Synthesis and reductive reactions of 2,3-dioxo-2,3-dihydrobenzo[b]furo[2,3-f]-, -[2,3-g]-, and -[3,2-e]indoles

Abstract

The Sandmeyer reaction was used to prepare 2,3-dioxo-2,3-dihydrobenzo [b]furoindoles which could be reduced with diborane or with complex hydrides to give the corresponding unsubstituted as well as the 3-hydroxybenzo[b]furoindoles.We have previously shown [1] that 3-aminodibenzo [b,d]furan undergoes a Sandmeyer reaction to give isomeric dioxodihydro-1H-benzo[b]furoindoles. These can be used in the preparation of unsubstituted indole containing tetracyclic systems and their hydroxy derivatives with varied linking to the pyrrole ring relative to the starting tricyclic system.Using the same scheme in this work we have prepared three other isomeric dioxodihydrobenzo [b]furoindoles and studied the mechanism of their formation.2 (7) were prepared by us under Sandmeyer reaction conditions [2, 3].The 1-and 2-aminodibenzofurans (1, 2) were used in the synthesis of the heterocyclic systems indicated. The starting amines were converted to the isonitrosoacetamidodibenzofurans 3, 4, cyclization of which in the presence of sulfuric acid allowed us to prepare the corresponding dioxodihydro-1H-benzo[b]furoindoles 5-7 in 16-88% yields.The cyclizations of the 2-isonitrosoacetamidobenzofuran led to a mixture of the two isomeric linear (6) and angular (7) structures.Separation of the isomers was brought about by the successive treatment of an alkaline solution of the mixture of compounds 6 and 7, initially with acetic acid to pH 3 and, after filtration of the crystals formed, by treatment of the filtrate with conc. HCl to pH 1. The first procedure gave compound 6 (66%) and the second gave compound 7 (16%).As was expected, compound 5 was formed as a single isomer. Through a study of the properties of the indicated heterocyclic systems it was found that compounds 5-7 undergo reduction to the corresponding benzo[b]furoindoles 8-10 and the yield of the latter depends both on the nature of the reducing agent and on the reaction conditions.