomitant Nglycosylic bond cleavage.In a number of methylation and glycosylation reactions of pyrrolo [2,3d]pyrimidines under liquid-liquid phase-transfer catalysis conditions with dichloromethane as solvent we observed a regioselective bridging of two chromophore moieties via N(7) by a methylene bridge [10].This reaction is then always favored when the alkyl halide is almost inert.
In contrast to this, the reaction of the heterocycle 1a with dichloromethane proceeded very slowly with formation of three reaction products.However, when dibromomethane was added to the reaction mixture compound 1a had disappeared completely with 60 min.Silica gel chromatography separated three methylenebis(4-methoxy-pyrazolo [3,4-d]pyrimidines) in a 8:16:1 ratio.Comparison of their 1 Hand 13 C-NMR spectra with those of 1a, as well as with those of its N(1)-and N(2)-methylated derivatives, revealed that the fastest migrating product is the N(1)-N(1) bridged compound 3a, as only one set of NMR signals could be observed.The second zone contained the unsymmetrically bridged compound 4a.In this case, the NMR spectra showed two sets of signals -one for the N(1)-and a second one for the N(2)-alkylated chromophore.The slowest migrating zone contained the N(2)-N(2) bridged compound 5a in low yield.This may be due to the low reactivity of the 2-nitrogen compared to N(1).b) The first value refers to the N(2)-alkylated and the second to the N(1)alkylated chromophore of compound 4a.
For unequivocal structural proof proton-coupled 13 C-NMR spectra of the three regioisomers 3a-5a were obtained (Table 1).While C(7a) of compound 3a appears as a complex multiplet, due to three 3 J(C,H) couplings with H-C(6), H-C(3) and CH 2 , the corresponding C(7a) of compound 5a shows only two 3 J(C,H) couplings with H-C( 6) and H-C(3) (12.8 and 6.8 Hz, respectively).Correspondingly, the CH 2 groups of both isomers exhibit different coupling patterns: the multiplicity of CH 2 of 3a is a triplet ( 1 J(C,H) = 155.4Hz), while that of 5a shows a triplet of triplets due to an additional 3 J(C,H) coupling with both H-C(3) atoms.In addition, the C(3) signal of the isomer 5a exhibits an additional fine splitting compared with that of 3a, due to a long range coupling ( 3 J(C,H)) with its methylene group.All these data confirmed the proposed structures of compounds 3a and 5a [18].
In the gated-decoupled 13 C-NMR spectrum of the unsymmetrically bridged isomer 4a the coupling patterns of both chromophore moieties behave additively.A positive indicator for the correct structure is the fact that only the C(3) signal of the N(2)-alkylated moiety shows an additional fine splitting due to a 3 J(C,H) coupling with CH 2 (3 Hz) while the corresponding C(3) signal of the N(1)-alkylated chromophore moiety exhibits only a doublet ( 1 J(C,H) = 196.2Hz).
Nucleophilic displacement of the 4-methoxy groups of the methylenebis(4-methoxy-pyrazolo[3,4d]pyrimidines) 3a-5a by 1N NaOH/MeOH (1:1, v/v) gave the corresponding methylenebis-(allopurinols) 3b-5b (Figure 1).Their structures were confirmed by UV-and 1 H-NMR spectroscopy as well as by elemental analyses.The different UV spectra of educts and products allow a simple quantitative spectrophotometric monitoring and the determination of pseudo first-order kinetic constants (half life τ and k) of the S N Ar reactions (Table 2).As can be seen from Table 2 the reaction rates of S N Ar reactions of the methylenebis(4-methoxy-pyrazolo [3,4-d]pyrimidines) 3a-5a are significantly faster than those of the N(1)-and N(2)-methylated compounds.In case of the N(2)-N(2)bridged derivative the half life is one order of magnitude higher than for the N(2)-methylated chromophore.
Table 2. Pseudo first order kinetic data of nucleophilic OCH 3 group displacement by hydroxyl of 4-methoxy-pyrazolo [3,4-d]pyrimidine derivatives.This leads to the assumption that a strong interaction exists between both chromophore moieties, probably caused by an orbital overlap of the two heterocyclic π-electron systems separated by the nonconjugating methylene group (homoconjugation).In this context it is interesting to see that for all three regioisomeric methylenebis(allopurinols) 3D-optimized molecular models [19] display structures with heterocyclic planes which are perpendicular to each other (Figures 2-4, Table 3); the heterocycles are not stacked one upon the other, as it has been shown for ethylenebis-or higher alkyl analogues [20,21].A mutual (-M) -effect of one chromophore on the other leads to an increase in electrophilicity at C(4), compared with the methylated heterocycles and, therefore, to a higher reaction rate for displacement reactions by hydroxyl.Even a weaker nucleophile such as ammonia (25 % aq.NH 3 ) is able to substitute the 4-methoxy group of compound 3a, yielding compound 3c.If the nucleophilic displacement reaction of the methoxy group by hydroxyl on compound 5a was extended for more than 1 h, an opening of the pyrimidine rings at C(6) was observed, and small amounts of compound 6 were formed [22].If the nucleophilic displacement reactions of 3a-5a are performed in a methanol-free medium (0.5 N NaOH), all reaction rates are enhanced (Table 3) which points to a weaker solvation of the reaction intermediates by methanol-water mixtures compared to pure water.It has been postulated that any bimolecular reaction of a small anion passing through a larger polarizable transition state will be accelerated considerably upon changing from protic to aprotic solvents [23].Based on this hypothesis, the rate of an ester saponification has been reported to be drastically effected by the solvent composition, e.g. when aqueous alcohol is substituted by aqueous dimethylsulfoxide [24][25][26][27][28][29].This prompted us to measure the influence of the DMSO concentration on the reaction rates of nucleophilic displacements on compounds 3a-5a in 1N NaOH/DMSO mixtures (Figure 5).This figure using 3a as an example how the reaction rate (rate constant k for pseudo first-order reactions) increases with the DMSO concentration.Analogous results were obtained for 4a and 5a (data not shown).Two reasons have been reported [23] for such hydrolysis rate enhancements in water -DMSO -sodium hydroxide mixtures: (i) with increasing DMSO concentrations the activity of the OH -ion is enhanced; at a mole fraction x DMSO of 0.35 the thermodynamic excess functions are maximal, and with increasing DMSO content the solvation of OH -is reduced.(ii) At lower DMSO concentrations the solvent lowers the energy of the reaction intermediate by specific solvation.This leads consequently to a rate enhancement.Because in our case the mole fraction of DMSO ranges between 0 and 0.3 (0 -60 vol-%), the second effect should be predominant.For all reactions measured under pseudo first-order conditions linear ln k versus x DMSO plots were obtained following the relation:

Reaction conditions
where m = sensitivity of the substrates to the dimethylsulfoxide content in the varying solvent compositions; ln k = natural logarithm of the pseudo first-order rate constant; ln k 0 = natural logarithm of the pseudo first-order rate constant in a DMSO-free medium and x DMSO = mole fraction of dimethylsulfoxide).The sensitivity values, m, are listed in Table 2.As can be seen, all S N Ar reactions performed show almost the same sensitivity toward the dimethylsulfoxide concentration in the solvent composition (7.9 -8.8).There is no significant difference between the N(1)-and N(2)-alkylated heterocycles.This becomes understandable under the assumption that in all cases similar reaction intermediates of the Meisenheimer complex type are formed.This is likely because the OCH 3 group is a rather weak leaving group.Both intermediates can be formulated by three mesomeric Lewis structures which are probably energetically equivalent as their negative charges are located on the nitrogen atoms (Scheme 1).Scheme 1. Mesomeric structures of the Meisenheimer complexes formed as intermediates during the nucleophilic displacement reactions of 3a and 5a with OH -and its solvation by dimethylsulfoxide.
This implies that for the nucleophilic displacement reactions of the methoxy groups of the N(1)-as well as for the N(2)-alkylated 4-methoxy-pyrazolo [3,4-d]pyrimidines the intermediates with a tetrahedral C(4) are energetically lowered by the same amount through solvation by dimethylsulfoxide (Scheme 1).

a)
Wavelength for spectrophotometric monitoring of the nucleophilic displacement reactions.b) m = sensitivity of the substrates to the dimethylsulfoxide content in the varying solvent compositions (1N NaOH/DMSO) according to equation 1 (correlation coefficients of linear regression, r 2 , better than 0.99.

Figure 5 .
Figure 5. Pseudo first-order rate constant k as well as ln k versus DMSO concentration for the reaction of 3a → 3b.