Syntheses of 7‐, 8‐ and 9‐Bromobenzotropolone, Cross‐Couplings of Their Methyl Ethers, and Deprotection to 7‐, 8‐ or 9‐Substituted Benzotropolones.

Unprotected 6,7‐benzotropolone and bromine were known to undergo an electrophilic substitution delivering the 7‐bromo derivative; it was carried on as its methyl ether. O‐Acetyl‐6,7‐benzotropolone added bromine at the C8=C9 bond whereupon an amine‐induced E1cb elimination led to the 8‐brominated benzotropolone acetate; ester cleavage with aqueous acetic acid and etherification with diazomethane delivered the corresponding methyl ether. O‐Methylbenzotropolone was brominated with N‐bromosuccinimide giving the 9‐brominated methyl ether. The mentioned bromobenzotropolone methyl ethers underwent Pd‐catalyzed C−C and C−B couplings. They comprised Suzuki‐Miyaura couplings (with R−BF3K, R=Me, Bu, vinyl, Ph), Negishi couplings (with BuZnCl), Sonogashira couplings (with HC≡C−R, R=SiMe3, CH2OH, Ph, Bu), Mizoroki‐Heck reactions (with methyl acrylate, methyl vinyl ketone, styrene, 3‐buten‐2‐ol), a carbonylative coupling, and Suzuki‐Miyaura borylations (with B2Pin2). Subsequent O‐demethylations with BCl3 succeeded in most cases; they furnished a library of 34 O‐unprotected Br‐free benzotropolones, 31 of which were unknown before. [ABSTRACT FROM AUTHOR]