Annelated furans. XVII. The Wittig reaction of benzofuranones

Abstract

Benzofuran-3(2H)-ones have been shown to undergo efficient Wittig reactions with resonance-stabilized phosphoranes at elevated temperatures, thus providing a convenient synthesis of benzo-furans functionally substituted at the 3-position. The initial reaction products, the 3-methylene-2,3-dihydrobenzofurans, were found to undergo facile thermal, acid- or base-catalysed rearrangement to the corresponding benzofurans. The reaction of benzofuran-3(2H)-ones with acetylmethylene-triphenylphosphorane were complicated by competing aldol condensations in which 2-isopropylidene-benzofuran-3(2H)-ones were formed.     Benzofuran-2(3H)-one displayed parallel reactivity with these phosphoranes and afforded the corresponding 2-substituted benzofurans. In these cases rearrangement of the initial Wittig products to the corresponding benzofurans was more facile and even when lower temperatures and shorter reaction times were employed only the rearranged products were observed.    The 3?-benzofurylacetonitriles, formed by the reaction of benzofuran-3(2H)-ones with cyano-methylenetriphenylphosphorane, were reduced by lithium aluminium hydride to give the physiologically active 2-(3?-benzofuryl)ethylamines. The formylation of methyl 6?-methoxy-3?- benzofurylacetate was also studied.