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 the expected adducts 11. While attempts to remove the N-Boc function were unsuccessful, desilylation and hydrogenolysis delivered the hydroxybenzyl derivative 14 which could be efficiently deprotected to give the amine 15. This then underwent smooth decomposition to the benzyne 16, upon exposure to N-iodosuccinimide, and intramolecular trapping by the phenol group, with incorporation of iodine, to give the iodoxanthene 17. A more efficient protocol featured condensation of dianion 1with 2-(benzyloxy)aryl aldehydes; hydrogenolysis of the initial products 19 and 22a served both to deprotect the phenol function and to effect hydrogenolysis of the benzylic alcohol group. A final acidic deprotection and exposure to N-iodosuccinimide delivered good yields of the iodoxanthenes 21 and 23, demonstrating for the first time a viable method for the intramolecular trapping of benzynes by phenolic groups.