Synthesis of espicufolin based on 6-endo ring closure of o-alkynoylnaphthols

Abstract

Regioselective halogenation of 3-acetoxymethyl-7-chloro-5,8-dimethoxy-1-naphthol 5 was achieved by DBH or I₂/N-methylmorpholine to give the corresponding 2-halogeno-1-naphthols 8 and 11, which were converted to 1-methoxymethoxy-3-(alk-2-ynoyloxymethyl)-2-halogenonaphthalenes 17 and 18 in good yields. An intramolecular acyl-transfer reaction of the 2-halogenonaphthalenes triggered by halogen–lithium exchange with BuLi at −78 °C gave 1-methoxymethoxy-2-alkynoyl-3-(hydroxymethyl)naphthalenes 21 in high yields. After protection of the hydroxymethyl group as a benzoate, formation of a γ-pyrone ring was easily achieved by deprotection of the methoxymethyl group followed by spontaneous 6-endo ring closure under mildly acidic conditions. The pyrone derivative having a 1-methylpropyl group was successfully converted to espicufolin 1.