A Regioselective Lithiation of ortho-Cresols Using the Bis(dimethylamino)phosphoryl Group as a Directing Group : General Synthesis of 2, 3-Dihydrobenzo[b]furans Including Naturally Occurring Neolignans

Abstract

A general synthetic method was developed for 2-aryl-2, 3-dihydrobenzo[b]furans via regioselective lithiation of ortho-tolyl tetramethylphosphorodiamidates followed by addition of aromatic aldehydes as a key step. ortho-Tolyl tetramethylphosphorodiamidates were regioselectively lithiated with sec-BuLi in tetrahydrofuran at -105°C to give benzylic lithio species. The resulting lithio species were reacted with aromatic aldehydes to provide 1, 2-diarylethanolderivatives. Reductive removal of the phosphoryl group with lithium aluminum hydride followed by acidic treatment led to 2-aryl-2, 3-dihydrobenzo[b]furans in good overall yields. The utility of this strategy was demonstrated in regioselective syntheses of highly substituted neolignan natural products, such as (±)-licarin B and (±)-carnatol, starting from O-bis(dimethylamino)phosphorylated eugenol or isoeugenol.