An Efficient Straightforward Synthesis of Benz[ a ]azulene

Benz[ a ]azulene ( 1 ) is synthesized in five steps ( cf. Scheme 2 ) starting from commercially available 2‐iodobenzyl alcohol ( 4 ) and tropylium tetrafluoroborate in an overall yield of 44%. The key step ( cf. also Scheme 1 ) is the intramolecular Heck reaction of the 8‐phenylsulfonyl‐substituted heptafulvene 7 , which leads in nearly quantitative yield directly to 10‐(phenylsulfonyl)benz[ a ]azulene ( 8 ). The desulfonylation of 8 can be accomplished by Julia 's method with Na 2 S 2 O 4 /NaHCO 3 in DMF/H 2 O at 85–90°, thus leading to pure 1 in 78% yield. The phenylation of 8 with PhLi or PhCul at −78° in THF occurs regioselectively at C(9). Dehydrogenation of the formed dihydroazulenes with o ‐chloroanil in toluene at room temperature gives 9‐phenyl‐10‐(phenylsulfonyl)‐benz[ a ]azulene ( 9 ) in 70% yield ( cf. Scheme 3 ), which, again, can be desulfonylated with Na 2 S 2 O 4 /NaHCO 3 in DMF/H 2 O in good yields. The addition of PhLi to 1 in THF occurs at temperatures ⩾ −25°. Ionic dehydrogenation (1. Ph 3 C + BF   4 − /MeCN; 2. Et 3 N) of the dihydro forms leads to 3 , as the main product, and its positional isomers.