Efficient Alkynylation of 2‐Oxindoles with Alkynyl Dibenzothiophenium Triflates: Total Synthesis of (±)‐Deoxyeseroline

We report here a highly efficient method for direct alkynylations of 3‐alkyl/aryl/indolyl 2‐oxindoles using a series of 5‐(alkynyl)dibenzothiophenium triflates ( 3 a – e ) under transition‐metal‐free conditions. This direct electrophilic alkynylation method gave access to a variety of 2‐oxindoles having quaternary center at the benzylic position in an operationally simple and inexpensive procedure. Further, chemoselective alkynylations could be realized in case of N−H free 2‐oxindoles and 3‐(indolyl)‐2‐oxindoles to afford a number of 3,3‐disubstituted‐2‐oxindoles. This methodology was then employed in the total synthesis of (±)‐deoxyeseroline ( 1 a ) as well as C(3a)‐indolyl furoindoline moiety ( 31 ).