Reaction of guaiazulene with 3-(N,N-dimethylamino)propenal in the presence of phosphoryl chlo-ride, followed by addition of sodium perchlorate, gave [3-(3-guaiazulenyl)propenylidene]dimethylam-monium perchlorate (91%). This salt, when heated with a mixture of sodium methylate and quinoline in methanol, yielded 9-isopropyl-1-methylcyclopenta[ef]heptalene(87%). In the same manner, the corresponding dimethyl salt (97%) was obtained from 4,6,8-trimethylazulene, and was converted into 3,5-dimethyleyclopenta[ef]heptalene(86%). The yield of the cyclization step was fairly improved compared with that (57%) of Hafner's method, in which N-methylanilinium salt and sodium methylate were heated at the boiling point of chlorobenzene. Nitration and trifluoroacetylation of 9-isopropyl-1-methylcyclopenta[ef]heptalene afforded 4-substituted products (20 and 23%), while its formylation led to 2-(13%), 4-(28%), 6-(14%), and 7-for-myl derivatives (2%). Moreover, dimethylaminomethylation of 3,5-dimethylcyclopenta[ef]heptalene occurred at 1-position (27%), but its nitration resulted in 7,9-dinitro compound (8%). Thus the sites of electrophilic substitution were dependent on the reagents, different from the case of azulene compounds. © 1996, The Chemical Society of Japan. All rights reserved.