Azirine-2-carbonyl Azides as C3-Synthons in the Reaction with Amidines toward 5-Aminopyrimidin-4(3 <i>H</i> )-ones

The azide group in azirine-2-carbonyl azides provides a unique electrophilic balance of the reaction sites, allowing these compounds to be used as all-carbon C3 synthons in the synthesis of N ′, N ′-unsubstituted 5-aminopyrimidin-4-ones via a (3 + 3) cyclocondensation reaction with amidines. Strongly basic conditions alter the reactivity of the key intermediate 2,4,7-triazabicyclo[4.1.0]hept-2-en-5-one, resulting in a formal (4 + 3) cyclocondensation reaction to afford 3,7-dihydro-6 H -1,3,5-triazepin-6-ones as the sole products.