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 Na2S2O4/NaHCO3 in DMF/H2O at 85-90 degrees, thus leading to pure 1 in 78% yield. The phenylation of 8 with PhLi or PhCuI at -78 degrees 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 Na2S2O4/NaHCO3 in DMF/H2O in good yields. The addition of PhLi to 1 in THF occurs at temperatures greater than or equal to 25 degrees. 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.