New C_2-Fragment for Ruthenium-Catalyzed Synthesis of Indoles

摘要

It is well known that indole moiety plays an important role as an intermediate for the synthesis of many pharmacologically and biologically active compounds. Thus, besides conventional popular routes, homogeneous transition metal-catalyzed synthetic methods have also been developed for the construction of indole framework because of the wide availability of substrates. In connection with this report, as the part of our ongoing studies on homogeneous ruthenium catalysis, we recently developed on the synthesis of indoles via a ruthenium-catalyzed alkanol group transfer from alkanolamines to N-atom of anilines (amine exchange reaction) (Scheme 1, route a) and ring-opening of epoxides by anilines (Scheme 1, route b). Watanabe et al. have also reported a ruthenium-catalyzed intermolecular cyclization of anilines with ethylene glycols as C_2-fragment leading to indoles (Scheme 1, route c). In these regards, it was suggested that theses reactions proceed via a sequence involving an initial formation of 2-anilinoalkanols shown in Scheme 1 and N-alkylation of anilines by 2-anilinoalkanols to form 1,2-dianilinoalkanes. These circumstances led us to seek a new C_2-fragment for such an intrinsic formation of 1,2-dianilinoalkanes. Herein we report on a ruthenium-catalyzed synthesis of indoles from anilines and 1,2-dibromo-ethane as C_2-fragment.