Efficient Synthesis of Cyclohepta[b]indoles and Cyclohepta[b]indole‐Indoline Conjugates via RCM, Hydrogenation, and Acid‐Catalyzed Ring Expansion: A Biomimetic Approach

Abstract

Cyclohepta[b]indoles, prevalent in natural products and pharmaceuticals, are conventionally accessed via metal or Lewis acid‐mediated cycloadditions with prefunctionalized substrates. Our study introduces an innovative sequential catalytic assembly for synthesizing cyclohepta[b]indoles from readily available isatin derivatives. The process involves three catalytic sequences: ring‐closing metathesis, catalytic hydrogenation, and acid‐catalyzed ring expansion. The RCM of 2,2‐dialkene‐3‐oxindoles, formed by butenyl Grignard addition to 3‐allyl‐3‐hydroxy‐2‐oxindoles, yields versatile spirocyclohexene‐3‐oxindole derivatives. These derivatives undergo further transformations, including dibromination, dihydroxylation, epoxidation, Wacker oxidation at the double bond. Hydrogenation of spirocyclohexene‐3‐oxindole yields spirocyclohexane‐3‐oxindoles. Their subsequent acid‐catalyzed ring expansion/aromatization, dependent on the acid catalyst, results in either cyclohepta[b]indoles or cyclohepta[b]indole‐indoline conjugates, adding a unique synthetic dimension. The utility of this methodology is exemplified through the synthesis of an A‐FABP inhibitor, showcasing its potential in pharmaceutical applications.