Exploration of Ring‐Closing Enyne Metathesis for the Synthesis of ­Azonino[5,4‐b]indoles

The use of the ring-closing enyne metathesis (RCEYM) as a methodology for the synthesis of the azonino[5,4-b]indole system, featuring the tricyclic substructure of the alkaloids cleavamine and quebrachamine, has been explored. Three series of enyne substrates were studied for their compatibility with the RCEYM reaction. In addition to the usual substrates bearing either a terminal or an internal alkyne, for the first time enynes with an alkynyl halide moiety were also considered. Although the metathesis cyclization allowed for assembly of the azoninoindole nucleus in all three series, an effective catalytic cycle was only noted for internal alkyne substrates. On the basis of the experimental results, the “yne-then-ene” pathway seems to be the mechanism at play in these reactions.