Time-Dependent Shifts in the Preferred Position of Hydration

Time-Dependent Shifts in the Preferred Position of Hydration 

In 1963, it was discovered that the neutral species of pterid-2,6-dione (26), which initially adds a molecule of water to the 3,4-bond, becomes (in the course of about an hour) hydrated exclusively on the 7,8-bond. This behavior was explained as follows the 3,4-bond has only a small 

Acidification of a freshly prepared solution (in D20) of pteridine showed, momentarily, a UV spectrum similar to that of the neutral species.39 This spectrum was rapidly replaced by that of the cation of the 3,4-hydrate, which was observed to come slowly to equilibrium with a new species, originally thought to be the product of ring-opening, but now known to be the dihydrate (27). The course of events was more clearly revealed by a PMR study,8 as follows. 

As can be seen from Fig 2, the isolated H-4 signal at r 0.11 in the monohydrate cation has moved strikingly upfield to r 3.40, indicating that this proton has taken on a more aliphatic character because the 3=4 bond has become saturated. That this was truly the cation of 3,4-dihydro-4-hydroxypteridine was confirmed by measuring a set of C-methylpteridines. In this monohydrate, the complex multiplet signal presented by the H(2), H(6), and H(7) atoms had moved slightly upfield with no loss of coherence. When this solution of the monocation was 

Because of the similar energy of the two possible resonances that can ensure hydration of the pteridine cation (namely, 4-aminopyridinium stabilization of 7,8-hydration and amidinium stabilization of 3,4-hydration3 ), a study of the effects of substituents on this delicately balanced system was undertaken. An initial survey showed that the majority of pteridine cations were subject neither to dihydration nor to shifting hydration. Only four examples were found in which 5,6,7,8-dihydration was preceded by 3,4-hydration, namely 2-methyl-, 6-methyl-, 7-methyl-, and 2-amino-4-methylpteridine, whereas the related cations of 6,7-dimethyl-, 2-amino-, 2-amino-6-methyl-, and 2-amino-7-methylpteridine remained stably hydrated in the 3,4-position.8,40 Further examples, including a reversed direction of shift, came to light from a 

---

III. Time-Dependent Shifts in the Preferred Position of Hydration. . 131... 

---