The diacid PA2 ( ) is the only Tz reported so far that allows efficient release (>90%) of primary amines. The two carboxylic acid functionalities direct post-click tautomerization of A and shift the equilibrium to B1/B2, as shown for the reaction of 6 and 8 (PBS, 37 °C) in Figure . Serial HPLC measurements confirmed this effect as the click product A was not detectable. Enhanced tautomerization leads to formation of instantaneously releasing B1 and slowly oxidizing B2, which can still tautomerize to B1 (via A), thereby enabling efficient release. In agreement with these mechanistic considerations, we observed 92% release of the uncaged phenol P and 8% of the respective oxidized dead-end (Fig. ). However, due to competition between the two directing CO2H groups, the overall release is significantly slower (approx. 20 hours to reach the endpoint) than observed for the reaction of 6 and DMT (7) (cf. Fig. ).