Further synthetic attempts towards calicene

First synthetic attempts towards the so-far-unknown calicene (= 5-(cyclopro p-2-en-1-ylidene)cyclopenta-1,3-diene) precursors 3-(cyclopenta-2,4-dien-1- ylidene)tricyclo[3.2.2.2.(2,4)]nona-6,8-diene (4; Scheme I), 1,4di(cyclopen ta-2,4-dien-1-ylidene)cyclohexa-2,5-diene (5; Scheme 2), and (2-bromocyclop rop-1-en-1-yl)cyclopentadiene (6; X = Pr; Scheme 5) are reported, which wou ld represent very attractive compounds for gas-phase pyrolysis (4), matrix photolysis (5), and low-temperature HBr eliminations in solution (5).