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).