Nitrosoureas have been widely studied from a mechanistic point of view. The presence of the nitroso group labilizes the nitrogen-carbon bond, leading to two electrophiles, an isocyanate 5.33 and a diazene hydroxide 5.34, which has been detected in some cases by electrospray ionization mass spectroscopy.45 This intermediate in turn generates a diazonium salt 5.3546 (Fig. 5.25). Alkylation seems to be the main reaction responsible for antitumor activity, while carbamoylation takes place primarily on amino groups in proteins, leading to inhibition of several DNA repair mechanisms. N-Nitrosoamides and N-nitrosocarbamates, which can behave as alkylating (but not carbamoylating) agents have also antitumor activity, which supports the above statement.47

The above mechanism was based mainly on studies of the thermal decomposition of nitrosoureas under anhydrous conditions,46 but in water solution the reaction is much more complex and has been explained by the mechanism shown in Fig. 5.26. Addition of a molecule of water to the nitrosourea, in its tautomeric

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