Coumarins from benzofuran derivatives

Miscellaneous Reactions. Palladium dba has been employed as a catalyst for effecting various annulation reactions. Mediumsized nitrogen heterocycles have been prepared from allenes and amino alkenyl hahdes in the presence of a Pd(dba)2/PPh3 catalyst system. 1,3-Dienes can be converted to benzofuran derivatives upon reaction with o-iodoacetoxy arenes and this reaction has been applied in the synthesis of new coumarins. Dihydro-quinoxalines and quinoxalinones have been obtained via reductive annulation of nitro enamines (eq 26). ... 

Benzo[6 ]furan (2), usually called simply benzofuran, was first prepared by Perkin, who obtained it from coumarin as in Scheme 2 and (not understanding its constitution) called it coumarone, an unsuitable name that still survives in the term coumarone resin , for the compound is now valued for copolymerization purposes. Later the compound was isolated by distillation of bituminous coal, which is also a source of several methyl and hydroxy derivatives and of dibenzofuran (the name diphenylene oxide is now discarded) <75AHC(18)337>. 

Transition from a chromenoid skeleton to a benzofuran has also been achieved by other means. We may mention the photooxidation of rotenone and the preparation of 5,6-dirnethoxybenzofuran by electronic impact on 6,7-dimethoxycoumarin. - The Koelsch reaction is kindred it converts coumarine into 2,3-dihydrobenzofuran derivatives. ... 

Psoralens, also known as furocoumarins, are naturally occurring or synthetic tricyclic aromatic compounds and are derived from the condensation of a coumarin nucleus with a furan ring [1-10]. Several new furocoumarins have been isolated from natural sources [10, llj. In addition, the synthetic methods for production of these molecules have been described and reviewed [10]. The synthetic methods can be organized on the basis of the key step used for the formation of the two different oxygenated rings, hi this respect, there are three possibilities (i) formation of the furan ring onto the coumarin, (ii) formation of the pyrone ring onto the benzofuran and... 

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