A Diversity‐Oriented Approach to Spiroindolines: Post‐Ugi Gold‐Catalyzed Diastereoselective Domino Cyclization

Abstract

Gold-catalyzed carbocyclization and heteroannulation strategies have recently attracted much attention owing to the selective and efficient activation of the C C bond towards a wide range of nucleophiles that these methods provide. Domino approaches involving gold-catalysis lead to complex heterocyclic compounds under exceedingly mild reaction conditions. Although gold-catalyzed approaches are rising to prominence, they suffer in terms of diversity and procedural length. Multistep sequences are usually required for assembling the starting material for cyclization. We have recently reported a concise route to indoloazocines by a sequential Ugi/gold-catalyzed intramolecular hydroarylation approach. Inspired by these findings and as a result of our continued synthetic interest in the indole core, multicomponent reactions and transition metal-catalysis, we have developed a post-Ugi gold-catalyzed domino cyclization method to generate spiroindolines. The Ugi four-component reaction (4-CR) of indole-3carboxaldehyde (1a) with p-methoxybenzyl amine (2a), 2-butynoic acid (3a) and tert-butyl isonitrile (4a) in methanol at 50 8C gave Ugi-adduct 5a in 71% yield. When this was treated with 5 mol% of Au[PPh3]OTf (OTf= trifluoromethanesulfonate) in CDCl3 at RT, the expected outcome of the reaction was indoloazepinone 6a’ through an endo-dig cyclization followed by rearrangement (Scheme 1). Surprisingly, an exo-dig cyclization followed by intramolecular trapping of the spiro intermediate occurred instead, resulting in the diastereoselective formation of tetracyclic spiroindoline 6a in 61% yield (Scheme 1). This observation was remarkable, as the attack on the a-position of an alkyne conjugated with an amide is rare, and trapping of the spiro intermediate by a sterically hindered tert-butyl amide is rather unexpected, as was the diastereoselectivity observed. Spiroindolines are prominent molecular motifs that are frequently encountered among the large family of alkaloids; for example, it is present in communesines and perophoramidines (Figure 1), which display distinct pharmacological properties. These fused polycyclic systems, which feature quaternary stereocenters, present a nontrivial challenge for organic chemists to develop synthetic approaches.