Enabling C7 Methylation and Arylation of the Indolyl Core via P(III)‐Directed C−H Functionalization

Abstract

AbstractPeripheral editing of complex molecules via C−H functionalization removes the pervasive retrosynthetic bias toward pre‐functionalization and taps into the innate, often subtle, stereoelectronic differences between C−H linkages. Using this approach, bioactive molecules and residues can be modified without being beholden to lengthy sequences, thus allowing biochemical hypotheses to be interrogated on accelerated timelines. Herein, the C−H functionalization paradigm is leveraged to tackle C7 C−H functionalization of the biologically important tryptophan core. The devised process, which was expanded to indolyl (and related) systems, utilizes an N‐bound unsymmetrical “designer” phosphine to direct C−H functionalization to the desired position while maintaining operational ease of directing group installation/removal. Quantum mechanically calculated steric properties and activation free energies suggest that this phosphine is hindered enough for favoring C−H functionalization over deactivation pathways but is still easily cleaved by nucleophiles. In addition, the process enables the direct C−H methylation, cyclopropanation, and arylation of tryptophan to yield unnatural amino acid (UAA) building blocks. As a testament to the versatility of this method, a solid phase peptide synthesis (SPPS)‐ready phosphinated tryptophan was incorporated into a pentapeptide and the C−H functionalization/dephosphination sequence was executed with ease. The utility of “on‐peptide” editing was exhibited through the late‐stage functionalization of several pentapeptides with a diverse set of C7 substituents on‐ and off‐resin.