The Claisen rearrangement of 1,4-bis(m-methoxyphenoxy-)2-butyne

Abstract

The Claisen rearrangement of 1,4-bis(m-methoxyphenoxy-)2-butyne (1b) gives four isomeric dimethoxy-6H-benzofuro[3,2-c]-6a,11a-dihydro-11a-methylbenzopyrans (3), three dimethoxy-4b,9b-dihydro-4b,9b-dimethylbenzofuro[3,2-b]benzofurans (4), and, in the presence of acid, three dimethoxy-5a,10b-dihydro-5a,10b-dimethylbenzofuro[2,3-b]benzofurans (5), which have been separated and identified by proton and 13C nmr spectroscopy. Based on the isolation of two chromene intermediates and their conversion to 3 and 4, as well as the isomerization of 3 to 4 and 4 to 5, a "double Claisen" mechanism is proposed. The reaction is not regiospecific but the two [3,3] sigmatropic shifts occur preferentially to the para positions of the methoxy substituents.