PivOH-promoted annulations of indoles and propargylic alcohols: construction of cyclopenta[b]indole scaffolds.

Synthesis of cyclopenta[b]indole scaffolds directly from propargylic alcohols and indoles under mild and metal-free conditions is described. The reactions are promoted by pivalic acid (PivOH) and the method is compatible with various indoles having both electron donating and withdrawing substituent groups including N-2-pyridyl indole. After the annulation of indole with propargylic alcohol, substitution follows with another molecule of indole making the system non-aromatic with the generation of a quaternary centre. The feasibility of the method has been demonstrated for scale up studies.