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Three 2,4‐diamino‐l(), 11‐dihydro‐9H‐benzo[3,4]cyclohepta[ 1,2‐d]pyrimidines (6a‐6c) representing the first examples of a new ring system were synthesized from 2‐benzosuberones and cyanoguanidine. Similarly, 2,4‐diamino‐6,7‐dihydro‐5H‐benzo[3,4]cyelohepta[2,1‐d]pyrimidine (24) was prepared from 1‐benzosuberone. The ultraviolet spectral properties of these compounds were examined with reference to those of the analogs in which the central ring is five‐ and six‐mernbered.