The first efficient method for the intramolecular trapping of benzynes by phenols: a new approach to xanthenes

Condensations between the dianion 1 derived from 1-(N-butoxycarbonylamino)- 1H-benzotriazole and silyloxysalicylaldehydes 10 give excellent yields of t he expected adducts 11. While attempts to remove the N-Boc function were un successful, desilylation and hydrogenolysis delivered the hydroxybenzyl der ivative 14 which could be efficiently deprotected to give the amine 15. Thi s then underwent smooth decomposition to the benzyne 16, upon exposure to N -iodosuccinimide, and intramolecular trapping by the phenol group, with inc orporation of iodine, to give the iodoxanthene 17. A more efficient protoco l featured condensation of dianion 1with 2-(benzyloxy)aryl aldehydes; hydro genolysis of the initial products 19 and 22a served both to deprotect the p henol function and to effect hydrogenolysis of the benzylic alcohol group. A final acidic deprotection and exposure to N-iodosuccinimide delivered goo d yields of the iodoxanthenes 21 and 23, demonstrating for the first time a viable method for the intramolecular trapping of benzynes by phenolic grou ps.