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The rate of hydrogen–deuterium exchange of the “aromatic” protons of hypoxanthine and some of its derivatives has been measured at various pH levels and a mechanism is suggested for these reactions.

The chemical shifts of 6-amino-5-formamidopyrimidin-4(3H)-ones show that two rotamers are present in solution, their formyl signals being of unequal intensity. The two forms are stable both as neutral molecules and as cations, in contrast to the behaviour of N-alkylformamides. Exchange rates and barriers to rotation have been calculated. The reasons for the unequal distribution of the rotamers are discussed.

In purines, including uric acids, steric interference between 3- and 9-methyl substituents leads to a marked down-field shift of their n.m.r. signals, of 0.20–0.40 p.p.m. In 1,8-dimethylpteridine-2,4,7-triones, the displacement of the methyl bands is less marked (0.12–0.18 p.p.m.). Two models are discussed to explain these downfield shifts: either moving the methyl groups out-of-plane, or spreading them apart within the plane of the heterocyclic structure. Calculations show that only the latter model is in accordance with the measured values. The smaller change of the δ-values in pteridine-2,4,7-triones can be explained by the observation that in 1- or 8-monomethyl derivatives the 7,8-or 1,2-lactam group, respectively, is partially lactimised.

N-Methyl derivatives of 8-(methylthio)hypoxanthine(1) were synthesised by N-methylation of various 8-(methylthio)-compounds or by S-methylation of appropriate 8-mercaptopurin-6-ones. In 8-(methylthio)hypoxanthines, protonation takes place at the same positions as in the corresponding hypoxanthines. However, the neutral forms of compound (1), its 1- and 7-methyl derivatives and the 1,7-dimethyl homologue are methylated preferentially at N-3, in contrast with the alkylation of the corresponding hypoxanthines which takes place in the imidazole ring. This difference is ascribed to the steric effect of the 8-SMe substituent.The chemical shift of the S-methyl group is influenced little by protonation of the molecule, unless mesomerism can involve the thioether group. Participation of sulphonium-like resonance forms is, however, strongly hindered in the 7,9-dimethyl derivatives of (1). The latter compounds also undergo ready demethylation, hydrolysis, and thiolysis of the 8-methylthio-group.

Implantation of enkephalins A or B into the ventral thalamus or injection into the lateral ventricle of rats evoked only weak signs of stereotyped behavior, but did not cause gnawing.

Pyrazolo [3,4-d]pyrimidines resemble purines by avoiding disubstitution at the peri-positions 1 and 7, but they also resist 1,2-disubstitution in the pyrazole ring. INDO calculations show that 1,2-disubstituted derivatives of allopurinol are the least stable.The anion of allopurinol is present as a mixture of tautomeric forms and yields upon methylation 1,5-, 2,5-, and 2,7-dimethyl derivatives. Alkylation of 4-methylthiopyrazolo[3,4-d]pyrimidine, either as the neutral molecule or as the anion, offers a convenient approach to the synthesis of many mono- and di-methyl derivatives, which serve as precursors for methylated allopurinols.

Thermal degradation of quaternary hypoxanthinium salts was achieved by heating their solutions in dimethylformamide. 1,3-Dialkylhypoxanthinium bromides or iodides lose the 3-substituent as alkyl halide. The latter then attacks the imidazole ring at N-7 or N-9. Thermolysis of 1,3-dimethyl-6,8-dioxotetrahydropurinium iodide proceeds along either of two pathways: (i) loss of the 3-methyl group as methyl iodide, leading to 7,9-dihydro-1-methylpurine-6,8-(1H)-dione; or removal of hydrogen iodide to give the corresponding betaine, which is methylated at N-9 to give the 1,3,9-trimethyl-6,8-dioxotetrahydropurinium salt. The latter in turn decomposes to 7,9-dihydro-1,9-dimethylpurine-6,8(1H)-dione. Similarly 7,9-dimethylhypoxanthinium iodide (16) is degraded mainly, by loss of methyl iodide, to 9-methylhypoxanthine, accompanied by a small amount of the 7-methyl isomer. Compound (16) can also lose hydrogen iodide to give the corresponding betaine, which suffers methylation at N-1 to give the 1,7,9-trimethylhypoxanthinium salt. The latter again undergoes thermolysis to give a mixture of 1,7- and 1,9-dimethylhypoxanthine.

The predominant tautomeric forms, the position of protonation in aqueous solution, and the course of methylation in aprotic solvents have been determined for all possible mono- and bis-methylthiopurines and for 2,6,8-trismethyl-thiopurine. As a rule, protonation creates resonating cations in which the charge is distributed over both rings. The site of methylation varies. Like purine itself, the 8-methylthio- and the 2,8-bismethylthio-derivatives are attacked by methyl iodide at N-1. In 6-methylthio-, 6,8-bismethylthio- and 2,6,8-trismethylthiopurine, N-3 undergoes alkylation. In 2-methylthio- and in 2,6-bismethylthio-purine, methylation takes place at both positions 7 and 9. These results are explained in terms of the combined influence of electronic and steric factors.

Purine-6,8-diones are divided by their physical properties into three classes: those of class (a) carry a hydrogen atom or a methyl group at position 1, and those of class (b) bear a 3-methyl substituent. The members of class (c), bearing both 1- and 3-methyl groups, exist either as betaine or, after protonation, as the conjugate acids, with the positive charge confined to the pyrimidine ring. Anions are formed predominantly by proton loss from position 9. Consistent with this, methylation of monoanions proceeds at position 9, except in the case of members of class (b), where alkylation at N-9 is sterically hindered by the 3-methyl substituent. Methylation of dianions follows in general the sequence of proton attachment, unless steric factors prevent this.Protonation and alkylation of neutral molecules proceeds predominantly in the pyrimidine ring. However in the case of 9-methyl derivatives, methylation at N-3 is sterically hindered.

6-Methylthiopurin-8-one (1) is present in aqueous solution as the 7-NH,9-NH-tautomer. Monoanion formation involves predominantly the 9-NH group (ca. 80%), and in protonation positions 1 and 3 participate about equally. In compound (1) and its N-methyl derivatives, as in other series of purines, formation of ‘resonance-stabilised’ ions is preferred over that of ‘fixed’ ions. The course of methylation of the neutral molecules or of the anions by electrophilic methylating reagents parallels in general the direction of proton attachment. However, methylation of the neutral molecule of (1) takes place exclusively at N-3, owing to steric interference of the 6-methylthio-substituent with alkylation at N-1.

U.v. and n.m.r. spectra have been measured for 6-thioxanthine and all its N-methyl derivatives. In aqueous solutions of 6-thioxanthines with a free NH-group in the imidazole ring, the 7-NH-tautomer is predominant, while the monoanion of 6-thioxanthine and its 1-methyl derivative are mainly present as the 9-NH-tautomers. Whenever possible, anion formation occurs first by dissociation of the 3-NH-group; the order of acidities is 3 > 7 > 1. The anions of 6-thioxanthine and its 1-methyl derivative tautomerize to the 9-NH form.Steric interactions between substituents (H and Me) at positions 3 and 9 are evident in the pK values of the 9-methyl-6-thioxanthines and in the large displacement to lower field of the n.m.r. signals of the 3- and 9-methyl groups in 3,9-dimethyl derivatives.

6,7-Dimethyl-4-methylthiopteridine (IV) resembles 6-methylthiopurine (I) in its dipole moment, and both compounds undergo alkylation in the pyrimidine ring, para to the thioether group. However, in contrast to the purine (I), the pteridine (IV) is readily amenable to thiohydrolysis, presumably under the influence of the π-deficient pyrazine ring.

Methylation of 6-methylthio-8-phenylpurines with methyl iodide in an aprotic solvent always yields a single product. The mother substance and its 7- and 9-methyl derivatives are substituted at N-3; the 1-methyl analogue is attacked at N-9 and the 3-methyl isomer at N-7. The most important difference between the 8-unsubstituted and 8-phenyl series is thus found in the case of the 9-methyl derivative; the structures of the 3,9-dimethyl compounds (XIX) and (XX) are established by n.m.r. and mass spectroscopic measurements. Similarly, 9-methyl-8-phenylhypoxanthine is converted into the corresponding 3,9-dimethyl derivative, in contrast to the behaviour of the non-phenylated compound.

6-Methylthio-2-oxopurine (1) is present in aqueous solution mainly as the 3,7-di-NH tautomer. This supports the general assumption that purines avoid 3,9-disubstituted structures. Anion formation in structure (1) takes place first at the 7- and then at the 3-position. Cations bearing a 1-methyl group are unstable, undergoing hydrolysis, even at pH 4·5, to the corresponding xanthines. In the n.m.r. spectra of the 1-methyl derivatives of (1). steric interference causes a marked downfield shift of the 6-SMe signal.

Anion formation in 2-(methylthio)- and 2,8-bis(methylthio)-hypoxanthine occurs first at the 1-NH group and then at 7(9)-NH. Protonation involves the imidazole ring, with the exception of the 3-methyl derivatives, which attach the proton to N-1.3-Methylhypoxanthine is methylated to form the 1,3-dimethylhypoxanthinium cation, but in the 3-methyl derivatives of 2-(methylthio)- and 2,8-bis(methylthio)-hypoxanthine attack at N-7 is predominant over that at N-1. Formation of the cations [RNC(SMe)NMe]+ is followed by S-demethylation.

The n.m.r. signal of the 8-proton in xanthines is shifted downfield more strongly by introduction of a 2- than of a 6-thioxo-group. The signals of N-methyl groups are also shifted to lower field, but the effect depends strictly on the distance between the methyl and the thioxo-group. Thus in 2-thioxanthines, the displacement decreases in the following order: 1-methyl = 3-methyl > 7-methyl, and in 6-thioxanthines the sequence is 1-methyl > 7-methyl > 3-methyl > 9-methyl.

A PROMINENT feature of all forms of nystagmus is the strict co-ordination of movement of both eyes, brought about by fibre systems which cross from one side to the other. We have observed a curious phenomenon in which both eyes move synchronously but in opposite directions.

All 6-methylthiopurin-8-ones (except those bearing a 1-methyl substituent, which decompose) are converted by chlorine in methanol at 0° into the corresponding sulphones. In a series of N-methyl 6-methylthiopurin-8-ones, only the 1-methyl, the 1,9- and 3,9-dimethyl, and the 3,7,9-trimethyl derivatives undergo thiohydrolysis. In contrast, all the corresponding sulphones are attacked by hydrogen sulphide anion. A 6-methylsulphonyl substituent weakens basicity and increases the acid strength of purines. In 6-methylsulphonylpurin-8-one, anion formation follows the order N-9 → N-7. A 3-methyl substituent, by virtue of reduction of the ring current in the pyrimidine system, causes a diamagnetic shift of the 2-H signal.

U.v. and n.m.r. spectra show that in aqueous solutions purin-8-one is present predominantly as the 7H,9H-tautomer. Anion formation takes place preferentially at N-9. Protonation of purin-8-one and its 7- and 9-methyl derivatives occurs at N-1 and causes splitting of the 6-H and 2-H n.m.r. signals. In purin-8-one and its N-methyl derivatives, the 2-H is more deshielded than the 6-H; the reverse is the case for purine itself.

The n.m.r. chemical shifts of the 8-protons in xanthines with an unsubstituted imidazole ring (class a) and in 7-methylxanthines (class b) are very similar; however the 8-H signals for 9-methyl derivatives (class c) occur at higher field. Thus, in aqueous solution, members of class a must be essentially present as the N(7) H tautomers.Methyl substituents in various positions may be distinguished by their n.m.r. signals; their δ values can be arranged in the following order: 7- > 9- > 3- > 1-methyl. The signals of individual N-methyl groups are influenced only slightly by introduction of additional alkyl substituents, with the exception of 3,9-dimethylxanthines, in the spectra of which both methyl signals are shifted to lower field because of steric interference.U.v. spectra establish the order of anion formation in 7- and 9-methylxanthine as 3 → 1 and in 3-methylxanthine as 7 → 1. However, the displacement of the 8-H signals in the n.m.r. spectra of xanthine and its 1-methyl derivative suggests that their monoanions are tautomeric mixtures in which both the N (3) H and N (7) H groups participate in ionisation.Cation formation is accompanied by a much larger displacement of the 8-H n.m.r. signal in class c than in class a or b.